Joshimath Landslide, Human Evolution, Early Animals

Sharing some of my readings over the past couple of weeks- 

1) Movement of Joshimath Landslide in India: The town of Joshimath in Gharwal Himalaya is built on an ancient landslide. Geological reports going as far back as the 1970’s had warned that excessive modification of the slope due to urbanization may result in slope movement and eventual failure. These warnings proved correct as slope movement since 2018 has caused major damage to houses and livelihoods. Landslide expert Dave Petley reports on a new study of the region that uses radar technology to track earth movements. 

It concludes that removal of vegetation, mismanaged groundwater seepage and blocked drainage paths contributed to accelerated movements of the Joshimath landslide system. There is unplanned and unregulated construction happening in other Himalayan towns. Authorities must take geological advice seriously and plan their growth accordingly.

2) The Olduvai Effect- New questions about meat eating in human origins: How about some food for thought?… I mean literally. Was meat eating connected to the evolution of larger brains in our human ancestors beginning around 2 million years ago? Paleoanthropologist John Hawks writes about recent work on East African sites across the period from 2.6 to 1.2 million years ago that throws doubt on the “meat made us human” hypothesis. 

The study he discusses finds no evidence for a systematic increase in meat eating across the studied period. The evidence also points to different hominin groups having flexible strategies for obtaining meat. But can we tie increased meat eating to one particular branch of the hominin tree? This is a very interesting article on how anthropologists retrieve and analyze evidence from sites and how the geography and time depth of sampling influence the conclusions that are drawn. 

3) Complex animals living millions of years before the Cambrian Explosion revealed by seabed tracks: What do we know about early animal life before the evolution of shells made their preservation more likely starting around 530 million years ago? That animals were present much before they acquired shells is inferred from molecular data that places their origin and diversification a good 50-100 million years before the Cambrian. But there is another way to understand animal evolution before body fossils appeared. It is through studying their movement on the sea bed. Tracks and burrows made by mobile animals start appearing in the rock record by 550 million years ago. 

James Ashworth describes some recent fascinating work that has decoded the morphology of these fossil trails and compared their shape with those made by some common modern sea floor animals. The researchers then propose that the changing shapes of trails across a 10 million year period is indicative of increasing complexity in animal locomotion. Early trails were made by simpler animals with short round bodies and limited sensory capabilities. Later in time, sinuous tracks made by worm like animals characterized by a slender anterior-posterior body profiles appear. A very clever way of understanding morphological changes during early animal evolution.

Early Animals, Hominin Diets, Groundwater Governance

 A few links to interesting listening and reading.

1) Tracking the first animals on earth:  Unequivocal evidence of animals is preserved in soft sediments from about 570 million years ago. The fossil record of the Ediacaran to early Cambrian times (570 to 500 million years ago) has yielded rich information about the patterns of animal evolution. Apart from fossils, comparative genetic studies have given insights into how different animal groups are related to each other and the order of branching of these groups. Amazingly, organic molecules recovered from enigmatic fossilized taxa have been used to differentiate between animal and non-animal remains. Zoologist Matthew Cobb explains all this and much more about early animal evolution in about 30 minutes. Give it a listen! 

2) Plant-eating and meat-eating in Australopithecus: What did our ancient relatives eat? By ancient, I mean going back a million years or more. We can use isotopes of nitrogen to tease out information about diets. Carnivores have more nitrogen-15 enriched tissue than plant eaters. Carbon isotopes (C13 and C12) also yield information about the diet of herbivores. Grazers munching on grass take in more of the heavier isotope of carbon than browsers eating leaves and stems. Paleoanthropologist John Hawks discusses some recent work on nitrogen and carbon isotopes of Australopithecines and how the patterns of isotopic variation extracted from tooth enamel can be interpreted in terms of diets and life history. Fascinating stuff. 

3) Addressing Depletion in Alluvial Aquifers: Why Context Matters in Participatory Groundwater Management: India relies a lot on groundwater for agriculture. There are signs from many parts of the country of acute groundwater distress. Participatory Groundwater Management initiatives have had some success in addressing this distress. Pratik Kumar and Veena Srinivasan point out that these cooperative movements have been more successful in hard rock aquifers from different parts of the country than alluvial aquifers of northwest India. Geology matters. Aquifer properties matter. Hard rock aquifers are more sensitive to abstraction and are rapidly de-watered and recharged seasonally. Alluvial aquifers are spread over vast areas and water levels are less sensitive to abstraction. The amount you can extract doesn't vary with lowering of water level. 

People depending on hard rock aquifers experience the limitation of the resource yearly and are more willing to join cooperative initiatives to manage the resource.

I have just given a gist of the more elaborate arguments in the paper. The graphic below very neatly compares hard rock and alluvial aquifers. 

 Source: Pratik Kumar and Veena Srinivasan 2025

The paper is open access.

Books: Animal Minds, India Language History, India Governance

 New on my book shelf:

1) This came highly recommended from science Twitter. Peter Godfrey-Smith has surveyed a wide section of the animal kingdom and writes about the evolution of sensory experiences in different species. Sponges, corals, worms and octopus all manipulate the environments in specific ways. Disparate evolutionary pathways to be sure, but they all inform us about the origins of our mental capacities. 

 

 

 

2) I had a brief introduction to this book over a chickoo milkshake when the author M. Rajshekhar had visited Pune couple of years ago. He has spent several years traveling across India, surveying both big cities and the rural regions. His Ear to the Ground project resulted in scores of articles on India's everyday economy and the general failure of governance in this country. Its good to see some of his work distilled into this book.

 

 

 

3) Live History India has a really good interview with Peggy Mohan about her new book on India's language history. This is always a fascinating topic, as it tells us so much about population history, their origins, migrations, and intermingling. There is a section on Marathi too, and I'm looking forward to learning about that.

Fossil Dickinsonia in Bhimbetka Sandstones: Nature India Article

 My short piece published in Nature India on the surprising report of Ediacaran age fossil Dickinsonia in the Bhimbetka caves near Bhopal, Central India, and its geological and biological significance.

an excerpt:

"The biological affinity of Dickinsonia is controversial. Most scientists tend to accept it as an early animal. Some like Gregory Retallack, co-discoverer of this fossil, think of it to be a large algae or lichen. He argues that the mainstream view that Dickinsonia was a marine animal is based on weak evidence, while his own detailed work shows that Dickinsonia was a land creature, forming biogenic crusts on soils. Interestingly, the Bhimbetka rocks were deposited in a mostly terrestrial setting, more in alignment with Retallack's interpretation. Importantly for geology, its restricted time span, being found only in rocks between 555-550 million years old, makes it a diagnostic age indicator. So far, no animal fossils have been found in the Vindhyan rocks. This finding may inspire geologists to start searching contemporaneous Indian basins afresh for such subtle clues".

Fossil from dawn of animal life found in India’s famous caves.

Sea Water Chemistry Triggers For Evolution Of Biomineralization

Geological Processes and Evolution #20

The bulk of the shells and skeletons of marine creatures are built out of aragonite or high-Mg calcite (> 4 mole% MgCO3) or low-Mg calcite. These three calcium carbonate minerals, along with dolomite (calcium magnesium carbonate), also occur as marine cements, i.e., they are precipitated from sea water as mineral grains in the open spaces between shell particles, resulting in loose sediment getting bound in to hard rock.

I came across this paper by Rachel Wood and colleagues from 2017 on the link between sea water chemistry and the evolution of biomineralization as evidenced in the limestone strata from Siberia. The time period is from 545 million years ago to 500 million years ago, a span in which early animals began secreting calcium carbonate skeletons. What were the main triggers for this evolutionary change?

Abstract:

The trigger for biomineralization of metazoans in the terminal Ediacaran, ca. 550 Ma, has been suggested to be the rise of oxygenation or an increase in seawater Ca concentration, but geochemical and fossil data have not been fully integrated to demonstrate cause and effect. Here we combine the record of macrofossils with early marine carbonate cement distribution within a relative depth framework for terminal Ediacaran to Cambrian successions on the eastern Siberian Platform, Russia, to interrogate the evolution of seawater chemistry and biotic response. Prior to ca. 545 Ma, the presence of early marine ferroan dolomite cement suggests dominantly ferruginous anoxic “aragonite-dolomite seas”, with a very shallow oxic chemocline that supported mainly soft-bodied macrobiota. After ca. 545 Ma, marine cements changed to aragonite and/or high-Mg calcite, and this coincides with the appearance of widespread aragonite and high-Mg calcite skeletal metazoans, suggesting a profound change in seawater chemistry to “aragonite seas” with a deeper chemocline. By early Cambrian Stage 3, the first marine low-Mg calcite cements appear, coincident with the first low-Mg calcite metazoan skeletons, suggesting a further shift to “calcite seas”. We suggest that this evolution of seawater chemistry was caused by enhanced continental denudation that increased the input of Ca into oceans so progressively lowering Mg/Ca, which, combined with more widespread oxic conditions, facilitated the rise of skeletal animals and in turn influenced the evolution of skeletal mineralogy.

Dolomite abundance through geologic time shows a positive correlation with periods of ocean anoxia. One reason could be that sulphate reducing bacteria which thrive in anoxic environments remove dissolved sulphate which interferes with dolomite formation. A 'shallow oxic chemocline' means that only the shallows were oxygen rich, while deeper water were oxygen poor or anoxic. These conditions changed after about 545 million years ago with increasing oxygen in even deeper waters thus increasing habitat suitable for the evolution and spread of oxygen demanding animals. Sponges may have played an important role in the ventilation of the water column by actively removing suspended organic matter during filter feeding, thus making more oxygen available to be transferred to deeper waters.

The terms "aragonite-dolomite seas", "aragonite seas" and "calcite seas" refer to geologic time-bound conditions facilitating the precipitation of marine cements of that mineralogy. Excessive magnesium is a hindrance to formation of calcite and a lowering of Mg/Ca meant a shift from "aragonite seas" to "calcite seas". From Cambrian to recent times, periodic swings in Mg/Ca of sea water has caused either aragonite or calcite to become the dominant marine precipitate.  

It is notable that the mineralogy of skeletons when they first evolve in a particular animal group seems to be determined by the prevailing sea water chemistry. Animal groups like the molluscs which acquired the ability to biomineralize during 'aragonite-high Mg calcite seas' of the late Ediacaran -Early Cambrian (550-520 million years ago) used these minerals to build their skeletons. Later in the Paleozoic, sea water chemistry changed to favor the precipitation of low Mg calcite. Animal groups like the trilobites, echinoderms, brachiopods and tabulate corals that first evolved skeletons during this time period (Early Mid Cambrian to Ordovician, ~520-450 million years ago) began using low-Mg calcite as their shell mineral.

The graphic shows the first appearance of carbonate skeletal groups with their inferred primary mineralogy plotted against the temporal distribution of aragonite and calcite seas (inferred from marine cements).

Source: Susannah M. Porter 2010: Calcite and aragonite seas and the de novo acquisition of carbonate skeletons.

Interestingly, once acquired, animals did not switch their shell mineralogy to match subsequent changes in sea water chemistry. Most aragonite shell secreting animals retained this mineralogy during later 'calcite seas' (e.g. Ordovician to early Permian and Jurassic-Cretaceous) and vice versa ('aragonite seas'- Permian-Triassic, Cenozoic). A wholesale change in skeletal mineralogy may require too many evolutionary steps and would be physiologically demanding. Conserving mineralogy even during changing ambient conditions is likely an evolutionary trade off.

One question remains unanswered. There is evidence as early as 560 million years ago of soft bodied animals making tracks and burrows on the sea floor. If sea water chemistry then was conducive for the precipitation of early dolomite, why didn't at least some early animal groups make skeletons out of dolomite? Perhaps the answer lies in mineral kinetics. Dolomite is slow to precipitate. Its atomic structure is made up of layers of calcium carbonate alternating with layers of magnesium carbonate. This is more difficult to build than the relatively simpler structures of aragonite and calcite which are made up of only calcium carbonate with a few magnesium ions substituting for calcium.

In latest Ediacaran-early Cambrian times, as oxygen levels rose and animal diversity increased, ecologic interactions became more complex. The rise of predators and predator-prey arms races would have favored the evolution of a protective shell that could be assembled rapidly. Faster precipitating minerals like aragonite and calcite became the fixed construction material.

Open Access.

Palaeontology: Some Recent Spectacular Fossil Finds

Sharing some news on exciting fossil discoveries of the recent past:

1) Early animal evolution is a topic that continues to fascinate. A fossil rich sedimentary deposit from China dated to about 518 million years ago reveals exquisitely preserved soft bodied animals of the early Cambrian. This find, termed the Qingjiang biota, compliments the well known Burgess Shale of Canada and the Chenjiang site in China. It contains representatives of early cnidarians (related to corals), comb jellies, sponges, and many other creatures, and is helping paleontologists answer questions about the evolutionary relationships and timing of branching of animal groups.

Link: Spectacular new fossil bonanza captures explosion of early life.

2) Before the early Cambrian diversification of animals, is fossil evidence of the roots of some animal lineages, contained in the Ediacaran biota of late Neoproterozoic age ( 600-542 million years ago). At one site in S. Australia, a farmer is conserving a rich Ediacaran fossil site, turning it in to an outdoor research museum.

Link: This Australian farmer is saving fossils of some of the planet’s weirdest, most ancient creatures.

3) A 4 foot sedimentary layer in South Dakota contains a jumble of fossils of animals and plants. This 'event deposit' formed instantaneously from material gathered and dumped by a tsunami triggered by a large meteorite crashing into the Yucatan Peninsula, Mexico. Readers will recognize this! It happened 66 million years ago and resulted in the end Cretaceous mass extinction.

Link: Fossil Site Reveals Day That Meteor Hit Earth and, Maybe, Wiped Out Dinosaurs.

..and there is a longer article in the New Yorker on this fossil site and the hard work paleontologists have put in to tease out its secrets..  (thanks to Hollis for the reminder! ).

Link: The Day The Dinosaurs Died

Happy reading!

550 Million Year Old Animal Tracks Preserved In Sediments

Late Ediacaran trackways produced by bilaterian animals with paired appendages - Zhe Chen, Xiang Chen, Chuanming Zhou2, Xunlai Yuan and Shuhai Xiao

Pretty exciting find this. One of the earliest evidence of the presence of bilateral animals on earth has been found in 550 - 540 million  year old sedimentary rocks from China. These are not body fossils. Rather a pair of pits impressed upon the sediment surface mark the movement of an animal with paired appendages.

Abstract:

Ediacaran trace fossils provide key paleontological evidence for the evolution of early animals and their behaviors. Thus far, however, this fossil record has been limited to simple surface trails and relatively shallow burrows. We report possible trackways, preserved in association with burrows, from the terminal Ediacaran Shibantan Member (ca. 551 to ca. 541 million years ago) in the Yangtze Gorges area of South China. These trace fossils represent the earliest known trackways. They consist of two rows of imprints arranged in poorly organized series or repeated groups. These trackways may have been produced by bilaterian animals with paired appendages, although the phylum-level phylogenetic affinity of the trace makers remains unknown. It is possible that the trackways and associated burrows were produced by the same trace maker, indicating a complex behavior involving both walking and burrowing. Together, these trackways and burrows mark the arrival of a new era characterized by an increasing geobiological footprint of bilaterian animals.

 Source: Zhe Chen et.al. 2018

To put this find in context to the broader trajectory of animal evolution. These sediments are 30-40 million years older than the Chengjiang Biota of China and the famous Burgess Shale of Canada. These two are known for their exceptional preservation of animal soft tissue, giving scientists a peek into the morphological diversity present at that time. However, animals did not suddenly originate during the time the Chengjiang Shale and the Burgess Shale were being deposited, as is sometimes misunderstood by some, or is deliberately and disingenuously argued by creationists.

Molecular clocks which can estimate the time of origin and divergence of organisms suggests that animals diverged from a common ancestor between 1000 million and 700 million years ago. Trace and body fossils start appearing by 600 million years ago.

I like to put up this figure which summarizes the fossil record of early animal evolution through the Late Neoproterozoic to Early-Mid Cambrian

Source: On The Origin of Phyla

You will notice an incremental increase in richness of the trace and body fossil record. The Cambrian "Explosion" was a geologically rapid diversification of the animal biosphere, fueled by a confluence of ecologic triggers. But in absolute years it took place over a 15-20 million year period.

Comments On The 1.6 Billion Year Old Red Algae From Central India

The Proterozoic Vindhyan sedimentary basin in Central India contains sediments ranging in age from 1.7 billion years to about 600 million years ago. Bengtson and colleagues report three dimensional preservation of cellular structures which they interpret as multicellular red algae. These fossils have been found in the Tirohan Dolomite dated to about 1. 6 billion years. Before this discovery, the earliest fossils of multicellular eukaryotes was the rhodophyte Bangiomorpha, dated to about 1.2 billion years.

The Tirohan Dolomite is exposed in the Chitrakoot region of Madhya Pradesh. The fossils occur in patches of carbonate sediment which was replaced by the calcium phosphate mineral apatite just after their deposition in a shallow marine setting. Phosphotization is often a very delicate process enablng the preservation of fragile cell structures.

Here is a picture of the cellular structures of red algae imaged by SEM (scanning electron microscope)

Source: Bengtson et.al. 2017

And another rendering of the three dimensional structure of the red algae imaged using Synchrotron-Radiation X-ray Tomographic Microscopy (SRXTM). The green objects inside the cell are interpreted to be organelles, components of eukaryotic cells which aid in different physiological functions. Prokaryotes (Bacteria) lack such organelles.

Source: Bengtson et.al. 2017

I don't want to dwell on this study too much. The paper is open access for those who want to explore further.

There are two side stories that I want to comment upon.

First. The Tirohan Dolomite and its fossil assemblage has a controversial past.

They were discovered about twenty years ago by Dr Rafat Azmi, a paleontologist working with the Wadia Institute of Himalayan Geology. He reported from the Rohtasgarh area in 1998 a rich trove of filamentous and spherical forms, and odd shaped mineral fragments. He interpreted the mineral fragments as "small shelly fossils" representing fragments of animal shells and the spherical forms as possible animal embryos. Later in 2006 he reported tubular forms which he interpreted as Cambrian animal taxa. The problem was that animals are thought to have evolved by the latest Neoproterozoic- early Cambrian (600 mya -540 mya), while the understanding then was that the Tirohan Dolomite is likely 1 billion to 1.5 billion years old. Azmi's interpretation carried two enormous implications; either a) the Tirohan Dolomite was much younger in age. This would have required a major revision of the ages of Vindhyan sediments or b) that the rocks were old (~1.5 billion years), but that animals evolved much earlier than the current fossil record indicated.

These very significant implications caught the attention of geologists and media alike. The Geological Society of India sent a team to investigate Dr. Azmi's claims. They reported that they were unable to find the fossils Dr. Azmi had claimed to have found.

 Memories of an earlier scandal in Indian palaeontology were still fresh. In the late 1980's Vishwajit Gupta of Punjab University was found guilty of fraud and plagiarism. He had been misreporting fossil discoveries from the Himalayas by using museum specimens from all over the world. He had  constructed an entirely fake narrative of Himalayan fossils and stratigraphy. Scientific journals were forced to retract his papers. The Paleontological Society of India produced a book authored by S. K Shah titled "The Himalayan Fossil Fraud".  Punjab University, disgracefully, allowed Dr. Gupta to remain in service till he retired in 2004.

Under this shadow, Azmi's fossils came under similar suspicion. Fortunately, Bengtson and colleagues in a study some years later confirmed that these fossils do exist in the Tirohan Dolomite. However, they sampled the Tirohan Dolomite at Chitrakoot and not its stratigraphic equivalent (Rohtas limestone) at Rohtasgarh where Dr. Azmi's initial claims came from. They established using absolute radiometric dating that the Tirohan Dolomite is 1.6 billion years old. And they showed that the forms, similar to those Dr. Azmi found, are not multicellular animals. The spherical forms were all likely gas bubbles. Some of the larger tubular forms were revealed in the present study as red algae. Animal evolution didn't take place that early after all. The claim of the "small shelly fossils" has not been resolved fully. Bengtson and colleagues work doesn't address them. Some other researchers though have interpreted them as non-biogenic mineral growths. The stratigraphy and broader fossil content of the Rohtas limestone from where Azmi collected his fossils firmly indicates that it is not Cambrian but Proterozoic in age. .  In this present paper, these scientists have named one of the red algal forms Rafatazmia chitrakootensis in honor of Dr. Razat Azmi.

The second comment I have is on multicellularity. These red algae are the oldest multicelluar eukaryotes found anywhere. Plants, Fungi, Protists (amoebas) are eukaryotes.  They share a common eukaryote ancestor which was unicellular. That means there was just one origin of the eukaryotic cell type. However, multicellularity has evolved many times independently in different branches of the eukaryote family.

Multicellularity comes in different flavors. In simple forms of multicellularity, organisms are made up of sheets and aggregates of cells sticking to one another. There is differentiation of somatic and reproductive cells. Communication between cells is limited. One important aspect is that all the cells are in direct contact with the environment, since in these organisms, nutrient transfer takes place by diffusion from the environment to the cell. More complex types of multicellularity require the evolution of not just cell to cell adhesion, but elaborate cell to cell communication systems and a division of labor i.e. cells specialized for different functions. Also, these organisms have a three dimensional arrangement of cells wherein only few cell types are in direct contact with the environment. Diffusion is not efficient enough to supply internal cells with all the necessary life support. Molecular conduits and tissues that facilitate bulk transport and circulation of nutrients need to evolve to build this type of multicellularity.

The figure below shows the many origins of the complex type of multicellularity (in red) in different eukaryotes branches.

Source: Andrew H Knoll 2011

Based on cell type, life is divided into two domains. The Prokaryotes (Bacteria and Archaea) have smaller simpler cells. Eukaryotes are generally larger and are made up of more complex cells. This cell type evolved by a symbiotic merger between two types of prokaryote cells. Prokaryote fossils have been found in rocks older than 3 billion years. The eukaryote fossil record begins in rocks younger than 2 billion years. The timing of the origin of eukaryotes is unclear. Estimates range from  2.5 billion to 1.5 billion years ago. These red algae fossils show that eukaryotes had already diverged into different branches by 1.6 billion years ago, which means that the unicellular ancestor of eukaryotes evolved before that. It also means that red algae took the road to multicellularity much earlier than animals.

Does complexity evolve necessarily whenever genetic potential is available or does it depend on ecologic opportunity? If the cellular machinery and the underlying genetic regulatory systems required for multicellularity evolved in the ancestors of red algae by 1.6 billion years ago, why did multicellular animals not evolve earlier as well? It could well be that there were ecologic conditions limiting the evolution of physiologically demanding creatures like animals. The end of Neoproterozoic ice-ages by about 650 million years ago and the break up of supercontinent Rodinia impacted sea water chemistry. Sea water oxygen increased to threshold levels permitting a more active life style. Increased weathering of continents brought into the oceans metals like zinc which are crucial for physiological functions. Creation of larger continental shelves and shallow water zones due to continental breakup provided varied ecologic spaces for diversification. Animal evolution was triggered in this ecological context.

Wormworld: Biological Transitions At The Precambrian-Cambrian Boundary

The earliest animals were worms and they had a profound impact on marine ecosystems.

The many theories and some new understanding on the always fascinating topic of early animal evolution has been summarized quite well in a paper by James Schiffbauer and colleagues.

Molecular divergence time estimates (e.g., Erwin et al., 2011;Peterson et al., 2008) suggest that the last common ancestor of all animals evolved in the Cryogenian (ca. 800 Ma; although see dos Reis et al., 2015, for caveats). The earliest interpreted stem-group animals, however, are the ca. 600 Ma Doushantuo embryo-like microfossils (Chen et al., 2014a; Yin et al., 2016), leaving a
200-m.y. interlude between the fossil and molecular records. This hiatus between the estimated origin of Metazoa and their first appearance in the fossil record highlights the growing realization that the earliest stages of animal diversification were neither truly Cambrian nor explosive—with the phylogenetic origin of animals temporally removed from their morphological and ecological diversification by a long fuse (e.g., Conway Morris, 2000; Xiao, 2014). 

In this case, the significant lag between the establishment of the developmental toolkits necessary for the origin of novelty and their later implementation and ecological success can perhaps be attributed to the uniqueness of newly developing animal ecosystems. Between the ignition of the fuse and the subsequent evolutionary boom, three major eco-environmental feedbacks (see Erwin et al., 2011) arose that helped to pave the way for the Cambrian Explosion: (1) linkages between the pelagic and benthic ecosystems; (2) expansion of ecosystem engineering; and (3) metazoan macropredation. These feedbacks are explored herein in the context of the terminal Ediacaran fossil
record of vermiform organisms. This “wormworld” biota— comprised of various tubicolous body fossils (Figs. 2A–2C), such as the cloudinids, and increasingly complex vermiform ichnofossils (Figs. 2D–2F)—critically occupied a fundamental phase shift from competition- to predation-governed marine benthic ecosystems.

What was the big change in macroscopic life habits from the Precambrian to Cambrian times? Macroscopic multicellular life of the Ediacaran was dominated by benthic sessile forms. Early Cambrian animals were mobile creatures engaged in predation, burrowing, grazing and reef building. These activities resulted in an ecosystem engineering of sorts. For example; a) grazing and burrowing activity churned up sediment and oxygenated it. b) the evolution of guts in bilaterians transferred nutrients from sea water to the sediment in the form of fecal pellets.  These life modes created new ecologic niches and opened up new potential evolutionary pathways.

..And what killed out the classic Ediacaran biota. Was is environmental changes or ecologic competition from early animals?

It is important to note that the suggested mass extinction of the Ediacara biota in the context of our wormworld model is an ecologically driven event rather than an environmentally driven cataclysm akin to more recent (Phanerozoic) mass extinctions, and thus may have been comparatively protracted—as evidenced by Ediacara holdovers in the early Cambrian (Conway Morris, 1993; Hagadorn et al., 2000; Jensen et al., 1998). Nonetheless, whereas the static synecology and comparatively passive feeding modes of the classic Ediacarans had once emplaced a boundary on evolutionary possibility, the successful expansion of innovative traits of herbivory and carnivory, and their causal ties to infaunalization, reef-building, and biomineralization, permitted a new scaling of this bounding “right wall” (sensu Knoll and Bambach, 2000) as realized by the organisms of the wormworld fauna. Over time, the evolutionary breakthroughs conveyed by these neoteric organisms, including novel strategies, behaviors, and physiologies, increased the heterogeneity of benthic ecosystems, allowed for enhanced exploitation of resources, and established insurmountable increases in ecospace that ultimately signaled the curtain call for the Ediacara-type guilds.

The question of extinction of Ediacaran biota though may be more open ended than that suggested in this paper. E F Smith and colleagues in a recent issue of Geology analyze carbon isotope signatures of a carbonate succession spanning the Precambrian-Cambrian boundary. They find the carbonate sediment have pronounced negative carbon isotope values signalling a collapse or significant decrease in primary productivity in the oceans. 

What is the link between ecosystem collapse and negative carbon isotope excursion in carbonate sediment? Organic tissue preferentially incorporates C12, the lighter isotope of carbon. That means in thriving ecosystems, life is using up C12 from sea water and less of it makes its way into growing CaCO3 crystals forming carbonate sediment on the sea floor. When ecosystems collapse due to a myriad of reasons resulting in mass extinction, there is more C12 available to get incorporated into carbonate sediment. This increase in the lighter isotope C12 is a negative excursion of del13C, the ratio of C13 to C12.

Additional environmental disturbances may also contribute C12 to sea water. Warming of ocean water may lead to thawing of gas hydrates trapped below the sea bed. Methane released from hydrates is isotopically light and may break down and contribute C12 that eventually makes its way into carbonate. On land, a collapse of vegetation may release pulses of lighter carbon to the sea. Such a scenario would be realized in post Silurian times after the evolution of land vegetation.  In short, environmental catastrophe is linked to disturbances of the carbon cycle, and many sources may provide C12 to marine carbonate being formed at that time.

Anyways, what that means is that the decline in Ediacaran biota may have been due to both an environmental calamity as well as by longer term persistent competition by early animal activity.

And here is an infographic that summarizes the significant geological, ecological and biological events spanning the Precambrian-Cambrian transition

Source: Schiffbauer et al 2016

Open Access.

Simon Conway Morris On The Burgess Shale

Don't miss listening to Prof. Simon Conway Morris on the Burgess Shale fauna on Paleocast hosted by Dave Marshall. The Burgess Shale is an important Middle Cambrian deposit in the British Columbia Rocky Mountains. It is a Lagerstatte, i.e. it contains exceptionally well preserved fossils and therefore gives us rich details about the animal life and biodiversity of the early Paleozoic oceans and some insights into the geologically rapid diversification of early metazoans. (the Cambrian "explosion").

Why is preservation so exquisite in the Burgess Shale? Reconstructions of the sedimentary basin indicate that the mud that became the Burgess Shale was deposited at the base of a high relief limestone reef which essentially formed a sort of an underwater sea cliff.  Periodic turbidity currents swept in fauna living in shallower  areas and buried them rapidly in the deeper  water at the base of the cliff. These currents form deposits a few cm thick, encasing animal remains a few mm in dimensions. The waters were oxygen starved, thus there was less aerobic bacterial degradation of soft tissue. Add to that were some peculiar geochemical conditions of the Cambrian ocean. One was a paucity of sulphate which retarded degradation by sulphate reducing bacteria. The other, as some recent work by Robert Gaines and colleagues suggest, was the high calcium carbonate saturation levels of the ocean, which lead to rapid cementation of the sea floor in between episodes of turbidity flows.

The image on the left shows CaCO3 cement rich layers in the Burgess shale (source: Gaines et al 2012). These cemented crusts on the sea floor formed an impermeable barrier and reduced the influx of sulphate and oxygen bearing sea water in to the sediment, further slowing down microbial activity. How do we know there was less activity of sulphate reducing bacteria? The researchers analyzed the patterns of sulphur isotopes in the fossil rich turbidity layers and the background sediment.  Sulphate reducing bacteria preferentially take up the lighter isotope of sulphur from sea water. Thus, background deposits with normal or enhanced microbial activity have a lighter isotope signature relative to the Cambrian sea water standard. On the other  hand, less microbial  activity means  less fractionation of the lighter isotope into bacteria and ultimately into the sediment matrix. In Burgess Shale type deposits the fossil rich turbidity layers capped by CaCO3 cements show an enriched or heavier sulphur isotope signal indicating less microbial activity. Finally, since the bottom  waters were anoxic, there was little benthic fauna living there. This meant that the cement crusts were not disturbed and broken by bioturbation and remained effective seals throughout the crucial first few weeks of burial when degradation is at its peak. Soft tissue does break down due to slowed microbial activity and fermentation and methanogenesis. The three dimensional carcass collapses into a nearly two dimensional carbon rich film. The final result is that recalcitrant extracellular organic material like cuticles, chaetae, and jaws are preserved as compressed thin carbonaceous films often just a few microns thick, the soft fine grained mud encasing the carcass helping preserve fine morphological details. This preservation style also meant that some animals lacking recalcitrant tissues like flatworms, mesozoans, nemerteans and unshelled molluscs are less well represented in the Burgess Shale style deposits ( Butterfield 2003). Peculiar preservational styles by their very exceptional and localized nature impose a bias on the fossil record that palaeontologists must recognize to understand true evolutionary patterns.

The examples on the left shows Burgess Shale style preservation of Arthropod (B), Polychaetae worm (C) and Arthropod (D) [source: Gaines 2014]. This style of preservation actually appears first in the early Neo-Proterozoic  and then disappears for about 150 million years until the earliest Cambrian. It again declines by late Cambrian with the earliest Ordovician being the last recorded example of this taphonomic style. A unique combination of geological conditions and early diagenesis of sediment prevailing in the latest Proteozoic and earliest Cambrian resulted in these fossil deposits. This time period also has other forms of detailed preservation of soft tissues, the two most important being the Edicaran style preservation wherein the remains of macroscopic plants and animals deposited in sandy and silty sediment were draped by microbial mats and compressed to form impressions (death masks) on the sediment surface. The other important style is the Doushantuo style preservation (named after the Doushantuo fossil beds of late NeoProterozoic age, China, containing preserved algae and putative embryos and larval stages of early animals ) where phosphate minerals are attracted to and precipitate around organic tissue preserving delicate cell outlines and internal organs. Very occasionally, the same fossil will show two different preservational styles, for example, the extracellular tissue preserved in the Burgess Shale style while  internal organs preserved in the Doushantuo style. These taphonomic "windows", as they are referred to, appear and disappear through the Neo-Proterozoic to Cambrian period. For example, the Edicaran style preservation first appears in the late NeoProterozic around 580 million years ago or so. Considering that microbial mats which play an important role in this style of  preservation are pervasive through the late Archaean and the Proterozoic, the first appearance of the Edicaran remains is then likely an evolutionary signal of the first appearance of  macroscopic multicellular  eucaryotes on earth. The disappearance of Edicaran style by the earliest Cambrian also suggests a biological feedback. The evolution of macroscopic benthic animals burrowing and grazing on bacterial mats may have destroyed the cover protecting the faunal remains. Preservational styles are controlled not just by geological conditions but due to contemporaneous evolutionary innovations too.

Coming back to the talk! Prof Simon Conway Morris describes the history of research on the Burgess Shale,  how he got into researching it, details of some of the animals found in it including the famous Pikaia. This has been interpreted as an early representative of  the chordates from which the vertebrates evolved. Overall, Conway Morris gives a masterly authoritative talk.

I would have loved to hear him talk a little more about the broader questions that arise from this deposit. Are the origins of the Burgess animals to be found in the earlier Edicaran fauna? Does the Cambrian have greater morphological disparity than later periods in earth history? Has life followed a contingent unique pathway or does examples of convergence tell us something deeper about the general principles of evolution? ..or an intelligence which frames the ultimate laws and guides evolutionary processes. Simon Conway Morris has indicated elsewhere his thoughts that the Universe is the product of a rational mind and that evolution is but a search engine and I wish Dave Marshall had pressed him on his theist beliefs. But I guess the topic was the fauna of the Burgess Shale and in particular that iconic quarry in British Colombia.

He did mention in passing something which I think  is an important aspect of this story. Just as he had finished his Master's degree from Bristol University in 1972, a project headed by Prof. Harry Whittington on the Burgess Shale was starting at Cambridge. Simon Conway Morris saw this as a good opportunity. At around the same time, the Chicago school of palaeontologists lead by David Raup (who died last week), Jack Sepkoski and Tom Schopf had started a program to broaden the scope of palaeontology to include rigorous quantitative methods on large sample sets to understand biodiversity and patterns of evolution, bringing the field of palaeontology, as John Maynard Smith famously said, to the "high table of evolutionary theory". These events underscore the important point that for all your brilliance in something, circumstances and timing matter. Simon Conway Morris was present at the right time at the right place. And he did the Burgess Shale fauna justice.

A Small Note On Animal Fossils Before The Cambrian "Explosion"

Every now and then there appears a news story about metazoan fossil findings that expresses great astonishment and surprise that there is NOW... THIS TIME.. new evidence that multicellular animals evolved long before their celebrated preservation in the Chengjiang and Burgess shale Lagerstatte.

But we have known that for a long time.  The Neo-Proterozoic and early Cambrian fossil record is so much better and is improving and paleo-biologists and palaeontologists have recognized in it the gradual increase in complexity of metazoans over a 50-60 million year period before the exceptional preservation windows of Chengjiang and Burgess shale gives us a false impression of a sudden appearance of complex multicellular animals. This artifact has been exploited by creationists who claim that the fossil record actually supports their creation story of a sudden origin, under some intelligent guidance, of complex animals in the Cambrian, summarized in books like Darwin’s Doubt: The Explosive Origin of Animal Life and the Case for Intelligent Design. The best rebuttal I have come across of the creationists many misunderstandings of early animal evolution is this excellent article by Nick Matzke.

Let me just post this invaluable figure below which summarizes the Neo-Protoerozic - Cambrian metazoan fossil record. This is from James Valentine's book On The Origin Of Phyla.  It shows clearly that metazoan complexity and diversity increased gradually over time. Molecular phylogeny which aims to reconstruct the last common ancestor of animals based on genetic similarities and differences also tells us that the origin of multicellular animals goes back at least 600 million years ago, maybe even more, a good 80-100 million years before the evolution of calcium carbonate skeletonization made their existence obvious in early Cambrian. Next time a news item appears that claims that somehow fossil embryos or fossil burrows from the Neo-Proterozoic times are some shocking new finding that will change our understanding of animal evolution - don't believe it.

Source: On The Origin Of Phyla

Metazoan Embryos From Terminal Neo-Proterozoic Early Cambrian Himalayas

In Current Science Sabhyasachi Shome et al  and in Journal of Geological Society of India V.K Mathur et al report fossilized cellular remains in phosphatic chert sediments from the Krol Group (Terminal Neo-Proterozoic ~ 590 -543 mya) and Tal Group (Early Cambian ~ 540 mya) from the Lesser Himalayan sequences exposed in the Kamlidhar syncline north west of Mussorie. These they interpret as metazoan (multicellular animals) eggs and embryos. The findings are significant because they are some of the earlier known examples of body fossils of animals and will add to our understanding of the timing and nature of early animal evolution.

The Krol Group and the succeeding Tal Group represents sedimentation taking place in shallow to mid shelf environments along a passive continental margin that would in the future after Gondwanaland split up become the northern edge of the Indian continent facing the Tethys ocean.

The stratigraphic sequence is depicted in the image below with the position of the reported metazoan remains marked in red. 

Modified from:  Jiang et al 2003

The Krol Group has been interpreted as representing an evolving carbonate platform that shows a change from an early sloping ramp style geometry to a rimmed shelf to an open flat shelf profile over the course of tens of millions of years. This platform was north north-west facing i.e. the shoreline was to the south and the open deep ocean towards the north. Only the facies representing the shallower environments have been preserved in the study area. The deeper water equivalents are likely to be found in the High Himalayas in what are called the Tethyan Sequence exposed at great heights. The reported embryos are preserved in phosphatic chert lenticular bodies in a sandstone from the lowermost unit of the Krol sedimentary sequence. Younger units also contain evidence of multicellular animal life in the form of Ediacaran biota and sponge spicules.

The Tal Group represents more restricted conditions with algal buildup and lagoonal facies with deposits of phosphatic limestone, black shale and fine sandstone. From the sediments, phosphatized globular to sub-oval metazoan eggs with distinctively ornamented covering and polar lobe forming embryos have been found. They are associated with Small Shelly Fossils, a name given to a diverse array of fossils forms which are probably the dis-articulated remains of organisms like sponges, brachiopods, echinoderms, significant because they represent one of the earliest examples of metazoan biomineralization i.e. the ability to secrete hard skeletons from calcium carbonate.

The picture below from the Krol Group sediments shows a CT scan of the metazoan embryos at a two cell division stage. These have been interpreted as blastomeres representing the blastula stage embryos of animals.

Source: Sabhyasachi Shome et al -2014

And here are some more pictures of interpreted metazoans embryos from an earlier study of the Krol Group.

Source: R. Babu et al 2013 - Open Access- Description - a, Embryo showing cytoplasm, arrow showing air chamber;  b, Embryo showing developmental stage comparable to gastrula, arrow showing multicelled structure; c, Lower enlarged part of (b) (embryo), arrow showing two-layered wall (epidermis and endodermis); d, Embryo, arrow showing cleavages of blastula stage infilling homogenous organic matter (? proteineous in nature); e, Embryo showing cleavages; f, Upper enlarged part of (b) showing multicelled structure;


Some time back a friend asked me whether there is any chance of finding a Burgess Shale like Cambrian fossil bonanza in India. The Burgess Shale fossil locality in the Canadian Rocky Mountains is a Lagerstatte.. which means it represents an event of sudden entombment of the animal and/or plant life. This may be due to a submarine mud and silt flows burying the animal and plant communities in soft sediment leading to a snapshot of life at that particular moment. Rapid burial prevents degradation on the sea floor of organic remains. This means that even details of soft tissue along with hard parts are preserved as impressions on sediment. 

My answer to the question was that in Peninsular India sedimentation stopped by terminal Neo-Proterozoic. There were no Cambrian basins. But along the northern margin of proto-India in a basin which in the future would be deformed and uplifted to become the Himalayas, sedimentation continued in the Paleozoic. Which means Cambrian sediments are present, although I remarked they may be metamorphosed and destroyed. That is not the case however. The Krol and Tal Group do contain a Neo-Proterozoic to Early Cambrian fossil record with Ediacaran biota, sponge spicules and small shelly fossils indicating presence of metazoans. Their deeper water equivalents may be found in the High Himalayas! So, there is always a change of a spectacular fossil find either in or near more familiar places like Mussorie or Nainital where the Krol and Tal Groups are exposed or along the frigid heights of the Himalayan snow giants -for explorers brave enough to venture there.

Creationists Don't Get The Cambrian Explosion

There is an excellent post by Nick Matzke on the Panda's Thumb on the Cambrian Explosion, which was the evolutionary radiation and diversification of animal life over a 30 million year period from about 540 mya to 510 mya. To clarify, the lower boundary set at around 540 mya does not mean there is no evidence of animal life before that and "poof" animals originated instantaneously at 540 mya. On the contrary, there is long earlier history of multicellular animal life preserved in various forms including the famous Ediacaran biota going back another 10's of million years or so,  but the fossil record does become more prolific beginning about 540 mya. The important point is that complex forms evolved and diversification occurred in recognizable stages, not all at once. So yes, there are "transitional fossils" preserved from this time period!  There are geological and ecological reasons for why animal life diversified and was preserved better by 540 mya, but that is another story.

Nick Matzke has written this post as a rebuttal to a book by Stephen Meyer titled Darwin’s Doubt: The Explosive Origin of Animal Life and the Case for Intelligent Design.

It is a long post but well worth reading. You will learn:

1) What is the nature of the fossil record beginning late Proterozoic and the Cambrian.

2) What methods of analysis are being used by paleontologists to make sense of this fossil record and what is the current thinking on how these ancient animal groups of the Cambrian were related to each other and to extant animal groups, with implications for how and when diversification from simpler forms took place.

3) How Creationists obfuscate, ignore and are ignorant of both, the nature of the fossil record and analytical methods, resulting in them misdirecting readers about the actual state of our knowledge.

Do read it.

My Rather Tenuous Connection With The Mars Rover Project Scientist

I have no connection with the Mars Rover program :)...but this @geosociety tweet a few days ago caught my eye:

@geosociety Fellow John Grotzinger, JPL geologist on Mars Curiosity rover mission, in LA Times. http://lat.ms/QzG81W  MT @earthmagazine

John Grotzinger was profiled in an article in the LA Times. He is project scientist for the Mars mission and in charge of directing the earth science effort to glean information about the geology of Mars. Here is what the article says about his work-

For much of his post-PhD career, the geologist kept his feet planted firmly on Earth. He combed ancient sedimentary rocks for signs of early life. He took trips around the globe, family in tow, to collect 550-million-year old specimens in Namibia and Oman.

What it left out was that Prof. Grotzinger is a carbonate sedimentologist. So.. I guess I can claim that I share an academic kinship with him :)

I am quite familiar with his work in carbonates. When I was working on my PhD in the mid 1990's he was already a faculty at MIT. His PhD research on Proterozoic carbonates of the Northwest Territories in Canada was directed by J. Fred Read at Virginia Polytechnic. During several GSA meetings I did get an opportunity to listen to his presentation on various aspects of Proterozoic carbonate platform evolution. He later moved to Cal Tech and JPL in Pasadena, California.

For long, carbonate sedimentologists gave much more attention to Phanerozoic carbonates and less attention to Proterozoic carbonate deposits. There was an economic incentive in that. Many Phanerozoic carbonate basins host prolific oil and gas deposits. The origin, growth and architecture of Phanerozoic carbonate sedimentary platforms,  a term for depositional basins in which hundreds to thousands of feet of calcium carbonate sediments accumulate, was studied quite intensely and we gained a very detailed understanding of these systems. All this work ultimately helps exploration geologists make reasonable predictions on the location and thicknesses of strata best suited to be oil reservoirs.

Earliest Examples Of Biomineralization

On Wired Science Brandon Keim summarizes the discovery of phosphatic biomineralization in Neo-Proterozoic protists, published in the June issue of Geology.

The microfossils were preserved in 750 million year old strata of the Fifteenmile Group in Yukon Territory, Canada. This makes it one of the earliest examples in eukaryotes of the ability to precipitate minerals from sea water and use them as structural support or as a protective casing around soft tissue.

In marine organisms, biomineralization has evolved independently many times in different eukaryotic groups. Preserved instances of biomineralization from the Neo-Proterozoic and the earliest Cambrian are of unicellular organisms like protists using mostly phosphatic minerals like apatite. That changed with the evolution of larger complex metazoans by early mid Cambrian times. These creatures preferred calcium carbonate in the form of either aragonite or calcite to build their skeletons.

This might reflect changing sea water chemistry, the increasing saturation of the Cambrian shallow water areas in calcium carbonate and the decreasing availability of phosphorus or it might reflect inherent energy efficiencies in large skeletal construction. Perhaps calcium carbonate molecules are easier to assemble into the larger edifices required than calcium phosphate is..that's just my speculation..

Brandon Keim ends his article quite evocatively:

Of course, predators eventually developed their own biomineralization strategies, as did other algae. Eventually it became ubiquitous in the marine world, to the point where what we now call limestone is simply a composite of microscopic fossil seashells. It’s also the primary ingredient in concrete. Their shells have become our own.

Sponges Again And A Good Example Of What Common Descent Means

On Cosmos and Culture 13.7 blog Marcelo Gleiser writes this about sponges:

Considering that sponges have been around for over 500 million years, possibly even a billion years, many scientists believe they form the base of the evolutionary branch in the tree of life that led to animals. In other words, don’t think of humans as coming from monkeys; we, and every other kind of critter out there, came from sponges, the cousins of the porous yellowy objects you use to scrub yourself in the shower.

A day later Ursula Goodenough on the same blog expands on that theme:

...we eukaryotes (non-bacterial life) trace our ancestry back to a single-celled Most Recent Common Ancestor (MRCA) that inhabited the planet some 1.5 to 2 billion years ago. This MRCA encoded all the core eukaryotic “ideas”: how to make membranes with channels, how to regulate gene expression, how to engage in meiotic sex. These ideas then moved through evolutionary time into numerous radiations, with particular ideas becoming expanded and elaborated, others degraded and lost, in particular lineages.....

she explains further about animal ancestry:

..An ancestral creature with larval globular cells gave rise to two lineages, one leading to modern sponges that have retained the globular-cell idea, and the other leading to modern animals whose “proto-neural” globular cells went on to acquire the capacity to differentiate into full-fledged neurons.

Spot the difference between the two posts?

Maybe the author didn't mean it but the first gives the impression that sponges being the oldest of all animals gave rise to subsequent lineages of animals. But as clarified in the second post, sponges may be the oldest of all animals, but the rest of the animal kingdom did not evolve from them. Rather sponges and other animals share a common ancestor.

Evolutionary diversity forms through a branching process.  Oldest simply means that the lineage that gave rise to modern sponges was the first to branch off from the root of the animal tree, root meaning the most recent common ancestor of all animals.

Oldest or "primitive" may also sometimes be taken to mean that the modern creature resembles the MRCA the most. But it certainly does not mean that sponges have stopped evolving since that early divergence. Modern sponges may have conserved certain ancestral traits, for example the globular cells sensitive to stimuli, but may also have acquired several new ones during their long evolutionary history.

Is My Fossil Not Spongeworthy?

The Australian reports on a recent announcement of possible sponge-grade metazoan remains from the Flinders range in south Australia claimed by a research group from Princeton University and hints at a darker controversy involving priority:

Paleontologists such as Jim Gehling with the South Australian Museum say it is no surprise that simple sponge-like animals lived 600-650 million years ago, as reported yesterday in the journal Nature Geoscience. But they are far from convinced they are what the Princeton University team has found.

"To argue these were sponges is a difficult proposition. They look like Coco Pops, " said Dr Gehling.

Moreover, Dr Gehling said better, older fossils had been found three years ago by University of Melbourne geologist Malcolm Wallace and his team. Dr Gehling suggested that competitive pressure might have been the reason Dr Wallace's group has been unable to publish their results.

The Australian understands that one of the co-authors of the contentious paper is a reviewer for the journal Science, to which Dr Wallace's group has submitted a paper. It is not clear whether the reviewer has read the paper but Dr Wallace acknowledged that "we've had difficulties getting our results published". He preferred not to discuss Dr Gehling's suspicions. He did affirm that his group's finds were roughly 20 million years older than those reported by the Princeton team, headed by paleontologist Adam Maloof.

Using a position of influence to suppress a rival groups study from being published would be a very serious ethical transgression but if the identity of the reviewer is that obvious would anyone take that risk?

Andrew Alden at Geology.about.com and Chris at Highly Allochthonous express their opinions about the study.

Multicellularity, Evolution, Life

On NPR's Cosmos and Culture blog biologist Ursula Goodenough has been writing some terrific posts on evolution. Two that recently caught my eye:

Unicellularity Vs. Multicellularity: Why We Bother With More Than One Cell

Time and Life

Both are worth spending some time on.

Coming back to the topic of the evolution of multicellularity the latest issue of Geology has a paper that does some analysis on continental reconfiguration during the early Cambrian and proposes that there was a major movement and rotation of Gondwana resulting in establishment of a new ecological landscape, new conditions that may have provided the impetus for the rapid radiation of metazoans otherwise known as the Cambrian "explosion".

Explanations for the Cambrian explosion have occupied two extremes. One view proposes that the fuse was a biological one. Some crucial biological innovation in terms of gene regulation and molecular cascades remained to be discovered until complex metazoans could evolve. According to this view these changes likely occurred beginning around 600 mya. The other view proposes that practically all the complex genetic machinery necessary for metazoans to function already existed in unicellular eukaryotes. The reason for the delay in the advent of metazoans (complex unicellular eukaryotes date back to more than a billion years) were ecological constraints such as a lack of enough oxygen in the atmosphere.

Lately I sense that the ecological argument is winning out not least because earlier experiments in multicellularity are coming to light. Life getting organized into agglomerates of cells is a  theme that has been independently invented several times and as far back as a couple of billion years ago if recent findings in Gabon and in India's Vindhyan basin hold true.

The thought of genetic potential waiting to be unshackled into new and varied forms is fascinating and one doesn't have to venture into the unfamiliar Cambrian terrain for an example. The transformation of the wolf into the myriad morphs of dogs is a strong example. A recent study (via Panda's Thumb) showed that the morphological variation in skull shape across the entire order of Carnivora is less that the variation seen within dogs. The ecological landscape experienced by ancestral wolves did shift dramatically in recent times not by continental movements as proposed for the Cambrian radiation but by the imposition of new selection pressures by humans. Wolves apparently always had the genetic potential to diversify into what could be described as downright weird forms. The changing human ecology provided the trigger and helped maintain these forms, many who may not have survived in the wild.

The evolution of complexity seems to be open ended. There must have been occasions when there was a significant change in genetic architecture. John Maynard Smith and Eors Szathmary document these steps in their book The Major Transitions in Evolution. Other times it was an exogenous influence that triggered a change, a  matter of filling a new ecological niche.

Cambrian Explosion: This Time Its The Calcium That Did It

Geological processes and evolution - 4

A short article in Science Daily describes research that suggests that a buildup of calcium in the oceans in the early Cambrian provided the necessary trigger for the Cambrian "explosion" - the pronounced expansion and diversification of multicellular groups of animals. Some of the best examples of this evolutionary radiation is preserved in deposits like the Burgess Shale and the Chengjiang strata.

Here is the one sentence summary-

The researchers succeeded to show that the massive and sudden surge in the calcium concentration of the Cambrian seawater -- that is believed to be the result of volcanically active mid-ocean ridges -- not only initiated the buildup of calcified shells, but was also mandatory for the aggregation and stabilization of multicellular sponge structures. This allows, on the other hand, to formulate a novel theory where the geologically induced increase of marine calcium might be the key for understanding the Cambrian Explosion of Life.

There is deja vu when I read another ultimate causative explanation for the Cambrian explosion. Over the years you could write a similar sentence but substitute the word calcium with the sudden increase in oxygen, the warming of the earth following the thawing of the snowball earth, the increase in shallow shelf areas following marine transgression, all proposed as a one point explanations of this evolutionary phase in earth history.

Reading the press release you get a sense that the Cambrian explosion coincided with the origin of multicelluarity. Take this sentence:

However, the causes of its origin have been the subject of debate for decades, and the question of what was the trigger for the single cell microorganisms to assemble and organize into multicellular organisms has remained unanswered until now.

Its important to make a distinction here between the origin of a system and its subsequent diversification. Groups like fungi, plants and animals have evolved multicellularity independently of each other.  Whatever triggered single celled animals to assemble into multicellular ones (metazoans), that transition did not happen in the Cambrian but much before in the late Proterozoic maybe as much as 50 -100 million years before.

The original study recognizes this point but it gets lost in the press release.

There is a good and improving fossil record of metazoans from the late Proterozoic Ediacaran fauna and other types of fossil preservation that indicates that  molecular mechanisms for cell to cell signaling and cell adhesion must have evolved well before the Cambrian. Multicellularity in animals originated long before the Cambrian explosion occurred and long before the rise of calcium in Cambrian sea-water.

The figure below summarizes the current fossil record of the evolution of metazoans. The Cambrian "explosion" corresponds with the Chengjiang fauna.

 Source: On The Origin of Phyla

Explanations of the origin of metazoans and subsequent evolutionary radiations like the Cambrian explosion involve long and intertwined causal chains. A rise in oxygen by late Proterozoic may have favored larger body size and one solution to achieve this large size was to aggregate into colonies of cells which eventually became integrated as one organism. The marine transgressions by late-Proterozoic early Cambrian expanded available shallow marine shelf areas and created large and diverse ecologic niches for evolutionary diversification to take place. The evolution of predation would have set forth selective pressures for skeletonization.  Many organisms would have taken opportunistic advantage of the rise of calcium in sea-water to boost skeletal production. Likewise calcium may have provided stability for even larger masses of cells to aggregate.

There were many geological and ecological factors at play feeding of each other that were responsible for one of the major transitions in the history of life. Insisting on just one cause as the most important to the exclusion of others is too simplistic.

How Do We Know Fossils Were Once Living Organisms?

In the September issue of Geology there is a good give and take (open access) on the origin of early Neoproterozoic carbonate rock textures. The debate revolves around an interpretation made by Neuweiler et al. (2009) that certain cement filled cavity and mud textures in Neoproterozoic carbonate bioherms (carbonate mounds formed by aggregating and colonial organisms) look very similar to younger sponge replacement fabrics found in Palaeozoic and modern bioherms.

Even though there is no recognizable fossil sponge body parts in the studied Neoproterozoic carbonate (little Dal reefs, Canada)  the authors interpret the diagenetic fabric as indicative of a metazoan origin (they say they cannot conclusively link it to any specific sponge taxon) pushing in their view the geological evidence for multicellular animals to around 875 million years ago. This is about 200 million years older than what most scientists acknowledge. Noah Planavsky in the comment section disagrees about the metazoan origins of the texture and suggests that microbiota can also form similar fabrics.


I found the study interesting in itself. I have worked with some pretty complex diagenetic fabrics and it is always a challenge to tease out components that are purely inorganic from those that have a biogenic origin.

I feel though that a study like this fulfills another important role in science - it is of epistemological value. I used to help my PhD adviser with the paleontology exhibit on Science Day at the local mall in Tallahassee and we had people coming up to us and asking " ...but how do you know that this fossil was once a living creature...?"

A study like this tells us about the methodology and chains of reasoning scientists use to gather a body of knowledge about how fossils form. In this case the authors compared sponge remains from modern bioherms with older and older deposits. In the modern bioherms the sponge organism had died relatively recently and the organic tissue and other skeletal parts were still joined together as a coherent organism. Some organic material had degraded and in its place were tiny carbonate crystals. In some internal body cavities carbonate mud had accumulated forming a sort of a cast of the body part. This is a clear indication that as organic material degrades its place is taken up by inorganic material which retains the same shape as the organic matrix.

The researchers then went further back in time and looked at Cretaceous and Paleozoic rocks. In these samples there was no organic matter, that had decayed away long back,  but other typical sponge skeletal hard parts like spicules were still preserved and recognizable as sponge remains. The shape and form of the interior of the sponge the characteristic canal system was now completely filled by cement and mud. So although there was no coherent organism with linked body parts the association of spicules with a cement and mud filled connected cavity system which had a shape just like the canal system of the sponge gives us confidence that we are looking at sponge fossil fabrics.

One can then go one step further as the authors have done and interpret fabrics with characteristic shapes but no sponge remains (no spicules or anything) as having formed by the alteration of a large multicellular creature. That specific interpretation may be right or wrong in this case but that's the way scientists "know" that fossils were once living creatures. It's a good example to use to explain - How do we know what we know?