3.66 Million Year Old Hominin Footprints From Laetoli Tanzania

I read this with a sense of wonderment:

New footprints from Laetoli (Tanzania) provide evidence for marked body size variation in early hominins (Open Access)

 - Fidelis T Masao, Elgidius B Ichumbaki1, Marco Cherin, Angelo Barili,Giovanni Boschian, Dawid A Iurino, Sofia Menconero, Jacopo Moggi-Cecchi, Giorgio Manzi 

Source: Fidelis T Masao et al 2016: Trackway L8 with four footprints (top). Relief map of the trackway L8 (bottom).

The tracks shown in the image above is the L8 trackway from site S, made by individual S1. Site S contains another set of tracks made by individual S2. Another site named site G, discovered some years earlier, lies about 150 meters to the north of site S. It contains tracks made by 3 individuals G1, G2 and G3. At both sites the tracks indicate that the hominins were walking in the same north- northeasterly direction. The tentative conclusion is that S1 is a large male, S2 and G2 are adult females and G1 and G3 are juveniles or smaller adult females. Based on fossils found in this stratigraphic succession it is believed that these tracks were made by individuals of the species Australopithecus afarensis.

Geological and stratigraphic reasoning lead to the conclusion that the trackways at site G and site S have been imprinted on the same tuff (volcanic ash) layer (Footprint Tuff) which contain a record of ash deposition over just a few weeks. Remarkably, this means that we could have a 3.66 million year old record of two contemporaneous trackways made by the same general population of hominins living in this area.

As the title of the paper suggests the important observation here is a marked body size variation interpreted from the morphology of the footprints. Below is a neat figure of stature estimates of hominins found from various localities in Africa ranging in age from 4 million years ago to 1 million years ago. The size range of A. afarensis overlaps that of later Homo.

 Source: Fidelis T Masao et al 2016

and an extract from the paper:

"These findings provide independent evidence for large body-size individuals among hominins as ancient as 3.66 Ma. Consequently, we may emphasise the conclusions by Grabowski et al. (2015) and Jungers et al. (2016), who reported that the body sizes of the australopithecines and of the early Homo representatives were similar, but also that certain australopithecine individuals (at least of Au. afarensis) were comparable with later Homo species, including H. erectus s. l. and H. sapiens. Thus, our results support a nonlinear evolutionary trend in hominin body size (Di Vincenzo et al., 2015; Jungers et al., 2016) and contrast with the idea that the emergence of the genus Homo and/or the first dispersal out of Africa was related to an abrupt increase in body size (McHenry and Coffing, 2000; Antón et al., 2014; Maslin et al., 2015). The identification of large-size individuals among the australopithecines – i.e. hominins commonly presumed to be small-bodied on average – shows also that the available fossil record can be misleading, resulting in an underestimate of the hominin phenotypic diversity in any given period".

.."Evidence for either marked or moderate body-size variation in Au. afarensis, based on data collected in a single site, was limited until now to the fossil assemblage from the Hadar 333 locality, dated to 3.2 Ma (with body masses ranging from 24.5 to 63.6 kg). The new estimates for the Laetoli individuals indicate an even more marked variation in body size within the same hominin population, at 3.66 Ma. Consequently, the combined records from Laetoli and Hadar suggest that large-bodied hominins existed in the African Pliocene for over 400,000 years, between 3.66 and 3.2 Ma. At the same time, these data contrast with the hypothesis of a temporal trend of body-size increase among Au. afarensis between the more ancient Laetoli and the more recent Hadar fossil samples (Lockwood et al., 2000)".

Another implication is that there was marked sexual dimorphism in Australopithecus afarensis possibly due to male to male competition. Their social structure may have been more similar to gorillas than chimpanzees or modern humans.

What a find!

The Shared Fossil Heritage Of Gondwanaland

Lovely infographic:

"As noted by Snider-Pellegrini and Wegener, the locations of certain fossil plants and animals on present-day, widely separated continents would form definite patterns (shown by the bands of colors), if the continents are rejoined".

via USGS

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.

Human Evolution: The Paleolithic In The Indian Subcontinent

Came across this article by anthropologist Sheila Mishra on the Paleolithic of the Indian subcontinent and its significance in understanding human evolution.

The Indian Subcontinent is one of the areas occupied by hominins since Early Pleistocene times. The Lower Palaeolithic in the Indian Subcontinent is exclusively Acheulian. This Acheulian is similiar to the African Acheulian and has been labeled "Large Flake Acheulian" (LFA). The Middle Palaeolithic in the Indian Subcontinent is a poorly defined entity and the author has suggested that this phase should be considered the final phase of the Large Flake Acheulian from which it evolved. Microblade technology has recently been shown to be older than 45 Ka in the Indian subcontinent and is certainly made by modern humans as it has a continuity from this time until the bronze age. Presently, the nature of the transition from Acheulian technology to Microblade technology is not well understood as few sites have been dated to the relevant time period.

The continuity of the Lower Palaeolithic in the Indian Subcontinent is due to its ecological features. The Indian Subcontinent extends from approximately 8°-30° N which would normally encompass equatorial, tropical and temperate latitudinal zones. However, the influence of the monsoonal climate and sheltering effect of the Himalayan mountains results in a sub-tropical grassland vegetation extending both northwards and southwards of its normal distribution. Rainfall, rather than temperature, is the most important ecological variable which has a longitudinal rather than latitudinal variation. Thus, the Indian Subcontinent has a more homogenous environment than any comparable landmass and one eminently suitable for hominins. In contrast, the African climate zones are strongly latitudinal in distribution. The Indian Subcontinent during the Early and Middle Pleistocene has close connections with Sundaland. The fauna associated with Homo erectus in Java is derived from the Indian Pinjor faunas. During low sea levels the area of land exposed in the Sunda shelf is equal in size to the Indian Subcontinent. Sundaland has an important buffering effect on the Indian Subcontinent, with favourable conditions for Hominins in Sundaland coinciding with unfavourable ones in the Indian Subcontinent.

She interprets the ecology and tool record as suggesting that Homo erectus evolved in the India-Sundaland region and not in Africa. This scenario implies there was a migration of Homo erectus into Africa from Asia by 1.8 million years ago or so.  She points out that a number of African mammal species appear in the Indian Siwaliks (Himalaya foothills)  by 3-2.5 million years ago and so presumably an ancestral species (Australopithecus? early Homo?)  may have migrated out of Africa at that time. There have been recent announcements of putative 2.6 million year old stone tools from the Siwaliks, but their significance is still up for debate. And given the paucity of skeletal remains in India, her theory is going to be a hard sell.

There is  also a really good description of the geological context in which Paleolithic stone tools are found in the Indian subcontinent. They have been often described as "surface" sites but Mishra points out that they have been eroding from fluvial sediments. Volcanism, sedimentation and tectonics in the African rift valley and parts of Java lead to conditions favoring both burial and preservation and later exhumation of fossils and tools. The situation in India is different. Since Mio-Pliocene most of Peninsuslar India has been an erosive landscape with sedimentation occurring in a few fluvial systems with a depositional regime. Thick fluvial successions are rare. Preservation potential on the Indian landscape was low. The implication is that India may have had a larger population of hominins through the Pleistocene than the rarity of remains suggest.  Caves are the other context in which hominin fossils have been found in Africa, Europe and Asia. Have caves been adequately explored in India?

A very interesting article. Open Access.

Jesus n Mo: Those Furry Eskimos

They nail it every time!

Absence of furry "eskimos" is an actual argument touted against evolution! :)

Photomicrograph: Treasure Inside A Brachiopod Shell

Couldn't help posting this picture. I am currently creating a catalog of carbonate textures and diagenetic fabrics for the geology department at Fergusson College, Pune, which I hope will be used as a teaching aid.

This photomicrograph captures the inside of a Mid Ordovician brachiopod shell. A complex cement sequence is present inside the pore space. The sequence represents passage of the sediment from depositional marine settings to later deep burial depths. During that long journey the sediment encountered fluids of different chemical make up resulting in the precipitation of different cement types.

Pure magic!

Evolution Is Still Misunderstood

Got this via @David_Bressan

Sigh... even Science gets it wrong here. I am assuming that the lighter colored, hairless and bipedal creatures shown in the figure are all hominins. Evolution is a branching process, that much is correct, but they bungled up the branching order.

Hominins are a group that consists of modern humans, all other extinct Homo and all members of the extinct ancestral taxa Australopithecus, Paranthropus and Ardipithecus to the exclusion of the chimpanzee.  In the branching tree starting from the primary branching node at the top, the left side branch contains some hominins and the chimpanzee. The right side branch contains other hominins. Since the diagram shows the chimpanzee splitting away from within the left side branch, it implies that the left branch hominins are more closely related to chimps than they are to the right branch hominins.

That can't be right!

The correct branching order should have been depicted like this:

Evolution is still misunderstood..

Photomicrograph: Super Mature Quartz Arenites From Proterozoic Cuddapah Basin

One of the vivid memories of my Master thesis fieldwork in South India were a series of brightly reflecting hills. In the afternoons, the bare slopes of the hills were a blinding white and you had to wear dark sun glasses to minimize the glare.

These hills were made up of the Paniam Quartzite. This sedimentary sequence is part of the Neoproterozoic Kurnool Group which represents one megacycle of deposition in the long lasting Paleoproterozoic to Neoproterozoic Cuddapah Basin.  In sedimentary petrology terminology these white and bright sediments are quartz arenites, rocks made up mostly of the mineral quartz. In fact, they were super mature quartz arenites, i.e. they were made up of more than 90% quartz. I  point counted several samples and the percentage of quartz was around the 95%-96% mark.

Here is what they look like under the microscope. Notice how rounded the quartz grains are.

The white arrows in the photomicrograph below points to quartz cement which has precipitated between the grains. These cements are called overgrowths. They maintain the same optical orientation as the substrate quartz grain and hence in cross polarized light the detrital grain and the overgrowth appears as a single crystal unit. The detrital quartz grain is outlined by iron oxide dust which helps demarcate the contact between the grain and the later cement.

Here is another example of a super mature quartz arenite. The contact between the detrital grain and cement is again marked by a coat of dust. Notice the planar crystal facets of the quartz cement (white arrow) which contrasts nicely with the rounded detrital particles.

 
The example I have presented show only one generation of quartz overgrowth cement. There are instances where two generations of quartz overgrowth cements are present. Like the detrital grain, the first generation overgrowth has a coating of iron oxide or clay and is abraded. This indicates that the quartz grains have been derived from the erosion of older silica cemented sandstones. The original source of the quartz in these older sandstones were igneous or metamorphic rocks. After being eroded from these rocks and then transported and deposited, the quartz grains were overlain by silica cement (the first generation cement) and lithified into a sandstone.

Later (perhaps tens of millions of years later), this sandstone was uplifted and eroded. Disaggregation of grains during weathering broke of quartz sand particles along with attached fragments of cement. This cement overgrowth then got abraded and rounded during transport and acquired a dust coat. In its final site of deposition it was overlain by new silica overgrowth (the second generation cement). Abhijit Basu and colleagues present an interesting example of such "second cycle" or "recycled" quartz arenites from the sedimentary sequences of the Bastar Craton from Eastern India (Image to left: source Basu et al 2013).

A careful examination of quartz arenites and generations of silica cements can reveal a lot of useful information about uplift, erosion and recycling history of the earth's crust.

Quartz arenites are not restricted to the Proterozoic. They are common in younger age Paleozoic, Mesozoic and Cenozoic deposits too. They occur only sporadically in Archean age deposits. Thick sequences of quartz arenites become more common in the Proterozoic. This increase in the occurrence of quartz arenites in the Proterozoic has to do with the changing tectonics of the earth.

Among the common rock forming minerals, quartz is relatively chemically inert and is more resistant to physical breakdown during weathering and transport. In the Archean, sedimentary basins were generally linear troughs formed in front of island arcs. Due to these tectonically active conditions, the basin floor subsided quickly and detritus derived from weathering of igneous and metamorphic source rocks was deposited and buried before physical attrition and chemical dissolution could remove unstable minerals. The result was a mineralogically "immature" sandstone with the framework of the rock made  up of  quartz, feldpsars and volcanic and metamorphic rock fragments in different proportions . The sediments and associated volcanic material frequently got metamorphosed to a low grade "green mineral" assemblage of chlorite, actinolite and epidote. These deformed and metamorphosed successions embedded in Archean gneiss terrains are known as greenstone belts.

There are a few instances of quartz arenites in the Archean from terrains of the Canadian Province, the Baltic Shield in Russia and from the Bababudhan Group of the Dharwar Greenstone belt in South India. Many of these have been interpreted as a product of intense chemical weathering in Archean soils, wherein unstable pyroxenes, feldspars and meta-igneous and meta-sedimentary rock fragments were leached away, leaving behind a quartz rich residue. Sedimentary structures like cross bedding and ripple marks indicate shallow water environments of deposition where the sand was further subjected to physical attrition leaving behind a quartz rich sand deposit.

Such conditions of longer residence time and more intense chemical weathering in soil profiles and long periods of attrition and physical sorting by wave and tidal action became more common in basins of Proterozoic age. Phases of prolonged magmatism and heat loss from around 3 billion years ago to 2 billion years ago resulted in a cooler earth and one that now was made up of large rafts of granite/granodiorite crust which was buoyant and tectonically stable. Although the boundary between the Archean and the Proterozoic is pegged at around 2.5 billion  years ago, basin tectonic styles do not change abruptly. These were evolving conditions.

In Peninsular India, Proterozoic age sediments were deposited in two types of basins manifesting different tectonic styles. "Mobile Belts" are reminiscent of the older Archean greenstone belts in that they were tectonically active elements of the crust, perhaps forming in subducting settings at the boundary between two cratonic blocks. They are depressions which contain abundant volcano-sedimentary successions made up of volcanic flow and ash beds interlayered with  immature sand and mud and chemically precipitated silica and iron oxide layers.  These are interpreted as deeper water deposits. Some basins contain stromatolite limestone/dolomite. There are a few quartzite deposits too.  These may be the metamorphosed equivalents of quartz arenites which were deposited in shallow water environments.  These successions were subjected to metamorphism, deformation and intrusion by granitic plutons during orogenic episodes forming the "mobile belts" or fold belts.  The Aravalli and Delhi Group of sediments which make up the Aravalli mountain ranges in Rajasthan are a good example of these Early to Mid Proterozoic mobile belts.

Overlapping in time with the mobile belts but extending into the Neoproterozoic are the epicratonic basins, also known as the "Purana" basins. These basins experienced less volcanic activity and were subjected to less deformation and metamorphism than that seen in the mobile belts. They contain thick sequences of quartz arenites and limestones.  These basins were initiated by extension and rifting of the continental crust resulting in extensive shallow marine shelf areas.  Low relief Archean to Early Proterozoic source terrains made up of granitic and metamorphic rocks were subjected to intense chemical weathering. Since the basin floor subsided slowly in these passive margin basins, shallow water conditions prevailed for long periods of time. Quartz rich residues were transported and deposited as sand sheets in beach and tidal flat settings and as sand shoals in more open waters away from the shorelines. Wave action further sorted them into a texturally mature sand.

The Paniam Quartzite, whose afternoon glare I tried in vain to avoid, is a remnant of one of these vast sand sheets that occur across many epicratonic Proterozoic basins in Peninsular India. Other examples of this stable cratonic style of deposition include the Vindhyan Basin in Central India and the Kaladgi and Bhima Basins of South India.

The satellite image below shows the brightly reflecting slopes (white arrows) of this quartz arenite deposit around the village of Gani in Andhra Pradesh. The black dotted line is the contact between the Archean basement and the overlying Proterozoic Cuddapah Basin sediments. The linear structure is the left lateral Gani Kalava fault offsetting the Cuddapah Basin.

And here is one final photomicrograph of the Paniam quartz arenite showing well rounded detrital grains with faceted quartz overgrowths meeting in planar contact in the pore spaces.

Bias In The Spider Fossil Record

Loved this one from xkcd comics:

Sea Water Chemistry And Shell Mineralogy: Tales Of Mesozoic Bivalves

Years ago when I was in the second year of college, I along with friends, went for a fossil collection tour to the town of Ariyalur in South India. Rocks of Cretaceous age outcrop all around, and these strata have now become one of the most famous fossil localities in India.

We collected ammonites, echinoids, plant leaf impressions on clay and bivalves... lots and lots of bivalves.. In the picture below are the remains of my collection of molluscs. On the top left is an oyster with a clam clinging on to one of its valves. Bottom left is another oyster with its jagged valve margin. In the middle is a largish clam and to the right is an oyster whose layered shell structure is clearly visible.

I have some photomicrographs too taken from thin sections given to me by a friend.

In the above image the foliated shell microstructure of a piece of a bivalve can be clearly seen in cross polarized light.

And in the image below, a coarser prismatic crystal structure of a shell fragment is visible in the center of the image.

Most molluscs groups (including bivalves) in today's tropical seas built their skeletons using the CaCO3 polymorph aragonite. I say tropical seas, because molluscs with calcite skeletons are more common in temperate waters, such as for example in the marine communities living on the continental shelf of the southern coasts of Australia. In the Cretaceous seas though, even at tropical latitudes, calcite bivalves were common. In this apparent puzzle lies a very interesting story of climate change, sea floor spreading, changing sea water chemistry, the evolutionary decline and success of different bivalve groups during the Mesozoic, the emergence of bivalve reefs and the localization of hydrocarbon reservoirs.

Interview- Rosemary And Peter Grant On Watching Evolution In Action

Source: Quanta Magazine; Courtesy Peter and Rosemary Grant

Daphne Major in the Galapagos chain.

Yes, 40 years of field research on that half a square km size island, tracking, generation after generation, changes in body and beak size of different species of ground finches.  Lately, they have supplemented their morphologic and bird song data with genomic analysis to get an understanding of the genetic underpinnings of morphologic change.

Rosemary and Peter Grant interviewed about their epic evolution watch:

"The diminutive island wasn’t a particularly hospitable place for the Grants to spend their winters. At less than one-hundredth the size of Manhattan, Daphne resembles the tip of a volcano rising from the sea. Visitors must leap off the boat onto the edge of a steep ring of land that surrounds a central crater. The island’s vegetation is sparse. Herbs, cactus bushes and low trees provide food for finches — small, medium and large ground finches, as well as cactus finches — and other birds. The Grants brought with them all the food and water they would need and cooked meals in a shallow cave sheltered by a tarp from the baking sun. They camped on Daphne’s one tiny flat spot, barely larger than a picnic table.

...They visited Daphne for several months each year from 1973 to 2012, sometimes bringing their daughters. Over the course of their four-decade tenure, the couple tagged roughly 20,000 birds spanning at least eight generations. (The longest-lived bird on the Grants’ watch survived a whopping 17 years.) They tracked almost every mating and its offspring, creating large, multigenerational pedigrees for different finch species. They took blood samples and recorded the finches’ songs, which allowed them to track genetics and other factors long after the birds themselves died. They have confirmed some of Darwin’s most basic predictions and have earned a variety of prestigious science awards, including the Kyoto Prize in 2009".

indefatigable to the end..

"Do you plan to go back to Daphne?

RG: We stopped intensive work after 40 years, but we do plan to go back.

PG: The oldest person died at 122 years old. That means we have 40 more years".

Ground finches are off course the birds that Charles Darwin famously observed when on tour to the Galapagos, but infamously didn't mention in his book since he never labelled his samples according to their island location. He did borrow correctly labelled samples from other sailors and then had the ornithologist John Gould classify them. Gould's finding was that the islands finches were a group of sibling species despite their widely varying body and beak size and shape.

Habitats varied on different islands. Darwin realized that sometime in the distant past one ancestral population of finches must have immigrated from the South American mainland and then diverged into several morphologically distinct species, each a fit to its habitat. That was one of the threads of reasoning he weaved into a more comprehensive theory of common descent and evolution through natural selection.

The book, Beak of the Finch by Jonathan Weiner, is a bit dated but is still a riveting account of Rosemary and Peter Grant's research.

Update: @avinashtn alerted me to the Grant's book "40 Years of Evolution".

No Population Continuity Between Pre Toba And Extant Humans In India

A few years ago stone tools were discovered in the Jurreru Valley region of Kurnool district, South India, in sediment stratigraphically below a volcanic ash layer dated to around seventy four thousand years ago. This was the deposit of the famous Toba eruption. Michael Petraglia, an archaeologist based at the University of Oxford, England, suggested that these tools were made by Homo sapiens. This would mean that our species had first migrated out of Africa and into India perhaps as early as hundred thousand years during the Marine Isotope Stage 5 interglacial phase when ecological corridors may have opened up between Africa, Arabia and the Indian subcontinent. This is much before the more commonly accepted dates of around fifty to sixty thousands years ago. Other scientists objected and argued that the tools were made by an earlier species of archaic Homo, perhaps descendants of Homo erectus who had migrated to India more than a million years ago. The various theories of the dispersal of Homo sapiens from Africa has been summarized well recently in an article by Huw. S Groucutt and colleagues.

The earliest unequivocal skeletal evidence of the presence of anatomically modern humans in the Indian subcontinent comes from Sri Lanka where these remains have been dated to be around thirty five thousand years old. They represent humans from the later wave of the out of Africa migrations.

A related question was left dangling. If these tools were made by people from an earlier migration of Homo sapiens, then is there population continuity between those older migrants and living Indians?  Did later migrants mix with the earlier inhabitants or did the earlier human populations go extinct without leaving a genetic legacy in us.

There were other hints of the presence of an older wave of Homo sapiens migration into India. The Indian Early to Mid Pleistocene hominin skeletal record is quite poor with examples only from the Narmada Valley at Hathnora and Nethankari . At the latter site, a humerus interpreted to represent a "short and stocky" early Homo sapiens has been found associated with delicate bone implements. The remains may be around seventy five thousand years old or even older. At Hathnora, two clavicles and a partial 9th rib was recovered from a layer of fluvial sediment. The materials are thought to be about one hundred and fifty thousand years old and have been interpreted to be an archaic Homo sapiens. What is the margin of error on these dates? Could they be a little younger and represent the early MIS 5 phase migration from Africa around one hundred to one hundred and twenty five thousand years ago? This population seems to have persisted for several tens of thousands of  years as is evidenced by the younger remains at Nethankari.

There is evidence in the form of tools  as well as skeletal material found in Israel, the Arabian Peninsula and China, that indicate that anatomically modern humans did migrate out of Africa as early as a hundred thousand years ago.  A series of DNA studies of global human populations published a few days ago seems to say that people from these earlier migrations died out without contributing ancestry to extant humans. The three studies say that all non-African humans have descended from a single wave of migration  that took place between fifty thousand and eighty thousand years ago.

The scientists, A.R. Sankhyan and colleagues, working on the remains of the short and stocky Narmada Valley hominin had suggested that this population may have contributed ancestry to later short bodied people of South Asia, for example the Andamanese tribes. This scenario now looks untenable. These older (putative) Homo sapiens  in India and elsewhere died out without leaving a genetic trace.

The exception to these findings seems to be in Papua New Guinea. One study finds that 2% of the genome of present day Papuans originated from an earlier expansion  of modern humans out of Africa.

Carl Zimmer has written a good summary of the results.

Here are the links to the papers -

1)  Genomic analyses inform on migration events during the peopling of Eurasia
2) The Simons Genome Diversity Project: 300 genomes from 142 diverse populations
3) A genomic history of Aboriginal Australia

Why didn't people from the two separate waves of modern human migrations mate? The answer likely is because they never met. These older Homo sapiens populations went extinct before the new settlers came. I say this because recent genetic work has shown that one almost inevitable outcome of the meeting of two peoples, however different they may be, is sexual intercourse. When modern humans left Africa fifty-sixty thousand years ago they met and interbreed with Neanderthals and Denisovans, two older hominin groups whose ancestors had left Africa about half a million years ago.

Consider also what happened much later in the Holocene. The end of last ice age and the advent of agriculture saw population growth and the migration and mixing of people. Many of these populations had diverged and remained relatively isolated for more than twenty thousand years, accumulating significant cultural, linguistic and physical difference between them. Yet, the result of the meeting of these people was mostly not the genetic disappearance of one group, but admixture and the formation of modern groups with multiple streams of ancestry.

Today's Europeans contain ancestry from three different groups. A small fraction from earlier resident hunter gatherers and the more substantial fraction from Near East farmers and from Central Asian steppe pastoralists. When Europeans began colonizing the Americas, the native populations suffered immensely from disease and subjugation. But there was also genetic admixture. Native Americans today, both from South and North Americas, contain a noticeable amount of  European and African ancestry.

In the Indian context multiple events of mixing in the Holocene took place between residents (the Ancestral South Indians) and migrants from the Eurasian regions (the Ancestral North Indians). Additions layers of ancestry to the Indian melting pot (but common more in the eastern parts of the country) were contributed by migration of the Tibeto-Burman and the Austroasiatic people from the north east.

Why did the older group of Homo sapiens go extinct? According to Dr Pagani, one of the scientists involved in the first study I listed above “They may have not been technologically advanced, living in small groups,”... “Maybe it was easy for a major later wave that was more successful to wipe them out.”

Or as I suggested, they went extinct before the new settlers arrived. Living in small isolated populations leaves people vulnerable to disease and environmental catastrophe. One such event could have been the Toba eruption which had considerable environmental impact in South Asia. Could that have played a role in the demise of older Homo sapiens groups in Asia?  It would be interesting to see if there is archaeological evidence of an overlap between the two groups of modern humans anywhere.

Mallick, S., Li, H., Lipson, M., Mathieson, I., Gymrek, M., Racimo, F., Zhao, M., Chennagiri, N., Nordenfelt, S., Tandon, A., Skoglund, P., Lazaridis, I., Sankararaman, S., Fu, Q., Rohland, N., Renaud, G., Erlich, Y., Willems, T., Gallo, C., Spence, J., Song, Y., Poletti, G., Balloux, F., van Driem, G., de Knijff, P., Romero, I., Jha, A., Behar, D., Bravi, C., Capelli, C., Hervig, T., Moreno-Estrada, A., Posukh, O., Balanovska, E., Balanovsky, O., Karachanak-Yankova, S., Sahakyan, H., Toncheva, D., Yepiskoposyan, L., Tyler-Smith, C., Xue, Y., Abdullah, M., Ruiz-Linares, A., Beall, C., Di Rienzo, A., Jeong, C., Starikovskaya, E., Metspalu, E., Parik, J., Villems, R., Henn, B., Hodoglugil, U., Mahley, R., Sajantila, A., Stamatoyannopoulos, G., Wee, J., Khusainova, R., Khusnutdinova, E., Litvinov, S., Ayodo, G., Comas, D., Hammer, M., Kivisild, T., Klitz, W., Winkler, C., Labuda, D., Bamshad, M., Jorde, L., Tishkoff, S., Watkins, W., Metspalu, M., Dryomov, S., Sukernik, R., Singh, L., Thangaraj, K., Pääbo, S., Kelso, J., Patterson, N., & Reich, D. (2016). The Simons Genome Diversity Project: 300 genomes from 142 diverse populations Nature DOI: 10.1038/nature18964

Groucutt, H., Petraglia, M., Bailey, G., Scerri, E., Parton, A., Clark-Balzan, L., Jennings, R., Lewis, L., Blinkhorn, J., Drake, N., Breeze, P., Inglis, R., Devès, M., Meredith-Williams, M., Boivin, N., Thomas, M., & Scally, A. (2015).               Rethinking the dispersal of
             
              out of Africa
             Evolutionary Anthropology: Issues, News, and Reviews, 24 (4), 149-164 DOI: 10.1002/evan.21455

Photomicrograph- Ooid Growth Over A Foraminifera Nucleus

This week's microscopic view is from the Mississippian limestones exposed in the southern Appalachian mountains of Alabama.

Ooid cortices made up of tiny radial crystals of calcite growing around a foraminifera nucleus.

I will be putting up a longer post on ooids and oolite facies as important contributors of carbonate sand through geologic time and their significance as indicators of sea water chemistry and ecologic disruptions.

Stay tuned.

#thinsectionthursday

Darwin: Househunting In London

via Darwin: The Life Of A Tormented Evolutionist: Adrian Desmond and James Moore

after his engagement to Emma Wedgwood in November 1838..

The couple took the Doctor's advice and opted for a London house until Charles said, 'I have wearied the geological public with my newly acquired cacoethes scribendi [itch to write].' Then they would 'decide, whether the pleasures of retirement & country... are preferable to society'. Charles scouted out houses, traipsing the misty November streets. The West End was out, the traffic noise was deafening (so much so that trials with wood block surfaces were about to start in Oxford Street). Bloomsbury, near the British Museum, was quieter and its leafy squares preferable. Emma gave Charley his marching orders: to investigate 'the back lanes about Regents Park' or nearer to Covent Garden 'if it is not too dear.'  But the astronomic rents came as a shock. The 'landlords are all gone mad. they ask such prices.' L150 a year was nothing. (And Charley remained canny, even with L15,000 in prospect). He and Eras decided that the Bloomsbury squares were the most affordable.

The great man obsessed over mundane matters too after all.

Review Paper- Proterozoic Basins Of Peninsular India

My friend Vivek Kale has written quite a good review of the status of our knowledge about the Proterozoic basins of Peninsular India. The review covers basin categories, basin initiation, sedimentation patterns, fossil evidence and tectonics with the context of global Proterozoic events.

This compilation is intended to present a snap-shot of the current status of the knowledge on the Proterozoic sediments and tectonic events that are preserved in Peninsular India; on the backdrop of the growing understanding of global events and environmental evolution during that period. Proterozoic sediments in Peninsular India are found in two contrasting categories of basins. Narrow linear intercratonic belts host terrigenous and marine sediments, often interbedded with volcanics and volcaniclastics; that are deformed, metamorphosed and occasionally intruded by granitic bodies. These belts abut with a tectonic contact with wide, unmetamorphosed platform sediments from epicratonic basins with limited igneous activity associated within them; clubbed as the Purana Basins of Peninsular India. Although traditionally the former (mobile belts) were considered to be older and different from the latter, emerging geochronological data demonstrates that they were coeval products of basins evolving adjoining each other in diverse tectonic setting. Available knowledge on these basins is summarised within the framework of the emerging understanding of the Proterozoic geohistory punctuated by assembly and break-up of supercontinents, progressive oxygenation of the atmosphere, changes in the sea-water chemistry; establishment of the continental free-board and the generic environments that laid the foundation of biotic evolution. Although significant advances have been made in the last decade in the knowledge of these sediments, much more is required to achieve the desired precision and resolution.

The paper is copiously referenced; Due to my interest in fossils and evolution I would have added Poornima Srivastava's excellent review of fossil eukaryotes from the Vindhyan Basin. Kale mentions the need to systematically document this fossil record as it has the potential of providing key insights into the Phanerozoic biotic explosion. I think for that the Neoproterozoic record (1000 million years ago to 541 million years ago) in particular will be of great interest. That is the time period that saw great changes in earth continental configuration (breakup of Rodinia) and formation of broader shallow marine environments. The earth warmed from a  deep freeze state - the Cryogenian Period - with perhaps consequential changes in sea water chemistry (increase in continental weathering).  Early animal evolution was triggered by these ecological upheavals.

The sediments from the Vindhyan Basin and from the Jodhpur Group in Rajasthan of Late Neoproterozoic age have yielded tantalizing Ediacaran grade fossils and some evidence of burrowing activity suggestive of a triploblastic grade worm like animals. Although molecular phylogeny show that the origin of animals goes back perhaps to between 800-700 million years ago, unequivocal body fossils of sponges, cnidarians and bilaterans (trace fossils) appear later in the Ediacaran (beginning 635 million years). And the great diversification of animals took place even later in the Early Cambrian fueled by the rise of predators and calcium carbonate biomineralization. Yet, over most of Peninsular India the very latest Neoproterozoic (sediments younger than ~ 600 million years) seems to be missing. Sedimentation had stopped in these basins by then or the section has been eroded. Except for the Rajasthan Basin (Nagaur Group, early Cambrian) there is no record of early Paleozoic sedimentation either. Most of Peninsular India thus lacks the sedimentary record bracketing the crucial 60-80 odd million years (~ 600 mya - 520 mya) over which early animal evolution unfolded.

Attention then, needs to given to the Late Neoproterozoic-Early Cambrian of the Himalayas. Late Neoproterozoic - Early Cambrian sediments from the Lesser Himalayan Krol and Tal Group of sediments not covered in this review contain Ediacaran impressions and putative animal embryos. The late Neoproteozoic Early Cambrian sections from the Tethyan Himalaya section in Spiti and Zanskar ranges also hold promising clues to our understanding of this interesting evolutionary period.

The infographic below summarizes the changes in the Neoproterozoic biosphere against the backdrop of plate tectonic events and the global carbon cycle.

Source: Butterfield N.J. 2015

Open Access

Kale, V. (2016). Proterozoic Basins of Peninsular India: Status within the Global Proterozoic Systems Proceedings of the Indian National Science Academy, 82 (3) DOI: 10.16943/ptinsa/2016/48461

Mathur, V., Shome, S., Nath, S., & Babu, R. (2014). First record of metazoan eggs and embryos from early Cambrian Chert Member of Deo ka Tibba Formation, Tal Group, Uttarakhand Lesser Himalaya Journal of the Geological Society of India, 83 (2), 191-197 DOI: 10.1007/s12594-014-0031-4

Photomicrograph: Botryoidal Silica And Dolomite Cement In Proterozoic Sandstone

This week, a gorgeous example of botryoidal and banded silica cement filling pore spaces in Proterozoic sandstones from Central India.

The sandstone has a complex history of cementation. Pore spaces are filled with dolomite or siderite, chalcedony and calcite.

Isolated dolomite rhombs (image above) were the first mineral to precipitate around quartz grains, growing inwards into pore spaces. Another strong possibility is that the rhombs are the mineral siderite which is the iron carbonate FeCO3. Siderite often alters into a mixture of hydrated iron oxides known as limonite which preserves the shape of the original siderite forming pseudomorphs.

Silica precipitation was either contemporaneous or succeeding the dolomite/siderite cements. Occasionally, silica cements cross cut the iron carbonate (image above, white arrow), indicating that at least some silica was introduced after the dolomite/siderite.

Finally, calcite cement filled the remaining open spaces.

 
It replaces the dolomite/siderite cement (top image, white arrows) but retains the iron oxide bands thus preserving the original shape of the dolomite/siderite crystals.

Calcite also cuts across (bottom image, white arrows) the silica geodes.

#ThinSectionThursday

Life Began As Clay Crystals

There is a fine article on BBC Earth by Martha Henriques on the work of chemist Graham Cairns-Smith and his theory that life may have begun as clay crystals. Cairns-Smith reasoned that clay minerals are made up of sheets of atoms bonded in a regular lattice pattern that is stacked in layers.  Pieces of this latticework break off, forming offspring crystals often with minor dislocations to the latticework. These offspring crystals grow ..break off with more minor changes... grow.. and so on. Organic molecules like the precursors of DNA might have used such a "replicating entity" as a scaffolding to build an organic replicating system.

His idea stood at the intersection of geology, chemistry and biology and his wife Dorothy recalls the reaction he got from his peers:

"He could never get funding," Dorothy says. A major stumbling block to securing research grants was that his work straddled too many different disciplines.

One time we went to California, and Graham gave lectures to the Menlo Park Geology Survey," says Dorothy. "They all said, well, your geology's fine but I don't think your chemistry's right. Then he gave a lecture to NASA on the chemistry side and they said, well, your chemistry's fine but I'm not sure about your biology. And then he lectured to Berkeley and they said, well, your biology's fine but I'm not sure about your geology".

Nowadays such grand problems are tackled by multi-disciplinary teams of sub sub specialists. If a chemist is asked to talk on the geology aspects,  he just forwards the email of his teammate.

Photomicrograph- Micro Fault Displacing Proterozoic Stromatolite Laminae

From the Paleoproterozoic Vempalle Dolomite near the village of Gani, Cuddapah Basin, South India,

This was my M.Sc dissertation area. Vempalle Dolomites got me fascinated with carbonate rock textures and diagenesis.

The image shows a micro fault displacing stromatolite laminae. Stromatolites are biosedimentary structures formed when sediment is either trapped within microbial sheets or when CaCO3 minerals like aragonite precipitate around the sheets that cover the sea floor. The microbial colonies grow in a variety of shapes and structures in response to the wave energy conditions. Flat sheet like structures like the one seen in outcrop from where I sampled this rock indicates a low energy regime.

Of interest here:

a) The presence of oolites associated with these lamellar stromatolites. Oolites form in high energy conditions where sediment grains are constantly rolled around and held in suspension for periods of time. This allows layers of calcium carbonate to precipitate around a nucleus resulting in a coated grain containing concentric rings of CaCO3. The presence of layers of oolites in a lamellar stromatolite rock suggests that oolites forming in high energy tidal channels and shoals were transported by storms onto adjacent lower energy settings such as these microbial covered tidal flats.

b) There is variation in the shape and size of dolomite crystals. This variation is not randomly distributed but is fabric selective. The fine grained stromatolite laminae has been replaced by fine grained dolomite. There is some patchy neomorphic (recrystallization) growth of this dolomitized mud into coarser irregular dolomite.  Pore spaces and sheet cracks and fractures are filled with coarser irregular shaped dolomite crystals.  Rhomb shaped dolomite crystals are associated with oolites. This suggests that the rock underwent multiple episodes of dolomitization. The fine grained stromatolite aragonite mud got replaced early by very fine grained dolomite crystals. Contemporaneously, sheet cracks and pores filled with a coarse irregular shaped dolomite crystals.  Both the saturation levels of the replacing fluid and the abundance of nucleation sites affect dolomite crystal shape and size. Finer grained substrates offer abundant nucleation sites resulting in finer grained dolomite. Crystals growing from supersaturated fluids form quickly and interfere with adjacent crystals resulting in irregular shaped interlocking textures.

Oolites made up of either aragonite or high Mg calcite crystals were replaced by rhomb shaped crystals. Rhombic shapes form when dolomite replaces coarser grained substrates or precipitates from fluids which are mildly saturated. In such instances there are fewer nucleation sites and individual crystals have a degree of freedom to grow crystal facets.


There is also chert (microcrystalline silica) in this rock. Its replaces oolites and is present in pores spaces and in fractures.

#ThinSectionThursday

Photomicrograph- Marine, Meteoric And Burial Carbonate Cements

JSR Paper Clips in their "A Look Back" series highlights an influential paper by J.A.D. Dickson on the use of staining of carbonate rocks to differentiate in a thin section the different mineral phases of calcium carbonate.

A staining procedure consisting of preliminary etching with dilute hydrochloric acid, treatment with a mixed solution of alizarin red-S and potassium ferricyanide, and a final treatment with alizarin red-S alone (Dickson, 1965) permits the distinction of orthorhombic carbonates and of calcite from other trigonal carbonates. The potassium ferricyanide stain reveals the distribution of iron in both calcite and dolomite. The use of the stains is illustrated by a discussion of the petrography of selected specimens and interpretations of the origin of various petrographic entities.

I am heartily thankful for this technique. I stained literally hundreds of thin sections of Ordovician carbonates for my PhD work. It helped me understand the changes in cement types and their chemical composition as the limestones passed from a marine setting to becoming a freshwater aquifer during sea level drops to their ultimate burial to depths of hundreds of feet where they encountered Mg rich brines from which precipitated the mineral dolomite.

Here is that sequence brought out so clearly by a mix of Alizarin Red S and Potassium Ferricyanide.

1) Bladed crystals of non ferroan marine calcite nucleated on a brachiopod shell (stained pink)
2) Equant crystals of ferroan calcite precipitated in a confined fresh water aquifer that formed during a late Ordovician sea level drop (stained purple)
3) Rhombic crystals of a non-ferroan dolomite precipitated during deep burial (not stained). This dolomite cuts across the early marine and later ferroan calcite cements.

... my series on photomicrographs of carbonates will continue...

Rhizome Structures Of Early Plants And Their Impact On Paleosols And Landscapes

From time to time it is instructive to move away from the subject of animal evolution that does tend to dominate media reports. From a sedimentology perspective, plant evolution too has played an extremely important role in shaping sediment composition and fabric, fluvial architecture and the structure of our landscape:

Belowground rhizomes in paleosols: The hidden half of an Early Devonian vascular plant- Jinzhuang Xue et.al. 2016

The colonization of terrestrial environments by rooted vascular plants had far-reaching impacts on the Earth system. However, the belowground structures of early vascular plants are rarely documented, and thus the plant−soil interactions in early terrestrial ecosystems are poorly understood. Here we report the earliest rooted paleosols (fossil soils) in Asia from Early Devonian deposits of Yunnan, China. Plant traces are extensive within the soil and occur as complex network-like structures, which are interpreted as representing long-lived, belowground rhizomes of the basal lycopsid Drepanophycus. The rhizomes produced large clones and helped the plant survive frequent sediment burial in well-drained soils within a seasonal wet−dry climate zone. Rhizome networks contributed to the accumulation and pedogenesis of floodplain sediments and increased the soil stabilizing effects of early plants. Predating the appearance of trees with deep roots in the Middle Devonian, plant rhizomes have long functioned in the belowground soil ecosystem. This study presents strong, direct evidence for plant−soil interactions at an early stage of vascular plant radiation. Soil stabilization by complex rhizome systems was apparently widespread, and contributed to landscape modification at an earlier time than had been appreciated.

My interest in this subject is a little tangential. It deals with how to recognize unconformities and disconformities in the field in carbonate sequences. During episodes of sea level falls, marine basins covered by layers of calcium carbonates shells and skeletons get exposed to atmospheric elements. Plants colonize this exposed surface and their root systems physically disrupt the layers of sediment. Rain water and organic acids released by plants dissolve sediments creating pore spaces. The disruption may be clearly visible as solution pits and collapse structures... a karst topography...

I have written a detailed post about this topic and so I won't repeat the lecture over here except to put up this image of a karst developing on Pleistocene limestones from South Florida. Notice how chemical dissolution and the action of roots have caused collapse pits on the limestone surface - 

Land Plants And Expression Of Disconformities in Limestone Sequences

Do read..

A World Made Of Coccolithophores And Foraminifera

A tweet by Andrew Alden sent me to this paper:

Factors regulating the Great Calcite Belt in the Southern Ocean and its biogeochemical significance- William Balch et al 2016

The Great Calcite Belt (GCB) is a region of elevated surface reflectance in the Southern Ocean (SO) covering ~16% of the global ocean and is thought to result from elevated, seasonal concentrations of coccolithophores. Here we describe field observations and experiments from two cruises that crossed the GCB in the Atlantic and Indian sectors of the SO. We confirm the presence of coccolithophores, their coccoliths, and associated optical scattering, located primarily in the region of the subtropical, Agulhas, and Subantarctic frontal regions.

Great Calcite Belt, Coccolithophores - tiny unicellular phytoplankton covering 16% of the global ocean...

how can one not go back to that wonderful essay by Stephen Jay Gould on Crazy Old Randolph Kirkpatrick

Kirkpatrick was an eccentric natural historian who in the early 1900's  proposed an outlandish theory that the earth was made up of Nummulites, a group of the protist organism Foraminifera. He saw nummulites everywhere he looked, in the global ocean, the entire crust, even in igneous rocks. He concluded that the earth's shell must have been made up of nummulites., Heat from the earth's interior fusing them together and fluids injecting them with silica to form the hard rock we recognize as the igneous variety..

Rocks are sometimes classified as fossiliferous and unfossiliferous, but all are fossiliferous... Really, then, there is, broadly speaking, one rock..... The lithosphere is veritably a silicated nummulosphere.

He thought that nummulites were one of earth's earliest creatures and gave them the name Eozoon and with a flourish wrote:

"After the discovery of the nummulitic nature of nearly the whole island of Porto Santo, of the buildings. wine presses, soil, etc., the name Eozoon portosantum seemed fitting one for the fossils. When the igneous rocks of Madeira were likewise found to be nummulitic, Eozoon atlanticum seemed a more fitting name."

"If Eozoon, after taking in the world, had sighed for more worlds to conquer, its fortunes would have surpassed those of Alexander, for its desires would have been realized. When the empire of the nummulites was found to extend to space a final alteration of name to Eozoon universum apparently became necessary."

We remain trapped in perceiving our world as one teeming with large multicellular animals. But the world is much more. It is a world full of microbes and unicellular eukaryotes too. These creatures occurs in numbers that dwarf our metazoan presence. They are ubiquitous in the surface ocean layers, in the sunlight plankton zone, and their skeletons blanket the depths, creating a layer of ooze covering the sea bed. Their life and evolutionary cycles modulate in large part the global carbon cycle.

Randolph Kirpatrick in his feverish imagination saw an empire of Nummulites.. not too far fetched from the Great Calcite Belt of Coccolithophores covering 16% of the global ocean.

Photomicrograph- Late Ordovician Calcite Cement Stratigraphy In Cathodoluminescence

Cathodoluminescence (CL) brings out beautifully the hidden growth history of calcite crystals. This photomicrograph is of a Late Ordovician pore space from the Fernvale Limestone, Georgia, Southern Appalachians. It is showing calcite cement grown syntaxially over echinoid fragments. Echinoid skeletons are monocrystalline. A syntaxial overgrowth means that pore filling precipitated calcite has maintained the same crystallographic orientation over this monocrystalline substrate. As a result, successive crystal masses even if precipitated at different times under different conditions appear to be one continuous block under polarized light and under crossed nicols. It takes CL to reveal these different growth phases.

The black growth zones were precipitated in oxidizing conditions by fresh water in the vadose zone (above the groundwater table). The black zones are pendant, hanging on the underside of skeletal grains. They are in essence micro-stalactites.

This was followed by another growth phase in suboxic conditions with the incorporation of divalent Mn(+2) in the calcite lattice. Divalent Mn is an activator of CL, hence the bright yellow growth bands interspersed with a thin black bands indicating periodic return to Mn poor oxidizing conditions.

The last phase is a pore filling phreatic ferroan calcite cement precipitated by reducing meteoric fluids in deeper burial conditions. Fe+2 is a quencher of CL. The cement appears dull brown.

The pore space is a couple of millimeters across.

#ThinSectionThursday

New Ancestor Of Man And Other Rants About Media Reports

I am ashamed to admit this, but these days I just shrug away the various instances of poor science reporting I notice in the Indian media. But enough outrage has been building up over a couple of  particularly bad misrepresentations of scientific findings to prompt this rant.

 1) Indian Scientists Find New Ancestor Of Man

One shudders with embarrassment at this jingoistic hyperbole. The study is an international collaboration. Why the chest beating?

The article in Deccan Herald on July 26 by Kalyan Ray completely misrepresents the evolutionary story of Homo sapiens. Here are the sentences which go badly wrong -

"Andaman’s Jarawas and Onges are descendants of a completely new family of early men unknown to science so far"..

"The discovery has the potential to open up a new window in the history of human evolution by suggesting that Homo heidelbergensis—the first group of men who came out of Africa—had given rise to multiple lineages and not just the Neanderthal and the Denisovan—the two known branches from which all modern human beings have evolved".

The writer is suggesting the modern humans evolved entirely from Neanderthals and Denisovans outside Africa and that this new research is showing that the Andamanese are descendants of a yet third branch of humans based outside Africa.

This picture given by Kalyan Ray is false. Take a look at the hominin family tree presented in the research paper.

Source: Genomic analysis of Andamanese provides insights into ancient human migration into Asia and adaptation

It presents our current understanding of human evolution and migration and admixing events between different branches of hominins. Modern humans migrating out of Africa about 60 thousand years ago met and admixed with the Neanderthals and Denisovans who were branches of an earlier wave of human migration out of Africa. This earlier wave of migration may have taken place about half a million years ago. This admixture between archaic and modern humans resulted in all living non -Africans having  2%-4% Neanderthal ancestry with additional Denisovan ancestry more common in Melanesians.  Now, this study is proposing that another unknown extinct hominid, a possible third diverged population from those earlier migrations, contributed a small amount of ancestry to south Asians. The Andamanese may be taken as an approximate proxy of the original modern humans who entered the Indian subcontinent from Africa since after diverging from a common South Asian population they have admixed less with other modern humans.

Another quibble is the sentence "Hominids are ancestors of the great apes and humans". Well, hominids is a grouping that includes both extinct and living great apes and humans. So yes, some extinct hominid would have been our ancestor, but modern humans are hominids too. As an aside, to confuse matters further, Hominin are the group that includes the extinct and living members of only the human family, excluding the chimpanzee, gorilla and orang-utans.

2) Before The Pharoah: Fresh Evidence Should Make Us Question Earlier Views Of Indus Valley Civilization

This piece which appeared in the Times of India on June 6 is referring to a paper about the link between Holocene monsoon record and the evolution of Harappan civilization. The authors also suggest a revision of the chronology of the various Harappan cultural stages.  Here is their proposed chronology.This is based mainly on the chronology proposed earlier by G.L Possehl. The authors of this study augment  that with new dates from two samples.

"The successive cultural levels at Bhirrana, as deciphered from archeological artefacts along with these 14C ages, are Pre-Harappan Hakra phase (~9.5–8 ka BP), Early Harappan (~8–6.5 ka BP), Early mature Harappan (~6.5–5 ka BP) and mature Harappan (~5–2.8 ka BP)"

And here is the conventional chronology

"Conventionally the Harappan cultural levels have been classified into 1) an Early Ravi Phase (~5.7–4.8 ka BP), 2) Transitional Kot Diji phase (~4.8–4.6 ka BP), 3) Mature phase (~4.6–3.9 ka BP) and 4) Late declining (painted Grey Ware) phase (3.9–3.3 ka BP). This chronology is based on more than 100 14C dates from the site of Harappa and nearby localities".

Here is the chronology Mr. Mehta presents:

The first line in the introduction section of the research paper makes it clear that all dates are presented in BP (Before Present). Yet Nalin Mehta in his article bungles up and without applying the necessary correction presents the chronology as representing dates in BC. The difference is 2000 years! For example, 5000 BP is 3000 BC.

Another big error he makes is lumping all the Harappan cultural stages into one mature phase spanning 8000 -2000 BC ! This gives an erroneous view of the evolution of Harappan society. The mature phase represents urbanization. The earlier cultural stages were rural antecedents represented by farming and pastoral communities and even earlier human settlements in this area. By terming the entire time span of Harappan culture as belonging to the mature phase, Mr Mehta gives an impression that Harappan cities were as old as 8000 BC. This is certainly not the case. This new study revises the mature phase of the Harappan culture from the accepted ~2600 BC-2700 BC (4700 BP) to ~ 3000 BC (5000 BP). This proposed revision at one cultural site should not be taken to mean that dates for cities like Harappa, Mohenjodaro, Dholavira will suddenly be changed to 3000 BC. Their chronology needs to be ascertained independently. As of now, large number of C14 and thermoluminescence dates have secured the age of these cities to be around 2700 BC or so.

One has to be careful with terminology. Mr Mehta uses dates as old as 8000 BP (wrongly presenting them as 8000 BC) to imply that the Harappan civilization is older than the Paraoahs of Egypt. Such a comparison is meaningless. These earlier dates represent a rural society. No doubt there was population and cultural continuity of these earlier people with the later urban phase, but you can say the same thing about pre-urban Egyptian and Sumerian cultures evolving into a full fledged urban civilization. There was a long pre-urban phase from 5-6 millenium BC in Eygpt and Sumer (synchronous to the Indus region) with central political consolidation and urbanism by around 3100 BC in Egypt when the first dynastic kings known as the Pharaohs seized power. In Sumer, the transition from rural to urban took place even earlier with cities like Uruk gaining prominence well before 3500 BC.

The differently named cultural stages of the Indus valley carry a specific meaning  in terms of societal complexity and cultural changes. You can't just call everything mature Harappan and then claim that the finding requires some kind of a fundamental rethink of Harappan society. 

As it happens, the dates presented in the paper that Mehta is ga-ga about are not new. Archaeologists have been aware of the alternate chronology presented by G.L Possehl for about 15 years now! In that sense there is nothing revolutionary about the chronology presented in this paper.

..rant over.