Friday, June 22, 2018

How Did Homo Sapiens Evolve?

Here is a very good summary of our evolving understanding of this question.

How did Homo Sapiens Evolve? - Julia Galway-Witham and Chris Stringer

Excerpt:

Over the past 30 years, understanding of Homo sapiens evolution has advanced greatly. Most research has supported the theory that modern humans had originated in Africa by about 200,000 years ago, but the latest findings reveal more complexity than anticipated. They confirm interbreeding between H. sapiens and other hominin species, provide evidence for H. sapiens in Morocco as early as 300,000 years ago, and reveal a seemingly incremental evolution of H. sapiens cranial shape. Although the cumulative evidence still suggests that all modern humans are descended from African H. sapiens populations that replaced local populations of archaic humans, models of modern human origins must now include substantial interactions with those populations before they went extinct. These recent findings illustrate why researchers must remain open to challenging the prevailing theories of modern human origins.

..and on the status of Recent African Origin (RAO) versus Multiregionalism, the two competing theories of modern human origins

The apparently more deeply rooted origin of H. sapiens adds to the problem of how scientists can delineate H. sapiens from other species in an evolutionarily meaningful way and suggests that the species did not evolve as recently as previously envisaged. However, evidence of evolutionary novelty in the fossil record of putative H. sapiens still appears relatively recently in Africa, compared to a multiregional view of human evolution (in which H. sapiens began to evolve at least 1.8 million years ago). Within Africa, it is currently unclear whether the origin of H. sapiens was localized, as in some early RAO formulations, or involved a process similar to multiregional evolution, but operating purely within the continent (5, 13).

With only a few dissenters, the strictest versions of both RAO (which denies interbreeding with other lineages or species) and multiregionalism (which argues for an interbreeding network of one species over the past ~1.8 million years) are now generally regarded as falsified. Instead, two intermediate theories best accommodate the complex interactions between hominin taxa ~40,000 to 100,000 years ago (8, 14): RAO with hybridization (RAOWH) and the assimilation model (AM) (see the figure).



Source: How did Homo Sapiens Evolve? - Julia Galway-Witham and Chris Stringer 2018

Open Access.

Sunday, June 17, 2018

Mars Geology

Came across a couple of cracking papers on Mars geology

1) Ancient Martian aeolian processes and palaeomorphology reconstructed from the Stimson formation on the lower slope of Aeolis Mons, Gale crater, Mars - Steven G. Banham et. al. 2017

During its travels, the Mars Curiosity Rover has been taking some exquisite pictures of landscapes and rock outcrops. This study uses field photos of a sandstone body and analyses its ancient depositional setting.  The sandstone is a dune field, created and shaped by aeolian processes. The overall aim is to better understand the sedimentary environments on Mars, how they changed over time, and whether they could have been habitable environments.

Just take a look at the spectacular cross bedding and bed sets of the dune.


Source: Steven G. Banham et. al. 2017

And here is a depiction of the regional setting of the dune field


Source: Steven G. Banham et. al. 2017

2) A Field Guide To Finding Life On Mars- S. McMohan et. al. 2018

Excerpt: This paper reviews the rocks and minerals on Mars that could potentially host fossils or other signs of ancient life preserved since Mars was warmer and wetter billions of years ago. We apply recent results from the study of Earth’s fossil record and fossilization processes, and from the geological exploration of Mars by rovers and orbiters, in order to select the most favoured targets for astrobiological missions to Mars. We conclude that mudstones rich in silica and iron-bearing clays currently offer the best hope of finding fossils on Mars and should be prioritized, but that several other options warrant further research. We also recommend further experimental work on how fossilization processes operate under conditions analogous to early Mars.

I got to relearn a good bit about how depositional settings, mineralogy and geochemistry influence organic matter preservation.

Both Open Access.

Monday, June 11, 2018

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.

Monday, May 21, 2018

W. Bengal Bangladesh- Geologic Controls On Arsenic Distribution In Ground Water

Science writer  Priyanka Pulla has written an excellent article exploring the geologic, socio-economic and technological issues related to the widespread arsenic contamination of groundwater in W. Bengal. Sadly, the government response to this crisis has been slow.

I thought I would elaborate on the geological question -  Why are Arsenic (As) levels much higher in shallower Holocene age aquifers and lower in the deeper Pleistocene age aquifer? The answer encompasses mineralogy, climate change, sea level changes and bacteria.

The ultimate source of As are high Himalayan rocks and Indo-Burman ranges with additional contributions from the Precambrian terrains of Peninsular India and the Siwalik hills.  Minerals like biotite, magnetite, illmenite, olivine, pyroxene, amphiboles contain As. These minerals release As when they undergo weathering in catchment areas and deposits of the alluvial plains. This As is absorbed on secondary minerals like Fe hydroxides like goethite. Such Fe hydroxides are authigenic, i.e. they grow in the shallow buried sediments of the alluvial plains. Under oxidizing conditions, As is immobile, sequestered in Fe hydroxides. However,  conditions may change, and these sediments may get overlain by or be redeposited in environments rich in organic material. Certain bacterial species living on this organic material break down these Fe hydroxides, using the oxygen for their metabolism, and releasing Fe and As into the groundwater. This is known as reductive dissolution of Fe hydroxides and is the principal mechanism for As entering the groundwater in the alluvial plains of Bangladesh and West Bengal.

During the Pleistocene.. 1) the high Himalaya was glaciated. Therefore, important sources of As like the Fe-Mg rich rocks of the Indus ophiolite belt (slices of oceanic crust that existed between India and Asia which have been thrust up during continental collision) and high grade metamorphic rocks such as schists and gneisses were covered in ice and not releasing sediment. Indian cratonic areas, the Siwalik foothills  and the Indo-Burman ranges were being eroded, but overall less As was making its way on to alluvial plains. 2) Since climate was cooler and drier, there was less organic material accumulating in sediment of alluvial plains. Conditions were oxidizing and As remained sequestered in Fe hydroxide minerals. 3) Sea level was much lower then. Almost the entire continental shelf was dry land. Ganga and Brahmaputra met the sea much to the south of present shoreline. Reducing environments like delta front marshes, ponds, estuaries, existed much to the south.

Sedimentary conditions changed by 12-15 thousand years ago. Glacial melt exposed As bearing rocks in high Himalaya. As a result, more As made its way on to alluvial plains. Importantly, sea level rose and flooded the continental shelf. The Pleistocene delta front reducing environments were drowned. Shorelines shifted northwards. The climate was warmer, encouraging vegetation growth. Reducing delta front environments like swamps, coastal marshes and lakes developed on previous alluvial plain sediments.

The map below shows the position of shorelines between 7 thousand and 4 thousand years ago along with the location of wells with high levels of As. This study focuses on Bangladesh but similar conditions existed in West Bengal as well. The sea has receded 2- 3 meters to its present location since 4 thousand years ago.  The delta front and shoreline belt that existed 4-7 thousand years ago is now a densely inhabited region .


 Source: Quaternary shoreline shifting and hydrogeologic influence on the distribution of groundwater arsenic in aquifers of the Bengal Basin- M. Shamsudduha, Ashraf Uddin 2007

Notice clustering of wells with high As along the past shorelines. Here, organic rich delta marshes and swamps developed. Bacterial reduction of Fe hydroxides released As in to groundwater.

As distribution also shows correlation with topography. This map shows high As levels in groundwater coinciding with topographic lows. Such low lying areas accumulate more fine sediment and organic material. Again, this will apply also to W. Bengal.


 Source: Quaternary shoreline shifting and hydrogeologic influence on the distribution of groundwater arsenic in aquifers of the Bengal Basin- M. Shamsudduha, Ashraf Uddin 2007

So, a change in climate and shifts in sedimentary environments in response to changing sea level from Pleistocene to Holocene exerted a strong control on As distribution in the alluvial plains of Bangladesh and W. Bengal. 

Monday, May 14, 2018

Dharwar Craton- Gooty Fort

I had quite a fruitful interaction today on Twitter, after I replied to a tweet about the location of Gooty Fort in Andhra Pradesh.

The fort is built on Archaean gneiss terrain just to the west of the Cuddapah sedimentary basin. I'm embedding the tweet below. It invoked quite a few questions about the terrain and the geology of that region.



And here is the link to the full discussion thread on Twitter - Gooty Fort, Andhra Pradesh