Saturday, September 24, 2016

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, 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.

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.

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 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. This scenario now looks untenable. These older (putative) Homo sapiens  in India and elsewhere died out without leaving a genetic trace.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.

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 immensly 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.

Thursday, September 15, 2016

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

Monday, September 5, 2016

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.

Attention then, needs to given to the Late Neoproterozoic-Early Cambrian of the Himalayas. Late Neoproterozoic 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

Thursday, September 1, 2016

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