Readings: Mars Geology, Human Diversity, India Rock Art

 A few interesting readings:

1) NASA's Mars Perseverence Rover is hard at work. It has an amazing collection of geochemical instruments which are probing the surface with the aim of categorizing the mineralogy and chemistry of surface materials. The hope is to pinpoint regions which could have hosted microbial life.

Signs of Life on Mars: NASA's Perseverance Rover Begins the Hunt 

2) How are Andaman Islanders closer to Swedes than to Africans?

Razib Khan explain in this informative essay on patterns of human diversity and what it tells us about human migrations and population admixture over the past 100,000 years.

Out of Africa's midlife crisis-on bottlenecks, crashes and what diversity really looks like: How are Andaman Islanders closer to Swedes than to Africans?

3) On the Aravalli ranges quartzite rock faces in the state of Haryana is art created as long as 20,000 years ago. The locals always knew about it, but the Archeological Survey has just begun studying it in detail. 28 ancient sites have been found. I hope all of them get protection immediately. Smithsonian Magazine has a summary describing these finds. The final photo of rock art in the article depicts mounted warriors. Are they mounted on donkeys/mules or horses? Curious to know what readers think. I am leaning towards them being donkeys or mules.

These Millennia-Old Cave Paintings May Be Among India’s Oldest. 

Books: Earliest Societies, Early Medieval India

 You didn't think I would stop at just three books, did you? More were delivered few days back.

1) Was the transition from a hunter gatherer lifestyle to agriculture a prerequisite for the formation of complex societies? James C.Scott explores this link between sedentism, domestication and state formation. The Sumerian Ur city state that formed around 3800 B.C is one example of this. New archaeological discoveries are hinting at complex societies of antiquity greater that the agriculture linked complexes that came up in the fertile crescent.  Sites like Gobekli Tepe in Turkey may make us reexamine our assumptions regarding the causes and timing of the formation of early states. Besides this book, I will recommend this essay by Samo Burja - Why Civilization is older than we thought. 

 

2) In the past year, I read four fine books on Indian history covering the time span from the 1000's to about the 1750's. India in the Persianate Age 1000-1765 and A Social History of the Deccan, 1300-1761: Eight Indian Lives, both by Richard M. Eaton. The Emergence of the Delhi Sultanate by Sunil Kumar who sadly passed away recently. And the fourth was The Mughal State 1526-1750. This is a collection of essays collated by Muzaffar Alam and Sanjay Subrahmanyam with a long introductory critical essay on Mughal historiography by the two editors. I thought it was time for me to explore the few centuries preceding the arrival of Central Asian Turkic invaders. The Making of Early Medieval India and The Early Medieval in South India look like good introductions to this time period. 

Books: Animal Minds, India Language History, India Governance

 New on my book shelf:

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

 

 

 

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

 

 

 

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

Mass Extinction, Peopling Of America, Tale Of The Horse

 Sharing some interesting items:

1) What was the impact of Deccan Volcanism on the end-Cretaceous mass extinction? Improved dating of the timing of volcanism shows that volcanism spanned the mass extinction. But what changes occurred to marine environments because of the outgassing wasn't well documented. A new study uses the oxygen isotope ratios in foraminifera shells to estimate ocean temperature changes before and after the mass extinction. The finding is that the oceans warmed well before the extinction but cooled back again. The warming event doesn't appear to correlate with marine extinctions. Rather the mass extinction coincides with evidence for a meteorite impact. 

Here is a figure from the paper on the estimated temperature changes collated using a variety of proxies:

Source: On impact and volcanism across the Cretaceous-Paleogene boundary

Joshua Sokol has written a good summary of the paper:

A Rapid End Strikes the Dinosaur Extinction Debate.

2) Anthropological geneticist Jennifer Raff has pieced together the genomic story of the peopling of the American continents in this really insightful article. Do read it!

Genomes Reveal Humanity’s Journey into the Americas.

3) And next, onwards to a bit of Indian history. A very interesting conversation between Live History India editor Mini Menon and author Yashaswini Chandra on Ms. Chandra's new book, The Tale of the Horse: A History of India on Horseback. Fascinating story of the horse trade from Central Asia into India and its assimilation as a war animal and into Indian society. 

The Tale of the Horse (video). 

Readings: India Dams, Geology Videos, Parsis in India

 Sharing some readings.

1) Neeraj Wagholikar, Parineeta Dandekar and Himanshu Thakkar weigh in on the dam building epidemic that is afflicting India. These three experts cover issues of environmental governance, destruction of fisheries and livelihoods, and a perspective on their irrigation potential and economic logic.

The deep political drive to push through permissions to build dams is best highlighted by an example of a malign recommendation in a report of the Parliamentary Standing Committee on energy published in January 2019. It seems to view in favor Himachal Pradesh's suggestion to the committee to help declare large hydropower projects as linear projects, thus enabling them to bypass Gram Sabha consent. The statement reads, “If it is done, then, to a large extent, the problem of FRA, which the Secretary also mentioned, will get resolved because the stringent provisions of FRA will get diluted. It is not our purpose to subvert them. Our only purpose is to get them more liberalised.” 

FRA is the Forest Rights Act which gives local forest dwellers a say in the site selection of infrastructure projects. 

Makes you despair and shake in anger, doesn't it?

India, Dammed.

2) Geology fans! I highly recommend Rice University Professor Cin-Ty Lee's YouTube Channel. He has a very informative collection of short videos on rocks and minerals and geologic processes. 

Here is one of my favorites.. Isostacy and what controls the elevation of mountains?

Email subscribers who can't see the embedded video, can view it here - Elevation of Mountains.

3) Like Sugar in Milk.. was the memorable assurance given by the Zoroastrian refugees to the King of Gujarat. We will assimilate in Indian society. And they have in many ways, while maintaining a distinct identity. 

What does genetics tell us? Fine post by Razib Khan.

Endogamy and Assimilation. Parsis in India.

Sedimentary Structures: Building Stones of Badami, Aihole And Pattadakal Temples

Is this sandstone slab in its original geological orientation (as when the sedimentary layers were deposited) or is it upside down? I'll answer this a little later, but first some background.

I recently visited the Chalukya style temples and rock cut monuments at Aihole, Pattadakal and Badami (6th -8th CE) in northern Karnataka and noticed some great sedimentary structures in the building stones. The term sedimentary structures refers to the shape and form sedimentary layers get sculpted into by the action of waves, currents, tides and wind during deposition of the sediment. The size of the deposited sedimentary particles and the orientation of layers are a reflection of both the vigor of the currents and waves and the direction of flow of water or wind.  

These monuments are made up of Neoproterozoic age (900-800 million year old) sandstones. Geologists have recognized using detailed sedimentological analysis that the sandstones formed mostly in a large braided river system that flowed in a northwesterly direction.

Between  roughly 1800 -800 million years ago, over the course of a billion years, the Indian continental crust sagged due to various tectonic forces to form several long lasting sedimentary basins. The Kaladgi Basin in which the Badami area sandstones were deposited is one such basin. The paleogeographic reconstruction below shows the position of the Indian continent at about one billion years ago and the location of the various sedimentary basins within it.

Source: Shilpa Patil Pillai, Kanchan Pande and Vivek S Kale: 2018: Implications of new 40Ar/39Ar age of Mallapur Intrusives on the chronology and evolution of the Kaladgi Basin, Dharwar Craton, India.

Much of this deposition took place in inland or epeiric seas that flooded the Indian continent. During intervals of sea level fall, rivers carved valleys and deposited coarse sediment. The Badami Cave sandstones are river deposits of the Kaladgi Basin. The stratigraphic column shows various sedimentary deposits of the Kaladgi Basin and their inferred environments of deposition.

Source: Shilpa Patil Pillai, Kanchan Pande and Vivek S Kale: 2018: Implications of new 40Ar/39Ar age of Mallapur Intrusives on the chronology and evolution of the Kaladgi Basin, Dharwar Craton, India.

The Badami braided river system was receiving sediment eroded from Archean age (>2.5 billion year old) rocks situated SE of the basin. These were granites, granodiorites, and low to medium grade metamorphic  rocks of the Dharwar craton (a large block of stable old continental crust). 

Land plants did not exist then. Weathered debris was moved quickly by surface flow into streams. Large sediment load, moving by traction i.e. by rolling and sliding on the stream bed, repeatedly choked the channels, forcing bifurcation of streams and formation of braids. Very broad braided rivers formed since there were no plants to stabilize banks.  The Badami sandstones (Cave Temple Formation) are technically known as arenites. This term indicates that the rock is made up of mostly coarse sand with very little finer sized mud. Accumulation of mostly coarser sand size and pebbly particles reflects a locale of repeated high discharges and vigorous currents which winnowed away the finer sized mud.  The braided river shown below as an example is from the Canterbury Plains of New Zealand.

 Source: Braided Rivers: What's the Story?

The Badami rocks preserve a record of  various subenvironments of this paleo-river. Picture shows channel and bar deposits in outcrop.

Source: Mukhopadhyay et. al. 2018; Stratigraphic Evolution and Architecture of the Terrestrial Succession at the Base of the Neoproterozoic Badami Group, Karnataka, India.

As river channels episodically migrated sideways and the basin floor subsided to accommodate more sediment, channel deposits and adjacent sand bars got stacked to form thick 'multi-story' sandstones. Each bed tells a story of a discrete depositional episode.

The arrangement of sand layers within each bed tells us about the subenvironments in which it formed and the energy and direction of water flow during deposition. I came across many types of these internal structures. I recognized tabular cross beds, trough cross beds, planar lamination and rippled beds. Water (or wind) can move & shape sand into piles or waves. Sand grains roll along the direction of flow, then avalanche down the steeper side (lee side) of the wave forming a layer inclined (cross) to the orientation of the main sand body. Successive avalanches form a set of cross beds. The graphic shows the formation of a set of cross beds.

 Source: Dr. Diane M Burns in Teaching Sedimentary Geology in the 21st Century.

Here is an example of cross beds from near the town of Badami.

And this one is from a building stone from Pattadakal temple.
 

Such cross beds were built by sediment avalanching on the lee side of migrating sand bars during high flow.

This picture show trough cross bedding from near the Badami cave complex. These represent the internal structure of migrating sinuous sand dunes on a channel floor. 

See this elegant explanation by Dawn Sumner, a sedimentologist at the University of California at Davis,  of how trough cross beds form.



Email subscribers who may not be able to see the embedded video, click on this link: Trough Cross Bedding Video.

And here is a beautiful example of trough cross bedding found in a Pattadakal temple building stone.

This is planar lamination on a slab at Pattadakal. The bed is constructed of parallel layers of coarse sand. It is interpreted to have been deposited in a high flow regime from sheets of water flowing over mid channel sand bars.
 

Ripples on a slab at Pattadakal. This is a rare preservation of a bedding surface showing rippled sand. Erosion usually cuts off the wavy upper part. These ripples indicate migration of small sand waves in a quieter flow regime on the channel floor.

Remember, cross beds are the inclined layers that form on the lee side of a ripple or wave or dune. Here are small cross sets on the floor of Aihole rock cut temple! These represent the cross beds formed by migration of small ripples. The ripples themselves have been eroded away. Arrows indicate the direction of water flow and cross bed accretion as ripples migrated.

Okay, let's go back to my first question. Is the slab I showed in the picture geologically upside down?

Yes it is. But how to tell?

As sand avalanches down the lee slope it forms a tail at the toe of the slope resulting in cross beds which become tangential to the floor. In picture the cross beds are tangential towards the top of slab i.e. that is actually the base.

Lets see at how the cross bed contact with the top and bottom bedding plane looks in an outcrop. Here is the original depositional orientation of cross beds manifest in this outcrop near Badami caves. They show a tail or tangential contact of the cross beds with the base. Since top of cross beds are not usually preserved they show a high angle contact truncated by upper bedding plane.

This slab is upside down too! Notice again the tangential contact of the cross beds (white arrow) is towards the top, which means that must have been the base. Yellow arrow points to high angle contact with the upper bedding surface. 

Towards the top of the exposed section of sandstone around Badami I came across some truly impressive examples of cross bedding. These particular exposures were on the crags opposite the four main Badami temples. There is a narrow passage past the archaeological museum and a short climb to the top. Take a look at these beauties!

These large cross beds reminded me of the inclined beds of wind blown sand dunes. Is it possible that abandoned sand bars were sculpted by wind in to big dunes? Or does this upper level sandstone represent, as a recent study suggests, the beginning of a marine incursion in to the basin? In this scenario, deposition of sand took place in high-energy shallow waters near the shore. These cross beds represent large migrating sand waves which were eventually shaped in to beach ridges and tidal bars.

The outcrops and building stones of these monuments mostly record the processes within the Badami braided paleo-river. 900 million yrs ago a complex of channels and bars, quieter pools and rippled sand beds existed where these temples stand today.

Do visit Aihole, Pattadakal and Badami and gaze at its splendid architecture and sculptures. But spare some time to appreciate the magnificent record of our natural history that these monuments preserve. 

Quiz- Is this slab upside down or in its true depositional orientation? 😉

Until next time....

Glacial Saraswati?: New Data On An Old Question

Did a perennial glacial fed river flow through the Indus Civilization region of what is now Haryana and Rajasthan? Previous work on the fluvial history of this region had indicated that a distributary of the glacially sourced Sutlej was flowing through a network of paleo-channels buried under the river now known as the Ghaggar until around 8,000 years ago. The Sutlej distributary system then died out, turning that river course into a smaller monsoon fed channel system.

For a more detailed history of research on this topic you can follow this link - Ghaggar /Saraswati Posts.

Recently, in November 2019, Anirban Chatterjee and colleagues published new data on deposits of grey sand in the subsurface of the Ghaggar channel and adjacent floodplains. The youngest of these deposits are 4, 500 years old. Geochemical fingerprinting points to High Himalayan granites and gneisses as their source. This likely extends the glacial phase of the Ghaggar to more recent times, until about the beginning of the urbanization of the Indus Valley Civilization (IVC).

Here is the abstract:

The legendary river Saraswati of Indian mythology has often been hypothesized to be an ancient perennial channel of the seasonal river Ghaggar that flowed through the heartland of the Bronze Age Harappan civilization in north-western India. Despite the discovery of abundant settlements along a major paleo-channel of the Ghaggar, many believed that the Harappans depended solely on monsoonal rains, because no proof existed for the river’s uninterrupted flow during the zenith of the civilization. Here, we present unequivocal evidence for the Ghaggar’s perennial past by studying temporal changes of sediment provenance along a 300 km stretch of the river basin. This is achieved using 40Ar/39Ar ages of detrital muscovite and Sr-Nd isotopic ratios of siliciclastic sediment in fluvial sequences, dated by radiocarbon and luminescence methods. We establish that during 80-20 ka and 9-4.5 ka the river was perennial and was receiving sediments from the Higher and Lesser Himalayas. The latter phase can be attributed to the reactivation of the river by the distributaries of the Sutlej. This revived perennial condition of the Ghaggar, which can be correlated with the Saraswati, likely facilitated development of the early Harappan settlements along its banks. The timing of the eventual decline of the river, which led to the collapse of the civilization, approximately coincides with the commencement of the Meghalayan Stage.

The geological work looks to be sound. The data on sediment fingerprinting overlaps with what we know about High Himalayan geochemical signatures and present day Sutlej sand composition.

I do want to comment on another sentence from the abstract (emphasis mine)-

"This revived perennial condition of the Ghaggar, which can be correlated with the Saraswati, likely facilitated development of the early Harappan settlements along its banks"

Saraswati is the name given to this river by the Vedic people. Correlation of the river's perennial phase between 9,000-4,500 years ago with Saraswati is valid only if  you can demonstrate that the Vedic people were inhabitants of this region from before 4,500 yrs ago. Geological studies cannot establish this. A combination of archeology, linguistics (cracking the Indus script would be nice!) and genetics will eventually answer that. The other scenario is that the Vedic people could have migrated into this region much later and began venerating a smaller monsoonal Ghaggar as Saraswati. Work by Liviu Giosan and colleagues suggests that stronger monsoons over the Siwaliks sustained sufficient flow in the old channels of the Ghaggar until the late IVC period (~1800-1600 B.C).

When did this river come to be called the Saraswati is still an open question.

Two recent genetics papers using ancient DNA recovered from the IVC site of Rakhigarhi and from Central Asia argue that people from the Pontic-Caspian steppes migrated into South Asia between 2000 -1500 B.C. bringing with them the Indo-Iranian branch of the Indo-European language family. These would presumably be the Vedic people.

Read these papers. They are interesting.

1) On the existence of a perennial river in the Harappan heartland.
2) The formation of human populations in South and Central Asia.
3) An Ancient Harappan Genome Lacks Ancestry from Steppe Pastoralists or Iranian Farmers.

Groundwater Must Be The Focus Of India National Water Policy

India's Water Management Crisis

A piercingly clear essay by Himanshu Thakkar on why India must realign its water resources priorities from big dams and river linking projects to protecting, managing, and regulating ground water.

Just take a look at the numbers:

"Most of the water that India uses today comes from over 30 million wells and tubewells. Irrigation is India’s biggest user of water and over two thirds of irrigated area gets water from groundwater. 85% of rural domestic supply, over 55% of Urban and Industrial water supply comes from groundwater. The graph of % of water in each sub sector coming from groundwater has been going up for at least four decades. In fact, some estimates show that over 90% of additional water India used in last four decades have come from groundwater. It sounds like an immitigable blessing. That’s not how blessings work, unfortunately.

Central Ground Water Board’s data shows that in about 70% of areas, groundwater is depleting and at many places it has exhausted or is on verge of exhaustion. The quality is deteriorating. Warnings have been available for decades now, but the government has done little to address the emerging crisis.

In fact, India’s water resources establishment, lead by the Big dam ideologues at Central Water Commission have ensured that the government do not even acknowledge that groundwater is India’s water lifeline"....

Scary.

Some States have taken initiatives to manage ground water. Maharashtra recently passed the Maharashtra Ground Water Act which provides a framework for management and regulation of ground water. How much diligent enforcement of the rules actually takes place remains to be seen.

Additional Reading:

The Maharashtra Groundwater (Development and Management) Act 2009 - Shashank Deshpande, Deputy Director GSDA.

A Decade Of The Maharashtra Ground Water Legislation: Analysis Of The Implementation Process - Sanjiv Phansalkar and Vivek Kher.

India Shale Gas: Environmental Concerns

Shale gas is natural gas trapped in very fined grained sedimentary rocks like shales. These rocks are not very permeable. To release the gas trapped in the tiny pore spaces, the rock is fractured by injecting water, sand and various chemicals into it at very high pressure. Several million gallons of fresh water is needed for such ' fracking' activity at any one site. 

Shashikant Yadav, Gopal K Sarangi and M P Ram Mohan in an essay in the Economic and Political Weekly explain the environmental concerns that shale gas production poses in India.

Regarding the guidelines for environmental management released by the government -

Further, the guidelines mention that water management is one of the key concerns. They state that the major and prime difference being in the hydraulic fracturing technologies requiring a large volume of water; the activities are likely to deplete water sources and cause pollution due to the disposal of produced water. However, instead of dealing with the water-specific issues, the guidelines (apart from explaining existing provisions) stated that the generic environment clearance process adopted by the Ministry of Environment, Forest and Climate Change (MoEFCC) will suffice to ascertain water-related issues posed by fracking. But, MoEFCC has not laid down any specific guidelines, policies, or manuals differentiating between conventional and unconventional gases to grant environment clearance.  More recently, despite the gaps, on 1 August, 2018, the cabinet approved a policy allowing companies to exploit shale gas in contract areas that were primarily allocated to exploit conventional gas.

..and this in the context of the ambiguous legal framework surrounding groundwater -

Considering the limited water legislation in India, the implementation of fracking may result in geopolitical and legislative complexities. For instance, shale rocks are usually adjacent to rocks containing useable/drinking water known as “aquifers.” While implementing the hydraulic fracking, the shale fluid can easily penetrate to aquifers leading to groundwater contamination. This contamination may result in methane-poisoning of water used for drinking and irrigational purposes. To avoid such contamination, as per industry standards, a project proponent must maintain a distance of 600 metres between aquifers and fracture zones (Davies et al 2012).

The Indian water legal regime is far away to make such specific observations, as aquifers are not defined in any of the Indian environmental regulatory or legal regime leading to a free pass for unregulated mixing of shale fluid and aquifers. Moreover, the landless have no right to groundwater, and accordingly peasants and tribal communities who have no ownership rights over land have no right on groundwater. Also, a project proponent may easily exploit groundwater while implementing the hydraulic fracking process with none or limited accountability of their actions.  In such a situation, the intent of “Public Trust Doctrine” is defeated, and the precautionary principle will be non-implementable.

Open Access.

Geoscience Education Woes In India

Dilip Saha of the Indian Statistical Institute, Kolkata,  has written an editorial in Current Science on the many problems with geology education in India.

He identifies a lack of attention to field work and the quality of teachers as the two major weaknesses that need correction.

I agree with many of the points he has made. I had a very poor quality field training experience during my Master's education at University of Pune (now Savitribai Phule Pune University). That was somewhat compensated for by some very good classroom teaching. Across State Universities and local colleges, the quality of teaching suffers not just because subject experts are not up to the task, but because many departments are understaffed and don't have subject experts.  Often, just two or three faculty end up teaching all the subjects.

I will also add that besides the obvious improvements in field courses, teacher quality and pedagogy, a module on research ethics is desperately needed. This is not a geology specific issue. Plagiarism is a big problem in Indian academia. I occasionally mentor students from local colleges. I have found out, to my dismay, that copying and pasting material from a research paper in to one's thesis seems to be commonplace. Students don't even realize that they are crossing serious ethical lines.

Open Access.

Environment Links: River Issues In India

Sharing a few interesting and informative articles I came across in the past few weeks on rivers.

Endangered Himalayan Rivers: This one is from 2012. A large number of dams are planned on the Alaknanda and Bhagirathi rivers in the state of Uttarakhand.  Parineeta Dandekar writes about the weaknesses and bias in the Environment Impact Assessment process.

Rally For Rivers Plan. Will It Help?: The Rally For Rivers campaign by the Isha Foundation is calling on creating a 1 km wide tree plantation along the river banks. This, they claim, will help rejuvenate India's dying rivers. Veena Srinivasan, Sharad Lele, Jagdish Krishnaswamy and Priyanka Jamwal with the Ashoka Trust for Research in Ecology and the Environment, Bengaluru examine their claims in detail and find them wanting.

Caution Warranted For River Linking Project: The gargantuan river linking project envisages a series of dams and canal systems to transfer water from Himalayan rain and snow fed river basins to the drier Peninsular rivers in the south. Is it worth it?

Reuter's Erroneous Reporting On The Ken-Betwa River Linking Project: Two rivers in Madhya Pradesh and Uttar Pradesh are to be linked. SANDRP clarifies that the permissions process has yet to be completed. The two states don't even have a water sharing agreement! Reuter's screwed up.

Environment Ministry Panel Reject's Uttar Pradesh's Religious Smart City Plan: I'm including this to give an example of the utter indifference to ecology and environment shown by "planners and developers". The plan is for a smart city to be built inside the Hastinapur wildlife sanctuary, along the banks of the Ganga, which would have destroyed dolphin habitat and river ecology along a 7 km stretch. How does one even come up with such ideas? Fortunately, the usually pliant Environment Ministry has balked at approving this outrageous plan.

Book: Indica- A Deep Natural History Of The Indian Subcontinent

I am not doing a general book review of Pranay Lal's book Indica: A Deep Natural History Of The Indian Subcontinent. For that, I recommend this fine literate piece by Pratik Kanjilal published in the Indian Express. And Prabha Chandran writes about it in the Huffington Post. Both are aimed at the general reader.

No one has, as far as I know, written critically about the science content of the book. I read through the book and have some comments on the geology.

Before I start, let me say that I enjoyed this book. Pranay Lal has read widely, traveled far, and has had immersive discussions with geologists and paleontologists.  The best sections of the book are when he is writing about the many fossil finds preserved in Indian sedimentary basins and their importance in interpreting paleo-geography, ecology and evolution. He certainly appears more comfortable writing about these themes than he is about geology.

There are many problems with the geology writing. Some are easy-to-fix errors, while others will, in my opinion, require some rethinking on the more effective presentation of ideas and processes.

Let's begin with the easy to fix errors-

1) Page 12: Ref: Structure of the earth-  "The innermost shell of the "core" was composed of iron  and nickel and was surrounded by a larger but less dense mass of molten iron called mantle". - The mantle which is the layer of the earth between the crust and the core is not molten. It is solid and is made up of silicates and not iron. The core itself has two layers, a solid inner core and an outer fluid layer made up of iron and nickel.

2) Page 13: Ref: Age of corals in Rajasthan and Kutch- " This coral colonized the seas about 380 million years ago". There are no 380 million year old sedimentary rocks in Rajasthan and Kutch (Devonian Period). This may be a typo. There are Jurassic age corals in Jaisalmer. They are about 170 million years old.

3) Page 45: Ref: Banded Iron Formations- "Both ferric iron and ferrous iron began to settle as successive bands at the bottom of the iron-rich seas and lakes as oxygen levels fluctuated. Once, deposited, the layers hardened one above the other and gave the appearance of a layered cake- thin strawberry-jam-coloured striations of highly oxidized iron (ferric oxide, Fe2O3) and dark coloured chocolate lines of less oxidized iron (ferrous oxide, FeO)" - In the vast majority of Banded Iron Formations the strawberry coloured striations are forms of silica, either chert or jasper. The dark coloured layers are hematite or magnetite ( ferric oxide Fe2O3). Ferrous iron (divalent) is usually in a dissolved state. Ferric oxides or hydroxide minerals and compounds form following oxidation of this dissolved ferrous iron. Some ferrous iron is trapped in carbonate and sulphide minerals.

4) Page 57: Ref: Coral mineralization- "When hard-bodied marine animals like corals evolved (around 2 to 1.7 billion years ago)" - Multicellular animals originated in the Neoproterozoic likely between 700 and 600 million years ago and acquired hard parts (mineral skeletons) by around 550 million years ago.

5) Page 57: Ref: Limestone formation- " ..the vast accumulation of shell and coral got pressed together into minerals like calcite and aragonite" - organisms combine Ca and CO3 ions to precipitate minerals like aragonite and calcite to build their shells. This accumulation of shells when pressed forms limestone rock.

6) Page 59: Ref: Picture of Cruziana- " This 565 million year old fossil is of Cruziana, one of the earliest multicellular animals and an ancestor of the trilobite which lived in shallow seas". Cruziana is an ichnofossil. It is a name for an impression of a particular shape made by trilobites disturbing the sediment on the sea floor (bioturbation). Cruziana is not an ancestor of the trilobite, it is evidence of the presence of trilobites. These ichnofossils from Rajasthan are in Cambrian age rocks and so have to be younger than 542 million years.

7) Page 60: Ref: Evolution of complex multicellular organisms and animals - " About 570 million years ago, a few enterprising organisms developed a new reproductive strategy - sex! Sex opened up a plethora of possibilities"  -  Sex evolved once in the unicellular eukaryote common ancestor of fungi, plants and animals more than a billion years ago. The oldest fossil evidence of a sexually reproducing multicellular organism is the protist Bangiomorpha pubescens. It is 1.2 billion years old. Preserved filaments show differential spore/gamete formation. So, sex evolved hundreds of millions of years before the evolution of animals.

8) Page 62: Ref: Animal family relationships- Comb jellies and jelly fish "evolved to become thin, pin-shaped worm like creatures with no arms or legs that wriggled on the bottom of the sea floor". The author is saying the creatures with bilateral symmetry arose from Cnetophores (comb jellies) and jelly fish (Cnidarians). Animal phylogeny reconstructed by genetic analysis shows that Cnetophores are a group which diverged from the animal family tree very early in its history. And Cnidarians and Bilaterans are sibling groups. They share a common ancestor. See this easy to understand essay by Jerry Coyne.

9) Page 154: Ref:  Bedaghat and  Makrana marble- The author says that the famous marble cliffs of Jabalpur (Bedaghat) and the Makrana marble used to build the Taj Mahal are Cretaceous in age.  He writes that Cretaceous sediment made up of calcium carbonate shells were deposited between 145 to 65 million years ago and were cooked by volcanic heat, which transformed these sediments into marble.  However, both these marble deposits are Proterozoic in age.  Calcium carbonate sediments accumulated in seas that covered Rajasthan and Central India in Proterozoic times. These deposits were then metamorphosed into marble during orogenic activity that took place during evolution of the Aravalli mountains (Makrana marble) and in Central India (Bedaghat marbles). Estimates are that deposition and metamorphism into marble took place between 2 billion to 1.5 billion years ago .

10) Page 131- Ref: Mid ocean ridges- "Deep sea trenches on the sea floor are the weakest points on the crust, made up as they are of a thin layer of rock and water above it.. " He goes on to explain that these are the spots where magma melts the crust and flows on to the surface creating new oceanic crust. Technically though, the term "deep sea trench" refers to places where tectonic plates are converging and oceanic crust is subducting underneath another tectonic plate. Lal on the other hand is describing regions where tectonic plates are spreading apart and new ocean crust is being generated. Such places are called "mid oceanic ridges".

11) Page 183: Ref:  Vivekanand rock as meeting place on Gondwana continents - "Geologists call the Vivekanand Rock memorial 'the Gondwana junction' because it marks a place where India, Madagascar, Sri Lanka, East Antarctica were once joined together". The Indian continental crust extends underneath the sea beyond the Vivekanand islet. The continental shelf edge, tens of kilometers away from the present day shoreline, is really the place where India would have been joined to Australia and Antarctica on the eastern margin and Madagascar on the western margin.

12) Page 210, 216, 222: Ref: Magma chambers in Deccan Traps- The authors points out examples of columnar jointing in basalts and calls them remnants of magma chambers. This is incorrect. Magma chambers are present several kilometers below the surface of the earth. If magma solidifies at this depth is won't be called a basalt (it will be called gabbro) and won't develop columnar jointing. These instances the author point out are either thick lava flows or volcanic plugs which have developed columnar jointing on account of cooling and shrinkage. Volcanic plugs are remnants of lava which solidifies in a volcanic vent at the surface.

13) Page 280: Ref: CO2 released by volcanism- " Most of the volume of CO2 in the atmosphere actually comes from volcanism and sea floor spreading. When sea floor spreading occurs, sediments on the ocean floor (including these shells) are dragged deep under the ocean floor where they heated and the trapped CO2 is released". At sea floor spreading centers volcanism releases CO2. Sediments and shells are dragged under the ocean floor at the other end of the plate at subduction zones. As they dive under, they get heated and release CO2.

Some longer discussions:

a) Page 18- Ore deposit formation- The author writes that collisions of meteorites during the early history of the earth up to 2.5 billion years ago kept puncturing the earth's crust releasing metals like iron from as deep as the core. These metals clumped together to form ore bodies. The early earth did go through a period of very heavy meteorite bombardment from 4.1 billion years ago up to 3. 8 known as the "Late Heavy Bombardment". Bombardment continued sporadically after that. No crust from this very early period is preserved as it kept getting smashed and recycled into the interiors. Any ore deposits that formed have also been destroyed.

The earth was much hotter then and after the easing of bombardment, intense magmatism from 3.8 billion to 2.5 billion years ago started forming the first continents. Geologists estimate that nearly 65% -70% of the present volume of the continental crust formed during this phase. The magmatism transferred metals from the mantle to the newly formed crust. 

A recent survey of  five years of research from 2011 to 2016 done by Indian geologists on ore deposits shows that not a single study invokes meteorite bombardment as the cause for ore concentration. Instead, internal forces like subduction zone magmatism, rift magmatism, hydrothermal circulation systems and near surface sedimentary processes are inferred. Now, there may be specific instances where meteorite impacts may have fractured the crust and initiated fluid circulation, but meteorite bombardment is a not a general explanation of metallogeny.

b) Chapter 7- Page 182 and subsequent pages- Pranay Lal discusses the breakup of Gondwanaland. How do continent breakup and what is the force that causes tectonic plates to move and drift for thousands of kilometer? He invokes volcanic eruptions as the cause of supercontinent breakup and the push exerted by magma upwelling through cracks as the force driving plate movement. He refers to a paper by Shanker Chatterjee and colleagues on the subject of India's epic northward journey after it broke up from Gondwana until it collided with Asia.

But an alternate view among geologists is that volcanic eruptions are the consequence of the break up of continents. And plate motion is driven not by the push of upwelling magma/lava but by the pull of cold dense lithosphere which sinks deep into the mantle at subduction zones. A perusal of the paper by Shanker Chatterjee shows that these scientists agree with this "slab pull" notion as the main force of plate motions. Mid oceanic ridge push is a secondary force. Unfortunately, the author does not even mention the slab pull force mechanism.

So, continents break up due to a variety of factors. Indeed, there could be an anomalous build up of heat underneath the continent, which thins and weakens the lithosphere (the rigid plate consisting of the crust and the upper part of the mantle). Hot buoyant mantle impinges the underside of the plate. At this point the mantle is still solid but can flow like silly putty. Continued stretching and thinning of the crust (caused by the slab pull force from a subduction zone at the other end of the plate) results in the underlying mantle decompressing. This results in the lowering of its melting point and magma generation. Magma rises through the fractures of the thinned crust and erupts on the surface.

In this view, volcanism did not prise apart fragments of Gondwanaland one by one. Rather, enormous episodes of volcanism like the Deccan Volcanic Event were triggered by rifting and Gondwana continents moving above anomalously heated portions of the mantle known as hot spots.

c) Page 261: Ref: Himalaya. Here is how Pranay Lal describes the rise of the Himalaya. "The Himalaya rose from below. The rubbing together of the immense plates and the monumental crushing and buckling of land produced a tremendous amount of heat and cause magma from below to ooze out of deep fissures which opened up on the surface. This melted and remelted granite, and pushed it upwards to the surface. As the granite slowly cooled, successive batches of molten granite thrust their way up,forcing the older granite slabs higher. Over time, this process created a pedestal for mountain building. Because the "cooking" process varied (different types of granite are cooked at various depths), the densities of rock slabs differed. This created large cracks or "faults" along places where the continental crust rasped, grinded and pushed slowly onward".

I didn't understand this at all.

Later he says that the Everest is made up of an initial four thousand odd meter foundation of granite overlain by another 3100 meter of sedimentary rock. The granite is 50-30 million years old while the sedimentary rocks are from the Paleozoic era (359 - 252 million years old).

During continental collision, there has been melting of the lower parts of the crust and this terrain has been intruded by pods and lenses of younger granite. Metamorphosed and partially melted Precambrian rock is the main component of the Greater Himalaya. In the Everest -Lhotse-Nuptsu region the granite intrusions are on a massive scale as described by Pranay Lal. These thick intrusive granite and high grade metamorphic rocks make up the base of Everest region. But, these younger granite intrusions are not this thick everywhere. They are present on a smaller scale along certain bands of the Greater Himalaya and are almost absent from the Lesser Himalaya.

Though the author may not mean it, phrases like "successive batches of molten granite thrust their way up, forcing the older granite slabs higher" may be misinterpreted by lay readers to mean that Himalayas formed as a result of magma pushing the crust up to form mountains. This is not how orogenic mountains like the Himalaya form.

As the Indian Plate collided with Asia it delaminated. You can think of this as the plate splitting into two tiers. The lower tier comprising the lower crust and upper mantle slid under Tibet. The upper tier impinged into Tibet and got squeezed, deformed and thickened. The Himalaya is this folded and faulted upper tier. The different Himalayan ranges are slices of the upper tier Indian crust stacked one on top of the other by a series of south moving thrust faults.

 The tectonic structure of the Himalaya with its geological divisions is summarized in the graphic below.

Source: Shankar Chatterjee et. al. 2013

What was the sequence of these thrust faulting events and how do they fit into the three pulses of mountain building that Pranay Lal mentions?  

Leaving the Tibet part aside, the Himalaya most people are familiar with are made up of four distinct geological terrains. I am listing them starting from the north and going south.  Tethyan sedimentary rocks (the ones making up the Everest and many other summits; These sediments  range in age from the Neoproterozoic to the Eocene- ~ 1000 million years to 40 million years, although the entire sequence is not exposed at one place), the Greater Himalaya Crystalline Complex (Proterozoic to Early Paleozoic, 1800 million years to 480 million years, with a younger imprint of metamorphism and granite intrusions), the Lesser Himalaya Sequence (Proterozoic to Cambrian; gneiss and low grade metamorphosed sediments, 1850 million years to 520 million years) and the Siwaliks which are Cenozoic sedimentary rocks deposited from around 15 million years to about 0.5 million year ago. The geological divisions roughly match up with the topographic divisions of the Greater Himalaya, the Lesser or Middle Himalaya and the Outer or Sub Himalaya.

The northern edge of the Indian plate was made up of  Proterozoic rocks, much as it is all across Peninsular India. This Proterozoic sequence was overlain by Paleozoic and Mesozoic sedimentary rocks. There is a more complete sequence of Paleozoic sediments in the Himalaya, since even though most of India was landlocked as part of Gondwanaland, the north edge of what was to become India was open to the Tethys sea all through the Paleozoic and Mesozoic.

As India collided with Asia:

1) Its continental crust impinged on the continental crust of Asia. The Neoproterozoic-Phanerozoic sedimentary cover was folded, faulted and scraped off and uplifted to form an early mountain range made up of the Tethyan sediments.

2) Horizontal shortening of the Indian crust during collision led to crustal thickening and rocks were subjected to high temperatures and pressures. They were metamorposed and partially melted into rocks known as migmatites and intruded by granites. Finally, compressive stresses broke the crust along a major fault known as the Main Central Thrust and uplifted this deeply buried terrain. The thrust moved crustal blocks upwards and southwards. Some geologists believe that the Great Himalaya Crystalline Complex is made up of hot soft rocks from the middle regions of the Indian crust which flowed towards the surface in response to the removal of  crustal cover by erosion. This ductile flow of rock from deep in the crust towards the surface is termed "channel flow" as hot soft rock is confined to a layer or channel between colder upper crust and a more rigid upper mantle.

Either way, with this thrusting and extrusion of high grade rock began the formation of the Greater Himalaya. The main activity of the Main Central Thrust is dated to between 16 million years to 25 million years or so. At about the same time the earlier uplifted Tethyan sediment detached themselves from the underlying crystalline basement and started sliding northwards along a major fault system known as the Southern Tibetan Detachment System.

3) As India continued to press into Asia, compressive stresses propagated southwards. Beginning around 16 million years to 11 million years, the terrain to the south of the Main Central Thrust began to get folded and faulted. Since it was further to the south from the collision zone, it did not experience the high levels of metamorphism and granite intrusions that the rocks of the Great Himalaya did. Eventually, these more distal rock formations were uplifted and moved southwards along the Main Boundary Thrust and associated faults to form the Lesser Himalaya.

4) The rise of the Greater Himalaya and the Lesser Himalaya loaded and depressed the crust in front of them in to a moat. In the alluvial plains, streams and lakes that formed were deposited sediments eroding from the rising Greater and Lesser Himalaya. These were the environments in which a lot of the mammalian evolution and diversification described in an earlier chapter took place. Beginning around a million years ago, maybe a little earlier, these sediments were folded, uplifted and thrust above the Gangetic alluvium along the Main Frontal Thrust to form the Siwalik ranges. The Main Frontal Thrust is still active, and Himalaya earthquakes which originate deep underground rupture along this fault plane. The Himalaya are growing southwards.

I am not writing a popular book for the lay public.  I realize I may have gone overboard with my Himalaya explanation and am not suggesting that Pranay Lal should include all this in his book. But any explanation should include at least the basic arrangement of the different lithologic terrains and their sequential uplift due to south progressing thrust faulting.

d)  Page 218 - Ref: Satpura mountains were uplifted due to the rise and push of magma leading up to the Deccan volcanism.- This is also a longer discussion but I'll stop on the geology aspects since this post is already too long. Let me refer to an article on the lack of pre- Deccan volcanic uplift in the Satpura region and elsewhere.  Many geologists have concluded that the uplift of the Satpura belt is not due to the push of magma. It occurred much later in the Cenozoic due to the various stresses on the Peninusular Indian crust.

e) I couldn't help elaborating on this:  Why do animals grow large? Pranay Lal mentions an intriguing evolutionary pattern seen in the fossil record. There is a trend towards an increase in body size in the early to mid Triassic following the Permian mass extinction. A second trend in increase in body size in seen in the Cretaceous when some lineages of dinosaurs evolved gigantism. The explanation given is that an increase in atmospheric oxygen levels favored an increase in body size.

Animals have a physiologically demanding lifestyle. That a certain threshold of oxygen will be required for them to prosper over the longer term is a given. It is too broad an explanation and it doesn't tell us why in a group one lineage evolved towards a larger size while a related lineage did not. In the two time periods that author points to, the reason why species evolved towards large size in the Triassic immediately after the Perman mass extinction may be different from why certain lineages evolved gigantism in the Cretaceous.

Let's take the case of body size trends during the early Triassic. Mass extinctions disproportionately cull larger bodied species . Since environmental condition are deteriorating rapidly, larger bodied species with  more energy expensive demands and slower reproduction rates cannot cope as well as species with a smaller body size. Survivor species in the aftermath of mass extinctions are small bodied, maybe as small as their biological limits. From this starting point, even if environmental conditions in the post mass extinction period are neutral in terms of favoring species with a particular body size, the only trend that will emerge is one towards a larger body size, since there is no room to get any smaller.

For Cretaceous too the oxygen hypothesis seems too pat. I could argue that Cretaceous was a time of high carbon dioxide levels. That would mean more food for plants. A lush healthy vegetated landscape means more food for herbivores, conditions favorable for evolution of larger size. Again this is a "just so story". Why did gigantism evolve in the Sauropoda? The answers may lie in phylogenetic heritage i.e. inheritance of ancestral characters which fortuitously proved advantageous, and evolutionary innovations that enabled them to acquire and utilize resources more efficiently than other groups. An avian style respiratory system enabled pnematization (air cavities in bones) of the axial skeleton. This evolved early in Sauropod history. A small head evolved because food was ingested without mastication. These two features enable a long neck to evolve (lighter head, lighter skeleton). A long neck enabled access to food not available to other herbivores.. a cascade of benefits due to inherited and newly acquired features.

Off course, these are ramblings about things that interest me. I have no expectation that Pranay Lal put all these details in his book.  

But, we need books like these to fire imaginations and inspire amateurs and students to go out and explore India's rich physical landscape. A good place to start will be the National Geological Monuments listed by the Geological Survey of India. A surge of visitor interest might put pressure on the government to expand protection to more sites of interest. The destruction of irreplaceable fossil sites and geological structures is a constant theme in Lal's book.

A second edition of this book will be welcome, but one that has gone scrutiny by a discerning geologist editor.

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.

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.

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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
             
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             Evolutionary Anthropology: Issues, News, and Reviews, 24 (4), 149-164 DOI: 10.1002/evan.21455