Tuesday, November 29, 2016

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.

Friday, November 18, 2016

Jesus n Mo: Those Furry Eskimos

They nail it every time!

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

Thursday, November 10, 2016

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!

Tuesday, November 8, 2016

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..

Tuesday, October 25, 2016

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.