Human puzzle

Out of Africa thinking

Let’s go back 5 to 8 million years ago. That’s when the human branch split off from our common ancestors with chimpanzees. The earth was beginning to cool. Dense forests were gradually being replaced with open woodland. Grasslands too began to appear. Our ancestors began to adapt to life on the ground while still at home in the trees. Over time they became efficient walkers. Sometime around 2.5 million years ago some of them acquired a large brain and began making tools. These were the pre-modern humans, placed in our own genus Homo. It was only 0.2 million years ago that the modern human, Homo sapiens, appeared, full of curiosity.

All this happened in Africa. It was through waves of migration out of Africa that our ancestors spread to other parts of the world. The first one was 2 million years ago when pre-modern humans colonised Asia and Europe. There they evolved into other species like Neanderthal. The last one was about 50,000 years ago, when modern humans quickly spread across the world. All the other Homo species were soon wiped out and only we the Homo sapiens thrive.

This is the mainstream view of human evolution in a nutshell. This was not always the dominant view. But ever since Africa disgorged an amazing number and variety of fossils of archaic and early humans in the later part of the 20th century, the continent has been established as the cradle of humankind.

Siwaliks throw a challenge

Early this year, an Indo-French team published papers in a French journal, claiming they have found butchery marks on 2.6 million-year-old fossils in the Siwalik Hills of India. These marks, they claimed, were left by someone who belonged to that human branch—a hominin—who broke the bones to eat the marrow. If the claim stands up to scrutiny, this will be the oldest sign of early pre-modern humans outside Africa, and hundreds of thousands of years before the first hominin is believed to have set foot outside Africa. Antiquity of this evidence “goes far beyond my daring”, writes Yves Coppens, French anthropologist of Lucy fame, in the special volume of the Proceedings of the French Academy of Sciences, Comptes Rendus Palevol. Honorary Professor at the College of France and patron of the Siwaliks project, Coppens is of the view that the pre-modern human (Homo) appeared as early as 3 million years ago and moved out about 2.5 million years ago.

“We never thought we would find a fossilised bone with scavenging marks,” says Mukesh Singh, president of the Society for Archaeological and Anthropological Research in Chandigarh, who co-led the team. The researchers were hoping to find stone tools close to fossils—a sign of hominin presence—from 1.8 million years ago in the low Siwalik Hills of sandstone and clay near Chandigarh. In this region with sparse vegetation the sedimentary layers have been raised into folds by tectonic activity. At places, the layers are broken or eroded by monsoon so that the inner, older layers stand exposed.

Luck struck in 2008. Singh found a chopping tool on an outcrop near Masol village, some 15 km from Chandigarh. Here the fossil-bearing sedimentary layer appears like an eroded dome. A seasonal rivulet has cut through the dome, evacuating the sediments.

Excited, the team visited the spot again in 2009 and collected several fossils, quartzite cobbles, choppers and flakes. Several steps away, one of them, Manjil Hazarika, picked up a fossil bone with distinctive marks on it. It belonged to the bovid family of mammals that includes bison and buffalo. The cut marks resembled signs of butchery. This was a turning point. Was this a scavenging site visited by early humans, and how early?

The team pored over publications on the Chandigarh Siwaliks and found that this particular fossil-rich zone was dated 2.6 million years old. It was then “I realised the importance of the discovery”, Anne Dambricourt Malassé, a palaeoanthropologist at the Institute of Human Paleontology in Paris who led the team, tells Down To Earth (DTE).

Later, they found two more bone fragments with similar cut marks and collected hundreds of animal fossils and stone tools. The fossil species belong to the transition period between two epochs, Pliocene that ends 2.58 million years ago and the next Pleistocene. One fossil of a giant turtle had to be carried on a camel. Nearly 46 kg of sediments were sent to France to study the rock layers, environmental variations and dating.

The team reconfirmed the age of the locality where the first cut-marked bone was recovered through palaeomagnetism. This dating technique relies on comparing the polarity of the sedimentary rock with the earth’s record of changing magnetic field. Experts studied the geology and geomorphology of the area and the speed of erosion and rivulet incision to ascertain that the fossils and tools were exposed recently by the dismantling of the old sedimentary layer. “The sediments are getting broken under tectonic pressure. If they (collected material) weren’t here at the time of deposition you could have found them further south of this locality,” says Singh. “The material is just left on the surface.”

This is the first detailed, multidisciplinary work on Masol since the 1960s, says G L Badam, former professor of palaeontology and scientific consultant to Palaeo Research Society in India.

The bones with cut marks were sent to Paris to study the traces on a scale of a thousandth of a millimetre (see ‘The cutting edge’,). Dambricourt Malassé then procured a pig foot from the market. She cut its skin using the sharp edge of a quartzite. The marks this experiment left on the bone were compared to the ones on the long fossil bone. They were strikingly similar.

Hominin in savannah

From the organisation of the cut marks and their trajectories Dambricourt Malassé is able to glean some information about the maker of these marks. “The gestures are that of a hand-held stone with a fine sharp edge (struck) to cut the tendons in very precise locations and then to break the bone in order to eat the marrow,” she says. “A long sequence of intentional choices and technical gestures reveals a high level of reflexive consciousness, and a social and cultural organisation around planned scavenging activity.”

“Scavenging activity is extremely rare. Asian Pongo (orangutan) can eat small carrion of squirrel but never do the wild chimp or Pongo create spontaneously a sharp edge in stone to cut the meat,” says Dambricourt Malassé. “As a specialist in palaeo-neuroanatomy and palaeo-psychomotricity, I do not need more data on the cut marks. They attest to a level of complexity/consciousness which matches to a hominin and in no case to a wild great ape.” Tool-making is associated with members of the Homo genus. Recent evidence shows probably their older relative, Australopithecus, also used tools.

The Masol hominin would have lived in a sub-tropical savannah among giant elephants, giraffes and hippos; small and large buffalo and equids (horses and related animals); and rare carnivores like Panthera. Giant terrestrial turtles would have occupied ponds and mud holes. At that time the Himalayas had formed but not the Siwalik Hills that run parallel to it. The Masol area was a floodplain of meandering rivers emerging from the Himalayas. These calm rivers would have turned torrential in monsoons. The researchers propose that torrential floods might have transported cobbles from the foothills and swept away the herbivores and giant turtles. After the water receded, the plain would have been littered with cobble beaches and cadavers. This would have attracted scavengers, among them early humans. Cobbles would have come handy for fashioning cutting tools.

Cutting edge   How cut marks on fossil bones were verified Researchers USED cutting-edge technology in Paris to observe the marks on three fossil bones on a micron scale and to compare them with other marks. The fossils included a long bone, a foot bone and a splinter of three animals of the bovid family that includes buffalo and bison. The fossil marks on the foot bone were observed using X-ray microtomography at the AST-RX platform of the National Museum of Natural History. It is a high-performance scanner that allows a 3D high-resolution view from the surface to the internal structure of the fossils. A binocular microscope was used to look at the surface features of the splinter. Then 3D digital Video Microscope Hirox helped capture 3D images of the surface of the long bone, bringing into focus the whole section of grooves and scratch marks. These technologies allowed an examination of the mineralisation of the marks. Mineralisation was found to be similar to that of bone tissue, distinguishing the marks from a fresh trace made by shepherds. The cut marks on fossils were first compared with animal marks on other fossil bones from Masol (carnivores, rodents) and a collection in Paris (bear, lion, wolf, hyena, Panthera, wild cat). The researchers then created cut marks on bones of extant species with quartzite stone collected at Masol for comparison. First they obtained flakes of quartzite by striking a stone with a ªhammerº (a quartzite cobble) on ªanvilº (another large quartzite cobble). Using this flake, they cut the surface of long bones of a deer skeleton collected at Masol in the area where the tendons are attached. Next, they procured a pig foot in France. A quartzite cobble from Masol was broken and its sharp edge used to cut the skin until reaching the hard surface of the bone. The topography of the cut marks on experimental bones was compared to the fossil marks of the long bone. They showed the same shape, profile and very fine details.

Caution, questions and imagination

But a hominin in Asia 2.6 million years ago is an extraordinary claim. It evokes caution, even scepticism, among other palaeoanthropologists, especially because of the context in which the cut-marked bones and stone tools were found. They were scattered on outcrops and their slopes, not extracted from an undisturbed buried layer (in situ). Unequivocal Early Pleistocene butchery evidence, say, in Africa and China, has usually come from excavations of archaeological layers little disturbed since deposition, says Parth R Chauhan, Assistant Professor of archaeology and palaeoanthropology at the Indian Institute of Science Education and Research in Mohali, who has also done research in the Siwalik Hills. The geomorphology of the Siwaliks is not similar to the large African plateau with horizontal layers. The slopes are eroding and rugged hills not easy to excavate.

“Good thing about their study is that they got the bones from a well-dated locality,” says A R Sankhyan, former palaeoanthropologist of the Anthropological Survey of India and now Visiting Fellow there. But “unless you find the stone tools at least 3-5 feet (1-1.5 metres) below the surface and beyond the gullies, it is doubtful to call them in situ. They must excavate the raised rock layers.”

This is the next step, says Dambricourt Malassé. The team first had to understand the characters of rock layers and the erosional process of the Masol dome to find the best locality to excavate. Then the priority was to study the cut marks, their exhumation, environment and date. This is done. “Now we know the best localities to open an excavation. The Indo-French collaboration can pursue the investigations.”

Not everybody is convinced of the butchery marks. The palaeoanthropologists DTE spoke to said contact with naturally sharp stones and gravel and trampling by animals, can also leave similar marks. “From what I have seen (research paper), I think the authors cannot discard that such marks were caused by natural processes such as abrasion or trampling. Remember these fossils derive from surfaces where abrasive sediments, such as gravels and clasts, are abundant,” says Manuel Domínguez-Rodrigo, a palaeoanthropologist at Complutense University in Madrid, Spain, who is an expert on cut marks. “More serious work is necessary before we are convinced that the marks are genuine and that the fossils have the age the authors claim.”

Chauhan says since with great claims comes great scientific responsibility, it would have been better to invite neutral taphonomists and experts on stone tool cut marks to have a look at the bones for objective interpretations. He also points out that pig bones were unsuitable for convincing comparisons. “The fossilised cut-marked specimens belong to a bovid, whereas pig bones are morphologically, dimensionally and compositionally different. While they rightly explain about avoiding experimenting on a cow carcass in India (to respect Hindu religions sentiments), they could have easily butchered a buffalo carcass,” he adds.

Dambricourt Malassé says both pigs and bovids are ungulates and the comparison is between homologous bones. She insists she is confident of the age and the hominin origins of the marks.

Future investigations may shed more light on this. As Sankhyan puts it, “The actual hominid (hominin in new classification) fossil evidence is important to establish any claim of hominid activities on the bones. However, the new findings give us new imagination...we work on that...that’s how we progress.”

Out of Africa questioned

And imagination they do stir. Masol findings are the latest in a growing number of discoveries that challenge the grand narrative of Out of Africa and force palaeoanthropologists to think outside the box (see ‘Asian challenge’,). Most of these are controversial stone tools found at various places in Asia that point to hominin presence earlier than 2 million years ago. The most obvious way to accommodate them is to revise the time of first migration out of Africa. In fact, debate has never ceased over exactly when the first hominin left Africa and exactly which species was it.

Widely held belief is that Homo erectus, with its big brain, long limb and sophisticated tools, was the first eligible migrant. But the oldest Homo erectus fossil is only 1.9 million years old. Some scientists propose it was perhaps an older hominin, Homo habilis or Australopithecus, who first colonised Eurasia. This doubt arises because of two fossil finds: one at Dmanisi in Georgia and the other on Flores Island in Indonesia. The Dmanisi fossils are dated almost as old as the oldest Homo erectus fossils in Africa. With an ape-like face and small braincase they seem to represent the earliest form of the species. Flores Man, nicknamed Hobbit because of its small size, is even more confounding. It has a brain half the average size of Homo erectus and several primitive features as if it evolved from an Australopithecus. Some scientists who have studied these fossils even propose an “out of Asia” hypothesis. It is feasible that Homo erectus evolved in Eurasia from a more primitive species and migrated back to Africa, David Lordkipanidze of the Georgian National Museum in Tbilisi has said.

Very few palaeoanthropologists entertain such a radical shift in the story of human evolution. “Unless we continue to entertain that possibility we are in danger of ignoring evidence just because it is not consistent with this dominant paradigm that evolutionary innovations in our own evolutionary history largely took place in Africa,” Bernard A Wood, University Professor of Human Origins at the George Washington University, tells DTE (see interview).

`It is perfectly possible that Homo genus evolved in Asia'   Bernard A Wood is University Professor of Human Origins at the Centre for Advanced Study of Human Paleobiology, George Washington University, USA. He was part of Richard Leakey's first expedition to the Koobi Fora site in Africa in 1968. A medically trained palaeoanthropologist, Wood speaks to Archana Yadav about alternative views on human evolution How do you make sense of the emerging evidence of hominin presence outside Asia more than 2 million years ago? Signs of hominin activity, 2.6 million years ago, have been reported in the Siwalik Hills of India and the age of fossil teeth at Longgupo cave in China was revised to 2.48 million years ago. I was one of the people who originally thought the Longgupo mandible might belong to a hominin, but I am increasingly persuaded that it is a fossil of orangutan. So I think the evidence from Longgupo is not as compelling as some of us initially thought. I am not a bioarchaeologist but I know that the people who are familiar with trying to analyse whether marks on bones are produced by stone artefacts remain to be convinced that those marks are made by stone tools and not by natural means. This is because when animals are trampling around they can leave marks on bones that really look like marks made by stone tools. There are lots of sites in Africa where people got very excited that they had found what they called cut-marked bones, and most of these turned out to be the results of natural processes and not human behaviour. There must be evidence of early hominins in the Indian subcontinent but at the moment the evidence is few and far between. I somehow doubt that the marks on those particular bones from the Siwalik Hills were made by stone tools. Is it possible that the Homo genus evolved in Asia? It is perfectly possible. At the moment everybody assumes that most of the evolutionary novelties in relation to our own evolution occurred in Africa, and the forms that evolved migrated out of Africa. I think it is perfectly possible, depending on how you define Homo, that Homo erectus itself might have evolved outside of Africa and then migrated back to Africa. Unless we continue to entertain that possibility we are in danger of ignoring evidence just because it is not consistent with the dominant paradigm that evolutionary innovations in our own evolutionary history largely took place in Africa. Could early hominins have crossed over to Asia before 2.6 million years ago? There isn't a good enough evidence to say that this is impossible. So my feeling is that even though there is not a lot of evidence in support of Homo originating in Asia, we need to keep that possibility in mind. Otherwise, when the evidence is discovered, we are not going to recognise it. If we just make the assumption that something like Homo habilis was incapable of exploiting habitats that would have been found in Asia then we are in danger of prejudging the evidence. The difficulty in palaeoanthroplogy and palaeontology is that the absence of evidence is not the evidence of absence. Just because there is no evidence of Homo habilis in Asia does not mean Homo habilis was not there. Could Asian great apes possibly have evolved into hominins just like African apes did? The problem with that hypothesis is that the molecular evidence suggests African great apes are much closer to modern humans than Asian great apes. So as much as I am willing to give Asia the benefit of the doubt, the people who continue to claim that modern humans are more closely related to orangutans than they are to the African apes are to my mind just crazy. The evidence against that hypothesis is so overwhelming, and it is not just molecular evidence. There is also morphological evidence. Anyone who continues to support the hypothesis that we are more closely related to the Asian than to the African great apes is indulging in an extreme version of wishful thinking.

“A more open-minded approach may be preferable, whereby early Homo erectus was simply one of several hominins that might have had the ability to disperse out of Africa at a time when environments were becoming more open,” writes Robin Dennell, a British scholar of South Asian prehistory who has done extensive research in the Pakistan part of Siwalik Hills, in the book Out of Africa I: The First Hominin Colonization of Eurasia. The hominin in Masol could be one of these pioneers.

Or could it have evolved in Asia from an Asian great ape? If the suggestion of the Asian origin of Homo erectus was radical, this one borders on heresy. It involves redrawing the family tree.

View from Asia

Before the weight of fossil evidence conferred on Africa the distinction of being the Mother continent, palaeontologists were looking to Asia as a possible birthplace of humankind. Siwalik Hills that span 2,400 km from the Indus river in the west to the Irrawaddy river in the east were the focus of attention for a major part of the 20th century. They were yielding a dazzling array of fossil fragments of apes, Sivapithecus and Indopithecus. Several ape species were found in China and elsewhere in Asia. Some Chinese scientists still believe early humans evolved parallelly in Asia.

In 2009, two US researchers stirred up a hornet’s nest by suggesting that humans are more closely related to orangutans, found only in Asia, than chimpanzees that are found only in Africa. Jeffrey H Schwartz, professor of anthropology at Pitt’s School of Arts and Sciences, and John Grehan, director of science at the Buffalo Museum, analysed the physical features of humans, great apes and fossil apes, and published the results in the Journal of Biogeography. Their conclusion flew in the face of by then widely accepted common ancestry of humans and chimpanzees. A comparison of genomes shows humans share 99 per cent of their DNA with chimpanzees and 97 per cent with orangutans. The two researchers also questioned the basic assumption that greater molecular similarity between two species means they are more closely related. Wood calls such suggestions of closer links with the Asian great apes “an extreme version of wishful thinking”.

Today, barring some Chinese scientists, few believe in parallel evolution of hominin. Dambricourt Malassé, however, does not hesitate to entertain such a possibility. She looks at human evolution from the perspective of embryogeny. She says she has observed that the origin of erected anatomy is the increasing complexity of the nervous system during embryogenesis (8 weeks after fertilisation) and not the choice to walk. In other words, it is primarily driven by internal factors rather than external environment. She has studied in detail the skull base of apes and hominins. A key bone of the base is the sphenoid. It is among the very first bones to form in the embryo. Its form dictates the position of other bones. Dambricourt Malassé believes it is the rotation of this bone at the end of the embryonic period and consequently the bending of the skull base that has marked human evolution among primates. Each time the base of the skull has flexed it has allowed a more vertical orientation of the central nervous system, leading to bipedality. This started with monkeys 40 million years ago, repeated with great apes, then Australopithecus and finally Homo sapiens 0.2 million years ago. Curiously, over the course of evolution the centre of the skull base has changed repeatedly in the same way. “As the African and Asian great ape species inherited the same genetic memory governing the axial embryogeny, I wonder if parallel evolution could be possible also in Asia. Properties with ecological stresses are maybe memorised in the sexual cells,” Dambricourt Malassé says.

It is a fascinating argument. But there is no fossil evidence of early hominin in Asia. No hominin fossil more than 1.85 million years old is known in Eurasia. In Africa, fossil evidence of human evolution goes back to 7 million years ago, from the earliest possible hominin, Sahelenthropus, to Australopithecus to the earliest Homo. Africa has also been lucky in this respect. Formation of Rift Valley there created an environment that favoured the preservation of hominin remains. As Dambricourt Malassé observes, “The rift is a future ocean, like a book open on its fossiliferous (fossil-bearing) pages…whereas the Himalayas result from the closure of an ocean, and are like a book closed on its oldest history.” (See ‘Masol threatens dominant views’.)

In the field of palaeontology fossil remains the final arbiter. Unless one finds well-preserved fossils, discoveries like the one in Masol will stir imagination, but not shake up the family tree.

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Asian challenge

The dominant notion is that the human lineage evolved in Africa. Right from our last common ancestor with chimpanzees to the emergence of the big-brained tool-makers (Homo) to the modern man (Homo sapiens). Homo erectus was the first one to venture out of Africa a little later than 2 million years ago and populate Asia and Europe. A growing number of discoveries do not fit into this view

 

Alternative views

Ways to explain the evidence that does not agree with the dominant view of human evolution

1. Early migration out of Africa. Several palaeontologists now believe that the Homo genus evolved 3 million years ago and very soon a member of it, older than Homo erectus, moved out of Africa. Scientists will have to consider whether conditions across the migratory routes were suitable for migration and the reason for moving out.
   
   
2. Homo evolved in Asia. A few paleontologists propose that probably an Australepithecus (slightly older than Homo), or a contemporary species, migrated out of Africa much earlier and gave rise to Homo in Asia, which then went back to Africa. Specimens of Homo in Africa around the time of its origin, between 2.5 and 3 million years ago, are few. This theory explains some of the anomalies in the Eurasian fossils but lacks conclusive proof.
   
   
3. Early human in Asia evolved from Asian great apes. This is an extreme view with very few takers, mostly in China. Some fossil teeth and jaw bones found in China appear ambiguous and are attributed to either Homo erectus or a mystery ape. But no unambiguous fossil of human forebears has been found in Asia.

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Masol threatens dominant views

Future discoveries may reveal scenarios more complicated than the linear vision of a single cradle of hominisation in Africa, with a single sense of migration

From the records of fossils and tools we know that walker and scavenger hominins, able to create sharp stone tools, lived in Ethiopia long before 2.6 million years ago. I think the question is not to know if a very old species of Homo lived among these hominins, but how tribes of hominin, with males and females, could cross the Red Sea. Salty water and marine currents form an ecological barrier, and the Arabian coast was not necessarily visible from Africa 3 million years ago.

Masol is in the sub-tropical zone of the Northern Hemisphere and bears comparison with the African Rift Valley. The cave of Longgupo in China, 3,000 km east of Masol, is at the same latitude. There, artefacts and fossils were recovered from a hominid settlement dated to 2.48 million years ago. In Pakistan, 500 km west from Masol, Robin Dennell found artefacts in stratigraphy at Riwat, south of Islamabad. He located them relatively to the position of the magnetic pole (north or south). Two dates are possible, 1.9 million years ago or 2.5 million years ago. Masol and Longgupo now allow to understand that the second date is quite possible.

Then the diversity of great ape species was important. Fossils have been recovered along the Himalayan foothills, in Myanmar, South China and Thailand. In the foreland basin along the Tibetan plateau Sivapithecus lived since 13.4 million years ago. Five species were living in the region of Masol 7-8 million years ago. Overall, it draws a picture of a consistent biogeographic unit similar to the Rift Valley. The difference is the tectonics. The rift is a future ocean, like a book open on its fossiliferous pages with the oldest great apes dated to 20-23 million years ago, whereas the Himalayas result from the closure of an ocean, and are like a book closed on its oldest history, with small surfaces offering, by chance, fossil-bearing pockets.

“Masol Man” was a hominin. Now it will be necessary to put in mind they could be the ancestors of more recent fossils. Other lithic assemblages in Asia belong to an evolved tradition. For example, the hand-axes (Acheulean) dated to 1.2 million years ago at Isampur in Karnataka. Paleanthropologists believed this is the result of migration of Homo into Asia around 2 million years ago.

Whatever the origins of Masol and Longgupo “Man”, African or Asian, if we accept migrations from Africa into Asia, there is no reason to reject migrations from India into Africa during the same period. The “white of the map” is Arabia. Future hominin fossils may reveal scenarios more complicated than the linear vision of a single cradle of hominisation in Africa with a single sense of migration since only 2 million years ago.

If the ancestors of “Masol Man” cannot be African hominins, is it possible that Asian great apes evolved like African species exposed to planetary cooling? This question concerns the evolutionary process which changed the orientation of the nervous system common to the African and Asian great ape species. I have studied this question for 30 years. These changes occurred during ontogeny in utero.