Bangalore: In the 200th birth year of Charles Darwin, the British naturalist who said the only way to understand the origin of man is to go and find his ancestors, a team of international researchers has found new fossils in Ethiopia that are pushing the limits of human evolution, suggesting that chimpanzees may not be our closest ancestors.
In Friday’s special issue of Science, 47 researchers in 11 different papers report that they have found new fossils of hominids—members of the Great Apes family Hominidae that includes humans, chimpanzees, gorillas and orangutans—from Aramis in Ethiopia. They have named it Ardipithecus ramidus, or Ardi, which, at 4.4 million years, is the oldest and the most complete of earliest specimens, taking us closer to the last common ancestor of humans and chimpanzees. In 1974, scientists unearthed 3.2 million-year-old skeleton of Lucy, the female skeleton of Australopithecus afarensis, which is the oldest hominid we knew until Ardi arrived. “Although clearly different from Australopithecus in many ways, the case is well made that this represents the ancestor of Australopithecus,” said David Pilbeam, professor of social sciences at Harvard University.
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In one stroke, but one that entailed 15 years of rigorous research, scientists have thrown a monkey wrench in the long-standing hypothesis that our last common ancestor, which is believed to have lived six or more million years ago, was more chimp-like than human-like. Instead, it suggests that hominids and African apes have followed separate evolutionary paths, the latter having evolved in a more specialized manner.
“This is a wonderful find, that although surprising in some ways, makes a very satisfactory animal for understanding the changes that have taken place along our evolutionary lineage,” said Andrew Hill, professor of anthropology at Yale University, in email.
Analyzing one Ardi skeleton and bones of 35 other individuals of the same species, which were carefully reconstructed, piece by piece, researchers say the fossils show a new type of early hominin (a subfamily of Hominidae) which was neither chimp nor human. “In Ardipithecus, we have an unspecialized form that hasn’t evolved very far in the direction of Australopithecus. So when you go from head to toe, you’re seeing a mosaic creature that is neither chimpanzee, nor is it human. It is Ardipithecus,” said Tim White, one of the lead authors from the University of California, Berkeley.
At about 120cm in height and 50kg in weight, Ardi was as big as a chimpanzee, even matching its brain size, but it differed in gait—it didn’t swing from tree to tree or knuckle-walk like apes, rather walked upright and lived in woodlands, not in grasslands. “It lacked the kind of sexual dimorphism that present day great apes have and bridges the gap between Australopithecines and later hominids such as Sahelanthropus and Orrorin,” said Rajeev Patnaik, a palaeontologist at Panjab University in Chandigarh.
Patnaik and others have been working in the Narmada valley and in the Siwalik forests of the Himalayan foothills looking for hominid fossils. India is currently at the crossroads of human evolution, even as its discontinuous palaeoanthropological record is improving with each new find. “One school of thought is that that the Homo erectus or later species originated in East Africa and migrated to Indonesia using the subcontinent corridor; another believes that they originated in Asia and migrated to East Africa. But there’s not enough evidence to either refute or confirm these hypotheses,” said Patnaik.
Researchers working in India expect to find evidence of early hominins from the Miocene (25 to 5 million years) or Pliocene age (5 to 1.8 million years). Around 13 million years ago, the Siwalik forests saw the emergence of a primate Sivapithecus which disappeared in the following five million years as the habitat changed. “We aim to find evidence of anything that has come from Sivapithecus or early Homo erectus or even Australopithecus. The present finding gives us hope and courage to keep looking for fossils in the Siwalik sediments,” said Patnaik.
As for Ardi’s case, the evidence is pretty conclusive, though experts disagree on its interpretation. For instance, Pilbeam isn’t convinced that human ancestors “never went through a chimpanzee-like phase”. However, researchers involved say further work will add more data to make this “clearer” and that they will continue to look for more fossils.
“Any place where there are fossil of the right age can be the source. If I may guess, any place between Kenya, Ethiopia and Chad, in that very wide triangle can be the place to look for,” said Berhane Asfaw of Rift Valley Research Service in Addis Ababa on phone, who thinks Ethiopia has simply been a “geological accident”.
Whenever and wherever the future fossils come from, experts say Ardi has provided enough data to keep the debate alive. This extraordinary discovery will keep the field busy “with discussion and argument for at least another 15 years”, said Pilbeam.