Appearance of the genus homo back by half a million years
A jaw fossil discovered in Ethiopia takes the appearance of the genus Homo back by half a million years, and together with other studies rewrites the history of our origins. What conditions caused Australopithecus to evolve into Homo?
Two fossil jaws discovered in 2013 shed new light on one of the deepest mysteries of human evolution: the appearance of the genus Homo. The two fossils – one recently found in Ethiopia, the other a specimen found half a century ago and later reconstructed – point to East Africa as the cradle of our evolutionary line.
The Ethiopian fossil, described in a study published by Science (https://doi.org/10.1126/science.aaa1343), takes the appearance of the genus to which we belong back in time by at least half a million years - to 2.8 million years ago.
It is a dating that is surprising as it differs slightly from that (about 3 million years ago) of the last documented presence of Australopithecus afarensis, the biped known above all for the famous fossil called Lucy, which according to many scholars is a direct ancestor of the genus Homo.
The jaw, called LD 350-1, was discovered in January 2013 just about twenty kilometers from the site where Lucy was found in 1974. “This is exciting news,” commented Donald Johanson at the time of the discoverer of the famous fossil.
The Afar region, part of the East African Rift Valley, has yielded many valuable hominin fossils, including a 2.3 million-year-old Homo jaw called AL 666-1, which until now was thought to be the fossil record older than the genus to which we belong.
Fossils attributed to the genus Homo dating back to between two and three million years ago are extremely rare.
Among the characteristics that place the new fossil in that box are a tapered molar, a particular arrangement of the dental cusps and the shape of the bony body of the mandible, all typical traits of the Homo genus. But the anterior part of the mandible presents a more primitive morphology, i.e. a poorly developed chin characteristic of Australopithecus afarensis.
“This narrows the time frame in which we can now focus our research on the emergence of the human evolutionary line,” says Kimbel at that time. "What we have is a transitional form, exactly what one would expect in a fossil of that age. The chin looks to the past; but the shape of the teeth looks to the future."
The cradle of humanity
The finding seems to disprove the theory, supported by other researchers, that the direct ancestor of our genus is a South African australopithecus, Australopithecus sediba.
The authors of the Science research underline that the only known fossil specimen of the species is about a million years younger than the fossil mandible found in Ethiopia to which it was supposed to give rise.
The site where the new fossil jaw was found, called Ledi-Geraru, was home to a mixed landscape of grasslands and shrubs very similar to today's Serengeti 2.8 million years ago, says a related study (https://doi.org/10.1126/science.aaa1415) by Erin DiMaggio and other researchers from the Penn State University.
The animal species present at the time of the fossil suggest an environment that was becoming more open and arid, which would suggest the hypothesis of a climate change that would have triggered evolutionary adaptation in many animal forms.
Despite this, however, said Kaye Reed, director of the Ledi-Geraru project at the Institute of Human Origins, "it is still too early to say that the origin of the genus Homo was caused by climate change".
The new life of a fossil
The Ethiopian jaw would be enough to justify the enthusiasm of paleoanthropologists, but its importance is somehow amplified by the new reconstruction of a Homo fossil that is a million years younger, presented in a study published by the journal Nature (https://www.nature.com/articles/nature14224).
The fossil jaw is attributed to the species Homo habilis, so named in 1964 by the discoverers Louis and Mary Leakey because was found in the Olduvai Gorge, in Tanzania, a layer of sediments that also contained the oldest lithic tools ever found until that time (later, older tools, dating back at least 2.6 million years, were found in Ethiopia).
Louis Leakey and colleagues stated that H. habilis was most likely the ancestor of all subsequent species of the genus Homo, including Homo sapiens. Since then H. habilis has remained perched on that branch of the human evolutionary tree, albeit in a rather precarious manner also due to the fragmentation of the fossil specimen that represented it: a strongly distorted jaw, various pieces of the skull and a hand.
Thanks to CT scans and an advanced 3-D imaging system, a team led by Fred Spoor of University College London and the Max Planck Institute for Evolutionary Anthropology in Germany has digitally reconstructed the original appearance of that jaw. Its elongated shape, with rows of teeth parallel to each other, recalls the australopithecines, ancestors of man who precede the appearance of the genus Homo.
A new reconstruction of this fossil skull of Homo Habilis, called Olduvai Hominid 7, shows a combination of primitive and more modern features, including a larger brain than previously thought, a sign that the various species of Homo may have had a common ancestor already endowed with a remarkable brain.
Although half a million years younger than the AL 666-1 jaw, the newly reconstructed jaw is more primitive. This suggests the existence of a "ghost" evolutionary line of the genus Homo well before 2.3 million years ago, and which would have given rise to both lineages. And the new Ethiopian jaw fits this hypothesis perfectly.
The jaw of Ledi-Geraru, according to Spoor, suggests a plausible evolutionary link between Australopithecus afarensis and Homo habilis.
But that is not all. Spoor and colleagues also digitally reconstructed the braincase of the first specimen of H. habilis, whose brain capacity had previously been estimated at around 700 cubic centimeters: more than australopithecines, but less than subsequent human species.
Their analyzes instead bring the volume to 800 cubic centimeters, which places H. habilis at the same level as two other Homo species that inhabited the savannahs of East Africa two million years ago, namely Homo rudolfensis and the early Homo erectus.
“What we have in front of us is an animal with a very primitive snout, but with a large brain,” Spoor said as he presented the new reconstruction of the fossil at the Turkana Basin Institute in Kenya last August.
It is very unlikely that the three contemporary species Homo habilis, Homo rudolfensis and Homo erectus evolved a large brain independently, so it is assumed that their common ancestor was already traveling in that direction, and earlier than previously thought.
If this were the case, the connection would be re-established between the appearance of a larger brain in hominids and the first stone tools.
