Original link: http://www.abc.net.au/news/2015-03-05/fossil-jaw-a-missing-link-in-the-human-evolution-puzzle/6281236
The discovery of a 2.8-million-year-old partial jawbone in Africa could rewrite the history of human evolution.
An international team of researchers found the lower jawbone, complete with teeth, at the Ledi-Geraru site in the Ethiopian Rift Valley, and published the finding in a report today in the journal Science.
The primitive jawbone is approximately 400,000 years older than Homo habilis, also known as Handy Man, which is the earliest-known species in the Homo lineage that led to modern humans.
Together with another study that examines the evolution of Homo habilis, the find could answer a gap in the fossil record that has frustrated scientists for nearly half a century.
"The fossil record for humans between two million and three million years ago is just very poor and has long been known as a gap in human evolution," said lead author and anthropologist Dr Brian Villmoare, from the University of Nevada, Las Vegas.
The fossil provides clues to changes in the jaw and teeth in Homo only 200,000 years after the last known occurrence of the Australopithecus genus that includes the fossil Lucy from the nearby site of Hadar.
While the front of the jawbone is very primitive — suggesting it comes from the Australopithecus — the back of the jawbone and teeth resemble those of Homo habilis.
"It does have some more primitive traits that come from Australopithecus, the more than three-million-year-old group, but also has a lot of traits that link it with the species that we see around two million years ago," Dr Villmoare said.
The teeth found in the jawbone suggest a change in diet from the mainly fruit-eating Australopithecus towards a more meat-based diet that would also have required the use of tools.
"At three million years ago, you have a very ape-like creature with long arms, living in the forest, probably eating fruit," Dr Villmoare said.
"And then at two millions years ago you have Homo with the tools and large brain and more modern body plan with shorter arms.
"The big gap between those two is really interesting because something must have happened that led to the evolution of us."
Homo habilis more primitive than first thought: researchers
The gap between Homo and Australopithecus was also of interest to another team of researchers, who set out to digitally reconstruct the jawbone of the original Handy Man skull discovered by Louis Leakey in 1964.They used CT scanning to carefully piece together a digital version of the damaged mandible in an attempt to clarify the key features that distinguish Homo habilis from its ancestors.
The scans show the mandible is far more primitive than first thought.
"In outline — not necessarily in exactly the details of what the teeth looked like — it was actually pretty similar to what you would see in the species [Australopithecus afarensis] that Lucy belonged to," said Fred Spoor, who works for the Max Planck Institute for Evolutionary Biology and is a professor of evolutionary anatomy at University College London.
What the authors have done is to just pick up all these individual specimens and toss them back into the mixing bowl, so we just don't know how many species there were, which belong to which.Professor Colin Groves, ANU anthropologist
The findings, published today in Nature, challenge the existing model of how genus Homo evolved and displace what was previously thought to be the ancestor of Homo habilis.
"It became interesting to ask the question, what would the true ancestor look like?" Professor Spoor said.
"We were musing about it, and at that moment, we heard from a colleague that they found this new lower jaw in Ethiopia."
The jawbone described in the Science paper is the "perfect missing link", Professor Spoor says.
Anthropologist Professor Colin Groves, who works in biological anthropology at the Australian National University, says the work by Professor Spoor and colleagues "really upsets the apple cart" in terms of the understanding of how all the Homo species fit together.
"What the authors have done is to just pick up all these individual specimens and toss them back into the mixing bowl, so we just don't know how many species there were, which belong to which," Professor Groves said.
While the new jawbone finding provides a nice intermediate specimen between Australopithecus and Homo, Professor Groves says the implications of the Homo reconstruction will take some time to digest.
