In ancient teeth, clues of human evolution — and perhaps a new species
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Human evolution has long been visualized as a simple linear timeline, a kind of evolutionary baton race in which one species evolves into another, from apes to modern humans.
The new finds reinforced that human evolution was more of a tangled-up bush. Different species and different branches of the hominin family tree overlapped with one another in time and space. Three smaller teeth, the researchers report, were from the group that modern humans belong to, Homo, and dated to 2.6 million and 2.8 million years ago. They were found at Ledi-Geraru in 2015 and 2018. The larger teeth were relatives of Lucy — an Australopithecus whose famed fossil find in Ethiopia 50 years ago changed the modern understanding of human origins.
Already, outside researchers are debating next steps and whether they agree that the teeth found by Reed's team represent a new species of Australopithecus. The study was supported by the National Science Foundation. Because of the uncertainties around federal funding, Reed isn't yet sure if the next year of grant funding will come through.
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'We don't have all the pieces of the puzzle,' Reed said. 'What's exciting to us is you want more pieces of the puzzle to put this [story] together.'
For years, paleoanthropologists have been piecing together evidence of a diverse world of hominins — the group that includes modern humans and our close ancestors.
In Kenya, 1.5 million-year-old preserved footprints show that Homo erectus and another branch of the hominin family tree, Paranthropus boisei,
walked along the shores of a lake within hours or days of each other. In and around a cave in South Africa, fossils reveal that Homo erectus, Australopithecus sediba, and Paranthropus robustus
coexisted 2 million years ago.
Now, in East Africa at sites within the Ledi-Geraru research area, a collection of ancient teeth show Homo and Australopithecus coexisting in yet another spot. But would they have interacted with one another? Would they have interbred?
John Hawks, a paleoanthropologist at the University of Wisconsin at Madison who was not involved in the work, said the find is exciting because it opens a window into a critical and mysterious period of human evolution between 2.5 million and 3 million years ago. He said he's eager to see the work published, but noted that such finds raise as many questions as they answer.
'The kinds of evidence that Ledi-Geraru is generating is evidence spanning hundreds of thousands of years of time, and potentially hundreds of square kilometers of space,' Hawks said. 'When several things are reported together like this, that creates a challenge of interpreting what that evidence together tells you.'
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Clément Zanolli, a paleoanthropologist at the University of Bordeaux, said in an email that he was not yet convinced that the teeth are a new species of Australopithecus and would be cautious about attributing the smaller teeth traced to Homo. Newer techniques that use CT scanning allow scientists to study the internal structure of teeth and could build a stronger case.
He said the finds would represent some of the oldest remains of Homo and the youngest specimens of Australopithecus in East Africa.
'These new findings are thus adding more pieces to the puzzle, but they are not decisive to understand human evolution,' Zanolli said.
When the famous Lucy fossil was discovered a half-century ago, she was popularized as a 'missing link' between apes and humans. Human evolution looked simpler then. Today, the tree of human evolution is full of species that may have been direct ancestors, distant cousins, or side branches that died off.
If anything, the science of human evolution has become a deeper mystery, as it has become clear that there were once lots of hominins walking around the planet, sometimes interacting with one another. As hallmarks of human innovation — such as using stone tools and butchering meat — have been shown to have been shared with hominin ancestors, the question of how and why humans alone succeeded has become more mysterious.
Zeray Alemseged, a paleoanthropologist and professor at the University of Chicago, said he thinks it's possible the Australopithecus teeth described in the paper aren't a new species, but they could be evidence that Lucy's species persisted. Australopithecus afarensis was previously thought to have vanished around 2.95 million years ago from East Africa.
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One model for how to understand human evolution is a concept called 'budding cladogenesis.' In this model, species don't progressively change into the next species and vanish. They give rise to different species at different times and in different places, and the ancestors may stick around and live alongside them. Lucy's species could have been one such long-lived species, and the teeth are one new piece of evidence to consider.
'The material is … highly fragmentary, but it does have important information in regard to a time period which is really critical,' Alemseged said. 'It really allows us to think about the complex question of how evolution has happened.'
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