While we know much of the story of how humans evolved, the puzzle is still missing critical pieces. For example, fossil evidence for human evolution between 2 and 3 million years ago is patchy. It's frustrating because we know that the branch of the hominin family tree that includes humans, or Homo sapiens, appears in the fossil record for the first time in this period.
Today, Homo sapiens (which anthropologists shorten to Homo), is the only hominin species alive. But in the past, Homo wasn't alone. We coexisted and competed with other branches of the human family tree. Research supported by the National Science Foundation and the Leakey Foundation and published in Nature now fills in a piece of the ongoing evolutionary puzzle, placing two early species of hominin side-by-side.
A team working in the Afar Region of Ethiopia, at the site of Ledi-Geraru, reports hominin fossils that date between 2.6 and 3.0 million years old. Lucas Delezene, an associate professor of anthropology at the University of Arkansas, was second author on a paper that incorporated the work of more than 20 researchers from North America, Africa and Europe.
The team found fossils of Homo that confirm the earliest evidence for the human lineage at 2.8 million years ago as well as evidence of Homo at 2.6 million years ago, solidifying the antiquity of Homo. Unexpectedly, the team also found evidence that Homo overlapped at the site with a different type of hominin, Australopithecus, at 2.6 million years ago.
This was a surprise because Australopithecus was thought to be extinct in the area by about 3 million years ago. The famous Australopithecus fossil known as Lucy was found at a nearby site, but her species disappeared from the fossil record at 3 million years ago.
"People often think evolution is a linear progression," explains Delezene, "like the March of Progress, but in reality humans are only one species that make up a twig of a bigger family tree — it's quite bushy and what we found is another twig that was previously unknown. The idea that Homo appears and immediately spreads around the planet and replaces all other hominin species is not accurate. Homo lived side-by-side with many other hominin species throughout Africa. What's neat is that Homo overlaps with different hominin species in different places."
For example, from southern Ethiopia to southern Africa, the earliest species of Homo overlapped with a hominin known as Paranthropus, which is well known for its massive teeth and chewing muscles and a diet reliant on grass in some parts of its range. However, in the Afar Region of Ethiopia, no Paranthropus fossils have ever been found.
Instead, the team working at Ledi-Geraru found that Homo overlap with a different type of hominin, Australopithecus. How all of these hominin species divided up resources is the question of ongoing research. Did Homo nod to the other hominin species on their way to hunting and gathering in the morning, or did the various species consume similar resources? Did Homo eat the same things in Ethiopia where it coexisted with Australopithecus as it did in the south where it coexisted with Paranthropus, or was its diet flexible?
We know that Homo eventually becomes a culturally reliant tool user and occasionally consumed meat. But the oldest Homo fossils at Ledi-Geraru predate any evidence of tool manufacture or meat consumption. Did Homo evolve those traits to avoid competing with other hominin species? Competition among these various hominin species likely set the stage for the evolution of the traits that ultimately made humans a globally widespread and successful species.
The fossils published in the Nature paper are all teeth. Teeth are often the best-preserved fossils because their enamel coating provides better protection from the ravages of time and the elements.
Delezene, a hominin dental expert, says, "When we get down to the picky details, the teeth of Homo and Australopithecus look different. The differences are subtle, but once you see them, you can't unsee them. They're very consistent."
While the new fossils fill in a piece of the puzzle, there is still a long way to go before we have a complete picture of human evolution. While there is evidence for the teeth of early Homo and the new Australopithecus, the team doesn't know what their heads or the rest of their bodies looked like. The multi-national collaboration, done in partnership with the local community of Afar people, will continue its work looking for more fossils, ideally with continued funding.
