Human Evolution Challenged by Fossils From a Hotly Debated Ancient Species

"I never could have predicted a species like this."

In 2008, a nine-year-old boy named Matthew Berger chased after his dog Tau near a site called the Cradle of Humankind in South Africa. As he ran past the Malapa pit, he tripped. Pausing to examine what caused the stumble, he called to his father, the paleoanthropologist Lee Berger, who confirmed what Matthew suspected. The boy had tumbled over fossilized bones. A decade later, these fossils are challenging the traditional, linear view of human evolution.

These bones belonged to a previously unknown hominin species, Australopithecus sediba. When Berger and his team announced the discovery of Au. sediba, they noted that it appeared to share features with both australopiths — a genus of extinct primates that includes the famous Lucy — and our own genus, Homo. Anthropology professor Steven Churchill, Ph.D., who was a part of the team that first analyzed these 2-million-year-old specimens, tells Inverse that among the known australopiths, he believes Au. sediba represents the best candidate for an ancestor of “true humans.”

“This is reflected in our choice of specific name, sediba, which means ‘source’ or ‘well spring’ in Sesotho,” Churchill says, referring to the language of the Basotho people of South Africa. “This interpretation hasn’t been universally accepted, because well, that’s the nature of science.”

This debate over Au. sediba’s place in human history is addressed in a special issue of the open-access journal PaleoAnthropology, published in December. To date, two confirmed Au. sediba skeletons, MH1 and MH2, have been analyzed. While scientists accept they are a valid species, critics have challenged the decision to include it in the genus Australopithecus, as opposed to Homo. This new analysis, in which scientists examined the last decade of research conducted on these ancient specimens, embraces the “mosaic nature” of *Au. sediba and supports its status as a unique species.

Australopithecus sediba
Life reconstruction of Australopithecus sediba.

But that doesn’t mean it’s a missing link, as some mainstream news outlets have reported. Instead, the existence of Au. sediba reinforces the idea that human evolutionary history looks more like a branching tree, in which evolutionary experimentation has birthed different forms.

Jeremy DeSilva, Ph.D., a professor at Dartmouth University and a co-editor of the new issue, tells Inverse that Au. sediba can be considered one such experiment, “in some ways more human-like than its predecessors, like Lucy, and in some ways more ape-like.” DeSilva notes that it’s still unclear whether Au. sediba is an ancestor to our own genus Homo, but the idea is still a testable hypothesis that researchers will continue to assess with more fossils and new methods.

“‘Missing link’ is a tired cliché that misrepresents how evolution actually works and just leads to misunderstandings and misconceptions,” DeSilva says. “Human evolution has been more complicated and more interesting than the all-too-common imagery of a human slowly evolving from a chimpanzee, with the ‘missing links’ positioned in between.”

Au. sediba
In the middle is a composite reconstruction of Au. sediba. 

Examinations of MH1 and MH2 suggest that Au. sediba was relatively petite — just 77 pounds with diminutive vertebrae. It has a unique combination of anatomies, unlike any other found hominin, with hands capable of powerful gripping and precise manipulation. The shoulder and forelimbs of Au. sediba are considered “primitive,” and its brain was small, but its pelvis is shaped more like a human’s. It walked with two feet, although somewhat differently than we do, and it couldn’t run long distances. It also spent its days climbing trees, perhaps looking for food or hiding from danger.

These anatomies, says DeSilva, force us to reassess the pathway by which we became human. He’s especially fascinated by the locomotion of Au. sediba — noting that for years, scientists debated whether it walked like humans today, or if it was still climbing in trees. Now, it seems like there is no right answer to this question. From the shape of its bones, DeSilva can tell that it “walked with a somewhat peculiar gait, precisely because they were still comfortable climbing in the trees.”

Scott Williams, Ph.D., a New York University professor and fellow co-editor of this new analysis, tells Inverse that the shared traits of this species means that either the characteristics it shares with Homo arose by convergent evolution, or that it’s more closely related to early Homo than it is to other species of Austroalopithecus.

“Figuring out which of those scenarios is correct is very tricky, and therein lies the debate,” Williams says. “It’s our hope that other research groups use the data we’ve published in these papers to test hypotheses — ours, theirs, and others — and generate new hypotheses.”

If Au. sediba does turn out to be an immediate ancestor or sister species to Homo, Churchill says, then it “should help us to understand the evolutionary processes and ecological context of our genus.” And if it turns out that the traits Au. sediba shares with Homo are the product of convergent evolution, we’ll still learn something about what was going on when our genus emerged because Au. sediba and Homo would have experienced the “same sorts of natural selection for them to converge in their morphology.”

And while these questions hang in the air, researchers will continue to hunt for more bones and are encouraged to examine an open database where one can download printable copies of the fossils already found. DeSilva emphasizes that these fossils tell the story of each of us, and because of that we should all have an element of access to them.

“I never could have predicted a species like this,” DeSilva says. “To me, the big lesson of Australopithecus sediba is that we clearly have more to discovery about our world, and ourselves.”