Berger is not claiming that he has found the missing link—a direct ancestor of modern humans who made a great evolutionary leap from australopith to Homo. "I don't like the use of the term," Berger said in a press conference Wednesday. "The term missing link is a Victorian term that implies some chain of evolution."
Evolution actually branches like a tree. Berger is arguing that a single hominin ancestor gave rise to two lineages. One produced the A. sediba fossils. The other gave rise to Homo—and, eventually, us. These two hominin branches grew over time, and 1.8 million years ago they were both still alive and well. But then the A. sediba lineage went extinct while the Homo branch kept growing.
In other words, the fossils Berger discovered cannot be our direct ancestors. Instead, they may be very informative cousins. If Berger's right, then the evolution of Homo happened in a surprisingly piecemeal way. Our legs got long, for example, well before our arms got short.
At least, that's what Berger is claiming. None of the experts I spoke to this week were ready to accept Berger's hypothesis about A. sediba's special place in the hominin tree. It might actually belong to a different branch of hominin evolution. It may have evolved its Homo-like traits independently of our own ancestors. "The origins of the genus Homo remain as murky as ever," said Daniel Lieberman of Harvard.
But that's OK. Even if A. sediba doesn't turn out to be a close relative, it gives us a glimpse at the remarkable diversity of hominins that walked the Earth 1.8 million years ago. And no matter which hypothesis wins out, A. sediba is evidence that these are exciting times to study human evolution. People have been digging up human fossils for more than 150 years, and yet the past decade alone has seen a string of spectacular discoveries, from fossils that push back hominin origins millions of years to a separate species of Hobbit-sized hominins who were alive just 17,000 years ago. And some of those fossils, incredibly enough, still have DNA in them.
But the study of cavemen can get very confusing if we cling to bogus terms such as "missing links." It's high time we ditched those canards so we can really get to know our history better.