Australopithecus sediba is an important find, but it's not the "missing link."

The state of the universe.
April 8 2010 10:47 AM

Yet Another "Missing Link"

A brand-new Australopithecus fossil is fascinating and important. But it's not one of our direct ancestors.

Illustration by Robert Neubecker. Click image to expand.

The news began bubbling out over the weekend: "Missing link between man and apes found," declared an April 3 story in the London Telegraph. When I saw that headline, I thought to myself, "Please, please, not again."

Whenever scientists make a major discovery about human evolution, we get treated to a lot of misconceptions. The most popular of them all is the myth of the missing link—the idea that paleontologists are on an eternal quest for ancestors linking us directly back to earlier forms of life. Last May, for example, scientists reported the discovery of a 47-million-year-old fossil of a primate called Darwinius. "Fossil is evolution's 'missing link,' " blared a headline in the Sun."The beautifully preserved remains—dubbed Ida—is believed to be a direct connection between humans and the rest of the animal kingdom," the article said—a sentence that makes no sense the first time you read it and then somehow manages to make even less sense the longer you look at it. The Sun was not alone in its delivery of nonsensical hype. Shortly after Darwinius was unveiled, the History Channel aired a show about its discovery, called The Link. The show itself may be long gone, but its elaborate Web site lives on, still "uncovering our earliest ancestor." To all our wormlike ancestors and primordial bacterial forerunners: You have my deepest sympathy for that slight.

This weekend's new missing link was much younger than Darwinius, just shy of 2 million years old. "Scientists believe the almost-complete fossilised skeleton belonged to a previously-unknown type of early human ancestor that may have been an intermediate stage as ape-men evolved into the first species of advanced humans," Richard Gray wrote in the Telegraph. Over the next few days, other news outlets picked up the story, repeating the promise that scientists had at last found the missing link in an unbroken chain leading directly back to our ape ancestors. They were all sketchy on the details, however, because the papers detailing the fossil were still in press at the journal Science. It was not until this morning that   the real story came to light. Lee Berger of the University of Witwatersrand in South Africa and his colleagues unveiled a pair of fossils dating back somewhere between 1.95 million to 1.78 million years. The world finally got to meet a creature called Australopithecus sediba. And while these fossils are certainly significant in a lot of ways, one thing they definitely are not is a missing link.

Berger and his colleagues argue that A. sediba offers us clues to a crucial transition in our evolution. About 6 million years ago, our ancestors began to spend some of their time walking upright on the ground. Their descendants evolved into many different species, known as hominins. For the first 3.5 million years of hominin history, hominins all had the same basic kind of body: a chimp-sized brain (about one-third the size of our own), a snoutlike mouth, long arms with curved fingers good for climbing trees, and short legs that allowed them to walk slowly on the ground. Scientists call these early hominins australopiths.

Then something curious happened, starting about 2.3 million years ago. New hominins arose—members of our own genus, Homo. The australopith snout flattened. The hooklike fingers became smaller and more agile. By 1.8 million years ago, a species called Homo ergaster was about as tall as living humans, with long legs and stiff feet that were only good for walking on the ground. Its brain was now two-thirds the size of our own.

Paleoanthropologists have been trying for decades to learn more about how australopiths evolved into Homo. They've wondered where that evolution took place and how it unfolded. But they've found relatively few hominin fossils from that period. Making matters worse, the fossils are not complete skeletons but scattered bones—a skull cap here, a forearm there.

A. sediba left behind at least two partial skeletons, giving Berger and his colleagues a spectacular look at its anatomy. And based on that anatomy, Berger and his colleagues argue that A. sediba is an australopith who's closer to Homo than any fossil ever found before. It had long australopithlike arms, but its hands were short like those of Homo. Thanks to its long, Homo-like legs, it stood about 4.5 feet tall. But its ankle bones looked like an australopith's. It had a projecting nose like us, but a tiny brain.

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.

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