Fossil Upends Theories about Evolution of Human Ancestors

Article excerpt

Scientists in Ethiopia have found a 4.4 million year old human predecessor that promises to upend long-held notions of what the common ancestor of African apes and humans looked like, how it lived, and how much both lineages have evolved since diverging.

The species, called Ardipithecus ramidus, is one million years older than "Lucy," the famous partial female skeleton of a hominid that lived 3.2 million years ago and was previously the closest scientists had come to finding the common ancestor of apes and humans. Modern genetic analysis suggests that the lineages of chimpanzees and Homo sapiens diverged more than 6 million years ago.

The discovery, published in Friday's issue of the journal Science, is a seminal find in a scientific quest that began when Charles Darwin first posited that humans and apes were descended from a common ancestor. Ever since, anthropologists have wondered what this progenitor might have looked like.

Now, the new find brings them closer than ever before. It also indicates that several assumptions about what that common ancestor looked like were off.

"What we're seeing here is something that we never could have predicted from either a modern human or a modern chimpanzee," said Tim White, a professor at the University of California in Berkeley and coauthor of several of the papers in Science, in an online video presentation. "The only way to learn about this creature is through the paleontological record."

"Ardi" inhabited woodlands in what's now Ethiopia during the Pliocene epoch. What is most remarkable about the find is that Ardi resembles neither apes nor humans. Previously, many anthropologists had thought that the apes of today were relatively unchanged from the common ancestor of 6 million years ago, and it was the hominid line alone that evolved.

As a result, it was frequently posited that the common ancestor would look like an ape. Now, it appears that approach may be more misleading than illuminating.

If Ardi resembles the common ancestor of humans and apes, then apes must have evolved much more than previously thought since separating from hominids.

"It really doesn't look like we evolved from chimpanzees," says Carol Ward, an anatomical scientist at the University of Missouri in Columbia. …