Disinherited Ancestor: Lucy’s kind may occupy evolutionary side branch

About 30 years ago, African excavations yielded the 3.2-million-year-old partial skeleton that became known as Lucy. The find, along with other fossils unearthed soon after, belongs to the species Australopithecus afarensis. Many scientists regard these creatures as ancestors of both the lineage that led to modern humans and of another, now-extinct evolutionary lineage known as robust australopithecines.

However, an analysis of an A. afarensis jaw from a skull discovered in 2002 near Lucy’s site in Ethiopia supports a longstanding minority viewpoint that Lucy’s kind occupied only a side branch of human evolution. A. afarensis evolved into the relatively small-brained, large-jawed robust australopithecines but didn’t contribute to the evolution of modern people, says anthropologist Yoel Rak of Tel Aviv University.

Rak and his coworkers base their conclusion on the size and shape of a horizontal bone that connects the lower jaw to the upper jaw. This bone, called the ramus, looks much the same in A. afarensis, in a roughly 2-million-year-old robust australopithecine species known as Australopithecus robustus, and in modern gorillas, the researchers report in the April 17 Proceedings of the National Academy of Sciences.

All other primates, including chimpanzees and fossil members of the human-evolutionary family, share a different ramus configuration, the team asserts.

These findings “cast doubt on the role of A. afarensis as a modern human ancestor,” Rak says.

Rak’s team examined 146 jaws from modern primates: 41 people, 31 gorillas, 29 pygmy chimps, 29 common chimps, and 16 orangutans. The researchers obtained 20 size and shape measurements from digital images of each ramus. They then used a computer program to calculate an average ramus contour for each primate group. People, chimps, and orangutans displayed a similar contour.

In the newly unearthed A. afarensis jaw and in a handful of previously discovered partial jaws from the same species, the ramus closely resembles that of the gorilla, Rak says. Key traits include an especially wide upper ramus and a relatively small notch where the bone attached to the upper jaw.

Two A. robustus specimens that retain part of the ramus also show a gorillalike pattern, the investigators hold. So does a 2.5-million-year-old South African fossil that had been attributed to Australopithecus africanus, in Rak’s view. That’s evidence that A. africanus was another robust australopithecine, he says.

Fossils from ancient Homo species, as well as those from a nearly 4.5-million-year-old human ancestor dubbed Ardipithecus ramidus, display a ramus configuration like that of modern chimps.

Rak theorizes that a chimplike ramus appeared in the first members of the human evolutionary family and then in later species. However, Lucy’s kind independently evolved a gorillalike ramus that was passed on to robust australopithecines, he asserts.

Other researchers disagree. The ramus doesn’t offer enough information for scientists to reconstruct broad evolutionary relationships among Lucy’s kind and other ancient species, remarks anthropologist Tim White of the University of California, Berkeley.

“Rather than trying to use the top edge of this jaw in such a dubious manner, [Rak’s group] would have done better to describe and analyze the important new skull that goes with it,” White says.

Donald C. Johanson of the Institute of Human Origins in Tempe, Ariz., and a codiscoverer of Lucy’s skeleton, had no comment on Rak’s report.