Lucy's Grandson

By MICHAEL D. LEMONICK

JUST ABOUT EVERYBODY HAS HEARD OF Lucy, the diminutive, apelike superstar of human evolution. The discovery of her fossilized partial skeleton in 1974 was startling evidence that humanity's ancestors walked the earth more than 3 million years ago, hundreds of thousands of years earlier than anyone had imagined. Since that find, paleontologists have unearthed many similar bones, some even older than Lucy's, in the same part of Ethiopia where she was found. Most believe that all the fossils come from a single species (scientific name: Australopithecus afarensis) and that this species was probably the forerunner of all later hominids, including modern Homo sapiens.

But there has always been a band of anthropological dissidents who subscribe to a different theory. A. afarensis was not a single species, they say, but a group of loosely related species. If that is true, then there must have been an even older species, still undiscovered, that was ancestral to them all. The debate has been difficult to resolve, because fossil hunters have never found a key piece of evidence: an intact A. afarensis skull. Skulls are the Rosetta stones of anthropology, bearing unique features that let scientists determine whether two fossil samples come from the same type of creature.

Now they have the evidence. Researchers from the Institute of Human Origins (IHO) in Berkeley, California, and from Tel Aviv University in Israel report in the current issue of Nature that they have discovered a nearly intact skull from a male A. afarensis who lived about 200,000 years after Lucy -- call him Lucy's Grandson -- along with several arm bones from other males. The new fossils virtually clinch the view that A. afarensis is one species, placing it more firmly than ever at the root of the human family tree. And because the specimens are nearly a million years younger than the very oldest A. afarensis bones, they argue that Lucy and her relatives were an extraordinarily long-lived species. Says Alan Walker, an expert on early human anatomy at Johns Hopkins: "Everybody's very excited about this."

The notion that A. afarensis might have been more than one species was largely based on the wide range of sizes of the specimens. Lucy, for instance, appeared to have been only about 3 ft. 6 in. tall and to have weighed 60 to 65 lbs., while some males would have topped 5 ft. and 110 lbs. Size variations between the sexes are common among the great apes, but they aren't usually this drastic among hominids. (Despite the extreme examples of, say, the towering Shaquille O'Neal and the tiny Dr. Ruth Westheimer, there's only about a 5-in. difference between the average modern man and woman.)

Moreover, the relatively few afarensis arm and leg bones that had been found seemed to show structural differences for different means of locomotion. The smaller females were evidently better at swinging through trees than the males, while the males appeared to be better at walking. It was hard to imagine that members of a single species could be built so differently.

But there were good arguments on the other side. While modern humans don't vary much in size, other early hominids did. Besides, argues William Kimbel of the IHO, principal author of the Nature report, if there really were two species, then we have just happened to find only females from one and males from the other -- an almost inconceivable coincidence.

Unfortunately, the few hundred afarensis bone fragments scientists have dug up over the past 20 years have been too few and too fragmentary to advance the argument very far in either direction. That is why Lucy's Grandson is a breakthrough. Says Walker: "We've had parts of afarensis skulls from different individuals, but now we know what a single skull looks like, and we have the proportions correct."

Those proportions, and comparisons between the grandson and other, more fragmentary skulls both large and small, convince Kimbel and his colleagues that afarensis was indeed a single species, as they had believed all along. The arm bones, too, appear to bolster this idea. According to Leslie Aiello, an anthropologist at University College London, they have exactly the robust, curving form you would expect from a tree climber. The two sexes didn't have different kinds of skills, she says, but were both "a mosaic, bipedal from the waist down and arboreal from the waist up."

As for what was above the neck, the skull confirms earlier constructs based on fragments: A. afarensis had an apelike face with a forward-thrusting jaw and an overhanging brow. The brain was no bigger than a chimp's, but it is now clear that Lucy and her kin were hardy enough to adapt to changing environments and thus to survive for some 9,000 centuries. And unless older hominid fossils are found -- always a possibility -- they will retain their distinction as the first evolutionary step that began to distinguish humans from other animals.

With reporting by Alice Park/New York