The Human Family Shrub?

The phrase "family bush" doesn't trip off the tongue the way "family tree" does, but anyone talking about human evolution had better get used to it. For years, scientists who study human origins have known that the simple model in which one human ancestor evolved into another in a nice, linear fashion is a myth. Instead, starting 4 million years ago, half a dozen species in the genus Australopithecus lived in Africa at the same time. Only one is our direct ancestor; the others were evolutionary dead ends, failed experiments. But experts thought that once the Homo lineage debuted about 2.5 million years ago in East Africa with Homo habilis, things settled down, with habilis evolving into Homo erectus who evolved into Homo sapiens—us—like biblical begats.

Two fossils discovered in Kenya suggest that evolution was a lot messier than that. One of the specimens, found just east of Kenya's Lake Turkana, is the upper jaw bone of a habilis from 1.44 million years ago; habilis was thought to have become extinct about 1.6 million years ago. The other is an erectus, say their discoverers, a well-preserved skull from 1.55 million years ago and the smallest ever found for this species. The more recent date for habilis shows that it and erectus were contemporaries for half a million years, from 1.9 million to 1.44 million years ago. The evidence that Homo habilis and Homo erectus lived at the same time in the Turkana basin makes it "unlikely that Homo erectus evolved from Homo habilis," says Meave Leakey, a lead author of the paper announcing the discovery in tomorrow's issue of the journal Nature. (A research associate at the National Museums of Kenya and research professor at Stony Brook University in New York, she is the wife of anthropologist and naturalist Richard Leakey; their daughter Louise, the third generation of her family to go into the fossil-hunting business, is a co-discoverer of the new specimens.)

Homo habilis first appeared in Africa about 2.5 million years ago, discovered by Louis and Mary Leakey, Richard's parents. Habilis was the first of our ancestors to have a brain bigger than a chimp's, and the first to make stone tools. Homo erectus, long thought to be a direct descendant of habilis, is best known for spreading beyond Africa, eventually venturing throughout Eurasia. Erectus was also the first of our ancestors to have an asymmetric brain, as modern humans do, and which is thought to underlie cognitive complexity. Homo erectus in Eurasia is not the direct ancestors of today's humans, genetic evidence suggests; instead, those who lived in, and eventually left, Africa more recently—possibly less than 100,000 years ago—are the common ancestor of everyone alive today, says Fred Spoor of University College London, a co-discoverer of the latest Homo fossils.

There is no evidence about how, or even whether, erectus and habilis interacted. But since they stayed reproductively separate for so long (there is no evidence of erectus-habilis mating) each probably had its own diet, habits and way of living. The teeth and jaws are larger in Homo habilis than Homo erectus. That suggests habilis ate tougher food, such as vegetation, while erectus ate more meat—just as gorillas and chimpanzees live in some of the same habitats today, with gorillas spending more time eating tough vegetation than chimpanzees do.

Another clue to how ancient species lived is the relative size of the sexes. The new erectus skull is so small, it may have come from a female, and the two sexes of dramatically different size within this species. In general, apes that show little difference in size and shape between males and females are monogamous; those that have multiple mates, such as gorillas and baboons, tend to have more sexual dimorphism. If Homo erectus was sexually dimorphic, their social groups may have been built around a few reproductively active males who took several females each as their mates.

As happens so often in the science of human evolution, researchers not involved in the find are not sure the discoverers got it right. Paleoanthropologist Ian Tattersall of the American Museum of Natural History, for instance, questions whether the specimens are indeed that of a habilis and an erectus. Neither "look anything like" the specimen that defines the species, he says. It's equally possible that yet more Homo species were popping up in East Africa, and that the erectus skull is actually from an entirely new species of Homo. "It just doesn't look like erectus as we know him," says Tattersall. "What they're calling erectus might be a new, as-yet-unnamed species."

The discoverers are sticking by their guns, and even Tattersall agrees that their conclusion—that erectus and habilis overlapped in time and that habilis was not the direct ancestor of erectus—is probably right. Which leads to perhaps the greatest puzzle of all. Throughout human evolution, several species of ancestors lived at the same time. The most recent, of course, were Neanderthals, which made their last stand in the Iberian peninsula about 35,000 years ago. Then why is Homo sapiens the one and only species of human on the planet today?