The first specimen, a jawbone assigned to the hominid (roughly: human ancestor) species Homo habilis, dates to a time 1.44 million years ago when the more recent species Homo erectus also roamed Ileret, a site east of Kenya's Lake Turkana where the fossil was excavated.
Researchers have often assumed that H. habilis evolved into the larger H. erectus, from which our species likely branched. But if the two early species coexisted, it is much more plausible that they evolved separately from a common ancestor, as opposed to habilis simply giving way to erectus, according to a report in Nature.
The oldest habilis and erectus fossils found in East Africa date to 1.9 million years ago, indicating that they cohabitated in the region for half a million years, the researchers say. "The problem is there was not a clear date for the last occurrence of Homo habilis," says study co-author Fred Spoor, professor of evolutionary anatomy at University College London.
Spoor says it is still possible that erectus evolved from habilis in places other than Lake Turkana, where the older species would have reencountered the newer one.
But if not, the species' shared ancestor would have had to live two million to three million years ago, he says. And the only remains from this time are fragments of stone tools and a few teeth, he notes, leaving up in the air whether the ancestor was much different from both species or more like the older habilis.
"Many of us have already abandoned this simple scheme" of habilis begetting erectus, says paleoanthropologist Philip Rightmire of Binghamton University in New York State and Harvard University, who was not involved in the study. "For me, it seems increasingly reasonable to suppose that a habilislike creature managed to disperse from Africa into Eurasia, sometime prior to 1.8 [million] to 1.7 [million years ago]."
The researchers also report discovering a 1.55-million-year-old erectus skull smaller than any yet found: 25.2 fluid ounces (691 cubic centimeters) compared with closer to 33.2 ounces (900 cubic centimeters) for other skulls. The difference implies that, unlike humans or chimpanzees, males and females of the species differed significantly in size, similar to gorillas and the australopithecines (even earlier hominids).
Gorillas and other apes with large male / female size differences have societies organized around a single dominant male, meaning erectus may have done the same, Spoor says. "We always thought," he says, "that this organization of a single dominant male had disappeared with the genus Homo."