Walking Small: Humanlike legs took Homo out of Africa

The earliest known human ancestors that trekked from Africa into Asia possessed legs, feet, and spines much like ours, even as they sported relatively apelike arms and small brains, according to an analysis of 1.77-million-year-old fossils unearthed in the central Asian nation of Georgia.

A team led by David Lordkipanidze of the Georgian National Museum in Tbilisi recovered 33 lower-body bones from at least three adults and one teenager at a site called Dmanisi. The researchers had previously found four skulls and four lower jaws, as well as simple stone tools, in the same sediment (SN: 5/13/00, p. 308). In several cases, skull and lower-body remains come from the same individual.

The researchers classify these ancient finds as early Homo. The fossils might be from an early form of Homo erectus that left eastern Africa for the Asian hinterlands, but a definitive species identity remains unclear, Lordkipanidze cautions. A description of the new finds appears in the Sept. 20 Nature.

“The Dmanisi individuals weren’t the first hominids [fossil ancestors of humans] to leave Africa,” Lordkipanidze says. “They must have had more-primitive ancestors that passed through the Near East before reaching Georgia.”

An intriguing mosaic of anatomical traits characterizes the Dmanisi folk. Their legs and spines closely resemble those of modern humans. In particular, Dmanisi leg and foot bones would have efficiently supported long-distance walking and running, the scientists assert.

However, the arms of Dmanisi hominids appear more like those of australopithecines, an earlier line of hominids. For instance, unlike people, the new specimens have upper arms that are straight rather than slightly curved, their shoulders are relatively narrow, and their palms are oriented forward rather than inward.

Moreover, the Dmanisi individuals are small compared with the oldest known African H. erectus. That specimen, a 1.5-million-year-old skeleton of a well-developed, roughly 10-year-old boy, stood tall at between 151 and 169 centimeters and weighed as much as 70 kilograms. At Dmanisi, adults reached estimated heights of between 145 and 166 cm and weighed between 40 and 50 kg.

Such estimates coincide with Dmanisi brain volumes that were one-half to two-thirds the size of modern human brains.

The Dmanisi fossils and early African H. erectus remains probably represent separate populations of a species that evolved variations of a common body plan as members settled different habitats, suggests anthropologist Daniel E. Lieberman of Harvard University in a comment published with the new report.

If the Dmanisi remains indeed belong to early H. erectus, members of that species must have returned to Africa and evolved into a larger, more modern-looking form by 1.5 million years ago, remarks anthropologist William L. Jungers of Stony Brook (N.Y.) University Medical Center. In his view, the Dmanisi foot bones were built for long-distance walking but show no convincing sign of having supported a fully modern running ability.

Jungers adds that the australopithecine-like arm traits in Dmanisi individuals also appear in Homo floresiensis, Indonesian ancestors that some researchers regard as modern humans with a developmental disorder (SN: 11/18/06, p. 330). Jungers, however, contends that H. floresiensis was a separate species that preserved some primitive skeletal features, just as H. erectus did at Dmanisi.