Encore for Evolutionary Small-Timers: Tiny human cousins get younger with new finds

Recently excavated remains of half-size human ancestors on the Indonesian island of Flores indicate that these ancient individuals belonged to a distinctive species that survived until about 12,000 years ago, which is longer than researchers initially estimated.

The growing cache of fossils of this species, Homo floresiensis, represents at least nine individuals, say archaeologist Michael J. Morwood of the University of New England in Armidale, Australia, and his colleagues. The same scientists announced their discovery of H. floresiensis last year (SN: 10/30/04, p. 275: Evolutionary Shrinkage: Stone Age Homo find offers small surprise), including a partial skeleton estimated to be around 18,000 years old and other fossils dating from up to 95,000 years ago. All the individuals represented by the fossils were no taller than about 1 meter.

Some scientists argue that the island population might not constitute a new branch of the human tree, but rather represent a population in which everyone had genetic defects that produced abnormally small brains and bodies. In the Oct. 13 Nature, Morwood and his colleagues hold their ground.

“It seems reasonable for [them] to stick to their original hypothesis that H. floresiensis is a new species,” remarks anthropologist Daniel E. Lieberman of Harvard University in an editorial published with the new report.

In their latest excavations, Morwood’s group recovered all three right-arm bones of the previously reported partial skeleton, the lower jaw of another individual, and leg, arm, shoulder, spinal, toe, and finger bones of yet other individuals. Radiocarbon dating places an arm bone, the radius, at about 12,000 years old and the jaw at around 15,000 years old.

H. floresiensis’ diminutive stature, long arms, and nearly chimp-size brain resemble body proportions of australopithecines, Morwood says. That group of human ancestors lived more than 2 million years ago. The Flores population may have directly evolved into a Homo species from an unknown Asian australopithecine, Morwood speculates.

In Lieberman’s view, however, the size and shape of the Indonesian creature’s teeth and bones signify a closer link to other Homo species than to australopithecines. Some other researchers agree and now suspect that Morwood’s team has mistakenly classified H. sapiens fossils as a new species.

Such a small-brained creature could not have made the sophisticated stone tools that have been found among its remains, contends anthropologist Robert D. Martin of the Field Museum in Chicago. Until now, such implements have appeared only at Stone Age H. sapiens sites, Martin says.

Martin proposes that the Flores skull comes from a H. sapiens individual who had microcephaly, a genetic condition that drastically reduces brain size and causes other developmental abnormalities.

Anthropologist Robert B. Eckhardt of Penn State University in State College, who examined the Flores partial skeleton and associated remains last February, agrees. Estimates of H. floresiensis’ brain and body size by Morwood’s team are too low, he says. Among the Flores fossils, the partial skeleton represents a short person who suffered from developmental problems that included microcephaly, Eckhardt argues.

“I’m absolutely, totally confident that H. floresiensis will not last,” Eckhardt says.