Friday, May 19, 2006

Hobbits are people, too

Major bummers on the science front this week. First we learn that our ancestors may have gotten funky with monkeys. Now it turns out that the Indonesian "hobbit" people were just diseased dwarfs, and not an unknown species (internal link removed):
The debate over whether the "hobbit” fossil found on an Indonesian island is a separate species has reignited, as a new study of dwarfing in a range of mammals suggests that Homo floresiensis was a modern human with a pathological condition.

The remains of a tiny woman were found in a limestone cave in Flores, Indonesia. Named H. floresiensis by the discoverers, she quickly became known as “the hobbit” by everyone else.

When the find was reported in 2004 some anthropologists disputed whether it was a new species of human, arguing that the skeleton had characteristics of a modern human with microcephaly, a condition that causes reduced cranium size. Microcephaly is relatively common in isolated populations and is associated with reduced brain function.

Peter Brown and Mike Morwood from the University of New England, Australia, proposed that the 1-metre-tall body (known as LB1) had evolved in an isolated population of Homo erectus as an adaptation to the restricted diet found on an island. But at 380 cubic centimetres, some thought that LB1’s chimp-sized cranial capacity was too small to be a dwarf H. erectus. Brown and Morwood denied this, but their conclusion has now been challenged again.

“As they dwarf, species’ brain sizes decline far more slowly than body size,” says Ann MacLarnon from Roehampton University, UK, who modelled dwarfing in a range of mammals from dogs to elephants with a team from the Field Museum, Chicago, US. “Brain size is key to a mammal species’ identity,” she says. There is, for example, hardly any difference in brain size between the smallest modern humans, the 1.4-metre Bambuti people of Congo’s Ituri Forest, and the tallest, the 2-metre Masai of east Africa.

The team calculated that a dwarfed H. erectus with a 400cc brain would weigh just 2 kilograms. “That’s one-tenth of what the Flores people must have weighed,” she explains. The only way to explain the discrepancy, the team believes, is microcephaly.

“It’s perfectly plausible that these were pygmy people. But there’s only one skull, and that is human and microcephalic,” says team leader Robert Martin. This, Martin believes, ties in with the abundance of sophisticated stone tools at the cave. “These were sophisticated people with a high level of mental development,” he says.

“Although we only have one cranium,” says Morwood, “the other bones we found show that LB1 was a normal member of an endemically dwarfed hominid population.” The distinctive traits of reduced body mass, reduced brain size and short thick legs mirror those found in other island endemic populations of large mammals, Morwood says. He calls the microcephaly explanation “bizarre”. It ignores other evidence from Liang Bua and the literature on island endemic evolution, he says.