Island of the Lost Hominids

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Get to know that little skull. Scientists are going to be talking about it for centuries.

As researchers report in tomorrow’s issue of Nature, the skull–and along with other parts of a skeleton–turned up in a cave on the Indonesian island of Flores. Several different dating methods gave the same result: the fossil is about 18,000 years old. (Additional bones from the same cave date back to about 38,000 years.) If all you had was the 18,000 year figure and this picture to go on, you might assume that the skull belonged to a small human child. After all, there is plenty of evidence that Homo sapiens had already been in this part of the world for 25, 000 years. But you’d be wrong.

The skull actually belongs to a previously unknown species of hominid, whose ancestors split off from our own some 2 million years ago. Homo floresiensis, as it’s known, stood three feet high as an adult and had a brain less than a third the size of our own.

To understand just how mind-blowing Homo floresiensis is, you have to consider it in the context of hominid evolution. Our closest living relatives (chimpanzees and bonobos) live in Africa, and both genetic and fossil evidence indicate that the common ancestor we share with them lived in Africa as well. The oldest known hominids–those species more closely related to us than chimps or other primates–date back 6 million years. They were short, probably could walk upright, and had brains about the size of a chimpanzee–about 350 cubic centimeters. It was only about 2.6 million years ago that hominids started using stone tools, and only about 2 million years ago that species emerged that stood as tall as we do. Its brain was also bigger–850 cc. The increase in brain size may not have been all that significant, since bigger mammals tend to have bigger brains, smart or not. But shortly after this evolutionary surge, the first hominids turned up outside Africa. Homo erectus moved as far east as China and Indonesia within just a few hundred thousand years. At the very least, their migration suggests an expanding population of meat-eaters who have to seek out much bigger ranges than their ancestors.

The Asian population of Homo erectus had little, if anything, to do with our own origins. The oldest human fossils, dating back 160,000 years ago, were found in Africa, and there’s a pretty good chain of evidence showing that Homo sapiens descends from hominids who stayed home on the mother continent while Homo erectus swept across Asia. For instance, African hominids underwent a massive burst of brain expansion around 500,000 years ago to close to our own capacity. Meanwhile, Homo erectus in Asia underwent a slight increase, if any. Humans only expanded successfully out of Africa about 50,000 years ago. They may have interbred with Homo erectus, but most of our genome still points back to a recent African origin.

Paleoanthropologists were first attracted to Flores when 800,000 year old tools were found on the island in 1998. Boats seem to have been essential for getting to Flores, which speaks of a pretty impressive mental capacity for Homo erectus . (On the other hand, lizards and elephants and other land animals got to the island without a boat–perhaps by swimming being swept away on logs during storms.) Researchers poked around on Flores, and last September they turned up something none of them had expected: Homo floresiensis. Homo floresiensis was not an ape–it had the signature traits of a homind, such as a bipedal anatomy and small canine teeth. But it wasn’t a pygmy human, either. Pygmy brains are in the normal range of variation for our own species. What’s more, the floresiensis brain wasn’t just small but had a drastically different shape than ours–a shape more like the brain of Homo erectus. This and other anatomical details have led the researchers to conclude that Homo floresiensis branched off from Homo erectus and evolved into a dwarf form.

Here is case-closed proof that today’s solitary existence of Homo sapiens is a fluke in the history of hominids. Even 18,000 years ago, at least one other species walked the Earth with us. Exactly how Homo floresiensis went extinct no one knows, but close to the top of the list would have to be ourselves. Neanderthals survived only a few thousand years after humans turned up in Europe, and Homo erectus seems to have disappeared from Indonesia around 40,000 years ago, just around the time humans came on the scene. Perhaps Homo floresiensis lasted longer on Flores because it was harder for humans to reach.

A dwarf hominid on an island is fascinating for another reason–islands are famous for fostering the evolution of dwarf animals, from deer to mammoths. It’s possible that the small territory of islands and the lack of competition and predators favors the small. For the first time, hominids have fallen under the same rule. Islands mammals have also been shown to sometimes evolve much smaller brains, and, incredibly, the hominid brain is subject to the same rule. Homo floresiensis’s brain shrank down to the smallest size ever found in a hominid. Did Homo floresiensis lose the mental capacity to use tools along the way? The researchers found stone tools in the same site where they found Homo floresiensis, but it’s not clear whether Homo floresiensis made the tools, or humans used them (perhaps to kill Homo floresiensis?).

One of the most interesting questions that comes to mind with the discovery of Homo floresiensis is how far back it goes in the fossil record. Just how long did it take for a lineage of hominids to lose half their height and two-thirds of their brain? It may have taken a million years, or a few hundred thousand, or maybe less. In a commentary in Nature, Marta Lahr and Robert Foley of Cambridge point out that it only took 12-foot high elephants on Malta only 5,000 years to shrink to the size of a dog. I’ve always been a bit skeptical when people forecast dramatic change for our species. But if evolution can produce Homo floresiensis, who knows what a few thousand years on Mars or another solar system could take our descendants?

Update, 11/1/04: Here’s a bundle of papers, interviews, and such on H. floresiensis from Nature. Much of it is free.