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University of Minnesota

Honey, I shrunk the cranium

March 2, 2009

Kieran McNulty.

University anthropologist Kieran McNulty studied the fossil Indonesian "hobbit" skull.

Photo: Kelly MacWilliams

Shrinking made Indonesian 'hobbit' fossils look deceptively like modern humans

When the remains of the pint-size 'hobbits' on the Indonesian island of Flores came to light in 2003, the discovery rocked the anthropology world.

Were they small modern humans, or a new species?

"The skeletons dated between 90,000 and 12,000 years ago, a period during which modern humans walked the earth," says University anthropologist Kieran McNulty. So the modern human theory was a possibility.

The answer hinged on the enigmatic nature of the one complete cranium discovered, which is named LB1 after the Liang Bua cave where it was found. Its facial features bear some resemblance to a modern human's. But the braincase is small, housing a brain only 30 percent the size of a modern human's.

And that's where McNulty comes in. In a study with Karen Baab of Stony Brook University Medical Center, he concluded that the hobbit people acquired their facial resemblance to modern humans as a consequence of shrinking in stature. And that's what it is: only a resemblance, and only in facial features.

The incredible shrinking method

If LB1 was a modern human, then modern humans that shrank over evolutionary time should end up with skulls bearing LB1's features. But if it was more closely related to an ancestral species of human, then that species, if shrunk, would develop skulls like LB1.

In other words, if a modern human and a fossil human species were both three feet tall, whose skull would more closely resemble LB1? The answer would point to LB1's next of kin.

McNulty and Baab could not, of course, actually shrink whole human generations. But they could simulate that. To do it, they compared cranial features in large modern humans to small ones like Andaman Islanders and the Khoe-San (pygmies) of Africa. They also compared skulls of large African apes (gorillas) to those of smaller chimpanzees and bonobos.

They noted that whether for humans or apes, the various parts of the skull rearranged their proportions in the same way with smaller body size. For example, as body size decreases, brains and eyes get relatively larger and the height of the cranium increases.

The researchers next pooled the cranial characteristics of several premodern human species to create an "average" skull. Then they "shrank" that skull, too, rearranging its proportions according to the pattern described above. This gave them a model of a fossil Homo species the size of a hobbit.

"The LB1 cranium fits very well a model of a fossil Homo at that size," says McNulty. "It's too different from the Andaman Islanders and the Khoe-San [to be a small modern human]. We think it's a closely related species to Homo erectus, or something even more primitive."

The researchers also noted that as fossil humans and African apes underwent simulated shrinking, their facial features looked more and more like those of a modern human. Such a resemblance on the part of LB1 could have thrown anthropologists a curve.

Why shrinking should result in predictable changes in skull shape is a complicated question.

"You can't just make things bigger or smaller," McNulty explains. "When things change size, they need different mechanical properties." Or, there could be certain spatial requirements for the brain. A host of factors determine the proportions of the "shrunken" skull.

Another baffling question about the hobbit people is how they got on an island like Flores in the first place, an island that also contained fossil dwarf elephants. Sea level drops dramatically during ice ages, but it was never low enough for the ancient Floresians to walk. And if they weren't modern humans, a boat seems unlikely, McNulty says.

"Maybe," he muses, "they got there by whatever event brought the elephants."

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