Hobbit News, Part 1
In an earlier post (http://blog.sciam.com/index.php?title=whatever_happened_to_the_hobbit&more=1&c=1&tb=1&pb=1) , I briefly summarized the state of the current debate over the tiny human remains from the Indonesian island of Flores, judged by their discoverers to represent a hominid species new to science (Homo floresiensis) that lived as recently as 12,000 years ago. I've just returned from the annual meeting of the American Association of Physical Anthropologists in Anchorage, where several presentations relating to the Flores material were given.
The most complete specimen from the excavation in Liang Bua cave is a skull and partial skeleton known as LB1. This individual, said to be a female, stood about a meter tall and had a brain that was on the order of 400 cubic centimeters--about the size of a grapefruit. The folks who conducted the original analysis concluded that it was a dwarfed species of Homo erectus that evolved its diminutive size in isolation as an adaptation to the limited resources its island home had to offer. Such dwarfing is a well-documented phenomenon among mammals larger than rabbits that live on small islands.
The problem is, LB1 does not appear to be a neatly scaled down version of the larger-bodied H. erectus. Tom Schoenemann of the University of Michigan-Dearborn addressed this issue in his talk. He surveyed brain size and body weight data for a bunch of modern and fossil humans, plotted them on a graph and concluded that among known hominid species, LB1's brain size relative to her body size more closely approximates that of the much older australopithecines than H. erectus. But an even better fit, he found, is with microcephalic H. sapiens. Viewed that way, of three possible explanations for what LB1 is--namely, a dwarfed descendant of H. erectus, a descendant of a gracile australopithecine, or a microcephalic modern human--the latter is the most parsimonious diagnosis.
Why is it the most parsimonious? Well, the most recent gracile australopithecines on record lived around two million years ago and their remains have never been found outside of Africa, so the idea that they persisted until far more recently--and got all the way to Indonesia--strikes many experts as improbable. And a lot of paleoanthropologists have trouble with the idea that selection might have downsized a Homo brain more than expected because it goes against the observed hominid pattern of increasing brain size over time. Even modern pygmies, with their small bodies, retain big brains. Large brain size is our adaptation--what we lack in speed or brute strength, among other abilities, we make up for in smarts--so it's hard to imagine natural selection favoring smaller brain size in humans.
But other findings discussed at the meeting, from a study conducted by Andrea B. Taylor of Duke University and Carel P. van Schaik of the University of Zurich, suggest that there may be a primate precedent for exactly this sort of selection. They looked at brain size variation in the four recognized subspecies of orangutan, measuring cranial capacity and skull dimensions. They found that individuals in one of these subspecies--Pongo pygmaeus morio, which resides in northeast Borneo--have a significantly smaller average cranial capacity than members of the other groups. It turns out that compared to the other subspecies, these orangutans contend with the longest and most unpredictable periods of food scarcity. Considering how metabolically expensive brain tissue is, it may be that natural selection has favored smaller brain size in these animals than in their counterparts who live in nutritionally richer environments. Perhaps brain size reduction was favored in the little Floresians for the same reason.
There were more presentations and discussions relevant to the Flores story, so as I go through my notes in the coming days, you can expect additional posts on the topic.
Source: http://blog.sciam.com/index.php?title=h ... &tb=1&pb=1
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