Thursday, March 16, 2006

Homo floresiensis: Think Small

The Grave Digger

The American Association of Physical Anthropologists met last week in Anchorage. Among the many topics discussed was Homo floresiensis, referred to by some as LB1, and many, many more as "the hobbit." She was a small bodied female, just one meter tall with the brain capacity of 400 cc, or about the size of a grapefruit. The discoverers assert that this individual was the descendant of Homo erectus or a small-bodied, possibly as-yet-undiscovered member of the human family. The reason given for its small size is an adaptation to the limited resources of the island it inhabited - this dwarfing phenomenon has been documented among mammals larger than rabbits living on small islands.

Skeptics have asserted that the hobbit is not a new species but a Homo sapien suffering from a pathological condition known as secondary microcephaly, as evidenced by the small chin, several unerupted teeth, and an arm bone that seemed to suggest a height of 1.5 meters, not 1 as originally reported. Tom Schoenemann of University of Michigan Dearborn discussed this issue in his talk:

[...Schoenemann] 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.

Scientific American (via John Hawks Anthropology Weblog). Parsimonious is because the last australopithecine walked the earth approximately 2 million years ago - extremely unlikely that this species persisted undetected in the fossil record for that time. Furthermore, most modern anthropologists believe that our large brains are our greatest adaptation, allowing for higher cognition to compensate for lack of size and strength when pitted against other species. Modern pygmies have retained their large brains - natural selection acting agaist our big brains is a tough bite to swallow.

However, a study conducted by Andrea B. Taylor of Duke University and Carel P. van Schaik of the University of Zurich, suggests 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.
As big brains require a lot of rich nutrients to keep them running (metabolically expensive), it is feasible that Floresians might have been better off with brain size reduction.

So where does this leave us? Big brained, broke, and confused? Searching for the Ebu Gogo to explain? Keep up with the debate at SciAm.


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