Is bigger always better? When it comes to brain size, that has long been the prevailing theory—at least among big-brained humans. But a new analysis shows that in the course of primate evolution, brains and brawn haven't always been on the rise.
Ever since a petite female Homo floresiensis (or "hobbit") was described in 2004, scientists have been debating whether this recently extinct hominin could have evolved to have such a small noggin or if this specimen—the only one for which a skull has been found—was an aberration.
The Indonesian individual stood at just about one meter high and had a brain about a third the size of modern humans. Despite these striking size differences, some H. floresiensis fossils date to just 13,000 years ago—which means they would have lived alongside brainy humans for some 187,000 years. Some researchers propose that she and her clan were modern descendents of Homo erectus and had devolved to their diminutive stature because they lived on an island (a phenomenon documented in other species known as insular dwarfism) or that her particular errant body and brain size were due to an underlying pathology, such as dwarfism or an oversized pituitary gland.
Others, however, posit that H. floresiensis is from a more primitive lineage and evolved—for whatever reason—to have this smaller brain.
A new study, published online January 26 in the journal BMC Biology, brings some new perspectives to the table, examining size trends in primate evolution and finding that brains and bodies don't always get bigger over time.
The researchers, led by Stephen Montgomery of the Department of Zoology at the University of Cambridge in the U.K., studied the brain and body sizes—changes that often go hand-in-hand—of 37 living and 23 extinct primate species. Montgomery et al. found that although "selection has acted to enlarge primate brains, in some lineages this trend has been reversed."
The primate brain, prized for being supersize among mammals, varies greatly today, ranging from the tremendous H. sapiens' encephalon, which typically weighs in at about 1,330 grams to the gray mouse lemur (Microbebus murinus), whose brain weighs only about 1.8 grams. And the earliest primates likely had a brain that was a mere 0.12 gram.
The authors found that although the overall trend has been a brainy explosion in primates (an average increase of 2.5 percent each million years), both brains and bodies have diminished in size along many branches of the primate tree. And given these corporeal and cerebral cutbacks, the researchers found that "under reasonable assumptions, the reduction in brain size during the evolution of Homo floresiensis is not unusual in comparison to these other primates," Nick Mundy, also with Cambridge, said in a prepared statement. Given the findings, the researchers concluded, "We should perhaps not be surprised by the evolution of a small-brained, small-bodied hominin."
Image of Homo floresiensis skull [left] and Homo sapiens skull [right] courtesy of Peter Brown/University of New England