Primate brains have not always gotten bigger as they evolved, according to new research. The findings challenge the controversial argument that Homo floresiensis, also known as the hobbit, had a tiny, chimp-sized brain because of disease.

"It was assumed that brain sizes generally get bigger through primate evolution," said Nick Mundy, a Cambridge University evolutionary geneticist and lead author of the study. While that may be true for most primates, "we find very strong evidence in several lineages that brain sizes actually have gotten smaller."

The brains of marmosets, mouse lemurs and mangabeys have shrunk significantly. The brain of the mouse lemur, a teacup-sized, nocturnal primate found in Madagascar, is 27 percent smaller than that of the common ancestor of all lemurs, Mundy said.

The paper, which appears Jan. 27 in Biomed Central, analyzed brain size and body mass from 37 current and 23 extinct primate species and used three different models to reconstruct how the brain evolved.

Though its not clear why smaller brains would be advantageous to some species, the brain's voracious energy consumption may have played a role, Mundy speculated. If food was scarce, it may have been better to sacrifice intelligence to use less energy.

The findings are more fodder for the debate about the mysterious H. floresiensis, a 3-foot-tall hominid discovered in a cave on the Indonesian island of Flores in 2003. Some have argued these "hobbits" were a distinct species, while others say they were simply stunted, sickly Homo sapiens.

In the second line of reasoning, the hominids may have suffered from cretinism, a pituitary gland disease that leads to stunted growth and small brains. Part of this camp's argument was that the hobbits' miniscule brain was too small to make evolutionary sense, Mundy said.

"We've just applied the reduction in brain size that we see across the rest of the primate phylogeny to the case of the Flores man," he said. "Under reasonable assumptions, it does look plausible that this brain-size massive reduction could have occurred."

Some scientists argue that there's no need to rely on either evolutionary brain shrinkage or pathology to account for the short stature of the hobbits.

"Arguments for H. flo being somehow pathological (one syndrome or another) have been totally refuted," Peter Brown wrote in an e-mail. Brown, a paleoanthropologist at the University of New England in New South Wales, Australia, first discovered the hobbit skeletons.

What's more, evidence suggests the diminutive island dwellers left Africa more than 1.8 million years ago, and "probably arrived on Flores already small-brained and small-bodied," he wrote. In addition, their skeletal and dental features most resemble the tiny-brained Australopithecus or Homo habilis. So, the brain of H. floresiensis could have started out small and stayed that way, rather than shrinking through evolution.

Images: 1) Pygmy marmoset. jwm_angrymonkey/flickr

2) Mouse lemur. Wikimedia Commons

Citation: "Reconstructing the ups and downs of primate brain evolution: Implications for adaptive hypotheses and Homo floresiensis*," Stephen H Montgomery, Isabella Capellini , Robert A Barton, Nicholas I Mundy,* BMC Biology*, 27 January 2010.*

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