Science has, over the centuries, humbled humans, gradually forcing us to abandon the illusion that our species represents the ultimate end of creation. Copernicus and Galileo displaced Earth from the center of the universe; Darwin dashed the conceit that humans originated in a special way, distinct from all other species. Now a group of researchers--Christopher B. Ruff of Johns Hopkins University, Erik Trinkaus of the University of New Mexico and Trenton W. Holliday at the College of William and Mary--throw cold water even on the notion of steady "improvement" within the human line.

In their recent study, Ruff and his colleagues thoroughly analyzed the fossil record to determine the evolving body mass and brain size of the Homo species leading up to us. The results, published in the May 8 issue of Nature, show just how far from the truth is the stereotypical image of a straight progression from small, pea-brained ancestors to the technologically adept egghead Homo sapiens who inhabit the world today. The truth is quite a bit more complicated.

Image: Natural History Museum, London

NEANDERTHAL SKULL housed a brain larger than that of a modern human, representing a kind of peak in homonid evolution.

Hominid brains appear to have remained fairly constant in size for a very long stretch from 1.8 million years ago until about 600,000 years ago--a "period of stasis" whose reality has long been debated by scientists. An abrupt break occurred during the Middle Pleistocene epoch (from 600,000 to 150,000 years before the present), when fossils show that the cranial capacity of our ancestors skyrocketed. This trend peaked roughly 75,000 years ago, when archaic Homo sapiens fossils (a category that includes the well-known Neanderthals) indicate a brain mass of about 1,440 grams. Since then, brain mass has actually drifted downward to the 1,300 grams that is typical today.

Brain size alone does not tell the whole story, of course. Intelligence seems to have less to do with brain size per se than with the brain's proportion to the body it must care for and control (and even that link is rather tenuous). Here, too, the results of the Nature paper are telling. Over the nearly two-million-year span that Ruff and his co-authors examined, ancient hominids were on average about 10 percent more massive than modern humans. Body size peaked about 50,000 years ago: Neanderthals were muscular brutes who weighed upwards of a quarter more than modern humans. Since that time, humans have been marching steadily downhill in both stature and cranial capacity (with the exception of some recent gains due to improved nutrition and reduced disease). The good news is that the steeper decline in body mass over the past 50,000 years has raised our ratio of brain to body above Neanderthal levels, even though total brain mass has dipped.

Calculating the size of our progenitors' brains and bodies from a few scattered bones is a tricky process. Many of the bones vary too much from one individual to another to use for such estimation. Teeth wear differently depending on diet, for example. Eye sockets have changed in proportion over the years, and skulls have grown thinner. Past estimates of the body masses of human ancestors have sometimes disagreed by as much as 50 percent. These disparities made it difficult to assess the changing nature of the human line. But in their methodical survey, Ruff, Trinkaus and Holliday found two variables that appear closely tied to body size in even the most ancient humans: the width of the ball joint on the top of the thighbone (which bears much of our weight when we stand) and the breadth of the pelvis. Measuring these dimensions for 163 fossilized hominids, the scientists were able to plot our genus's changes in brain and brawn.

These improved data are already prompting anthropologists to re-evaluate their assessment of the environmental and cultural transformations that shaped human evolution. In an accompanying commentary in Nature, John Kappelman of the University of Texas at Austin offers some intriguing speculations along these lines. The long, dry spell of constant brain size suggests to him that among our ancestors, as in modern apes, competition among males for access to females may have created an evolutionary pressure favoring continued large bodies. Behavior that was "more dependent on brawn than brains," Kappleman writes, evidently was successful enough that there was little evolutionary pressure toward a bigger cranium.

JOHN KAPPELMAN of the University of Texas ponders why our bodies are so small rather than why our brains are so large.

In considering the new reconstructions of Homo over the past 90,000 years, Kappelman is struck less by the roughly constant brain size than by the rapid decrease in body size, which runs quite counter to the earlier steady or upward trends. He suggests that this decrease in overall bulk was favored "by a social structure that relied on more cooperative foraging and better communication skills." At the same time, a better and more reliable food supply could support the metabolic demands of a large brain. "The increase in relative brain size of modern humans may then be, in part, an effect of selection for smaller body mass," Kappelman rather ignominiously concludes.

So this is what it has come to. The favored son of the Garden of Eden has been demoted to the incredible shrinking human.