Ann Gibbons When paleontologist Stephen J. Gould teaches his history of life course to Harvard undergraduates, he flashes slides from popular media that depict human ancestors marching through time. The earliest members of our genus, Homo, appear short and stooped, but as time passes, our forebears gradually grow taller and bigger brained. The march of progress culminates in the implicit masterpiece of evolution--big-brained, upright, modern humans. But this popular view of human evolution is wrong, says Gould, who is the most visible critic of the long-standing notion that our lineage evolved gradually and inexorably toward a bigger, brainier human. Compelling proof that he's right has now come from the fossil record. In the 8 May issue of Nature, a new study of the bones of 163 early members of Homo who lived 2 million to 10,000 years ago suggests that our bodies--and brains--have gotten smaller lately, not bigger. Anthropologists have long thought that some members of the Homo lineage, the Neandertals, were brawnier than we are, but the new study, based on skull volume and two skeletal indicators of body mass, shows that the same was true of our direct ancestors. What's more, the study shows that the recent downsizing trend is just the latest twist in a complex history of brain and body size. Evolution apparently favored brawn early in human history: At least one early human already stood 1.85 meters (6 feet 1 inch) tall 1.8 million years ago. But brains stayed relatively small until 600,000 years ago, when they underwent a tremendous growth spurt that lasted until 50,000 years ago. It has been downhill ever since, with our brains and bodies shrinking by about 10% on average--perhaps, the authors speculate, because changes in technology and lifestyle have rendered muscular bodies unnecessary. "The bottom line is that body size did vary through human evolution," says paleoanthropologist John Kappelman of the University of Texas, Austin. That variation "challenges the traditional view that living humans are the epitome of large body and brain size," says Christopher Ruff, a biological anthropologist at Johns Hopkins University and the study's lead author. ----------------------------------------------------------------------------
Brawn and brain. Two skeletal measures yielded a history of body mass, shown below with brain size. After expanding for half a million years, both have declined in recent millennia.
Putting that view to the test has been difficult because anthropologists can't measure early humans from head to toe, or "even from pelvis to toe," says Ruff, as the fossils from any one skeleton are too fragmentary. As a result, researchers have tried indirect means of estimating body size, such as skull thickness or tooth and eye-socket size. These methods, however, have proven to be unreliable, probably because factors such as activity, diet, or climate can also influence them, says Ruff. For the past decade, he has been trying to find better measures. One feature that seems to fit the bill is the head of the femur, or thighbone. In studies of living humans, Ruff and others have found that the breadth of this femoral head is proportional to the mass of a person's body--the bigger the body, the bigger the femoral head supporting its weight--and they have developed equations that express the relation. A team including Ruff and paleoanthropologists Erik Trinkaus of the University of New Mexico and Trenton Holliday of the College of William and Mary in Williamsburg, Virginia, has now applied this equation to fossil femora from 93 individuals. To check their results, Ruff and colleagues used a second method for estimating body mass based on stature, which they gauged from the length of limb bones, and on the breadth of the pelvis, as measured (or estimated) between the two widest points of the flaring iliac bones. They applied this measure to 96 fossils (including 26 for which they also had femoral heads) and got results that closely matched those based on the femoral head. The agreement "helped increase my confidence that we were getting fairly unbiased estimates of body weight," says Ruff. Finally, they compared those estimates with their own and others' published measurements or estimates of cranial capacity to get brain size relative to body size. In an earlier, less comprehensive study with Pennsylvania State University paleoanthropologist Alan Walker, Ruff had found that one H. erectus fossil, the 1.5 million year old Nariokotome boy who lived near Lake Turkana in Kenya, would have stood 1.85 meters (6 feet 1 inch) tall and weighed 71 kilograms (156 pounds) if he had reached adulthood. The new study confirmed that six-footers were already striding around east Africa at that time, but their brains were about two-thirds the size of ours--and stayed that size for a million years. The stasis ended when "there was a truly extraordinary increase in brain size from about 600,000 to 30,000 years ago," says Trinkaus. This coincides, he notes, with the expansion of early humans to colder climates, which could have reinforced selection for larger brains to plan the use of seasonal resources. The trend in brain size continued over the past 100,000 years through the Neandertals to early modern humans. Brain size peaked at about 10% larger than ours in early modern humans, such as the people who lived in caves at Skhul and Qafzeh in Israel 90,000 years ago and the cave painters at the Cro-Magnon rock shelter in France 30,000 years ago. Meanwhile, body size continued a slower increase, peaking within the last 100,000 years. Indeed, the new analysis shows that Neandertals were the champions of brawn, outweighing contemporary humans by 30%. Their brains were also larger than ours in absolute terms, but their ratio of brain size to body mass was about 10% lower. This finding solves an important mystery--why Neandertals' activities don't look particularly intelligent in the fossil record, despite their big brains: "People always go on about Neandertals having larger brains than ours, but this disproves that if you take into account body size," says Leslie Aiello of University College London. In a commentary in Nature, Kappelman suggests that the result will require "critical re-thinking" about the behavior of Neandertals, implying that it "was probably decidedly non-modern--and more dependent on brawn than brains." The decline in both brain and body size since the days of the Neandertals and Cro-Magnons may be due to tools or social skills that reduced our ancestors' reliance on sheer brawn, says Ruff. And as the body shrank, so did the brain. Trinkaus points out other factors that may have contributed to the trend in recent millennia: for example, poor nutrition as agriculture replaced the varied fare of hunter-gatherers with a poorer diet. Other researchers have found that stature was smallest in the Neolithic and Middle Ages, although Ruff suggests that better nutrition has allowed some populations to bounce back to their Pleistocene heights, including Americans and northern Europeans. Kappelman and Richard Smith of Washington University in St. Louis believe that the trends in brain and body size that the Ruff study has traced are real. They are less convinced by Ruff's absolute values for body mass, however, because he calibrated his equations on living humans. The modern, sedentary lifestyle may have thrown off the relation between body mass and skeletal features. Kappelman suggests that athletes might be a better basis for the equations. But those concerns, he adds, won't affect the most important conclusions. Body and brain size reflect the different ways our ancestors adapted to their environments--suggesting that "they were behaving differently than us," says Kappelman. And, as far as the human physique goes, the march of progress is definitely a myth. Volume 276, Number 5314 Issue of 9 May 1997, pp. 896 - 897 ©1997 by The American Association for the Advancement of Science.