New research suggests that humans became the large-brained, large-bodied animals we are today because of natural selection to increase brain size.

New research suggests the larger bodies of modern humans, shown here in the Spitzer Hall of Human Origins, are genetically linked to their bigger brains.

© AMNH/D. Finnin

The work, published in the journal Current Anthropology , contradicts previous models that treat brain size and body size as independent traits responding to separate evolutionary pressures.

Instead, the study shows for the first time that brain size and body size are genetically linked, and that selection to increase brain size will “pull along” body size, a phenomenon that may have played a key role in the increase in both traits that occurred near the origins of modern humans and other species in the genus Homo.

“Over the last four million years, brain size and body size increased substantially in our human ancestors,” said paper author Mark Grabowski, a James Arthur postdoctoral fellow in the Museum's Division of Anthropology. “This observation has led to numerous hypotheses attempting to explain why observed changes occurred, but these typically make the assumption that brain- and body-size evolution are the products of separate natural selection forces.”

Human ancestors like Australopithecus afaresnsis (shown here in the Spitzer Hall of Human Origins) had much smaller statures—and brains—than species in our genus, Homo.

© AMNH/D. Finnin

In the study, Grabowski created a number of models to examine how underlying genetic relationships and selection pressures likely interacted across the evolution of our lineage. His findings demonstrate for the first time that strong selection to increase brain size alone impacted both brain- and body-size increases throughout the course of human evolution and demonstrated in fossils of species like Australopithecus and Homo erectus.

While there are many scientific ideas explaining why it would be beneficial for humans to evolve bigger bodies over time, the new work suggests that those hypotheses may be unnecessary; instead, body size just gets pulled along as the brain expands.