A new analysis of the body size of early humans and our relatives suggests that the first members of the genus Homo—which includes Homo sapiens—were not larger than earlier hominin species. The study, published in the Journal of Human Evolution, challenges the long-standing notion that the origins of early humans coincided with, or were driven by, an increase in body mass.
“An animal’s body size is directly related to how it interacts with the natural world, influencing factors such as energy requirements, brain size, growth rates, diet, locomotion, and more,” said Brian Richmond, a co-author on the new work and the Museum’s curator of human origins. “But the last major analysis of body size in the evolution of our own family took place more than 20 years ago. We’ve learned so much about human evolution that we were overdue for an updated study.”
Richmond worked with lead author Mark Grabowski, an assistant research professor at George Washington University and soon-to-be James Arthur Postdoctoral Fellow in the Museum’s Anthropology Division, and other colleagues to build the most comprehensive set of body mass estimates, species averages, and species averages by sex for fossil humans and their close relatives to date.
Using the largest sample of individual early hominin fossils available—220 specimens—the team showed that early hominins were generally smaller than previously thought, and that the earliest Homo species did not differ significantly in size from their immediate ancestors.
The new results challenge the evolutionary model that argues that body size increased from Australopithecus individuals to early Homo species. Increased size came later with H. erectus, the first species widely found outside of Africa.
“One of our major results is that we found no evidence that the earliest members of our genus differed in body mass from earlier australopiths,” said Grabowski. “In other words, the factors that set our lineage apart from our earlier ancestors were unrelated to an increase in body size.”