A significant evolutionary leap in human ancestor body size, a rapid growth spurt occurring between 2 and 2.5 million years ago, may have been driven by increased meat consumption and more efficient bipedal locomotion, according to a study published in the Proceedings of the National Academy of Sciences.
The research analyzed 386 hominin specimens spanning 21 species, from early Australopithecus to modern humans, employing 1,000 statistical models to map changes in body weight over millions of years. This comprehensive approach aimed to resolve discrepancies in prior studies regarding whether ancestral size increases were gradual or occurred in distinct phases.
Previous research had often focused on specific species or extrapolated data from incomplete fossil records. The new study integrated various methods to provide a more complete picture of hominin evolution. This analysis revealed that while earlier ancestors like Australopithecus saw a steady, incremental increase in size, a more dramatic, approximately 50-pound jump occurred with the emergence of species such as Homo erectus and Homo ergaster around two million years ago.
These later species exhibited enhanced bipedal efficiency, covered greater distances in their search for food, and more actively hunted or scavenged for animal protein. Researchers suggest that this larger physique facilitated more effective travel, conserved energy expenditure, and improved the ability to defend against predators, marking crucial milestones in human development.
"This change coincided with broader developments in how our ancestors moved across landscapes and exploited their environments, pointing to a close relationship between body size and major ecological and behavioral transitions," said co-author Dr. Thomas Puschel of the University of Oxford. In essence, a diet richer in meat appears to have played a direct role in the physical expansion of our lineage.
However, the evolutionary path was not uniform across all hominin branches. While species like Homo erectus achieved sizes comparable to modern humans, others, such as Homo floresiensis, often referred to as "hobbits," maintained a more diminutive stature, illustrating divergent evolutionary trajectories within the human family tree.
"The human story is not simply one of constant growth, but also of a major change that happened later, within our own genus, while other branches of the family, including some surprisingly small relatives, went their own way entirely," stated lead author Jacob Gardner of the University of Reading.