A statistical analysis of our closest extinct relatives has revealed that our prehistoric ancestors experienced a modest growth spurt between 2 and 2.5 million years ago, coinciding with an increased consumption of meat and the transition to bipedal locomotion. The findings of this new study were published in the journal Proceedings of the National Academy of Sciences.
Data from 2005 indicates that the average weight of a modern human is around 62 kilograms, but based on fossil remains, the earliest members of our evolutionary group—which includes all species bearing the genus name Homo—would have weighed only about 40 kilograms, give or take.
So it is clear that we have grown larger over time, but according to the researchers behind this new study, paleontologists have yet to reach a consensus on how exactly this size increase occurred: whether it was gradual and continuous or marked by abrupt jumps.
“For many years, different studies have arrived at different conclusions about whether the size of our ancestors increased gradually over time or changed dramatically at some key moment,” said Jacob Gardner of the University of Reading in a statement. “We believe this is because each examined a somewhat different part of a much more complex puzzle. When you bring all the fossils together, consider multiple competing ideas, and account for the evolutionary relationships between species, a clearer picture emerges. The answer is likely a combination of these ideas.”
In their work, Gardner and his colleagues used 386 specimens covering 21 hominin species, ranging from early representatives like Homo habilis, which was widespread in Africa around 3.5 million years ago, all the way to ourselves.
Other researchers had previously estimated the body mass of our ancestors from these samples, but since many of them are only fragments of a complete skeleton, the estimates are based on different bones or teeth and use different methods, introducing considerable uncertainty into the data.
To try to account for this, Gardner’s team ran a series of 1,000 statistical models that included many different plausible mass values, while also accounting for disagreements about the evolutionary links between samples. Some of these models incorporated stepwise jumps, while others did not.
When a jump was included in the genome after Homo habilis—one of the earliest and most primitive members of our genus—it explained on average 11 percent more of the observed trend in the mass data than the model without it.
Learn more
This suggests that around the same time that Homo erectus and Homo ergaster emerged—some of our first ancestors to spend considerable time on two legs about 2 million years ago—a noticeable increase in their size took place.
“This change coincided with broader shifts in how our ancestors moved across landscapes and adapted to their environments, indicating a close relationship between body size and major ecological and behavioral transformations,” said study co-author Thomas Püschel of the University of Oxford.
Although the modeling results were less conclusive, they also supported an overall ongoing increase in size across all hominin species, averaging up to 0.99 kilograms per million years.
This pattern aligns with what biologists call Cope’s rule—a long-observed tendency for animal size to increase over evolutionary time. This is often explained by the advantages larger bodies provide in competing for mates, defending against predators, and regulating temperature, though the rule is far from universal and its underlying causes remain debated.
Indeed, even in this study, some key outliers were found. Homo floresiensis—sometimes called the “hobbit”—and Homo naledi turned out to be significantly smaller than the model predicts, suggesting that body size evolution in our lineage was far from a one-way street.
“The story of humanity is not just one of steady growth, but also one of major changes that occurred later within our own genus, while other branches of the family, including some remarkably small relatives, went their own way,” Gardner said.
About the Author
