Jan Bartek – AncientPages.com – Our ancestors experienced the largest increase in body size about 2 to 2.5 million years ago, when Homo rudolfensis or Homo erectus/ergaster appeared. This change did not happen gradually across all human species.

Recent research shows that some species did not follow this trend. Homo floresiensis and Homo naledi remained small, while early Australopithecus averaged 40 kg and reached the height of a child. In contrast, other Homo branches grew larger. Homo erectus/ergaster were the first hominins to average 60 kg or more, a weight similar to that of many modern humans.

Findings from the University of Reading and University of Oxford challenge the view that body size increased steadily over time, ultimately resulting in modern humans.

“For years, different studies have come to different conclusions about whether our ancestors steadily grew bigger over time or jumped in size at some key point in our Homo ancestors. We think that’s because everyone was looking at slightly different pieces of a much bigger puzzle. When you put all the fossils together, examine multiple competing ideas, and account for how species are related to each other, a clearer picture emerges. The answer is most likely a combination of these ideas.

“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,” Dr Jacob Gardner, lead author at the University of Reading, said.

Piecing Together The Human Puzzle 

Researchers analyzed body weight data from 386 fossils representing 21 hominin species, including humans and extinct relatives. They applied statistical models to track changes in body size over millions of years.

Hypotheses of hominin body size evolution through time. (A) Body mass increased linearly through time (solid line) or did not change on average (dashed line). (B) Alternatively, body mass may have increased separately in Homo or later-occurring Homo (black H. ergaster silhouette and lines) relative to earlier hominins (gray Australopithecus africanus silhouette and lines), with or without a change in slope. All silhouettes are from phylopic.org (T. Michael Keesey, public domain).

Previous studies reached different conclusions because they focused on different hominin groups, used different methods to estimate body weight, and did not fully consider species relationships or uncertainties in the fossil record. By integrating these factors into a single model, researchers found that the studies were examining different aspects of a complex pattern. Body weight increased gradually in early hominins such as Australopithecus, then rose sharply at a key stage in Homo.

This increase in body size coincided with other changes in later Homo, including more efficient bipedalism, increased meat consumption, and wider geographic range. A larger body likely supported these adaptations by enabling longer travel and a more varied diet. The findings indicate that increased body size was closely linked to broader shifts in early human behavior.

“Our results suggest that human body size evolution was not simply a story of steady growth over time. Although body mass generally increased throughout our evolutionary history, the most significant shift occurred later within the genus Homo.

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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,” Dr Thomas Puschel, co-author from the School of Anthropology and Museum Ethnography, University of Oxford, said.

The study was published in the journal Proceedings of the National Academy of Sciences

Written by Jan Bartek – AncientPages.com Staff Writer