Early human ancestors experienced a significant increase in body size approximately two million years ago, a development that likely facilitated longer-distance travel and adaptation to a more varied diet, according to a study published in the Proceedings of the National Academy of Sciences. This finding challenges the notion of a gradual, uniform increase in human body size throughout evolutionary history.
Researchers examined body weight data from 386 fossils representing 21 different hominin species. Hominins include modern humans, extinct human species, and immediate ancestors post-dating the split from chimpanzees and bonobos. Statistical models were employed to trace changes in body size over millions of years. The analysis indicated that while earlier hominin relatives saw a steady increase in body weight, a pronounced jump occurred around two million years ago with the emergence of species within the genus Homo, notably Homo erectus.
This period of growth saw hominin body weights increase from an average of 88 pounds (40 kg) to 132 pounds (60 kg), reaching weights comparable to many modern humans. In contrast, other ancient human relatives remained considerably smaller, comparable to the size of children.
"Our results suggest that human body size evolution was not simply a story of steady growth over time," stated Dr. Thomas Puschel, co-author and researcher at the University of Oxford. "Although body mass generally increased throughout our evolutionary history, the most significant shift occurred later within the genus Homo. 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 behavioural transitions."
Dr. Jacob Gardner, lead author from the University of Reading, commented on the historical debate surrounding human evolutionary size changes. "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," he said. "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 study posits that the timing of this growth spurt aligns with other evolutionary developments in the Homo genus. These ancestors exhibited more efficient bipedal locomotion, consumed more meat, and ranged over larger territories in search of sustenance and suitable habitats. This suggests that increased body size was closely linked to a broader shift in the lifestyle of these early humans.
Homo erectus, for example, was the first human species to walk exclusively upright on two legs, a characteristic that facilitated more efficient movement and freed the hands for tool development. This species originated in Africa and was the first hominin to migrate outside the continent, spreading across parts of Asia and Europe. Ranging in height from under five feet to over six feet, Homo erectus represented a key evolutionary step, possessing a smaller brain and heavier brow ridge compared to modern humans. While previously thought to have disappeared around 400,000 years ago, more recent estimates suggest their extinction occurred approximately 140,000 years ago. They are believed to have given rise to other extinct human species, including Homo heidelbergensis and Homo antecessor.