When the first Homo erectus walked the East African savanna 2 million years ago, they stood out—not just for their upright stride, but for their size. At an average of 60 kilograms (132 pounds), they were a dramatic departure from earlier hominins like Australopithecus, who weighed just 40 kilograms (88 pounds) and stood no taller than a modern ten-year-old. For decades, scientists debated whether human ancestors grew steadily larger over time, but new research analyzing 386 fossils reveals a more complex story—one of sudden leaps, surprising detours, and evolutionary experiments that defied the trend.
The idea that humans simply got bigger in a straight line has long shaped how we see our evolution. But this study, led by Dr. Jacob Gardner at the University of Reading and co-authored by Dr. Thomas Puschel from the University of Oxford, shows that the real story is far more dynamic. By combining fossil data across 21 hominin species and using advanced statistical models that account for evolutionary relationships and uncertainties in classification, the team found that body size did increase gradually among early hominins—but the biggest leap happened suddenly, within the genus Homo.
That jump coincided with a wave of transformation. Homo erectus didn’t just grow larger; they walked more efficiently, traveled greater distances, and incorporated more meat into their diet. A heavier body may have supported these new behaviors—helping them regulate energy over long treks and thrive on a variable, nutrient-rich diet. "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," Gardner explains.
And those other branches were truly unexpected. Homo floresiensis, discovered on the Indonesian island of Flores, and Homo naledi from South Africa, both remained small-bodied, challenging the assumption that bigger was always better. Their existence shows that evolution didn’t push all human relatives toward larger size—instead, different environments and lifestyles led to different outcomes.
The findings, published in the Proceedings of the National Academy of Sciences, underscore a deeper truth: human evolution was not a march of progress, but a branching tree with many paths. The shift in body size wasn’t just about growth—it was tied to how our ancestors moved, ate, and explored. As Dr. Puschel puts it, "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."
Looking ahead, this research opens new doors for understanding how physical change and lifestyle evolved together. It reminds us that our lineage wasn’t destined to look like us—many versions of humanity once walked the Earth, each shaped by the world they lived in.