The biggest jump in body size among our ancestors happened around 2 to 2.5 million years ago, with the appearance of Homo rudolfensis or Homo erectus/ergaster, rather than gradually across the whole human family tree.
New research published today (Monday, 22 June 2026) in the journal PNAS, found that some species bucked the trend completely. Homo floresiensis and Homo naledi stayed small, with the early hominin Australopithecus weighing 40kg, on average, and reaching the height of a child. Other branches of Homo grew larger. Homo erectus/ergaster were the first hominins to weigh around 60 kg or more, on average, achieving weights similar to many modern humans.
The University of Reading and University of Oxford findings challenge the idea that bodies simply got bigger and bigger over time in a steady line, eventually leading to modern humans.
Dr Jacob Gardner, lead author at the University of Reading, said: "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."
Piecing together the human puzzle
Researchers reached these conclusions by looking at body weight from 386 fossils across 21 different species of hominins, the group that includes humans and our extinct relatives. They used statistical models to track how body size changed over millions of years.
Previous studies disagreed because some focused on early relatives such as Australopithecus, others on later members of Homo, and some used different methods to estimate body weight from fossil bones. These studies also did not account for how hominin species were related to one another or the various uncertainties that come with an incomplete fossil record, such as which fossils belong to which species. Bringing all of this together in one model shows that these studies weren't actually disagreeing with each other, they were just looking at different parts of a more complicated story. Body weight steadily increased over time in our earlier hominin relatives, like Australopithecus, but then jumped in size at a key point later in Homo.
The timing of this growth spurt lines up with other changes in later Homo. These ancestors were walking on two legs more efficiently than earlier hominins, eating more meat, and roaming over much larger areas in search of food and suitable habitat. A bigger body may have helped with all of these things, making it easier to travel long distances and survive on a varied diet. The findings suggest that growing larger was closely tied to a wider shift in how these early humans lived.
Dr Thomas Puschel, co-author from the School of Anthropology and Museum Ethnography, University of Oxford, said: "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. 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."
