Early human ancestors underwent a dramatic growth spurt around two million years ago that made long-distance travel easier, according to researchers from Oxford and Reading.
Experts discovered that our ancient ancestors jumped in body mass from around 88lbs (40kg) to 132lbs (60kg), attaining weights comparable to many modern humans.
In contrast, other species of ancient humans remained far smaller throughout their existence, reaching only the height of a typical child.
The sudden increase in size may have enabled early humans to roam over much larger geographic areas in search of food and suitable habitats. The change could also have helped ancestors survive on a more varied diet.
The findings challenge conventional scientific theories that human body size expanded gradually across the entire evolutionary family tree.
Published in the Proceedings of the National Academy of Sciences (PNAS), a leading peer-reviewed journal of the US National Academy of Sciences, the research provides new insights into early hominin development.
Analysis of hominin fossil data
To analyze body size shifts, the research team from the University of Reading and the University of Oxford examined estimated body weights from 386 fossils across 21 different hominin species.
Hominins comprise the evolutionary group that includes modern humans, extinct human species, and all immediate ancestors following the evolutionary split from chimpanzees and bonobos millions of years ago.
Using statistical models, the scientists tracked how body mass evolved across different lineages over vast periods of time.
The analysis revealed that while body weight increased steadily in earlier relatives, a major size jump occurred around two million years ago with the emergence of Homo erectus, a direct ancestor of modern humans.
Dr Thomas Puschel, co-author of the study from the School of Anthropology and Museum Ethnography at the University of Oxford, emphasized that human evolution did not follow a simple linear path.
"Our results suggest that human body size evolution was not simply a story of steady growth over time," Dr Puschel said.
"Although body mass generally increased throughout our evolutionary history, the most significant shift occurred later within the genus Homo."
Dr Puschel added: "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."
Decades of evolutionary debate resolved
For decades, palaeoanthropologists have debated whether human ancestors grew larger at a steady rate or experienced sudden growth spurts at critical points in evolutionary history.
Dr Jacob Gardner, lead author of the study at the University of Reading, explained that previous conflicting studies resulted from narrow analytical scopes.
"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," Dr Gardner 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."
Dr Gardner noted that the reality combines elements of both long-held scientific perspectives.
"The answer is most likely a combination of these ideas," Dr Gardner said.
"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."
Emergence and traits of Homo erectus
Researchers explained that the timing of this dramatic growth spurt aligns closely with other major biological and behavioural changes within the Homo genus.
"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," the researchers said.
"The findings suggest that growing larger was closely tied to a wider shift in how these early humans lived."
The study concludes: "Overall, these findings bring clarity to a fundamental question in human evolution."
Homo erectus was the first ancient human species to walk exclusively upright on two legs in the manner of modern humans.
First evolving in Africa around 1.9 million years ago during the Early Pleistocene epoch, Homo erectus stood taller than previous hominins, featuring longer legs that facilitated efficient walking across open terrain.
An upright posture freed their hands, enabling big-brained Homo erectus to manufacture stone tools, such as handaxes, and become the first hominin species to master fire.
Global migration and species survival
Homo erectus became the first early human species to leave Africa, becoming a true global traveller.
Populations migrated into Eurasia, spreading into regions that include modern-day Georgia, Sri Lanka, China, and Indonesia.
Individual sizes varied significantly within the species, ranging from just under five feet tall to over six feet in height.
While possessing a smaller brain capacity and heavier brow ridge than Homo sapiens, Homo erectus represented a crucial evolutionary bridge to modern humans.
Scientists previously believed that Homo erectus became extinct roughly 400,000 years ago, but more recent fossil re-evaluations suggest the species survived until around 140,000 years ago.
The species lived in hunter-gatherer social structures and is thought to have given rise to later extinct human species, including Homo heidelbergensis and Homo antecessor.
Recent fossil discoveries, including ancient skull finds, continue to reshape timeline estimates, with some findings pushing back the origins of Homo sapiens to over one million years ago.
