For decades, paleoanthropologists have largely agreed that somewhere along the evolutionary course from apelike ancestors to modern humans, our bodies got dramatically larger. However, one lingering mystery has been when this intense growth began.
Did our ancestors gradually grow bigger over millions of years? Did the shift coincide with the emergence of the genus Homo? Or did it occur later, after some of the earliest human species had already appeared?
A new study published in the Proceedings of the National Academy of Sciences (PNAS) argues that the answer is more complex than any single theory has suggested.
Rather than supporting a single long-standing hypothesis, researchers used a sophisticated statistical method to show that human body size likely followed two evolutionary courses simultaneously: a slow, steady increase spanning millions of years, punctuated by a much larger leap in later members of the genus Homo.
The findings help reconcile decades of conflicting research while offering one of the clearest pictures yet of how, and perhaps why, our ancestors evolved into larger-bodied humans.
“Despite its central role in hominin evolution, there is little consensus on how body size increased through time,” researchers write. “We provide strong evidence for a marked size increase in later-occurring Homo, alongside a moderate general increase over time among all hominins.”
Body size is one of the defining characteristics of an organism. In humans and our extinct relatives, it has been linked to everything from brain expansion and walking efficiency to metabolism, hunting behavior, and the ability to travel across increasingly large landscapes. Understanding when body size changed, therefore, helps researchers reconstruct not only what our ancestors looked like, but how they lived.
Yet despite decades of fossil findings, scientists have struggled to agree on the evolutionary pattern behind those changes.
Some previous studies suggested that hominins simply became larger over time. Some contended that body size remained relatively stable until the emergence of Homo. Still others have proposed that the real transformation arose only after early species such as Homo habilis, often regarded as one of the first members of our genus.
According to researchers, one reason for the disagreement is that earlier analyses typically examined these competing ideas independently. They also often failed to account for uncertainties inherent in the fossil record, including disagreements over species classification, differences in body mass estimation methods, and uncertainties in evolutionary relationships among fossil species.
To address those limitations, researchers compiled one of the largest hominin body-size datasets assembled to date, incorporating body mass assessments from 386 fossil specimens representing 21 different taxa.
They then analyzed the data using Bayesian phylogenetic generalized linear mixed models—a statistical approach that accounts for evolutionary relationships while encompassing multiple layers of uncertainty.
The team repeated its analyses across 1,000 different phylogenetic trees and 1,000 separate datasets, allowing them to test whether the overall pattern remained consistent despite uncertainties.
The results pointed to two simultaneous trends.
The first was a modest but persistent increase in body size across the hominin family as a whole. On average, body mass increased by roughly 2 pounds (0.9 kilograms) every million years, suggesting that larger bodies gradually became more common throughout human evolution.
However, the second trend proved far more dramatic.
Instead of all members of Homo becoming substantially larger, the researchers found the strongest evidence for a distinct jump in body size among later members of the genus, particularly species that appeared after Homo habilis. That pattern regularly emerged regardless of how the fossil data were analyzed.
In other words, early members of Homo may not have represented the major biological turning point scientists once assumed.
This finding disputes a common assumption that simply belonging to the genus Homo meant possessing a significantly larger body. Instead, the study suggests that some hallmark features traditionally associated with humans, including larger body size, may have evolved after the genus’s origin.
That interpretation also corresponds to broader changes that occurred later in human evolution.
Researchers note that this period coincides with evidence for more efficient upright walking, greater reliance on meat consumption, and expanding home ranges. Larger bodies may have been both a consequence of and a contributor to those behavioral and ecological shifts.
The study likewise reinforces another growing theme that evolution rarely follows a straight line.
Species such as Homo floresiensis, often nicknamed the “Hobbit” because of its diminutive stature, and Homo Naledi frequently appeared as outliers with unusually small body sizes despite belonging to relatively recent branches of the human family tree. Their existence demonstrates that body size did not simply increase in a uniform march toward modern humans.
Instead, evolution repeatedly experimented with different solutions depending on local environments and ecological constraints.
The researchers also argue that part of the longstanding disagreement over body-size evolution stems from the fossil record itself. Different studies have estimated fossil body masses using different skeletal elements, comparison populations, and statistical techniques, sometimes producing substantially different results for the same individual.
By expressly incorporating those uncertainties into their analysis, rather than treating them as fixed values, the new framework seeks to capture the range of plausible evolutionary scenarios rather than relying on a single interpretation.
Even so, researchers caution that important questions remain unresolved.
Improved fossil discoveries, more accurate body-mass estimation methods, and a better understanding of species relationships could all further refine the picture. The study also does not explicitly account for sexual size differences between males and females, which may have been greater in some early hominin species than in modern humans.
Nevertheless, the wider conclusion is intriguing.
Rather than choosing between competing theories, the evidence suggests both contain elements of the truth.
Body size did increase gradually across the hominin lineage. But the most considerable leap appears to have occurred later, after the earliest representatives of Homo had already evolved.
That conclusion may help settle one of paleoanthropology’s longest-running debates while showing how advances in statistical methods are transforming scientists’ ability to extract new insights from fossils that have often been studied for decades.
“Overall, these findings bring clarity to a fundamental question in human evolution and show how the application of rigorous phylogenetic comparative tools can unlock patterns previously hidden in the fossil record while accounting for multiple sources of uncertainty,” researchers write. “By reconciling conflicting findings within a single framework, our results clarify key evolutionary transitions and highlight the complex nature of body size change in human evolution.”
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com