Summary: A new study overturned a long-standing anthropological assumption: that human ancestors grew steadily and progressively larger over millions of years. Instead, an analysis of 386 fossils representing 21 distinct hominin species reveals that human body mass evolution occurred via a dual-mechanism model. While body size increased gradually among early hominin ancestors like Australopithecus, a massive, non-linear evolutionary “leap” occurred between 2 and 2.5 million years ago.
This dramatic growth spurt coincided directly with the emergence of Homo rudolfensis and Homo erectus/ergaster, marking the first time human ancestors reached modern adult weights of 60 kg or more. The study accounts for missing fossil data and complex family-tree relationships to explain why previous localized studies arrived at conflicting conclusions.
Key Facts
- The 2-Million-Year Jump: The primary transition to modern human body proportions was a sudden evolutionary leap 2 to 2.5 million years ago, driven by Homo erectus/ergaster and Homo rudolfensis, rather than a slow, gradual accumulation across the human lineage.
- Species That Bucked the Trend: The human family tree branched dynamically rather than moving in a straight line. Species such as Homo floresiensis and Homo naledi actively diverged from this growth spurt, remaining remarkably small and maintaining child-sized stature.
- The Australopithecus Baseline: Prior to the Homo growth spurt, early hominin relatives like Australopithecus stagnated at an average weight of roughly 40 kg (approx. 88 lbs), matching the height and weight profiles of a modern young child.
- Ecological and Behavioral Catalyst: The rapid growth spurt matches critical neurobehavioral and ecological milestones: the transition to high-efficiency bipedalism (walking on two legs), a dramatic dietary shift toward meat consumption, and the expansion of geographic home ranges.
- Unifying Methodological Conflict: By analyzing 386 distinct fossil specimens using sophisticated statistical models that weight evolutionary relationships and species classification uncertainties, the researchers unified decades of seemingly contradictory anthropological data.
Source: University of Reading
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.”
Key Questions Answered:
A: Past disagreements were primarily a byproduct of narrow sample parameters. Some teams focused exclusively on early hominins like Australopithecus, finding a slow, gradual increase, while others looked solely at the later genus Homo, observing sharp size differences. By compiling 386 fossils across 21 species into a singular statistical model that factors in structural family-tree relationships and historical gaps, this new study proves these two concepts coexist: growth was linear at first, but erupted into a massive jump later.
A: A larger body mass altered how early humans interacted with their landscapes. It dramatically enhanced the biomechanical efficiency of bipedal walking, allowing Homo erectus to travel immense distances with lower energy expenditure. Furthermore, a larger physique was essential for a shifting lifestyle that relied on hunting and scavenging meat, defending resources from large apex predators, and successfully adapting to highly varied, challenging new habitats.
A: Absolutely not. Human evolution is a highly branched bush rather than a straight ladder. Even as our direct ancestors grew to modern weights, distinct evolutionary cousins like Homo floresiensis (often called the “Hobbit” hominin) and Homo naledi branched away from this trend entirely. They retained small body sizes and child-like statures long after Homo erectus had established a larger, 60 kg physical framework across the globe.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
About this evolutionary neuroscience research news
Author: Ollie Sirrell
Source: University of Reading
Contact: Ollie Sirrell – University of Reading
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Competing models of hominin body size evolution” by Jacob D. Gardner, Thomas A. Püschel, Suzy White, Manabu Sakamoto, and Chris Venditti. PNAS
DOI:10.1073/pnas.2521732123
Abstract
Competing models of hominin body size evolution
The evolutionary trajectory of hominin body size remains contested, with prior studies suggesting a gradual increase or lineage-specific shifts.
Here, we apply Bayesian phylogenetic generalized linear mixed models to a dataset of 386 specimens across 21 taxa and find strong evidence for a marked body mass increase in non-habilis or later-occurring Homo, with moderate support for a general increase over time.
Contrary to some existing hypotheses, we find less support for a distinct size increase across all Homo. By accounting for phylogenetic nonindependence, intraspecific variation, and multiple sources of uncertainty, our analysis examines competing views in a single framework, supporting a more complex explanation for hominin body size evolution and clarifying key transitions in Homo.
