For most of our evolutionary history, human ancestors grew in size gradually, generation after generation, slowly and step by step toward greater height. Then, sometime between 2 and 2.5 million years ago, this pattern completely broke down.
New research indicates that body sizes in our family tree increased sharply and rapidly, rather than continuing the steady growth that scientists had long assumed. The findings are published in the journal Proceedings of the National Academy of Sciences.
This shift occurred around the same time as the emergence of Homo rudolfensis and Homo erectus/ergaster. The study was led by the University of Reading and the University of Oxford. This research firmly challenges the old notion that hominin bodies simply kept growing larger along a straight line, smoothly transitioning to modern humans.
Many relatives entirely skipped this period of intense growth. Homo floresiensis and Homo naledi remained genuinely small. Australopithecus, one of our earliest relatives, weighed on average around 40 kilograms and stood about as tall as a modern child.
Meanwhile, other branches of the genus Homo moved in the opposite direction. Homo erectus/ergaster became the first hominins whose average weight reached 60 kilograms or more—a weight that could easily match many modern humans.
Instead of a single, clear trend toward larger bodies, the paleontological record actually shows several distinct strategies unfolding in parallel across different branches of the family tree.
“For years, various studies have reached different conclusions about whether our ancestors’ sizes increased gradually over time or whether growth surged at some key point in the history of the Homo lineage,” said lead author Jacob Gardner of the University of Reading. “We believe this happened because each study looked at a slightly different part of a much more complex puzzle.”
The researchers found that a clearer picture emerged when they examined all fossils, considered competing hypotheses, and accounted for evolutionary relationships between species. The answer most likely lies in a combination of these different ideas. Human evolution is not simply a story of constant growth.
Instead, significant changes occurred later within our own genus, while other branches of the human family—including some surprisingly small relatives—took a completely different path.
To sort this all out, the researchers gathered body mass data from 386 fossil remains belonging to 21 hominin species. The analysis essentially covered the entire extended family, including humans and our extinct relatives.
The team performed the analysis using statistical models designed to track changes in body size over millions of years. It turned out that earlier studies were not entirely wrong about this. They were simply looking at only part of the overall picture.
Some previous studies focused on early relatives like Australopithecus, while others delved into later species of the genus Homo. Different research teams also used different methods to estimate body mass from fossil bones.
Furthermore, none of them fully accounted for how these species are related to one another. They also failed to consider the enormous uncertainty tied to the incomplete fossil record, including which specific species a given set of bones belongs to.
Once the researchers combined all the data into a single model, the disagreements largely disappeared. These studies were not contradicting each other. They were simply looking at different fragments of a more tangled and complex history. Body mass increased gradually among earlier relatives like Australopithecus, and then jumped sharply at a specific, later stage in the development of the genus Homo.
The increase in human size is closely linked to a number of other important changes that occurred in later species of the genus Homo around the same time. These ancestors walked upright far more efficiently than their predecessors, consumed much more meat, and traveled much greater distances in search of food and habitat.
Larger stature likely contributed to all of this at once, making long-distance travel easier and allowing for a more demanding and varied diet. Thus, the increase in size was tightly connected to far broader changes in the lifestyle of these early humans.
“Our findings show that the evolution of human body size was not simply a story of gradual growth over time,” said co-author Thomas Puschel of Oxford. “While body mass did increase overall throughout our evolutionary history, the most significant changes occurred later in the genus Homo. This change coincided with broader shifts in how our ancestors moved across landscapes and adapted to their environments, indicating a close relationship between body size and major ecological and behavioral changes.”
About the Author
