Early relatives such as Australopithecus weighed around 88 pounds (40 kilograms) on average and stood at a height close to that of a modern child. Millions of years later, Homo erectus was reaching weights closer to those of many humans today. In between, studies kept reaching different conclusions about how and when bodies in the human family tree actually got bigger.
A new study published in PNAS found that human body size did not simply increase linearly. Instead, there was a gradual increase across all hominins over millions of years, followed by a pronounced increase in size in later members of the genus Homo around two million years ago, coinciding with some of the most significant behavioral and ecological changes in human evolutionary history.
“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,” said lead author Jacob Gardner in a press release.
Read More: Ancient Tooth Proteins Reveal Possible Link Between Homo erectus, Denisovans, and Modern Humans
Hominin Body Size Jumped in Later Homo Species
The biggest change came with species such as Homo ergaster and Homo erectus, which appeared around 2 million to 2.5 million years ago. These were among the first of our relatives to regularly weigh around 132 pounds (60 kilograms) or more, putting them in a weight range that would be familiar to many modern humans.
Working out how much an ancient hominin weighed from fossilized bones is not an easy task. Researchers estimate body mass from limbs, pelvises, and other skeletal elements, and different studies have used different bones, time ranges, and estimation methods — helping explain why they often reach different conclusions.
The new study analyzed body mass estimates from 386 fossils across 21 hominin species and ran its models across 1,000 different phylogenies (evolutionary family trees), trying to account for as many of those uncertainties as possible.
The increase in size aligns with other changes in how these ancestors lived. They were walking upright more efficiently, eating more meat, and covering much larger distances in search of food. A bigger body appears to have been part of that shift, making long-distance travel easier and supporting a more varied, demanding diet.
“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 behavioral transitions," said Thomas Puschel, study co-author in the press release.
Some Human Relatives Stayed Small
While some branches of the human family were getting bigger, others were doing something completely different. Homo floresiensis, the so-called Hobbit from the Indonesian island of Flores, and Homo naledi, discovered in South Africa, both remained significantly smaller than the general Homo trend throughout their existence.
And Homo habilis, one of the earliest members of our own genus, didn't fit the body pattern either, leading some to question whether it belongs in the genus Homo at all. Why these species diverged from the broader trend remains not fully understood.
Human Body Size Evolution Was Not a Straight Line
The size increases in later Homo also appear to have gone hand in hand with a wider transformation in how these ancestors lived, moved, and found food.
What the study makes clear is that the human family tree was never moving in one direction. Some relatives grew larger as they ranged farther and ate more meat, while others stayed small, and at least one early member of our own genus may have had less in common with later Homo than its name suggests.
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- This article references information from a study published in PNAS: Competing models of hominin body size evolution
