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As someone who writes a lot about human evolution and archaeology, I鈥檝e seen a great many artists鈥� impressions of prehistoric people. Some are remarkably believable, closely tied to scientific findings as much as possible. Others, not so much. I twitch every time I see a reconstruction of an African or tropical hominin with northern-European-style pale skin, and the twitches escalate whenever I see hairless hominins wandering around naked in temperate regions like Britain. Put something on or you鈥檒l die.
However, the thing that has come to bother me most is that a lot of our images of prehistoric people are just too big.
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Sometimes, this is about musculature. You can find pictures showing prehistoric people (generally men) with ripped physiques, muscles bulging, as if they鈥檝e been doing reps at the gym and downing protein shakes. This just isn鈥檛 how practical everyday fitness works. If your body looks like a movie superhero鈥檚, that is a cosmetic choice 鈥� one that is achieved using physiological trickery, like artificially dehydrating yourself so your muscles stand out. Genuine speed, strength and endurance don鈥檛 look like that.
Beyond these surface aesthetics, there is also a deeper issue. Prehistoric people were smaller than us, both in body mass and in stature. At the extreme end of this, we have the truly diminutive groups like Homo floresiensis (known as 鈥渉obbits鈥�), but even our direct ancestors were probably, on the whole, on the small side compared with people today.
The story of how humans got heavier and taller is slowly coming together, as our fossil collection expands. And it looks like it was an important part of our origins.
Small folk
To find out how hominin body size has changed over time, I spoke to evolutionary biologist at the University of Reading in the UK. He鈥檚 lead author of a study published in PNAS on 22 June, which looks at .
Let鈥檚 start by putting some concrete numbers on this. Earlier hominins like Ardipithecus and Australopithecus, which lived earlier than 2 million years ago, had an average body mass of 40 kilograms, with a range of 30 to 50 kg. Homo habilis, an early member of our genus, was a bit bigger: 45 kg on average, ranging from 35 to 55 kg. Homo erectus, the first hominin known to have migrated out of Africa into Eurasia, was bigger still, averaging 60 kg with a range of 50 to 75 kg. Finally, prehistoric Homo sapiens were up to an average of 75 kg and a range of 55 to 80 kg.
Gardner and his colleagues compiled estimates of body mass for 386 specimens belonging to 21 hominin groups, dating from 4.5 million years ago to 30,000 years ago. If that seems a little arbitrary, there are reasons why. They excluded more recent specimens because there is evidence of our species having shrunk in the past 30,000 years, which would have made a mess of the analysis. They also left out the earliest hominins, Sahelanthropus and Orrorin, because of ongoing uncertainties over how they are related to later groups.
The team then ran 1000 models to see what would best explain the data. Did body size increase within each group, meaning earlier H. erectus were smaller than more recent ones? Or did each group pretty much stay the same size, with the changes coming when new groups evolved? Did body size increase gradually over the whole 4.5-million-year timespan, or were there long periods of stasis and occasional bursts of growth?
The researchers ultimately identified two trends, which between them explained most of the changes.
The first was 鈥渢he gradual progression from Ardipithecus to later hominin species鈥�, says Gardner. This was a 鈥渕odest鈥� increase in body mass of about 1 to 3 kg per million years. If you remember, average body mass went from 40 kg to 75 kg in 4.5 million years: this gradual increase can only explain about a third of that.
Hence the second trend, which was a 鈥渂ig jump around the appearance of Homo erectus and other later species in our genus Homo鈥�, says Gardner. The first H. erectus appeared in Africa around 2 million years ago.
We鈥檒l get to why this might have happened in a minute, but first let鈥檚 look at what happened to people鈥檚 height.
Short kings
Gardner and his colleagues didn鈥檛 analyse height, but a team led by at the University of T眉bingen in Germany did in . This one compiled 204 stature estimates from 4.4 million years ago until nearly the present day. The variation is dramatic. One especially dainty Australopithecus specimen clocked in at just 105 centimetres, while a 24,500-year-old H. sapiens from Barma Grande cave in Italy was almost 189 cm tall.
Will鈥檚 team found 鈥減hases of relative stasis intermitted by periods of rapid increases鈥�. Before 2.2 million years ago, most hominins were less than 140 cm tall. There was then a noticeable increase between 2 million and 1.6 million years ago. By 1.6 million years ago, some individuals grew taller than 170 cm for the first time. However, such heights only became commonplace much more recently, around 500,000 years ago.
This loosely maps onto what Gardner鈥檚 team found. Hominins got significantly bigger, both in mass and in height, from around 2 million years ago when H. erectus came on the scene.
What was going on? The answer, as usual, is 鈥渓ots of things鈥�.
One factor may be sexual dimorphism. In early hominins like Australopithecus, males were significantly bigger than females 鈥� 鈥� and had larger canine teeth. We see the same pattern in chimpanzees and other animals where males compete violently for females, with successful males mating with multiple females while unsuccessful males don鈥檛 mate at all. But in early Homo species, males and females were much more alike (and in H. sapiens today, the height and mass differences are smaller still).
Exactly how this played out is unclear. 鈥淭here鈥檚 some hypotheses that perhaps males got smaller through time, and then there鈥檚 hypotheses that it鈥檚 the females that got bigger through time and the males stayed about the same,鈥� says Gardner. If the latter is true, that would account for some of the average increase.
Diet may also have played a big role. Early Homo ate more meat than Australopithecus, and meat is calorific. Furthermore, 鈥渨e are finding some of the earliest evidence of cooking meat in Homo erectus鈥�, says Gardner. Cooked food contains more calories than raw food, so this was another way of getting more nutrition from a given food source.
Cooked food may also have been a factor in another important change in our bodies: the growth of our brains. The extra calories from cooked food may have enabled our brains to evolve to be larger.
It鈥檚 also crucial to bear in mind that different hominin groups evolved in different ways, depending on their circumstances. So while there was this overall tendency to get bigger, not everyone did. H. floresiensis, who lived on Flores in Indonesia until just 50,000 years ago, were barely more than 100 cm tall. Homo naledi from South Africa, who lived 335,000 to 236,000 years ago, were taller but still not much more than 140 cm. One of the shortest known adult hominins was a female Paranthropus robustus, who was probably just 103 cm. We don鈥檛 know why these hominins became so small, and it may be that each one did so for different reasons.
鈥淎ll these different species are finding their own evolutionary path,鈥� says Gardner. Each hominin group had its own unique set of environmental contexts, and this was reflected in their varying body sizes, he says.
Finally, let鈥檚 turn the question around. Once early Homo groups like H. erectus evolved larger bodies, how did that change their lives? 鈥淭he larger you are,鈥� says Gardner, 鈥渢he longer the strides you can take across the landscape.鈥� That鈥檚 especially true of H. erectus, whose legs were proportionately longer than those of earlier hominins. Gardner suggests they had larger home ranges than previous groups.
This may even help to explain why H. erectus were the first hominins (that we know of) to roam outside Africa. That鈥檚 a long-standing puzzle: how come they travelled all the way to Java, when earlier hominins seemingly never left Africa? If Gardner is right, it may be partly because they were bigger, so they needed more room.
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