How humans evolved to be twice as big as our ancestors | New Scientist

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‘A lot of our images of prehistoric people are just too big’…<br>Chuang Zhao

This is an extract from Our Human Story, our newsletter about the revolution in archaeology. Sign up to receive it in your inbox every month.

As someone who writes a lot about human evolution and archaeology, I’ve seen a great many artists’ 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’ll 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’ve been doing reps at the gym and downing protein shakes. This just isn’t how practical everyday fitness works. If your body looks like a movie superhero’s, that is a cosmetic choice – one that is achieved using physiological trickery, like artificially dehydrating yourself so your muscles stand out. Genuine speed, strength and endurance don’t 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 “hobbits”), 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.

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Small folk

To find out how hominin body size has changed over time, I spoke to evolutionary biologist Jacob Gardner at the University of Reading in the UK. He’s lead author of a study published in PNAS on 22 June, which looks at the trends in body size over the past 4 million years.

Let’s 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.

Read More<br>What really happened when ancient humans migrated out of Africa

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 “the gradual progression from Ardipithecus to later hominin species”, says Gardner. This was a “modest” 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.

The grapefruit-sized skull of Homo floresiensis<br>Shutterstock

Hence the second trend, which was a “big jump around the appearance of Homo erectus and other later species in our genus Homo”, says Gardner. The first H. erectus appeared in Africa around 2 million years ago.

We’ll get to why this might have happened in a minute, but first let’s look at what happened to people’s...