70,000 years ago humans underwent a major shift – that’s why we exist

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Humans come from Africa. This wasn’t always obvious, but today it seems as close to certain as anything about our origins.

There are two senses in which this is true. The oldest known hominins, creatures more closely related to us than to great apes, are all from Africa, going back as far as 7 million years ago. And the oldest known examples of our species, Homo sapiens, are also from Africa.

It’s the second story I’m focusing on here, the origin of modern humans in Africa and their subsequent expansion all around the world. With the advent of DNA sequencing in the second half of the 20th century, it became possible to compare the DNA of people from different populations. This revealed that African peoples have the most variety in their genomes, while all non-African peoples are relatively similar at the genetic level (no matter how superficially different we might appear in terms of skin colour and so forth).

In genetic terms, this is what we might call a dead giveaway. It tells us that Africa was our homeland and that it was populated by a diverse group of people – and that everyone who isn’t African is descended from a small subset of the peoples, who left this homeland to wander the globe. Geneticists were confident about this as early as 1995, and the evidence has only accumulated since.

And yet, the physical archaeology and the genetics don’t match – at least, not on the face of it.

Genetics tells us that all living non-African peoples are descended from a small group that left the continent around 50,000 years ago. Barring some wobbles about the exact date, that has been clear for two decades. But archaeologists can point to a great many instances of modern humans living outside Africa much earlier than that.

At Apidima cave in Greece, there is a single skull of a modern human from 210,000 years ago. A jawbone from Misliya cave in Israel is at least 177,000 years old. There are some contentious remains from China that might be modern humans. “And there are debates swirling around the earliest colonisation of Australia,” says Eleanor Scerri at the Max Planck Institute of Geoanthropology in Germany. Some researchers claim people were on the continent 65,000 years ago.

What is going on? Is our wealth of genetic data somehow misleading us? Or is it true that we are all descended from that last big migration – and the older bones represent populations that didn’t survive?

Scerri and her colleagues have tried to find an explanation.

African environments

The team was discussing where modern humans lived in Africa. “Were humans simply moving into contiguous regions of African grasslands, or were they living in very different environments?” says Scerri.

To answer that, they needed a lot of data.

“We started with looking at all of the archaeological sites in Africa that date to 120,000 years ago to 14,000 years ago,” says Emily Yuko Hallett at Loyola University Chicago in Illinois. She and her colleagues built a database of sites and then determined the climates at specific places and times: “It was going through hundreds and hundreds of archaeological site reports and publications.”

There was an obvious shift around 70,000 years ago. “Even if you just look at the data without any fancy modelling, you do see that there is this change in the conditions,” says Andrea Manica at the University of Cambridge, UK. The range of temperatures and rainfalls where humans were living expanded significantly. “They start getting into the deeper forests, the drier deserts.”

However, it wasn’t enough to just eyeball the data. The archaeological record is incomplete, and biased in many ways.

“In some areas, you have no sites,” says Michela Leonardi at the Natural History Museum in London – but that could be because nothing has been preserved, not because humans were absent. “And for more recent periods, you have more data just because it’s more recent, so it’s easier for it to be conserved.”

Leonardi had developed a statistical modelling technique that could determine whether animals had changed their environmental niche: that is, whether they had started living under different climatic conditions or in a different type of habitat like a rainforest instead of a grassland. The team figured that applying this to the human archaeological record would be a two-week job, says Leonardi. “That was five and a half years ago.”

However, the statistics eventually did confirm what they initially saw: about 70,000 years ago, modern humans in Africa started living in a much wider range of environments. The team published their results on 18 June.

Jacks of all trades

“What we’re seeing at 70,000 [years ago] is almost kind of our species becoming the ultimate generalist,” says Manica. From this time forwards, modern humans moved into an ever-greater range of habitats.

It would be easy to misunderstand this. The team absolutely isn’t saying that earlier H. sapiens weren’t adaptable. On the contrary: one of the things that has emerged from the study of extinct hominins is that the lineage that led to us became increasingly adaptable as time went on.

“People are in different environments from an early stage,” says Scerri. “We know they’re in mangrove forests, they’re in rainforest, they’re in the edges of deserts. They’re going up into highland regions in places like Ethiopia.”

This adaptability seems to be how early Homo survived environmental changes in Africa, while our Paranthropus cousins didn’t: Paranthropus was too committed to a particular lifestyle and was unable to change.

Instead, what seems to have happened in our species 70,000 years ago is that this existing adaptability was turned up to 11.

Some of this isn’t obvious until you consider just how diverse habitats are. “People have an understanding that there’s one type of desert, one type of rainforest,” says Scerri. “There aren’t. There are many different types. There’s lowland rainforest, montane rainforest, swamp forest, seasonally inundated forest.” The same kind of range is seen in deserts.

Earlier H. sapiens groups were “not exploiting the full range of potential habitats available to them”, says Scerri. “Suddenly, we see the beginnings of that around 70,000 years ago, where they’re exploiting more types of woodland, more types of rainforest.”

This success story struck me, because recently I’ve been thinking about the opposite.

Splendid isolation

Last week, I published a story about local human extinctions: groups of H. sapiens that seem to have died out without leaving any trace in modern populations. I focused on some of the first modern humans to enter Europe after leaving Africa, who seem to have struggled with the cold climate and unfamiliar habitats, and ultimately succumbed. These lost groups fascinated me: why did they fail, when another group that entered Europe just a few thousand years later succeeded so enormously?

The finding that humans in Africa expanded their niche from 70,000 years ago seems to offer a partial explanation. If these later groups were more adaptable, that would have given them a better chance of coping with the unfamiliar habitats of northern Europe – and for that matter, South-East Asia, Australia and the Americas, where their descendants would ultimately travel.

One quick note of caution: this doesn’t mean that from 70,000 years ago, human populations were indestructible. “It’s not like all humans suddenly developed into some massive success stories,” says Scerri. “Many of these populations died out, within and beyond Africa.”

And like all the best findings, the study raises as many questions as it answers. In particular: how and why did modern humans became more adaptable 70,000 years ago?

Manica points out that we can also see a shift in the shapes of our skeletons. Older fossils classed as H. sapiens don’t have all the features we associate with humans today, just some of them. “From 70,000 [years ago] onwards, roughly speaking, suddenly you see all these traits present as a package,” he says.

Manica suggests that the expansion into new niches may have enabled this, by bringing previously separate populations into more regular contact. For instance, if two populations were separated by a desert, they would never have met, never exchanged ideas and genes – until someone figured out how to live in the desert.

“There might also be almost a positive feedback,” says Manica. “You connect a bit more, you become more flexible… You break down some of those barriers, you become even more connected.”

With apologies, here is a pat conclusion. In that story about lost populations, I said that one of the biggest threats to human groups is isolation: if you don’t have neighbours you can call on and your group is small, even a minor misfortune can mean apocalypse. If Manica is right, the exact opposite played out in Africa. Populations grew and became more connected, and that enabled an explosion of creativity that sent our species all over the planet.

In which case, the reason the last out-of-Africa migration succeeded so wildly is: people need people. Without other people, we’re stupid and doomed. Any doomsday preppers hoping to ride out the apocalypse alone in a well-provisioned bunker: you may have the wrong approach.