Ancient humans’ extraordinary journey to South America

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A central plot point in the human story is our species’ worldwide spread. From our homelands in Africa, our ancestors went to Europe, Asia, Australia and, eventually, to the Americas. The last continent they reached was South America (apart from Antarctica, but let’s not worry about that).

This is a curiously understudied chapter in our story. Huge amounts of research effort have gone into figuring out when and where humans first entered Europe, Asia and North America, but there’s been less attention to the first arrivals in South America.

That’s reflected in my own output: looking back through the archive of Our Human Story, I realised the last time I wrote in any depth about South America was June 2023.

However, that’s starting to change. On 15 May, Science published a massive genetic study of South Americans, which sheds a lot of light on the early peopling of the continent. It reveals a four-way split in the population as groups dispersed to different regions of the landmass. It also fits into an emerging story of extraordinary journeys – and the tremendous risks that were sometimes involved in moving to a new continent.

The long and winding road

If you know that Homo sapiens first evolved in Africa, and then look at a globe, it becomes apparent that getting to South America would be a significant undertaking. The vast span of the Atlantic Ocean lies between the two continents and was presumably an impassable barrier. So, humans ended up going the long way around.

Of course, it wasn’t a planned thing. Nobody back then knew South America even existed. People just kept wandering over the horizon to the next place. That took them out of Africa into south-west Asia, and from there to every corner of Eurasia. Some people even ended up in the far north-east of Asia, in the region we now call Chukotka in the Russian Far East.

From there, it was a relatively short hop to what we now call Alaska, in the far north-west of North America. Humans arrived there at least 16,000 years ago. Today, there is a sea crossing of about 82 kilometres, called the Bering Strait. But thousands of years ago the climate was colder and sea levels were lower, so more land was exposed – including an area called Beringia, linking Asia and North America. People may have simply walked across, without realising they were doing anything monumental.

In line with this, a study published in May found that horses regularly moved between North America and Asia via Beringia between 50,000 and 13,000 years ago. If horses could make this journey, people presumably could as well.

So did a species of bacteria that causes leprosy. In late May, we learned that Mycobacterium lepromatosis has been living and evolving in the Americas for almost 10,000 years.

Somehow, groups of the first Americans then made their way south. Some may have used boats to travel down the Pacific coast, while others went inland. Either way, people eventually found their way to the southernmost tip of South America.

These populations have left a rich archaeological record. A study published in February described a large collection of artefacts from the Tacuarembó Department of Uruguay from 10,000 to 11,000 years ago.

Who were these early South Americans? That’s where the new genetic analysis comes in.

On the move

Researchers led by Hie Lim Kim at Nanyang Technological University in Singapore compiled genomic data from 1537 people belonging to 139 ethnic groups. Some were from northern Eurasian populations potentially related to those who first entered the Americas, and some were from the Americas, including South America.

“We showed this humongous migration history,” says Kim.

Between 13,900 and 10,000 years ago, the first people in South America split into four groups with distinct genetic variants. All four genetic patterns can still be found in South Americans today.

It’s “very difficult” to come up with terms to describe this, says Kim. The study identifies genetic differences between populations, but these don’t necessarily correspond to cultural traits. “We didn’t define them as their culture or languages,” she says, but purely by ancestry.

With that caveat in mind, Kim’s team has labelled the four groups Amazonians, Andeans, Chaco Amerindians and Patagonians. The names relate to the regions where the genetic signals are strongest today. For example, the Amazonian ancestry is detectable today in people living in the Amazon rainforest, the Andean in the Andes mountain range, and the Patagonian in, well, Patagonia in southern Argentina. The Chaco Amerindian ancestry is today found in the Dry Chaco, a region spanning parts of Argentina, Bolivia and Paraguay. “They are hunter-gatherers living in the desert,” says Kim.

After the groups diverged, there is no sign of significant gene flow between them. It’s very possible that “they never met again”, says Kim. Geographical barriers like the Andes may have contributed to this isolation.

This almost certainly isn’t the full story, though, says Kim. There could well be more groups than just these four. “We have a very limited sample from Brazil,” she says, “And then there are still a lot of ethnic groups in the Amazon jungle.”

Other studies from the last few months hint at the richness of stories still to be uncovered. One, from March, looked at archaeological evidence from the “Southern Cone”: the region, south of the 22nd parallel, that includes the south of Brazil, Paraguay, Argentina, Chile and Uruguay. There, humans hunted large megafauna, such as giant ground sloths and giant armadillos called glyptodonts, for food and to make bone tools.

Another study published in March described how a people called the Guaraní made a huge journey across South America, travelling 2500 km from south-western Amazonia to south-eastern South America. After hundreds of years, they eventually reached the Río de la Plata estuary on the east coast, which today is the site of both Buenos Aires and Montevideo.

The most recent study, published in late May, highlights the dangers of moving to a new area. Ancient DNA from Colombia revealed a hitherto-unknown population of hunter-gatherers who lived on the Bogotá Altiplano, a plateau that is on average 2600 metres above sea level, around 6000 years ago. By 2000 years ago, they had been replaced by populations from Central America, and today there is no trace of their genetic makeup in any population that has been sampled – suggesting that, for some unknown reason, the group didn’t survive.

South America is a big place, and we’ve only just scratched the surface: there will be many, many more stories like these.