麻豆传媒

DNA evidence from four ancient skeletons uncovered in western Cameroon has revealed a long-lost mystery branch of early modern humans, suggesting we may need to rethink our species鈥� family tree.

The skeletons all belonged to children who were buried at a rock shelter at a site called Shum Laka. Two of the skeletons are 8000 years old, and the other two are about 3000 years old. Despite living 5000 years apart, genetic analysis by David Reich at Harvard Medical School and his colleagues suggests that they were from the same population of modern humans.

The team was interested in the skeletons because linguistic studies have suggested that the Bantu languages, which are spoken today by around 30 per cent of people in Africa, originated in this region. However, the genetic signature of these remains look nothing like those of the people known to have spread the Bantu languages across the continent.

By comparing the Shum Laka people鈥檚 genetic signatures with those in databanks of modern African populations, the team found that a third of the children鈥檚 DNA was similar to that of Central African hunter-gatherers 鈥� but the rest is a mystery (Nature, DOI: 10.1038/s41586-020-1929-1). 鈥淭wo-thirds of their ancestry is related to a modern human lineage that we鈥檝e never seen before. That鈥檚 exciting,鈥� says Reich.

Previous genetic studies have suggested that the very earliest modern humans split into three branches between 250,000 and 200,000 years ago. These were the southern African hunter-gatherers, the Central African hunter-gatherers and a lineage that leads to most other modern humans including most East and West Africans. But the bulk of the Shum Laka skeletons鈥� genetic make-up doesn鈥檛 match any of these three groups. This suggests there was a fourth branch early on in our species鈥� history.

鈥淲hat we think is that most of the Shum Laka ancestry is from a lineage that hadn鈥檛 been identified before, but was present in very high proportions as recently as 3000 years ago at the site and presumably other sites too,鈥� says Reich. He thinks this ghost lineage was later displaced by the Bantu expansion and other population changes.

鈥淎 much more diverse landscape of very different humans were living a few thousand years ago鈥�

The evidence for the three early branches of our species has come mainly from looking at the genomes of modern-day Africans, which may be why we hadn鈥檛 detected this fourth group, as this ancestry has been mostly lost from today鈥檚 populations.

鈥淚t鈥檚 exciting to get ancient DNA from Africa, especially central Africa where ancient DNA doesn鈥檛 preserve well because it鈥檚 a tropical region,鈥� says Sarah Tishkoff at the University of Pennsylvania.

One of the skeletons, a 15-year-old boy from 8000 years ago, was found to have a variant of a rare genetic signature on his Y chromosome, known as A00. First discovered in 2013, this signature showed that our species鈥� last shared male ancestor lived 200,000 to 300,000 years ago.

The analysis also found that the Shum Laka genomes contain traces from an archaic hominin species that no longer exists. Many people around the world carry DNA from extinct hominins like Neanderthals or Denisovans. But less is known about archaic hominins that contributed to the genomes of some African populations.

About 2 per cent of the DNA of the Shum Laka people seems to have come from an unknown extinct species that one of their ancestors interbred with.

A great diversity

There are also signs of much interbreeding with other human populations. All this points to a time in our history when Africa was inhabited by a much greater diversity of humans. 鈥淚f you go back in time to a few thousand years ago, what you see is a much more diverse landscape of very different groups of humans living in the region,鈥� says Reich.

Tishkoff says the hypothesis that the Shum Laka people were different from any modern populations is interesting, but isn鈥檛 conclusive yet. 鈥淎dditional samples and analysis will be needed to be able to reconstruct the complex demographic history of Africa,鈥� she says.

Joshua Akey at Princeton University says the study has implications for our understanding of the origin of humans. 鈥淧erhaps modern humans did not originate in any particular location in Africa, but evolved across a broad swathe of the African continent,鈥� he says.

Ultimately, it may have been genetic 鈥渁dmixture鈥� 鈥� mixed up DNA that is the result of many populations interbreeding 鈥� that predominantly contributed to the make-up of modern humans, says Akey.

Further insights are expected later this year when Tishkoff and her colleagues publish the largest ever analysis of genomes from across contemporary Africa.

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