New Scientist - USA (2020-01-25)

8 | New Scientist | 25 January 2020

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’ 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’s genetic signatures with

those in databanks of modern

African populations, the team

found that a third of the children’s

DNA was similar to that of Central

African hunter-gatherers – but the

rest is a mystery (Nature, DOI:

10.1038/s41586-020-1929-1). “Two-

thirds of their ancestry is related

to a modern human lineage that

we’ve never seen before. That’s

exciting,” 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’ genetic

make-up doesn’t match any of

these three groups. This suggests

there was a fourth branch early

on in our species’ history.

“What we think is that most of

the Shum Laka ancestry is from a

lineage that hadn’t been identified

before, but was present in very

high proportions as recently as

3000 years ago at the site and

presumably other sites too,”

says Reich. He thinks this ghost

lineage was later displaced by

the Bantu expansion and other

population changes.

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’t

detected this fourth group, as this

ancestry has been mostly lost

from today’s populations.

“It’s exciting to get ancient DNA

from Africa, especially central

Africa where ancient DNA doesn’t

preserve well because it’s a tropical

region,” 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’

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. “If 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,” says Reich.

Tishkoff says the hypothesis

that the Shum Laka people

were different from any modern

populations is interesting, but

isn’t conclusive yet. “Additional

samples and analysis will be

needed to be able to reconstruct

the complex demographic

history of Africa,” she says.

Joshua Akey at Princeton

University says the study

has implications for our

understanding of the origin

of humans. “Perhaps modern

humans did not originate in any

particular location in Africa, but

evolved across a broad swathe of

the African continent,” he says.

Ultimately, it may have been

genetic “admixture” – mixed up

DNA that is the result of many

populations interbreeding – 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. ❚

News

Ancient humans

ISA

BE

LL

E^ R

IBO

T

Our ghost lineage

A pioneering analysis of ancient African DNA has discovered an unknown

branch of our species’ family tree, reports Alison George

Excavating the

8000-year-old remains

of two boys in Cameroon

Discover the story of human evolution

Come to a masterclass in London on 29 February

newscientist.com/science-events

“A much more diverse

landscape of very different

humans were living a

few thousand years ago”