New Scientist - USA (2020-04-04)

4 April 2020 | New Scientist | 41

Jebel Irhoud, Morocco

(~315,000 years ago)

One species, many origins

The idea that Homo sapiens evolved from a single population in eastern Africa has been undermined by

discoveries of human skulls across the continent. The huge variation in their features and dates suggests

that our species was born of the occasional mixing of many isolated populations

Herto, Ethiopia

(~160,000)

Florisbad, South Africa

(~259,000)

Eliye Springs, Kenya

(200,000 - 300,000?)

Olorgesailie, Kenya

(490,000 – 1.2 million)

Acheulian and prepared core tools

Iwo Eleru, Nigeria

(~14,000)

Ishango, Democratic Republic

of the Congo

(22,000)

Omo I and Omo II, Ethiopia

(~195,000)

Laetoli, Tanzania

(~120,000)

Guomde, Kenya

(180,000+)

HOW DOES DNA ANALYSIS

REVEAL OUR PREHISTORY?

Genetics is now at the leading edge

of human evolution research and is

perhaps even more important than

bones and stones. But extracting

information about the past from

ancient or modern DNA isn’t

straightforward. The techniques rely

on arcane statistics and computational

biology. However, the basic principles

are easy to grasp.

One key thing that DNA can tell us

is how long ago two lineages diverged

• for example, when the ancestors of
  Neanderthals and Homo sapiens split.
  These analyses are based on mutations,
  says Carina Schlebusch at the University
  of Uppsala in Sweden. The simplest
  method uses mitochondrial DNA, a
  self-contained mini genome found
  inside cells’ mitochondria, which
  produce energy. This is passed on
  from mothers to their offspring.
  “We sequence different mitochondrial
  genomes and then compare and count
  the different mutations between them,”
  she says. “Then, using the rate at which

mutations accumulate, we date how

long ago the mitochondria diverged

from each other.”

Extracting information from DNA

on the Y-chromosome – which is

passed from fathers to sons – is also

quite straightforward because, like

mitochondrial DNA, it is inherited intact.

Other chromosomes, however, undergo

a process called recombination during

the formation of sperm and eggs,

in which pieces of DNA are shuffled

around. This makes them trickier

to analyse. But mitochondrial and

Y-chromosome DNA don’t tell the

whole story, so methods have been

developed to estimate divergence

times using other chromosomes.

There are two approaches. “One is to

take recombination into account, using

recombination maps to know where

the hotspots are,” says Schlebusch.

“Another is to ignore recombination

by using small pieces of DNA

randomly sampled across all of the

chromosomes.” If these pieces are

small enough, researchers assume

they have been passed down the

generations pretty much intact.

DNA can also reveal when separate

populations – or species – interbred.

“When populations split from each

other back in time, they evolve

independently and accumulate patterns

of mutations that are distinct,” says

Arun Durvasula at the University

of California, Los Angeles. Then, when

interbreeding happens, the resulting

genomes will be a mixture of these

distinct patterns. “We can look for

these patterns across the thousands

of genomes we have sequenced from

individuals around the world,” he says.

The time of these interbreeding

events can be estimated, too. “When

[interbreeding] happens, large stretches

of DNA come from one population or

the other,” says Schlebusch. “Then over

generations, because of recombination,

these become smaller and smaller. This

also happens at a certain rate, and that

is what we use to date admixture times.”

Bones found in Ishango in the

Democratic Republic of the Congo

are among many finds suggesting

that early human populations were

living across Africa

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