New Scientist - USA (2019-06-15)

8 | New Scientist | 15 June 2019

News Our human origins
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“Images are converted into

a 3D design that is used

to make a life-sized replica

of a skull and face”

IT IS the oldest case of

interbreeding found so far in the

tangled tale of human origins, and

one of the most extreme. About

700,000 years ago, a group of

ancient humans that would later

give rise to both the Neanderthals

and the Denisovans interbred with

an unidentified group of hominins.

This mystery group had evolved

separately from their new sexual

partners for over a million years.

Despite this, the two groups

were able to have offspring.

The Neanderthals and

Denisovans belong on a distinct

branch of the human family tree

that split away from our “modern

human” branch within the past

million years. Neanderthals lived

in Europe and west Asia, and

Denisovans lived in east Asia.

Previous studies have revealed

that both species interbred with

modern humans and with each

other. They also suggest that

Denisovans interbred with

a mysterious population of

“super-archaic” hominins.

To find out if any other

interbreeding went on, Alan

Rogers at the University of Utah

and his colleagues compiled DNA

from Neanderthals, Denisovans,

modern Europeans and modern

Yoruba people in Africa.

The team looked to see if the

known instances of interbreeding

were enough to explain how

various genes were distributed

among the four populations.

Many anomalous gene patterns

remained, but Rogers’s team

could get a good fit by adding

an extra episode of interbreeding

involving the shared ancestors

of Neanderthals and Denisovans,

a group he calls “Neandersovans”.

Their sexual partners came from

the same mystery population of

super-archaic hominins that the

Denisovans later interbred with

(bioRxiv, doi.org/c63t).

However, Serena Tucci

at Princeton University says the

finding needs to be confirmed

using multiple methods.

Most anthropologists agree

that hominins evolved in Africa

within the past 13 million years.

The first species to migrate to other

continents, about 2 million years

ago, was probably Homo erectus.

“My results are consistent with

the view that these super-archaics

are descendants of that original

out-of-Africa migration,” says

Rogers. In that case, the super-

archaics may have been H. erectus.

However, it is also possible

that the super-archaics are a

group not yet identified in

the fossil record, says Tucci.

When humans and

Neanderthals interbred, their

two lineages had been evolving

separately for at most 750,

years. In contrast, Neandersovans

and super-archaics had been

evolving separately for perhaps

1.3 million years. That makes

them among the most genetically

distinct hominin groups known

to have interbred, and may mean

that the offspring experienced

poor health as a consequence. ❚

RADIOTHERAPISTS are practising

cancer treatments on 3D-printed

heads before they do the real thing.

The replica heads were approved

as safe to use by the US Food

and Drug Administration last year

and are now being used to practise

the treatment of people in Greece.

Radiotherapy employs high,

targeted doses of radiation to kill

cancerous cells. Using the technique

for brain cancer is tricky because

a difference in aim of just a few

millimetres can damage healthy

brain tissue. To accurately target

the treatment, you have to take into

account a person’s anatomy and

the size and shape of their tumour.

Using replica heads can help.

Before a person with brain cancer

undergoes radiotherapy, they have

a CT scan to image their brain, the

tumour and surrounding bone and

facial structure. The images are

converted into a 3D design that is

used to make a life-sized replica of

their skull and face. It is 3D-printed

from a material that absorbs

and scatters the radiation used

for radiotherapy in the same way

as human tissues.

“The aim is to focus a large

number of very small radiation

beams so they can converge

exactly on the spot where the brain

tumour is,” says Evangelos Pappas

at RTSafe, a firm in Athens that

has developed the technique.

Each replica head is filled with

water and contains inserts that

detect the radiation dose at precise

locations within the head. Using the

models, radiologists can have a trial

run of focusing on the tumour and

make any necessary tweaks before

performing the procedure for real.

“The medical team treats

this head as if it is a living patient,”

says Pappas. This allows doctors

to make sure the radiation

won’t hit and damage vital

structures nearby, such as the

brainstem or the optical nerve. ❚

Donna Lu

Cancer treatment

Human evolution

Michael Marshall

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Ancient sexual encounters

Mystery humans bred with ancestor of Neanderthals

3D-printed replica

heads give cancer

therapies a trial run

Homo erectus may

have bred with other

archaic human species