New Scientist - USA (2020-03-21)

16 | New Scientist | 21 March 2020

Human evolution

COMETS appear to have 10 times

less nitrogen than they ought to,

based on our understanding of how

they form, but a finding from comet

67P/Churyumov-Gerasimenko

might reveal how they have been

hiding this element.

The European Space Agency’s

Rosetta spacecraft orbited 67P

between 2014 and 2016. Analysis

of the light bouncing off the comet

revealed that some compound

was absorbing light from its

surface, but researchers couldn’t

immediately identify this chemical.

Olivier Poch at the University

of Grenoble Alpes in France and his

colleagues compared the properties

of that light with that of light

bounced off artificial asteroid dust

in the lab. This contained dust

grains similar to those known to

exist on 67P, as well as various

other compounds. There was

a close match with ammonium

salts, which contain nitrogen and

hydrogen (Science, doi.org/dprs).

This could help explain where the

missing nitrogen is.

The team found similar

observations from asteroids, which

means these rocks could also have

such salts on their surfaces.

The discovery could also help

explain how some of the solar

system’s giant planets formed:

measuring how much nitrogen

they have and comparing it with

the amount in comets could reveal

if comets made up young planetary

cores, says Kathleen Mandt at Johns

Hopkins University in Maryland.

“It could be valuable information

for understanding how the ice

giants formed and how many

comets might be in there,” she says.

That would help us understand

what proportion of the early planets

were made of solids like comets and

asteroids as opposed to gas.

Ammonium salts are important to

the chemical reactions that form the

amino acids that living organisms

are made from, as well as the

building blocks of DNA, says Poch.

“There is a tantalising scenario

in which you can imagine that if

comets such as 67P fell on the

early Earth and the ammonium salts

dissolved in water, you may have

had all these prebiotic reactions

happen,” he says. If that did occur,

comets could have been important

not only to the formation of planets,

but also to the evolution of life. ❚

THE ancient human species

Australopithecus afarensis may

have been the earliest hominin

to run on two legs. Although it

had relatively short, ape-like legs,

A. afarensis may have had shared

a musculoskeletal feature with

modern humans that enables

us to run efficiently.

Conventional thinking

is that early hominins like

A. afarensis – the species to which

the famous Lucy fossil belonged –

walked on two legs long before

they could run. Lucy was an

ape-like bipedal hominin seen

by some as a likely direct ancestor

of the earliest species of human.

Some evidence places the

origin of bipedal walking more

than 10 million years ago. But

many researchers think it was

only with the appearance of the

human genus Homo, between

2 and 3 million years ago, that

hominins began to run.

Ellison McNutt at the University

of Southern California thinks

the story is more complicated

than that. Some earlier hominins

should have had some ability

to run when faced with a predator,

for instance.

McNutt looked for evidence

of running ability in A. afarensis,

because this species appeared

about 3.9 million years ago

and disappeared a million

years later, about the same

time as the first humans,

such as Homo habilis, evolved.

She and Jeremy DeSilva

at Dartmouth College in New

Hampshire focused on the

Achilles’ tendon, a band of

tissue connecting the calf

muscles to the heel.

Modern humans have a long

Achilles’ tendon that extends

more than halfway up our lower

leg. It stretches when we run to

store elastic energy that it then

releases explosively. This helps

us to use 35 per cent less energy

when running. “A long Achilles’

tendon is helpful for efficient

walking, but it is especially

critical for efficient running,”

says McNutt.

Tendons rarely fossilise. But

by studying the shape of the heel

bone in humans and 11 other

living primates, McNutt and

DeSilva discovered that the size

of one part on the rear of the

heel bone scales with the length

of the Achilles’ tendon.

When they measured two

A. afarensis heel bones, they

calculated that the species

might have had an Achilles’

tendon that extended more

than halfway up its calf, just as

we do (The Anatomical Record,

doi.org/dprh). For comparison,

chimpanzees – which can’t run

well on two legs – have an Achilles’

tendon that stretches barely

higher than the ankle.

“Currently, I think A. afarensis

is the earliest hominin for which

we have good evidence for some

of the key adaptions necessary

for modern human-like running,”

says McNutt.

But we know there were

earlier bipedal hominins, such as

Ardipithecus. McNutt says as we

learn more about these species, we

might conclude that even earlier

hominins were also born to run. ❚

“ Nitrogen-rich comets could

have made up planetary

cores and may help explain

how giant planets formed”

Solar system

Heel bones suggest that

Australopithecus afarensis

could have run if necessary

Colin Barras

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News

35%

The running efficiency gained due

to the human Achilles’ tendon

When our ancestors started to run

Hominins may have begun running a million years before we thought

Comet 67P’s hidden

nitrogen may solve a

solar system puzzle

Leah Crane