New Scientist - USA (2021-12-11)

12 | New Scientist | 11 December 2021

News

ANCIENT footprints that were

once thought to have been made

by a bear walking on its hind legs

were actually made by an extinct

human species. The discovery

means there are now three known

sets of hominin footprints from

the same locale in Tanzania.

It isn’t clear which hominin

species made the prints. The

authors of the new study say they

don’t match the other sets of

footprints at the site in Tanzania –

called Laetoli – so were probably

made by a different species. If this

is true, it would mean that two

hominin species coexisted in the

same region at the same time.

“Not only are they not a bear,

they are hominin and they are not

the same hominin as those that

made [the other footprints],” says

Ellison McNutt at Ohio University

Heritage College of Osteopathic

Medicine in Athens.

The five footprints were

discovered at Laetoli’s “site A”

in 1976. The tracks had been

left in soft volcanic ash that

subsequently hardened into rock.

They were initially thought to have

been left by a hominin, but later

studies suggested that they were

actually made by a bear walking

on its hind legs. As a result, site A

fell into obscurity.

Meanwhile, more footprints

were found at “site G” in Laetoli,

a few kilometres away. These were

definitely made by hominins.

The trail stretches 24 metres

and includes prints (one of which

is pictured above) from three

individuals walking together. Both

sets of footprints are 3.66 million

years old and are thought to have

been made by Australopithecus

afarensis, the species to which the

famous Lucy fossil belonged.

Now, McNutt and her colleagues

have re-excavated the footprints

at site A. “The bear hypothesis was

very reasonable at the time,” she

says, because the prints do look

unusual. But the sediment in

them was never properly cleaned

out, so their true shape wasn’t

known. McNutt’s team cleaned

the prints thoroughly and

produced 3D scans of them.

The researchers compared the

tracks from site A with footprints

made by humans, chimpanzees

and American black bears. “There

are a lot of things that make it

distinctly hominin,” says McNutt.

For example, the big toes were

proportionally much larger than

the second toes, which is seen

in hominins but not in bears

(Nature, doi.org/g775).

The team was “very clever”

to re-excavate site A, says Marco

Cherin at the University of Perugia

in Italy. “I think it almost definitely

is a hominin.”

McNutt and her colleagues

also argue that the prints don’t

match the more famous tracks at

site G, so were therefore made by

a different hominin. For instance,

the hominin seems to have cross-

stepped, meaning it brought its

feet across its body’s central line,

a gait seen in its most exaggerated

form when fashion models walk

down runways placing one foot

directly in front of the other.

“It’s not afarensis,” says McNutt.

“It is certainly Australopithecus

or something very like it.”

She adds that elsewhere in

Africa, there is clear evidence

of multiple hominin species

coexisting in the same regions,

so it wouldn’t be surprising if

the same was true at Laetoli.

Cherin isn’t convinced about

that. “For the moment, I would

be most cautious about the

possibility of having two

hominins in Laetoli,” he says.  ❚

Human evolution

Michael Marshall

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Fossil footprints hint at mystery

hominin with unusual walking style

A footprint left by a

hominin 3.66 million

years ago

Astrophysics

POWERFUL outbursts of energy

from young sun-like stars could

be central to how they and their

planetary systems form, according

to a modelling study. The results

hint at a possible explanation for a

decades-old astrophysical mystery.

One idea for how stars like our

own are born – the steady-state

concept – makes predictions about

how bright young stars should be

when they are under construction,

continuously gathering gas and

dust. Yet when we look into the

Milky Way, these objects aren’t

anywhere near that bright and don’t

appear to grow quickly enough. This

is called the luminosity problem.

A competing idea examined

in the new research – the outburst

model – suggests that nascent suns

brighten dramatically during flurries

of feeding as they accumulate, or

accrete, material. “Outbursts solve

the luminosity problem by providing

episodes of much higher accretion,”

says Jason Steffen at the University

of Nevada, Las Vegas, one of the

researchers behind the work.

Because this type of episode is

transitory, that would help explain

why we don’t see young stars

blazing brightly all the time.

As part of the study, the team

modelled the physical properties

and composition of a disc of

material around a young sun that

had undergone outbursts. This

tallied better with observations

than the steady-state theory.

For example, the outburst model

more closely replicated the mix of

ingredients measured in primitive

meteorites from our solar system,

known as carbonaceous chondrites

(arxiv.org/abs/2111.03798).

The team also suggests that

heating from outbursts by the infant

sun could shed light on the origins

of features in some meteorites

called calcium-aluminium-rich

inclusions, which appear to have

been forged at high temperatures. ❚

New look at infant

suns may solve

galactic mystery

Will Gater

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A star-forming

cloud captured

by NASA’s Spitzer

Space Telescope

5

footprints were left by a mysterious

hominin at a site in Tanzania