14 | New Scientist | 27 February 2021
THERE are microbes near the
bottom of the third deepest hole
in the world. The cells, recovered
from rocks almost 5 kilometres
below the surface in China, are the
deepest so far found on land – and
they may extend the known heat
tolerances of life on Earth.
Until now, the deepest known
microbes on land were nematode
worms found 3.6 kilometres
below the surface in a South
African gold mine.
A team led by Hailiang Dong
at the China University of
Geosciences and Li Huang at the
Chinese Academy of Sciences has
now discovered bacterial cells at
greater depths. The team studied
rocks from a 5.1-kilometre-deep
borehole in eastern China, made
as part of the Chinese Continental
Scientific Drilling (CCSD) project.
Microscopic analysis confirmed
the presence of cells in CCSD rocksamples extracted from a depth
of 4.85 kilometres. The team also
recovered bacterial DNA from
rocks at this depth.
Dong and Huang say that, to
the best of their knowledge, these
are the deepest known microbes
ever found on land (Geobiology,
doi.org/fwdx). Demonstrating
that the cells are living will be a
challenge as microbes that live
deep below the surface often
operate on such a slow timescale
that they show few typical signs
of life such as movement or
reproduction.
But there are reasons to suspect
the microbes may be alive. Most
importantly, they are intact rather
than just cell fragments, which
might hint they are carrying out
basic cellular repair.
The researchers think microbes
may be found at greater depths
elsewhere in the world. Theysuspect that conditions in the
CCSD borehole become too hot at
4.85 kilometres for life to survive
at deeper levels. But in regions
where the local geology means
temperature rises more slowly
with depth, life might survive
several kilometres deeper.“The research community is
beginning to consider that the
limits are actually down to a
complex interplay between a
variety of parameters including
temperature, pressure and the
physical nature of the rocks – their
porosity, for instance, and the
water penetrating the system,”
says Barbara Sherwood Lollar at
the University of Toronto, Canada.Regardless of whether these
microbes have hit a thermal limit,
confirming living microbes that
deep would be significant. At 4.
kilometres down in the CCSD hole,
the temperature is about 137°C, far
above 122°C, the threshold above
which no known organism can
survive. Dong and Huang say
theoretical calculations suggest
life might be possible up to 150°C.
“I don’t think any of us would
be hugely surprised if there’s an
organism that grows at 130°C or at
135°C,” says Sean McMahon at the
University of Edinburgh, UK. Such
a microbe would widen the search
for alien life. “Astrobiology defines
habitability as an environment
that can support the growth of at
least one known organism,” says
McMahon. Finding an organism
that grows at higher temperatures
means our definition immediately
changes, he says. ❚News
“The limits on life are down
to a complex interplay
between temperature,
pressure and other factors”A LIFE-SIZE kangaroo painted in red
ochre around 17,300 years ago is
Australia’s oldest known rock art,
indicating that early artists in the
country focused on animals.
Rock art sites are found all over
Australia, with the Kimberley region
of Western Australia containing a
particularly rich record. But dating
the images is challenging without
the minerals or organic material
used to determine an artwork’s age.
Now, Damien Finch at the
University of Melbourne and his
colleagues have dated the images
in eight rock shelters in Balanggarra
Country, in the north-eastern
Kimberley region. Finch and his
colleagues worked with theBalanggarra Aboriginal Corporation,
which represents the Traditional
Owners of the land. Members of
the corporation reviewed the
team’s paper.
Finch’s team dated the images by
measuring the radiocarbon signal
from ancient wasp nests that liebeneath and on top of the artwork.
They discovered that a kangaroo
image (pictured) on the ceiling of a
rock shelter containing thousands
of ancient mud wasp nests was
painted between 17,500 and
17,100 years ago. “Wasps have
been building nests at this siteconsistently for 20,000 years“, says
Finch (Nature Human Behaviour,
doi.org/fwsg).
“The dating of this oldest known
painting in an Australian rock
shelter holds a great deal of
significance for Aboriginal people
and Australians and is an important
part of Australia’s history,” said
Cissy Gore-Birch, chair of the
Balanggarra Aboriginal Corporation,
in a media statement.
Analysis of other images in the
rock shelters showed that these
naturalistic animal paintings were
common until 13,000 years ago.
Younger art from elsewhere in
Australia predominantly features
images of humans. ❚
Alison GeorgeColin BarrasMicrobiologyDeepest land microbes ever found
Intact cells discovered near the bottom of a 5-kilometre hole in China
This image of a kangaroo is
the oldest of its kind so far
discovered in AustraliaArchaeologyAustralian rock
painting is at least
17,000 years oldDAMI
EN
FINCH