The Economist May 28th 2022 Science & technology 73class of biosignatures would be “indirect
proxies for a living organism”.
One example would be to search for gra
dients in an environment—zones of sharp
change in, for example, heat or electrical
voltage or chemicals. According to Dr Gir
guis, “all living organisms that we know of
establish gradients of one kind or another
to maintain themselves at a kind of dis
equilibrium from the environment.”
Some of these gradients occur at cellu
lar and microscopic scales, and can be in
credibly sharp and therefore distinguish
able from nonbiological processes. Others
are largerscale. In marine sediments on
Earth, for example, microbes work togeth
er to oxidise methane, a process tied to the
chemical reduction of sulphate ions. “We
see gradients in methane and sulphate
concentration over centimetres, and
they’re really pronounced,” says Dr Gir
guis. “This is a biological manifestation of
their activity and yet this is detectable by
simply making abiotic measurements.”
Another tactic would be to study the
complexity of the molecules at a particular
location. Biological molecules are selected
and shaped by evolution to do specific jobs
within an organism, such as assembling or
disassembling other molecules, or signal
ling between cells. That often requires
unusually energetic chemical processes,
which in turn need the help of catalysts.
On Earth, these catalysts are protein mole
cules called enzymes which are, them
selves, the product of evolution. Finding
complex molecules of any sort might thus
be considered a potential biosignature.
A related concept is what Chris McKay, a
planetary scientist at nasaAmes Research
Centre, calls the “Lego principle”. The idea
here is that life is recognisable by its use
and reuse of a selected set of molecules.
Abiotic samples scooped up from an alien
world would be expected to contain a wide
array of organic molecules, some of them
in fairly small amounts. A biological sam
ple, by contrast, would contain large num
bers of just a few distinctive molecules.
Molecules that are chemically similar (left
handed and righthanded versions of an
amino acid, for example) might have mark
edly different concentrations if they came
from a biological sample, whereas they
would probably be present in nearequal
numbers in a nonbiological one. Spotting
patterns like these would be independent
of the specific biochemistry involved.
The past as a clue to the present
Such methods would widen the astrobio
logical search wherever it was possible to
obtain a sample—in other words any world
in the solar system to which researchers
can send a probe—and apply to it tools
such as miniaturised, spacehardened
mass spectrometers. For planets going
around other stars, though, things are ob
viously trickier. Few people think human
beings or their machines will visit any of
the rapidly expanding population of these
exoplanets anytime soon. Astrobiologists
are instead considering other ways to
search for new agnostic biosignatures. Mi
chael Wong, an astrobiologist at the Carne
gie Institution for Science, in Washington,
dc, presented a technique that applies
what is known as network science to data
about exoplanets’ atmospheres. These data
can be gathered using telescopes on, or or
biting, Earth.
Any chemical system, the chemicals
within an atmosphere included, can be
represented by a socalled network dia
gram, in which molecules that react with
each other in some way are connected by
lines. Dr Wong showed that, when com
pared with those of other planets in the so
lar system, Earth’s atmospheric network
stands out like a sore thumb. In fact Earth’s
network more closely resembles those of
biological systems, such as marine food
webs.Thistechniqueisa workinprogress
andDrWongsaiditwouldneeda lotmore
developmentbeforeastrobiologistscouldinclude it in their lifedetection toolkit.
But it is an intriguing approach.
Dr Girguis told the meeting that future
searches for exotic life in the universe
would do well to learn from mistakes made
by explorers searching for life in Earth’s
oceans in the 19th century. In one expedi
tion, for example, Edward Forbes, a promi
nent naturalist from the Isle of Man, was
dredging in the Aegean Sea. He noticed
that the farther plants and animals were
from the water’s surface, the less well they
thrived. In 1843 he extrapolated his incom
plete data to propose his azoic hypothesis,
which stated that life would not exist at all
below 550 metres.
It took several decades to prove him
wrong, an effort that involved some of the
first scientific endeavours designed to ex
plore the deep ocean—such as the Chal-
lengerexpedition that sailed from 1872 to1876. These, said Dr Girguis, were some of
humanity’s earliest lifedetection mis
sions. “Let’s not be too quick to extrapo
late,”hewarnedhisfellowastrobiologists.
“Andlet’sneverunderestimate the capaci
tyoflivingorganisms.”n
GenebanksA close-run thing
I
t was amoment of horror. In a video
posted on the internet on May 14th Ser
gey Avramenko, a researcher at the Nation
al Gene Bank of Plants of Ukraine, the
world’s tenthlargest such facility, ran his
fingers through bags of charred seeds.
“Everything turned to ashes,” he grieved. It later emerged that only an outpost of
the bank had suffered the shelling which
caused this destruction. The main trove of
seeds remains safe in an underground
vault. But it may have been a closerun
thing. The bank in question is in Kharkiv,
Ukraine’s second city, and that city’s de
fenders have only now repelled the Rus
sian forces which were besieging it.
The Kharkiv gene bank’s precarious sit
uation underscores the importance of pro
tecting and conserving genetic material
from crops, as climate change and a grow
ing, prospering human population drive
demand for novel approaches to plant
breeding. It started as an experimental sta
tion in 1908 and is now one of more than
1,700 such repositories around the world.
The purpose of gene banks is to archive
crop biodiversity. Mostly, this is done by
dehydrating and freezing seeds. The un’s
Food and Agriculture Organisation esti
mates that, over the 20th century, the di
versity of planted crops shrank by 75% as
commercial farmers concentrated their ef
forts on a few reliable varieties. But the va
rieties abandoned as a consequence may
still conceal valuable properties, and mod
ern genetic techniques, such as genomeUkraine’s agricultural-research establishment is threatened by the warAlso good for towing tanks