16 November 2019 | New Scientist | 43

of all: that they be capable of generating energy

from their environment and putting it to use.

Organisms on Earth do this in three

main ways. “You eat light, you eat organic

matter or you eat rocks,” says Jennifer

Macalady, an astrobiologist at Pennsylvania

State University and leader of the Frasassi

expedition. Plants and other organisms that

photosynthesise are powered by the sun,

while others get their energy from chemicals

stored in other life forms or within the

geology of our planet. “Piece by piece, we’ve

uncovered all sorts of amazing metabolisms

that we didn’t even imagine,” says Penelope

Boston, director of NASA’s Astrobiology

Institute in California.

These diverse metabolisms almost all

rely on the same fundamental chemistry

to generate energy. In each case, the power

comes from the transfer of a single electron

from one molecule to another. The molecule

that donates the electron is said to be oxidised,

while the electron receiver, paradoxically, is

said to be reduced. These processes, known

as redox reactions, can release energy and

bring stability to the system.

One of the factors that may explain the

importance of redox reactions to life is their

extreme sluggishness. Reactions that take

Stranger things

Could we be about to redefine life?

Donna Lu joins an expedition hunting for

alien creatures deep beneath Earth’s surface

I

T’S the smell of science!” says Heidi

Aronson, her face dimly lit by the beam

of her head torch. In that case, science

smells like an egg sandwich that’s been left

out past its use-by date and then rolled in mud.

Some 300 metres above our heads is a

bucolic Italian landscape of rolling sunflower

fields, Verdicchio wineries and winding

mountain roads. Here in the Frasassi caves,

the air reeks of hydrogen sulphide and the

walls are slimy with slow-growing microbial

deposits. My mind keeps drifting upwards,

but there is nowhere Aronson, a researcher

at the University of Southern California,

would rather be. Down here, far from the

light, she is hunting aliens.

Virtually anywhere you look on Earth,

you find life. It can be in sites dominated

by heavy metals that are toxic to humans or

on the plateau of the Atacama desert, where

soils are so dry they are Mars-like. It can be

found feeding on nuclear waste, as well as

at both extremes of temperature and pH.

But if Aronson is right, then the Frasassi

system could be crawling with life unlike

anything we’ve ever seen: microbes that gulp

down sulphur compounds the way we breathe

air. These would be evidence of a biology

radically different from all other life on Earth.

Such a discovery would have dramatic

consequences. An organism capable of

generating energy in this way would not

only shed light on the origins of life on our

own planet, it could also hint at the nature

of life elsewhere in the universe. To find

this new life form, she just needs to follow

the sulphurous stench.

The diversity of life on Earth is astonishing.

You don’t need a brain to be alive, or a heart,

or a spine. You can survive without oxygen,

without sunlight, even without two cells to

rub together. You can live without feeling

the effects of ageing, or in total isolation, or

through centuries of hibernation. As soon

as scientists come up with a definition of

what makes a living creature, it seems that

something comes along to challenge it.

What is life, anyway?

But in the absence of a robust definition, there

are a few things that most biologists agree

all life forms must have. They should all be

capable of passing on their genetic material

to a new generation of organisms through

reproduction, as well as being able to grow and

shed waste. But none of this would be possible

without the most fundamental requirement >

Features

“