40 | NewScientist | 8 September 2018that made methane, and oxygen levels were
at an all-time low. This was fortunate – if they
had been any higher, life as we know it might
not have emerged at all. “Oxygen poisons
some of the prebiotic chemistry that we think
culminated in the origin of life on Earth,”
says Stephanie Olson at the University of
California, Riverside.
A billion or so years later, the planet
was entering a time called the Proterozoic.
Photosynthesis was already well under way,
but it was at this time that the ability to convert
carbon dioxide into oxygen had lasting
consequences for the planet. For the first
time, oxygen, carbon dioxide and methane
coexisted in the atmosphere, leading to the
accelerated evolution of multicellular life.
During this time there were two “snowball
Earth” events, when the entire planet was
covered in ice. The trigger may have been
something as trivial as a brief drop in global
temperatures, allowing the polar ice caps
to expand. This reflected more sunlight,
reducing temperatures even more in a
feedback loop that eventually froze the whole
planet. The trouble is, we don’t know how
freezing over would have affected the chance
of spotting life from afar. “The composition
of the atmosphere during the snowball Earth
events is actually not terribly well known,”
says Reinhard.
The Proterozoic’s defining feature, however,was a billion-year period of apparent stability
from 1.8 billion years to 800 million years ago.
After the frenzied emergence of multicellular
life, and the chaos of the first snowball Earth
event, the planet relished the opportunity to
take a breather. The climate was constant, life
appeared to be evolving very slowly, if at all,
and oxygen remained at low levels. Little
wonder that geologists have taken to calling
this the “boring billion”.Oxygen masked
Some think that name is a tad unfair. After
all, this was when sexual reproduction first
evolved, and when the first eukaryotes –
organisms with complex cells that ultimately
gave rise to ourselves – appeared, “which
is a big deal”, says Nick Butterfield at the
University of Cambridge. From the
perspective of an alien Galileo space probe,
however, little would appear to change.
By the time the boring billion ended, life
was really stepping on the accelerator pedal.
At this time, called the Phanerozoic, the
diversity of life forms skyrocketed in what is
known as the Cambrian explosion, oxygen
finally reached levels that would be remotely
detectable and plants began to flourish on
the planet’s surface. This is when the red
edge would have first been visible.
This complicated history offers a stark
lesson for those hunting exoplanets based on
Earth’s current appearance. “For about four-
fifths of Earth’s history we would not be able
to see evidence of life on the surface,” says
Reinhard. Whether searching for high oxygen
levels, oxygen-methane coexistence or a red
edge, you would mostly come up empty-
handed. “The obvious candidates for
biosignatures aren’t going to work as well
as we thought,” says Olson.
Part of the problem is how little we know
about early Earth. It is no exaggeration to
say that, in some ways, we will soon know
more about planets billions of kilometres
away than we do about our own world
billions of years in the past. With so little
surviving evidence – and each piece so open
to interpretation – reconstructing Earth’s
history is a major challenge. Until the gaps
are filled in, what life-hunting astronomers
need is a broader-brush way of figuring out
which planets to investigate further.
For Enric Pallé at the Institute of
Astrophysics of the Canary Islands, one
promising avenue is the ancient equivalent of
red edge. Ever since the days of the Archaean,
long before the continents became hotbedsThe emergence of vegetation dramatically
changed Earth’s appearance from spaceVITALIJ CEREPOK/EYEEM/GETTY“ For about four-fifths
of Earth’s history, we
would not have been
able to see evidence
of life as we know it on
the surface”