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TRACES of methane-eating bacteria that lived in a pond over 2.7 billion years
ago may help to explain the puzzle of how the early Earth kept warm. The
discovery suggests that methane, a powerful greenhouse gas, was abundant at that
time and could have kept the Earth from freezing.

About 2.8 billion years ago the Sun was 20 per cent fainter than it is now.
With today’s atmosphere, that would have left the Earth’s average surface
temperature below freezing, says climate modeller Jim Kasting of Pennsylvania
State University in University Park. But we know that liquid water was present
then, so something else must have been warming the planet. This enigma has been
dubbed the “faint young Sun paradox”.

Today’s most important greenhouse gases are carbon dioxide and water vapour.
But ancient soils show that the air contained too little CO₂ to have
kept the planet from freezing, says Rob Rye of the California Institute of
Technology. So attention has turned to methane.

Last month, Kasting showed that 100 parts per million of methane in the
atmosphere could have done the trick. Now Rye and Heinrich Holland of Harvard
University have found organic carbon trapped in ancient soil from Mount Roe in
Western Australia, which indicates that methane was present 2.76 billion years
ago.

The carbon contains unusually low levels of carbon-13, a stable isotope that
is depleted in living cells. Rye says that the low level is the hallmark of two
successive depletions: the first by methane-producing bacteria, the second by
methanotrophic bacteria, which gobble up the gas in the presence of oxygen.
Methane producers “were probably some of the very earliest organisms”, says
Kasting.

The carbon cycle began shifting away from a methane-rich atmosphere after
photosynthesis evolved, says John Hayes of the Woods Hole Oceanographic
Institution. However, methane and oxygen could not both have come from the air,
because they would have reacted. Rye believes that photosynthetic bacteria in
the water supplied the oxygen, while the carbon-13 isotope levels suggest that
the methanotrophs were feeding off an atmosphere with at least 20 parts per
million of methane.

Other studies of ancient soils support the idea that the atmosphere contained
very little oxygen at the time. If methane was being produced at present levels
and released into an oxygen-free atmosphere, Kasting says, methane levels of 100
to 1000 parts per million are plausible. That would have kept the early Earth
warm.

As photosynthetic bacteria continued producing oxygen, methane concentrations
gradually fell, eventually triggering the first widespread glaciation 2.4 to 2.3
billion years ago, Rye believes. The Earth then thawed as increasing oxygen
levels caused more oxidation, raising CO₂ levels and strengthening the
greenhouse effect.