44 | NewScientist | 8 June 2013Ripping water apart to release oxygen is incredibly
hard, but we wouldn’t be here if life hadn’t learned
to do it. Colin Barras has the scoop
Dawn of the
T
HE planet is in crisis. The stench of
death is everywhere as whole branches
of the tree of life are pruned almost
to oblivion – and all because of the waste
gas pumped into the atmosphere by one
incredibly successful species. Welcome to
Earth, 2.4 billion years ago.
This was arguably the most tumultuous
episode in life’s history. It had been thriving
for well over a billion years when a new kind of
cell appeared on the scene, one that harvested
the sun’s energy using a process that generates
a highly toxic by-product – oxygen. These
cells were soon growing in such unimaginable
numbers in the primordial oceans that they
transformed Earth’s atmosphere.
At the time, this was a catastrophe. The
rise of oxygen may have wiped out a greater
proportion of life than in any other mass
extinction. But the very property that makes
oxygen so dangerous – its high reactivity – also
makes it a rich source of energy. Life soon
started to exploit this, including, of course,
our animal ancestors.
In the past decade, our view of this crucial
episode has been turned upside down. The
textbooks will tell you that oxygen levels
began climbing soon after photosynthesis
evolved, but we now know that some cellsstarted photosynthesising as long as 3.4 billion
years ago, long before oxygen levels began to
rise. The question is, why did it take so long for
them to start pumping out oxygen?
At its heart, photosynthesis is about
harvesting the sun’s energy. Plants use this
energy to make food, by building chains of
carbon from carbon dioxide. The process
produces sugars that can be used as an energy
source or to make more complex molecules,
from proteins to DNA. But contrary to what
you might expect, it does not necessarily
produce oxygen. In fact many bacteria turn
light and CO 2 into food without producing
oxygen. What’s more, recent discoveries
suggest they have been doing so for nearly
as long as there has been life on Earth.
In 2004, Michael Tice and Donald Lowe,
both then at Stanford University in California,
were studying rocks in South Africa that
formed in shallow water 3.41 billion years ago.
They found fossil structures rather like the
microbial mats formed by photosynthetic
bacteria today, but no sign that any oxygen
was produced (Nature, vol 431, p 549). The
most likely explanation, they think, is that
these cells were carrying out anoxygenic
photosynthesis.
Since that discovery we have actually comewater eaters
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