Summoned by the Sun
an excerpt from Lane, Nick (2009)
Life Ascending: the Ten
Great Inventions of Evolution
magine a world without photosynthesis.
It wouldn't be green, for a start.
planet reflects the glory of plants and algae, and ultimately their green pigments, which
absorb light for photosynthesis.
First among pigments is the marvelous transducer that is
chlorophyll, which steals a beam of light and conjures it into a quantum of chemical
energy, driving the lives of both plants and animals.
The world probably wouldn't be blue either, for the azures of the heavens and the
marines of the oceans depend on clear skies and waters, cleansed of their haze and dust
by the scouring power of oxygen.
And without photosynthesis there would be no free
In fact there might not be any oceans either.
Without oxygen there is no ozone; and
without that, there is little to cut down the searing intensity of ultraviolet rays.
split water into oxygen and hydrogen.
The oxygen is formed slowly and never builds up
in the air; instead it reacts with iron in the rocks, turning them a rusty-red colour.
hydrogen, the lightest of gases, evades the tug of gravity and slips away into space.
process may be slow but it is also inexorable: the oceans bleed into space.
radiation cost Venus its oceans, and maybe Mars too.
So we don't need much imagination to picture a world without photosynthesis: it
would look a lot like Mars, a red dusty place, without oceans, and without any overt signs
Of course, there is life without photosynthesis, and many astrobiologists seek it
But even if a few bacteria are found hiding beneath the surface, or buried in an
icecap, the planet itself is dead.
It is in near-perfect equilibrium, a sure sign of inertia.
could never be mistaken for Gaia.
Oxygen is the key to planetary life.
No more than a waste product of photosynthesis,
oxygen really is the molecule that makes a world.
It is let loose by photosynthesis so fast
that it finally overwhelms the capacity of a planet to swallow it up.
In the end, all the
dust and all the iron in the rocks, all the sulphur in the seas and methane in the air,
anything that can be oxidised is oxidised, and free oxygen pours into the air and the
Once there, oxygen puts a stop to the loss of water from the planet.
when released from water, inevitably bumps into more oxygen before it finds its way out
Swiftly it reacts to form water again, which now rains back down from the
heavens, drawing to a halt the loss of the oceans.
And when oxygen accumulates in the
air, an ozone shield forms, ablating the searing intensity of the ultraviolet rays, and
making the world a more habitable place.
Oxygen doesn't just rescue a planet's life: it energises all [well, almost all - ed.] life,