Past climate change - Climate Change: Biological and Human Aspects - page 66

contemplated that life arose on Mars, which had a far more benign environment at that
time and presented a small target for asteroids, as well as having a lower gravitational
well attracting them. Indeed, even if Mars had had an impact large enough to affect
life (which it frequently did) the early Martian seas were so much shallower than
Earth's that the damage would have been less, with the resulting steam condensing
out in just decades. Life would have had a better chance of arising on Mars first
and then hitching a ride to Earth; ejecta from an asteroid impact on Mars could have
carried life to seed Earth. (Again, Mars' shallower gravitational well would have more
easily facilitated material leaving it, compared with the Earth's.) Conversely, others
engaged in this scientific whimsy argue that as we know that life got off the mark
quickly in geological terms, so the Martian hypothesis (albeit theoretically possible)
is not necessary, and in any case would only have given a few hundred million years'
(about half a billion years at the most) advantage over any life arising on Earth.
Furthermore, life needed to arise somewhere, be it Mars or Earth, and so the point
is moot (unless one considers a non-planetary origin for life such as panspermia
theories). Either way, as we shall see, consideration of the climate in which life arose
remains central to discussion of its immediate subsequent evolution.
The exact temperature of the environment in which early live arose is not known,
other than it was much warmer than the temperature of tropical seas. In 2006 French
researchers Fran¸ois Robert and Marc Chaussidon, using 18 O/ 16 O and 30 Si/ 28 Si iso-
tope analysis of cherts (siliceous geological strata often including the remains of
siliceous organisms), found that the ambient temperature of sea water then was
roughly 70 ◦ C and that this temperature cooled to around 20 ◦ C about 800 mya. So
it does seem as though there was a time when sea water was around 70 ◦ C, and this
fits with the Boussau team's idea that there was an early life phase living in an envir-
onment of 50-80 ◦ C. Nonetheless, the evidence is being challenged - as in all good
science - and work to develop a firm picture continues. For example, one challenge
came in 2009 when the US team of Hren, Tice and Chamberlain used a combination
of deuterium and oxygen isotopes to analyse South African Buck Reef Cherts from
3.42 bya in an attempt to get around some of the assumptions made by many earlier
workers (including that the early Earth was essentially ice-free). They concluded that
the sea temperature was 40 ◦ C or less. I mention this because there is (rightly) debate
in the literature, although of course it is important not to make the 'It rained on
Mongo' mistake 2 of assuming that the sea temperature everywhere in the primordial
Earth was the same.
What is known is that our RNA/DNA bioclade arose in a biosphere very different
from the one we know today. As stated above, 3.8 bya the Sun was dimmer than it
is today: this is an important point, as atmospheric greenhouse gases have played a
major role in helping to stabilise the planet's surface temperature within a window
enabling life to survive if not flourish. If the Sun was dimmer then something had
to be making the early seas warmer than today for thermo- and mesophilic (warmth-
loving) early organisms to thrive. (Even if the very early Earth had a thinner crust
and bombardment, which would have caused warming, there were still billions of

2

This is writer and space engineer Jerry Pournelle's way of explaining the oversimplification that science
fiction authors sometimes make of treating planets where local or regional conditions apply as happening
to the whole world. It is a lesson to be kept in mind when considering climate and environmental proxies.