Past climate change - Climate Change: Biological and Human Aspects - page 65

theoretical temperature-sensitive proteins that early bacteria may have used, suggest-
ing an environment of around 65-73 ◦ C and that this bacterial environment cooled
over the next billion years or so to 51-53 ◦ C. Then a French and French Canadian
team led by Bastien Boussau looked at ribosomal RNA and protein sequences (as did
Gaucher and colleagues) from evolutionarily ancient but extant bacteria and, seeing
how these differed between species, created a model taking evolution backwards
and deducing likely early proteins (Boussau et al., 2008). This work suggests that
the earliest forms of living bacterial cells may have thrived in an 'optimal growth
temperature' of up to 50 ◦ C. A second thermophilic phase of evolution then may have
taken place in which the precursors to today's bacteria and then later Archaea and
Eukaryota thrived in a thermophilic environment of 50-80 ◦ C. This tentative theory
has the benefit of reconciling previous, seemingly contradictory, views.
Of relevance to the question of biology and climate change, temperature appears
to be a factor in life arising in the first place. Having said that, the common notion
that life began around deep-sea hydrothermal vents is not as cut and dried as much
popular science would have it. If many parts (if not most) of the Earth's seas and
oceans were far warmer than today then this would have increased the chance of life
arising compared to localised hydrothermals. Furthermore, with the thin crust of the
early Earth, there were many locations other than deep-sea hydrothermal vents where
magma was close to and in contact with sea water. Having said that, it is important
to be aware of nuance. There is a reasonable case that serpentinite hydrothermal
vents provided a chemical energy source for proto-life and/or early life as well as
iron-, nickel- and sulphur-bearing minerals of catalytic value that early life could
have exploited. (Serpentine is a magnesium siliceous mineral mix with, among other
things, magnesium and iron hydroxides, and which is formed by the reaction of
olivine-rich rocks with its magnesium and iron silicates.) This vent environment, in
addition to being contained, had a pH gradient that could have been exploited by
life for energy (Sleep et al., 2011). Even so, it is a leap to say that this environment
is where life actually started and even if hydrothermal vents are required for the
chemical energy gradient they may not have been deep-sea vents. Today we see
Archaea around deep-sea hydrothermals possibly because this is the refuge to which
they retreated as the Earth cooled: it is not because they necessarily evolved there.
It is not known whether life based on RNA and DNA (and its RNA world precursor)
was the very first bioclade 1 ; it may well have been that some other system did arise
first (with a preference for 12 C that we may even eventually isotopically detect), but
if it did this non-RNA/-DNA life failed to become established. Equally, it may be
(we simply do not know) that large impacts capable of sterilising large areas of the
Earth's surface, and even the smaller impacts that continued up to 3.8 bya, made the
surface too perilous for sustained life, a view that was firmer prior to the work of
Oleg Abramov and Stephen Mojzsis (2009) mentioned above. Conversely, microbes
sheltering deep in the Earth's crust would have survived: perhaps this is why the genes
of living microbes suggest that the ancestor of all life had much in common with
the hyperthermophiles that thrive in hot springs of 113 ◦ C or more. It has even been

1

A bioclade is a related ensemble of life arising from a common ancestor. As noted in the text, the accepted
view is that life on Earth arose once from a single common ancestor (although simple pre-biotic chemical
precursors could have formed independently many times).