Life of the Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 130

after the catastrophic impact of Theia (see Chapter 2), and an estimate
of geothermal gradients in early continental and oceanic crust, sug-
gest that such habitats existed on Earth as early as 4.4 billion years
ago, less than 200 million years after the formation of the Earth. The
first surface-dwelling organisms may, therefore, have had ancestors in
the underworld: chemoautotrophs subsisting on chemical energy
rather than photoautotrophs using the Sun's energy. Chemical sig-
nals from the isotopic ratios of oxygen and silicon preserved in
ancient marine silica deposits suggest that these subterranean micro-
organisms would have emerged on to a surface where temperatures
remained high compared to the present, and possibly as warm as 70
degrees Celsius. This notion of a warm early Archaean is also sup-
ported by evidence from the 'tree of life', in which more ancient
groups of organisms have proteins that are stable at higher tempera-
tures, consistent with a thermophilic 'heat-loving' setting for their
origins. Life at the surface could only really get going after the Late
Heavy Bombardment was complete, and it may have had to survive
intermittent periods of darkness caused by the continuing impacts of
large meteorites. Earth's atmosphere at this time contained no oxy-
gen, and there was no dissolved oxygen in the oceans either (Fig. 11).
Life then had to contend with higher surface temperatures than today,
and used metabolic pathways that did not involve free oxygen.
Life in a Noxious World
We can glimpse the ancient environments of Earth through the lives
of microbes whose ancestry stretches back some 4 billion years. The
Archaea include forms with an ability to tolerate temperatures as
warm as 120 degrees Celsius, acidity as low as pH 2, waters that are
supersaturated with salt, and the complete lack of oxygen. They
evolved on a warm Earth, where the surface temperatures may have
reached 70 degrees Celsius and the atmosphere was made of methane,