We may reasonably conclude that oxygen was a cornerstone of Precambrian evolution. While nobody would propose that oxygen itself stimulates evolution there can be little doubt that rising oxygen levels opened new horizons for Precambrian life. Not one important evolutionary step took place without an associated rise in oxygen, and no rise in oxygen was divorced from a rapid increase in biodiversity and in the complexity of life. Curiously, though, the major injections of oxygen were not brought about by biological innovations, as had been tacitly assumed for many years (with the exception of guts), but by non-biological factors, such as glaciers and plate tectonics.
Left to its own devices, life on Earth dawdled for billions of years. If the stimuli for change and evolution were little more than accidents of tectonics and glaciation, a quiet world untroubled by geological strife would almost certainly fail to accumulate much free oxygen. The Earth stagnated for two prolonged periods, which between them account for half its history. From 3.5 to 2.3 billion years ago, the world was dominated by bacteria. Then, after the violent upheavals of 2.3 to 2.0 billion years ago, another equilibrium was established, in which the oxygen levels remained between 5 and 18 per cent of present atmospheric levels. This new equilibrium stimulated a blossoming of genetic diversity among the early eukaryotes, but could not provide enough energy for the evolution of large animals. At such low oxygen levels, life is denied size and complexity; and without these, a brain is unthinkable.
The deadlock was broken by a second series of snowball Earths, which started 750 million years ago and catapulted oxygen to modern levels. Now the evolution of large animals was only a matter of time, and it didn’t take long. The Vendobionts, the Cambrians, the whole plethora of modern life, exploded into being in a period less than that taken up by the proceeding glaciations. If nothing else, this relationship between life and environmental conditions should sound a note of caution to those who seek intelligent life elsewhere in the Universe. We must look beyond the mere presence of water to the presence of volcanoes, plate tectonics and oxygen. Perhaps, if life once existed on Mars, it died out as the fires of vulcanism faded within.
Whether oxygen was linked with opportunities or extinctions in the modern age of plants and animals, the Phanerozoic, is a question for the next chapter. I can find no evidence to support the idea that free oxygen between our modern oxygen level of about 21 per cent, and the postulated Carboniferous high point of 35 per cent, around 300 million years ago. Our experience with diving gas mixtures alone suggests that prolonged exposure to high levels of oxygen can provoke lung damage, convulsions and sudden death, to say nothing of the raging infernos and stunted plant growth predicted by most biologists. Did oxygen really reach fever pitch? If so, how did life cope? And if life flourished, what does this say about our health today as we pop another multi-antioxidant pill to stem the ravages of ageing?
Oxygen: The Molecule That Made The World | Nick Lane; Image via fas.org