Oxygen's Impact on Evolution

Did a rise in oxygen levels in the atmosphere lead to the rise of complex life forms? The question doesn’t make sense at first. We know life adapts to whatever is available. By that token, oxygen levels can at most be the backdrop for however life evolved, but surely they cannot be called a causative factor…

 

In case of oxygen though, things are very different, writes Nick Lane in his book named (what else?) Oxygen:

“The most obvious basis for a causal link is energy production.”

Huh?

“Oxygen releases more energy from food than do sulphur, nitrogen or oxygen compounds acting as oxidants… The length of any food chain is determined by the amount of energy lost from one level of the chain to the next.”

 

Without oxygen, that efficiency is less than 10%. Put differently, it means each step of the chain gets only 10% of the previous step. Here is the key point:

“Below a 1% threshold, there is not enough energy available to eke out a living.”

Do the maths, and you see that food chains can’t exist in the absence of oxygen:

“(Life during low oxygen levels would) compete with each other for scarce resources, rather than ‘eating’ each other.”

 

But once the oxygen level in the atmosphere went up, respiration as the way to extract energy from food becomes possible. And the energy efficiency of oxygen based respiration is 40% at each step. Do the maths, and up to 6 levels in a food chain are now sustainable:

“Suddenly carnivorous food chains pay and the predator is born.”

 

The advent of the carnivore then sets off the next set of events in the complexity of life forms. How?

“Predation is a powerful stimulus to weight gain in both predators and prey, either to eat larger prey or to avoid being eaten.”

Which in turn creates other evolutionary pressures:

“Large size requires structural support.”

And those structural elements have to be built using what’s available:

“The two most important structural components of plants and animals, lignin and collagen, require oxygen for their synthesis.”

 

Oh, and one last thing about the effect of rising oxygen levels on life. Some of that oxygen formed the ozone layer:

“Ozone is good at absorbing ultraviolet rays, so once a thick ozone layer builds up, the penetration of damaging ultraviolet rays into the lower atmosphere is cut by a factor of 30 or more.”

 

Who’d have thought one molecule would have such a big impact on life on earth?