As recently as 600 million years ago, oxygen levels were extremely low on Earth, only about a tenth of what they are today. So how did ancient animals move around and avoid mass asphyxiation?
We're not talking about very big animals, of course - just tiny organisms that made small burrows into rocks during the Ediacaran Period some 600 million years ago. But even that would have required lots of oxygen, roughly the same amount that we have in the air and oceans today. At the time, the oceans only had about 10% of their current oxygen supply, which should have meant any organisms living within it would have been utterly incapable of energetic activity.
To figure out what was going on, researcher Murray Gingras of Edmonton's University of Alberta looked to these ancient oceans' modern-day equivalent, seeking out low oxygen lagoons in the islands of the coast of Venezuela. Throughout most of these largely oxygen-free waters, there was no life at all. But in a few select locations, there was about a fourfold oxygen spike, and that increase was just enough to let worms and larvae live a relatively energetic existence.
The oxygen was coming from tiny microbial mats, and Gingras says there's good evidence that the 600 million year old rocks show fossil evidence of the very oxygen-producing structures. We can then imagine these microbial mats as life-giving oases in the middle of a vast, ancient oxygen-free desert. In much the same way that a few isolated sources of water can help animals survive in modern deserts like the Sahara, these microbial mats allowed these early animals to get much-needed energy.
And yet, as Ediacaran Period expert Jim Gehling points out, this would all make for a very tenuous existence. While he praises Giungras's research, Gehling says this would mean that these creatures would be risk of suffocation every single night, as the microbes would stop photosynthesizing whenever the Sun set, putting them at renewed risk of asphyxiation.