By about two billion years ago, Oxygen had risen to a level as high as four percent of the atmosphere. This was enough Oxygen for the earliest Eukaryotic cells to begin to thrive, but not yet enough for any large multi-cellular organisms.
In OXYGEN: The Molecule That Made The World, author Nick Lane says that, long before plants and animals split from each other to form two of the main branches of the Tree Of Life, Eukaryotic cells very early-on absorbed into themselves a strain of Purple Bacteria that would eventually become the Mitochondria which today live inside our cells. And not just human cells: “All true multi-cellular organisms contain Mitochondria,” says Lane.
There is some debate about what advantage Mitochondria gave our little ancestors. Lane thinks that maybe early Eukaryotes used Mitochondria as a way of combating the rising levels of Oxygen in the atmosphere. At that early stage of evolution, even a tiny amount of Oxygen would have proven toxic to cells— cells as of yet unevolved to cope with the air’s increasing level of Oxygen. What the Mitochondria Bacteria received from their new host cells is easier to explain: living inside a Eukaryotic cell, a lucky little Purple Bacteria would have been tucked away from many of the dangers of the harsh outside environment.
Over the eons, Mitochondria and their host Eukaryotes have evolved into an interwoven dependency. Even though the Mitochondria have kept their own bacterial DNA and still replicate themselves by asexual fission inside our cells (just as their bacterial ancestors did), our own cellular DNA now provides the Mitochondria with many of the component parts they need to carry on their work. Which, believe me, is no burden for us since it is the Mitochondria that provide us with most of the energy we need to keep our bodies functioning.
So, with so much multi-cellular, Mitochondria-super-powered plants producing tons of Oxygen everyday… what was to stop these new life-forms from producing too much of a good thing? Why did the Earth not become a world poisoned by an overabundance of Oxygen?
Some scientists believe that there existed no environmental mechanism to check the production of Oxygen in the early days, and that the Earth was indeed visited by an “Oxygen holocaust” which wiped out much of the life on the planet in mass extinctions. However, Lane does not believe an Oxygen holocaust ever actually occurred.
According to Lane, when Oxygen in the atmosphere gets too high, plant growth is suppressed, thus reducing the amount of photosynthesis taking place, thus bringing back down levels of Oxygen. Is he right?
Scientists have several ways of reading the past from the present. Lane mentions two ways particularly that we can use to determine if there was ever an Oxygen holocaust.
The first way is to take a look at the Carbon signature left in rocks over geological time. If an Oxygen holocaust had ever actually occurred and most of the world’s plants had been knocked out, then we should see evidence of this in the story of the stone.
Photosynthesis distorts the natural ratio between two different isotopes of Carbon: Carbon-12 and Carbon-13. Photosynthesizers prefer to use Carbon-12 because its chemical bonds are weaker and easier to break. This means that organic matter is enriched with Carbon-12 compared to Carbon-13. This in turn means that there is less Carbon-12 available to incorporate into the formation of Carbonate rocks. Therefore, when there is abundant Life present on Earth, Carbonate rocks are heavy-loaded with the other isotope, Carbon-13.
If Life had suffered severely due to an Oxygen holocaust (or due to any catastrophic event), we could read of event in the story of the stone: we’d see more Carbon-12 in the Limestone and such. However, as Lane tells us, for the period in which the Oxygen holocaust is thought by some to have occurred, there is no sign at all that it or anything like it occurred.
The dismissal of the Oxygen holocaust is backed up by another way of reading the past from rocks. If most of Earth’s plant-life had died out from over-abundant Oxygen, the rate of erosion from land into the oceans would have significantly increased. If this had occurred, the Carbonate rocks of the oceans would have become richer in Continent-based materials (as opposed to Oceanfloor-based materials). The prime example here is Strontium, which comes often in the form of Strontium-86 on the Continents and in the form of Strontium-87 at the Ocean floor.
When the story of the stone is examined for the relevant period, no evidence has been found of any greatly increased Continental erosion… further evidence that no Oxygen holocaust ever occurred.