The amount of oxygen in the atmosphere has fluctuated a billion years ago. Researchers believe that this fluctuating oxygen level may greatly speed up the evolution of the first animal life.
The first atmosphere, which formed 4.5 billion years ago after the formation of the Earth, contains hardly any molecular oxygen (O2). Today, more than 20% of the atmosphere is made up of this gas, which we can get out of in less than ten minutes. This current amount of oxygen may have arisen in three steps.
The first time the atmosphere was enriched with molecular oxygen was during the oxygen crisis over two billion years ago. Perhaps it was the blue-green algae (or cyanobacteria) that produced this oxygen. This caused a mass extinction of anaerobic microbes, for which oxygen is toxic.
Food lessons from space
The last step, which raised the oxygen level to 20 percent today, occurred about 400 million years ago. At this time, the first wild plants appeared. It is still unclear what exactly happened to the level of oxygen during the second step, during an era called the Neoproterozoic.
In any case, it was an exciting period for life on Earth. At that time, about 1 billion to 500 million years ago, the first animal life forms appeared. The remains of the oldest known animals were found in the rocks from 630 to 642 million years ago.
To find out if and how the level of oxygen during that period affected the evolution of these first animals, researchers in the UK examined limestone from that period. Using carbon-14 dating, which measures the ratio between different forms (isotopes) of carbon, they were able to determine the extent of photosynthesis. Since photosynthesis is the main source of oxygen in the atmosphere, they were able to derive the oxygen level from it. For example, they have mapped the level of oxygen in the atmosphere over the past 1.5 billion years.
“Our research shows that oxygen levels were much more dynamic than previously thought,” said Alex Krause, a biogeochemical modeller at University College London, who led the study. “It turns out that it oscillates back and forth between high and low oxygen levels for a long time. We see periods when the ocean environment, where the first animals lived, should have had an abundance of oxygen and we see periods when it wasn’t.”
According to the researchers, these changed conditions may have caused it to end up developing into some kind of pressure cooker. A drop in the oxygen level always causes a (small) extinction wave. When the amount of oxygen increased again, the survivors were given the opportunity to expand their habitat. In those moments there was room for new life forms to emerge. All these first animals lived in the sea.
This gradual growth of habitable areas, with enough oxygen, progressed two steps forward and one back for millions of years. This ended when land plants stabilized oxygen levels 400 million years ago by emitting oxygen. This paved the way for the evolution of larger, oxygen-requiring marine animals, and eventually land animals.
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