In a new paper published in Nature Communications, Alexander Krause from the University of Leeds and colleagues show that about 450 million years ago, oxygen concentrations in Earth’s atmosphere increased from an average of less than five percent to more than 15 percent. Significantly, that was around the same time the first vascular land plants appeared on our planet.
The authors claim the oxygen increase was related to a rise in both the number of organisms and their diversity. They also believe it’s related to the appearance of large predatory fish, which would have required high oxygen levels for their energetic lifestyle.
A few thoughts come to mind. First, there already were macroscopic animals, even predatory ones like trilobites, on Earth before that huge increase in oxygen. So perhaps increased oxygen doesn’t tell the whole story. An alternative hypothesis is that the Cambrian explosion—the emergence of so many different animals with a huge variety of body plans about 550 million years ago—had more to do with genetic developments than oxygen. Also, there appear to have been other oxygen spikes in the atmosphere before 450 million years ago, but in each case concentration levels always returned to under five percent.
We do know that high oxygen levels are a requirement for highly energetic and advanced animals, including humans. They are also needed to make fire. Thus, if life on some other planet took a similar evolutionary trajectory as on Earth, it would mean that intelligent—and particularly technologically advanced—life would only be possible after a long, long time of planetary evolution. Thus, SETI programs may be advised to single out fairly old planets in their search for possible radio signals from a highly advanced alien species.
Finally, if the previous assumption is correct, photosynthesis by algae and marine organisms may not be sufficient to increase oxygen levels or stabilize them enough for the evolution of highly advanced animals. If so, land life may be a requirement for the evolution of intelligent, technologically advanced life. That, in turn, would mean that we would not expect to find such advanced life on ocean planets covered entirely by water.