How Animals Came to Roam the Earth

The rise of complex life came after a rise in oxygen levels, according to new research.

A trilobite, one of the first hard-shelled fossils found after the Cambrian explosion.

One of the major innovations in the history of life on Earth is the appearance of multi-cellular life and animals about 600 to 750 million years ago. The big question has always been: What took so long? Terrestrial life had been pretty dull up to this point, consisting only of microbes for more than three billion years. Did animals and plants need that long to develop the required genetic programs? Or was life constrained by environmental factors on a planet where oxygen levels were low?

A new study by Noah Planavsky from Yale University and colleagues indicates that the rise of animals was closely linked to the rise of oxygen. By analyzing isotope data for the element chromium in old sediments they found that prior to 800 million years ago, oxygen levels were below 0.1 percent of present atmospheric concentrations. That wouldn’t have been enough to support the metabolic requirements of even primitive animals. But the researchers detected a major shift in the chromium record between 750 million and 800 million years ago, which they attribute to an enormous rise in oxygen concentrations. Correlation does not necessarily mean causality, but if their interpretation is correct, it would explain why the first animals did not appear much before the so-called Cambrian explosion, the rapid diversification of life about 540 million years ago.

Also, their work does not exclude the possibility of environmental niches in which higher oxygen levels might have accumulated even earlier. However, the evolution to more sophisticated life forms—with hard body parts that preserve well in the fossil record—would be difficult to envision unless animals were widespread on Earth.

This study also has implications for complex life elsewhere in the universe. If animal life is only possible when oxygen levels are unusually high (nearly all animals, or at least complex animals, require oxygen on Earth) then it will take a long time on any given planet—in the range of billions of years—for animals and plants to appear. The age of an exoplanet (or exomoon) and the length of time it can provide a stable environment for life will be critical in this regard. And that may mean the number of worlds harboring animal-like life could drop significantly—perhaps well below a previous prediction by Louis Irwin and colleagues that there are up to 100 million habitable planets in our galaxy alone.

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