Did the First Animals Live in a World Without Oxygen?

A new study suggests the answer may be yes.

Animal life in the Ediacaran era, from 635 to 541 million years ago, was strictly “low energy.”

According to current scientific thinking, complex life on Earth needed oxygen to evolve.

Now Daniel Stolper and Brenhin Keller from the University of Berkeley, California, have called that thinking into question. In a new paper published in Nature, they report on their analysis of submarine lava flows to determine the oxidation state of iron in the rocks. Today the deep ocean contains a lot of oxygen, which circulates in fluids within the basaltic rock that oxidizes iron. The researchers found that this oxygenation effect dates back to only 540 to 420 million years ago, long after the first animals evolved on our planet. Before then, there must have been much less oxygen in the deep ocean, and by extension in the atmosphere and shallow water habitats.

This finding challenges the traditional idea, advanced by David Catling and others, that oxygen is a key requirement for the rise of complex life. After all, animals need oxygen to satisfy the high-energy metabolism that allows them to move, jump, fly, and hunt.

So how can these two contrasting ideas be reconciled?

Perhaps the first animals on Earth were filter feeders that didn’t require much energy. These creatures date back to 800 million years ago, perhaps earlier. They would have stayed mostly in one place, like sponges, or may have moved with the water current, like jellyfish. For these species, tiny amounts of oxygen would have been enough. Perhaps they didn’t need any at all. Low-oxygen tolerance still exists among some species today, including fish such as the Crucian Carp. Some turtles also can go without oxygen for a long time, and it’s been claimed that some species of the phylum Loricifera—tiny microscopic animals that live between the grains of gravel and mud on the seafloor—don’t need oxygen.

This implies that high oxygen content wasn’t a requirement for the origin of complex animals, which occurred well before the Cambrian explosion about 541 million years ago. Instead, animals may have originated right after the appearance of genetic control systems that allowed the developmental program necessary for complex life, as suggested in my new book with William Bains, The Cosmic Zoo. In this scenario, animals would have remained low energy until the oxygen content spiked at the beginning of the Cambrian explosion, allowing fast-swimming predators such as trilobites to evolve.

It’s intriguing that dates for the Cambrian explosion coincide quite closely with dates for the first oxygenated basalts analyzed by Stolper and Keller. The ensuing diversity of animal species during that time, including both predators and prey, would be unthinkable without oxygen. But some animals can do without it, apparently. And that has far-reaching implications for life on other worlds, where we might find organisms thriving in the absence of oxygen.

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