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A thornyhead fish swimming through oxygen minimum zone waters in the Southern California Bight.

Nautilus Live (Ocean Exploration Trust)

When oxygen disappeared, early marine animals really started evolving

Animals need oxygen to survive, but a relative lack of oxygen in Earth’s ancient oceans helped early marine creatures evolve, a new study claims. Indeed, the “Cambrian explosion”—the burst of evolution about 540 million years ago that included the birth of most of the major animal groups we know today—was enabled by oxygen deprivation, the researchers say. The finding comes in the wake of a better understanding of how oxygen levels in the oceans and the atmosphere fluctuated in the deep past, and may shift how scientists think animal evolution can proceed.

The study shows that low-oxygen episodes “charged the pump” for animal evolution to get going, says Timothy Lyons, a biogeochemist at the University of California, Riverside, who was not involved in the work.

Today, depending on the area, typical surface ocean waters consist of between 5.4 and 8 milliliters of dissolved oxygen for every liter of seawater. But waters with low oxygen—or near-anoxic—levels exist as “oxygen minimum zones” (OMZs). This includes places like parts of the eastern Pacific Ocean where small animals like nematodes and specially adapted fish live on the fringes of habitability, subsisting in waters where oxygen concentrations can be only about 1% of normal surface water levels. In some ancient eras, according to other recent work on ocean chemistry, marine animals lived in “worlds of lower oxygen,” Lyons says. “Much of the ocean during these time periods may’ve been like OMZs today.”

Paleontologists Rachel Wood of the University of Edinburgh and Douglas Erwin of the Smithsonian Institution National Museum of Natural History in Washington, D.C., set out to study how the animal kingdom responded to those low oxygen levels. They charted how fluctuating oxygen concentrations correlated with the emergence of new animals as seen in the fossil record and from genetic data. They noted three phases when, after oxygen in the seas dipped and then rose again, animal diversity flourished.

Very early on in animal evolutionary history, during the Ediacaran period between 635 million and 540 million years ago, low-oxygen conditions permeated the oceans; in the subsequent Cambrian period, which began about 540 million years ago, more oxygenated waters appeared—and so did many key animal traits, including a heart, a central nervous system, mouthparts, and the ability to produce limbs and skeletons. As oxygen levels became more tolerable, groups with these traits boomed, filling the fossil record with what’s called the Cambrian explosion. But it was before the explosion itself, “during these anoxic phases … that a lot of morphological novelty arises,” Erwin explains, likely in small, soft-bodied animals that existed on the sidelines of ancient ecosystems and which left little to no fossil record.

The same thing happened in two other, later periods. At the end of the Cambrian, the oceans became devoid of oxygen for about 3 million to 4 million years; afterward, animal life flourished in what’s called the Ordovician radiation, during which diversity in the major animal groups skyrocketed. “The corals really get going, the sponges get going … [in] an enormous wave of diversification,” Wood says.

Then about 252 million years ago, another anoxic event was associated with the Permo-Triassic extinction, the largest mass extinction on record. But afterward, the fossil record shows new coral and sponge species, and animals like the ichthyosaurs—extinct dolphinlike marine reptiles—diversify. These new forms likely appeared during the low-oxygen times, but boomed once oxygen levels recovered, the researchers report in Biological Reviews.

Scientists say that anoxia remains bad for modern-day ecosystems. But over very long timescales, it can enable evolution. “People previously thought that you needed some threshold level of oxygen for evolution to work really well,” says Carl Simpson, a paleobiologist at the University of Colorado in Boulder who was not involved in the work. “What this says is that it’s possible for animals to be diversifying like normal even at much lower oxygen levels.”

What remains unknown is exactly how low oxygen episodes drove animal evolution. Did anoxia simply remove larger, more dominant animals, leaving space for smaller animals to take over? The answer is not clear, but, Wood explains, studying how animals evolve in modern day OMZs may shed some light.