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Ancient seafloor diggers, like the extinct worm Ancalagon minor depicted here, may have altered planetary chemistry.

You can credit your existence to tiny wormlike creatures that lived 500 million years ago, a new study suggests. By tunneling through the sea floor, scientists say, these creatures kept oxygen concentrations at just the right level to allow animals and other complex life to evolve. The finding may help answer an enduring mystery of Earth’s past.

At the dawn of the Cambrian period about 570 million years ago, multicellular organisms were just beginning to appear, largely in the oceans. But for animals to evolve, the concentration of oxygen in the ocean and atmosphere had to be just right. Too little oxygen, and nascent animals would have suffocated. Too much, and lightning strikes would have created catastrophic fires, torching the primordial land vegetation. “How come oxygen levels didn’t crash or double?” says Tais Dahl, a geochemist at the University of Southern Denmark (SDU), Odense. “Something [regulated] oxygen in relatively narrow limits.

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