The best spot for evolving radically new marine creatures has seemed to be in shallow waters, where storms and fierce battles for resources wipe out the competition. Now two researchers say in today's Proceedings of the National Academy of Sciences that the deep sea can also give birth to new life forms--if it contains enough life-sustaining oxygen.
Previous studies of the fossil record have suggested a straightforward picture of how new types of marine invertebrates evolved: New creatures first appeared in shallow water, diversified in deeper water, and then moved to deep-sea environments. The most usual explanation is that hurricanes and other stressful conditions near the shore caused the extinction of some animals--and so cleared the way for new groups to evolve in the absence of competition.
But the previous analyses of this pattern may have failed to take into account a simple biological fact: Most marine organisms can't live in oxygen-poor water. Since the deep-sea bottom has frequently been plagued by a low levels of oxygen, two paleontologists--David Jacobs of the University of California, Los Angeles, and David Lindberg of the University of California, Berkeley--took a closer look at the fossil data. They found that novel groups did originate onshore through much of the Mesozoic Era, 245 to 90 million years ago, when the deep sea was frequently oxygen-poor. But after that, new orders of marine invertebrates were just as likely to originate in deep water as shallow. They attribute this change to the cooling of polar regions, which pumped the deep sea with dense, oxygen-rich water.
Other experts are pleased with the suggestion that oxygen levels rather than location spur evolutionary novelty. "There's been this notion in the past that there's some fundamental reason why [orders of invertebrates] should appear nearshore, and I've never been satisfied with this explanation," says paleontologist Steven Stanley of Johns Hopkins University.