Half a billion years ago on Earth, after the Cambrian explosion had created an astonishing array of new species, there was still no life on land. No complex life anyway. No plants, no animals, certainly nothing that even compared to the great diversity of life in the sea, which teemed with trilobites, crustaceans, bristly worms, and soft squid-like creatures. Most major animals groups that exist today originated in the sea at this time.
Fast forward to the present, and it is now the land that has a dizzying array of species. In particular: flowering plants, fungi, and insects, so many damn insects. By one estimate, there are five times as many terrestrial species as marine species today. So how did biodiversity in the ocean—despite its head start, despite its larger share of the Earth’s surface area—come to fall so far behind biodiversity on land?
Why more species live on land than in the ocean has puzzled biologists for a long time. Robert May, a ecologist at the University of Oxford, appears to be the first to put the conundrum down in writing in a 1994 article titled, “Biological Diversity: Differences between Land and Sea (and Discussion).”
The question has held for the two decades since, even as humans have explored more and more of the deep ocean. Scientists now estimate that 80 percent of Earth’s species live on land, 15 percent in the ocean, and the remaining 5 percent in freshwater. They do not think this difference is entirely an artifact of land being better explored. “There are oodles and oodles of species in the sea, but to make up that difference would take an awful lot,” says Geerat Vermeij, a marine ecologist and paleoecologist who has written about the land-sea species discrepancy with his collaborator Rick Grosberg, another ecologist at the University of California, Davis. So this seeming lack of ocean diversity is not just the bias of us land-based creatures, Vermeij and Grosberg argue—a bias that they as marine researchers are all too keenly aware of.