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They do not think this difference is entirely an artifact of land being better explored. 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. The question has held for the two decades since, even as humans have explored more and more of the deep ocean. 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).” Why more species live on land than in the ocean has puzzled biologists for a long time. 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? By one estimate, there are five times as many terrestrial species as marine species today. In particular: flowering plants, fungi, and insects, so many damn insects. Most major animals groups that exist today originated in the sea at this time.įast forward to the present, and it is now the land that has a dizzying array of species. 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. 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.