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.

What then is intrinsically different about the land’s ability to support biodiversity?

(We’re going to put aside microbial diversity in this discussion, which is not meant as a slight to microbes. But rather, they are too different to generalize together with multicellular life. Single-celled microbes are governed by different forces and even the concept of “species” is different. They deserve their own discussion.)

One reason May and others since have suggested is the physical layout of terrestrial habitats, which may be both more fragmented and more diverse. For example, as Charles Darwin famously documented in the Galapagos, islands are hotbeds for diversification. Over time, natural selection and even chance can turn two different populations of the same species on two islands into two species. The ocean is, in contrast, one big interconnected body of water, with fewer physical barriers to keep populations apart. It also doesn’t have as many temperature extremes that can drive diversification on land.

Land may also be “architecturally elaborate,” to use May’s term. Forests, for example, have covered much of the Earth’s land surface, and the leaves and stems of trees create new niches for species to exploit. Coral do the same in the ocean, of course, but they do not cover nearly as much of the seafloor.

Plants definitely play a major role. The Earth’s tipping point from predominantly marine to terrestrial life came around about 125 million years ago, during the Cretaceous period, where early flowering plants evolved to be extraordinarily successful on land. Plants need sunlight for photosynthesis, but there’s little sun in the ocean outside of shallow coastal areas.  For this reason, land is simply more productive that the cold, dark depths of the sea. “The deep sea is basically a big fridge with the door closed for a long time,” says Mark Costello, a marine biologist at the University of Auckland, who recently published an overview of marine biodiversity. Sure there is life in the deep sea, but not nearly as much as on the sunny coast and land.

Interestingly, Costello notes, increased productivity on land after the diversification of flowering plants also seems to have fed back into increased diversity in the oceans. Pollen, for example, can be an important source of food on the floor of the deep sea. A recent study found pollen likely from pine plantations in New Zealand in a deep sea trench 35,000 feet below the surface of the Pacific Ocean.

The diversification of flowering plants also has everything to do with their coevolution with insects. Plants, for example, evolved features like flowers with long tubes that could only be reached by long-tongued bees that pollinate them. “It’s kind of a big race between the plants and the insects,” says Costello. This coevolution helped create an astonishing number of species: The vast majority of plants on Earth are flowering plants, and the vast majority of animals on Earth are insects. By one estimate, insects alone account for 80 percent of all species on the planet.

Yet insects, so successful on land, are marginal in the sea. Vermeij and Grosberg trace the lack of diversity among small animals to the differences in air and water as a medium. Small animals, like an insect, have more difficulty moving around in water because it is thicker than air. (This applies less so to bigger animals due to the laws of physics.) Mating scents and even visual information don’t travel as well through water—limiting the potential for sexual selection to drive diversification. Sexual selection drives traits that may not seem beneficial but for whatever reasons are preferred by mates. The peacock’s tail is a classic example.

Drawing on the work of Richard Strathmann, Vermeij and Grosberg also try to get at why something like the relationship between flowering plants and insects could not exist in the ocean. The seawater is teeming with potential food sources like zooplankton. While going from one hypothetical sea flower to another, a marine creature would encounter plenty of food floating in the water along the way. Why swim all the way to the other sea flower? On the other hand, an insect flying from one flower with nectar to another would just be flying through air. There is no food floating in the air. And this has evolutionary consequences: A hypothetical sea flower would have to offer much more nectar to attract pollinators lazily feeding on floating food—so much so that it’s not worth it.

Like any question that encompasses all of the history of life on Earth, it’s not quite feasible to run an experiment that proves any of these possible explanations. Instead, there is a whole lot of theorizing. When May and his coauthor asked why there was so much more biodiversity on land than in the sea in their 1994 paper, they put forth some of these possible answers. Though, they admitted, “It is more a list of questions than answers.” It still is.