All life on Earth began in the sea some 3.5 billion years ago. Yet there is a twist to this story. New research shows that almost all fish species that inhabit the oceans today moved there from rivers and lakes. This sheds new light on the importance of freshwater ecosystems for life on Earth. And it suggests that by damming and polluting rivers, we may destroy the seed banks of future generations.
Terrestrial environments occupy 30 percent of the Earth's surface, but contain 75-85 percent of all species. Freshwater ecosystems only make up about 2 percent of the Earth's surface, but count as many fish species as the sea, which covers 70 percent of the planet. Why are there so few fish in the sea? Greta Carrete Vega and John J. Wiens, two researchers at Stony Brook University near New York, have found a fascinating answer to this question.
In a paper that appeared in the Proceedings of the Royal Society of Biological Sciences today, Carrete Vega and Wiens explore the evolutionary history of ray-finned fishes, one of the dominant species on Earth. Ray-finned fishes make up almost half of the world's vertebrates, and more than 95 percent of all fish species. An estimated 15,150 of these species inhabit freshwater ecosystems -- slightly more than the 14,740 species that live in the much larger marine environments.
Using a variety of scientific methods, the two researchers have found that all ray-finned fish species, whether they live in lakes, rivers or oceans, derive from a common freshwater ancestor. Important and well-known fish species such as the tilapia, the European perch and the guppy left the oceans to invade rivers and lakes over the past 100 million years. Yet when they did so, they repopulated environments from where their ancestors had originally set out to the seas. "Family histories don't come much more bizarre," a reviewer comments on the topsy-turvy evolution of fish in the New Scientist.
Carrete Vega and Wiens found that the common ancestors of all existing ray-finned fishes lived in a freshwater ecosystem approximately 300 million years ago (or possibly much earlier). Their first marine descendants only appear about 180 million years ago. This evidence does not question that fish first evolved in the seas. Yet rivers and lakes must have acted as a refuge where the ancestors of almost all existing fish species survived waves of marine extinction before some of them returned to the seas.
Rivers are the arteries of our planet. By transporting water, sediments and nutrients, they connect terrestrial, freshwater, coastal and marine ecosystems. The groundbreaking study by Carrete Vega and Wiens suggests that rivers and lakes have acted as a genetic repository, a freshwater Noah's Ark of sorts, which repopulated the seas after catastrophic waves of species loss in the distant past.
Today freshwater ecosystems continue to be treasures of biodiversity. Yet as the third Global Biodiversity Outlook has found, "rivers and their floodplains, lakes and wetlands have undergone more dramatic changes than any other type of ecosystem." As a consequence, they suffer from a higher rate of species extinction than any other major ecosystem. By damming, diverting and polluting our rivers, we not only destroy branches of the tree of life that may have existed for hundreds of millions of years. The new study shows that we may also deplete the seed bank of future generations.