If fish evolved on land, where did they all go? Evolution and Biodiversity in the Ocean

This ray-finned fish was my dinner last night. Photo by Andrew David Thaler

When Carl Sagan described our planet as a “pale blue dot” he was invoking the fact that, despite being called Earth, our world is mostly Ocean. The surface of the Earth is a little more than 70% water and the ocean accounts for 98-99% of our total biosphere–the volume of the planet that can support life. Most contemporary theories point to ocean ecosystems–like deep-sea hydrothermal vents–as the launching point for the emergence and evolution of life. Ocean processes dominate biological interactions, even among unwitting terrestrial actors. A new paper, published in the Proceedings of the Royal Society: Biological Sciences, revisits an old debate about the ocean biodiversity and challenges the notion the ray-finned fishes have a marine origin.

In Why are there so few fish in the sea? the authors begin with the seemingly innocuous question–why are there so many more species in terrestrial environments than in marine environments?  From there, they look at species counts, phylogenetic relationships, and diversification rates to determine the ancestral state of the most recent common ancestor of one fish class, Actinopterygii, the ray-finned fish. What they found was that, despite the vastly smaller habitat available for freshwater fish, the number of actinopterygian species found those ecosystems was roughly equivalent to the number of species found in marine systems. In both systems, the dominant groups are relative newcomers on the evolutionary stage, with superorder-level radiations happening between 111 – 150 million years ago.  Most surprising, the authors discovered that the most recent common ancestor of actinopterygians may have been a freshwater, not marine, fish. Ray-finned fishes may have invaded the ocean from lakes and rivers.

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