Source: New Scientist
Science is always good for an awesomely counter-intuitive finding, and this one has to be the mother of them all. Despite the fact that life itself originally came from the sea, the same can't be said of the ocean's current occupants.
Stony Brook researchers Greta Vega and John Wiens have found that about 75% of all living fish species trace their evolutionary history back to freshwater sources, not saltwater ones. If you go back 170 million years a time contemporaneous with the dinosaurs the ancestors of most fish alive today had never even ventured into the ocean, instead restricting themselves to the rivers and lakes where they had first evolved about 300 million years ago.
The scientists say we can't be absolutely sure about what happened before that, but we know of some pretty famous examples of sea creatures that got their start in freshwater: dolphins and whales. These mammals most likely evolved from wolf-like creatures that at one point lived mostly, if not exclusively on land before beginning the slow evolutionary transition to a life at sea. The same might well be true for most fish.
If that's the case, then it raises the obvious question of where all the original sea creatures went. According to the researchers, mass extinctions likely played a role. We know extinction events like the Permian-Triassic event 250 million years ago killed up to 95% of all marine species, and generally the seas suffer the most in any mass extinction.
While life was all but wiped out in the ocean, the freshwater habitats might have proved more conducive to survival, allowing these one-time landlubber fish to survive and eventually repopulate the oceans, becoming the seas' new dominant form of life. Maybe this whole "living on land" thing really was just a huge mistake after all.