Lake Baikal in Siberia and the Caspian Sea each has its own species of seal that evolved from marine species a few million years ago. But which species they evolved from, and how they got there, has remained controversial. A new study by Jukka Palo and Risto Vainola at the University of Helsinki challenges existing ideas, and adds new mysteries.
It was previously thought that the Baikal seal Phoca sibirica and the Caspian seal P. caspica were most closely related to the Arctic ring seal P. hispida. But analysing mitochondrial DNA sequences, the two Finns find that the Caspian seal is in fact more closely related to the grey seal Halichoerus grypus, and that several other seals, including the boreal harbour seal, are just as, if not more closely related to the landlocked seals than the Arctic ring seal. It looks as if a bunch of species evolved in a bit of a burst, and that the old triad of Caspian, Baikal and Arctic ring seals — the three species were lumped together in their own sub-genus — does not reflect evolutionary history.
The DNA suggests that the two landlocked seals evolved in the Arctic Ocean about 2-3 million years ago (although DNA dates are controversial), in the Late Pliocene. This raises another problem, of how the seals could have made their journey from the Arctic to their current homes. Previous hypotheses suggested that the seals got to the Caspian first, and then spread out, or hopped across the continent in glacial lakes during an ice age. But the DNA doesn't fit with the paleoclimate data. We have no idea what water features may have existed at this time that let the seals' ancestors make the journey south.