Lake Baikal is over 600 kilometres long, holds around a fifth of the world's unfrozen surface fresh water, and contains a seal population usually given in the tens of thousands. Nothing else in the pinniped family lives permanently in fresh water; the Saimaa and Ladoga ringed seals come closest but are subspecies of a marine species, and other seals visit rivers rather than living in them.
Genetic work places the Baikal seal close to the ringed seal, and the divergence is generally estimated in the region of a few hundred thousand to a couple of million years. The colonisation route is disputed. The usual suggestion is passage from the Arctic Ocean along the Yenisei and Angara river systems during a period of higher water, but no direct evidence of the route survives.
The lake's food web supports the seal in an unusual way. Its main prey are the golomyanka, small oily endemic fish that bear live young and lack a swim bladder, which is why Baikal seal diet studies read so differently from those of any marine seal.