Early attempts to deduce how these various species relate to one another identified a number of different sub-groups among the seals, based largely on anatomical resemblances. On this basis, it was thought for a while that the Arctic and Antarctic seals were each others' closest relatives, with the more temperate species representing older, and perhaps more ancestral, forms. But this picture began to change in the 1990s, and its replacement has subsequently been confirmed by a number of different genetic analyses. While the exact details have still not been fully settled, we do now know the broad pattern of what happened, and I've provided one consensus family tree below. Using estimated rates of evolutionary change, calibrated with the age of known fossils, we can also get a rough idea of when some of the key events in this history unfolded.
Perhaps the first thing that's notable from the family tree is that the true seals fall into two natural groups, which diverged from one another very early on. These two groups are now recognised as subfamilies, and can be loosely referred to as the "northern" and "southern" seals. The "northern" seals (Phocinae) live exclusively in the Northern Hemisphere, and most of them live in the Arctic. Within this group, the bearded seal has the longest separate history, its earliest ancestor having appeared around 12 million years ago, around the end of the Middle Miocene.
The line that leads to the hooded seal, with its weird inflatable nose, splits off next, around 8 million years ago, and shortly after that, the ribbon/harp seal line. These latter two species live on opposite sides of the American Arctic, becoming split from one another during the Pliocene, possibly as the Arctic Ocean began to freeze over. A final flurry of evolutionary change occurs during the Ice Ages, with the remaining species arising in such rapid succession that the exact sequence isn't entirely clear; as noted in my chart, the positioning of the grey seal is particularly obscure, with different studies giving different answers.
The "southern" seals (Monachinae), however, are by no means restricted to the Southern Hemisphere, and their history is one that involves some significant long-distance migration. Seals first appeared in the north, and it was here that the "northern" and "southern" seals first split, around 16 million years ago, at the end of the Early Miocene. This probably occurred somewhere around Europe, either in the north-east Atlantic, or in the Tethys or Paratethys seas (now the Mediterranean Sea/Persian Gulf and the Black/Caspian/Aral Seas, respectively). The crucial change was likely the increasing adaptation of the "northern" seals to colder waters, leading them to head towards the Arctic while the "southern" sort stayed behind.
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Before that happened, though, the warm-water seals of the Americas diverged into the elephant seals and a second group that headed south down the Pacific coast of South America, re-adapting to cold climes. This second group rapidly diversified in the Late Miocene, eventually giving rise to the various species of Antarctic seal that we have today. It's unclear whether the elephant seals initially followed them, before some decided to head back again, or whether they were once widespread across the American side of the Pacific and subsequently became separated; either way, we now have two species in opposite hemispheres with quite a wide gap between them. This seems to have occurred during, or shortly before, the Ice Ages.
The picture that I have described so far, is, however, derived from molecular and genetic data. To check its accuracy, we really need fossils, and fortunately, we do have quite a few.
A number of these come from the North Sea, primarily off the coasts of Belgium and Denmark. While we obviously don't have the genetic data to know for sure, the anatomical features of many of these species, such as the Pliocene Phocanella, suggest that they are "northern" seals, some of which must surely date from before those creatures became so adapted to cold waters. Close relatives have been found in Pliocene and Miocene deposits from Hungary to Ukraine, in places that were once submerged under the Paratethys Sea.
One of the first fossil seals to be described was "Phoca" vitulinoides from Antwerp, in 1871. Although none of the fossils of this species are particularly complete, there are a great many of them, and enough to initially identify the animal as a very close relative of the living harbour seal, despite living much earlier, during the Middle Miocene. A recent re-analysis of the remains, however, has shown that they were more distantly related to harbour seals than once thought, easily sufficient to warrant a new name, Nanophoca. This seal was notable for its unusually small size; it was probably no more than a metre (3 feet) in length when fully grown, making it smaller than any living species.
This doesn't add much to the genetic picture, but we can, perhaps, gain rather more from examining fossils that appear to belong to the "southern" group. For example, while genetic evidence tells us that monk seals travelled across the Atlantic before they crossed the Pacific to Hawaii, it can't tell us which direction they were going in; it's as likely that they started out in the Caribbean and travelled both east and west as it is that they first appeared in the Mediterranean and hopped westward in two stages. However, fossil monk seals from Denmark and the Mediterranean are old enough to make this the likely origin area for the group, not the Caribbean. Pliophoca etrusca, first described in 1941 from deposits in Tuscany, first appeared during the Pliocene, but survived through into the Ice Ages, presumably post-dating the migration of some of its relatives to the west, but the closely related Pontophoca is much older.
Similarly, we can tell that the Antarctic seals reached their current home by travelling down the west coast of South America because of the number of seal fossils found there - today true seals no longer live in that part of the world, although sea lions and fur seals are common. Most of these appear to be particularly closely related to the four living Antarctic species, although there has been some debate on this point. Whatever they were, they may once have been more widespread than today, with a notable example being Homiphoca, an Antarctic-type seal that actually lived along the coast of South Africa during the Pliocene.
Unusual examples of such seals from late Miocene and Pliocene South America include the diminutive Australophoca and Hadrokirus, which had unusually large teeth and strong jaws, hinting at a diet similar to that of the modern leopard seal. Some of these fossils are unusually complete, and allow us to determine that, for example, the Pliocene genus Acrophoca had an unusually long neck. This would likely have made it less streamlined than living seals (although, with just bones and no blubber or muscle, it's hard to know for sure) but perhaps made it easier for the animal to lunge at its prey.
Fossils, unfortunately, don't help us much with elephant seals, because, while we do have a few fossils of (probably) the living species dating to the late Ice Ages, their record prior to that point is sparse to non-existent. One fossil described last year from the late Pliocene of New Zealand suggests that they were probably in the Southern Hemisphere before they reached the north; other proposals focus on a purported close relationship with Callophoca from Pliocene Europe, but the identification here is perhaps less certain.
We know, again because we have the fossils, that there was once a time when "northern" and "southern" seals lived alongside one another, somewhere broadly around the north-east Atlantic and what is now the Mediterranean. But what about before that time? The most "primitive" seal fossil that we know of is that of Devinophoca, which lived on the shores of what is now Slovakia around 15 million years ago (Slovakia still had shores then; the sea level was higher). It had a curious mix of skeletal features as well as some unique ones, which, taken together, strongly imply that it was neither a "northern" nor a "southern" seal, but belonged to some third group that is now extinct. Even so, in life, they wouldn't have looked all that different from seals today.
Molecular evidence puts the origin of the seals back to at least 23 million years ago, at the very dawn of the Miocene, and perhaps a little earlier. Seals were once thought to have entered the water independently from sea lions, and their similarities just a case of parallel evolution. While this proposal still pops up from time to time, the overwhelming consensus today is that they are ultimately descended from a single aquatic ancestor.
Seals, so far as we can tell, first appeared in the North Atlantic, close to the Oligocene/Miocene boundary. Their own ancestors had probably travelled from further west, swimming round the south coast of North America from the Pacific. But that, I fear, is a story for another time.
[Photo by Ryan Somma, from Wikimedia Commons. Cladogram adapted from Higdon et al. 2007 and Fulton and Strobeck, 2009.]