1. Why Did Scientists Think Seals, Sea Lions, and Walruses Had Different Ancestors?
Seals, sea lions, and walruses have very different body shapes and ways of moving. Because they look so unlike each other, scientists in the 1960s through the late 1980s often concluded that their ancestors were also different [6]. This idea is called the "two-lineage hypothesis."
In this old theory, seals were thought to be related to weasels (part of the Musteloidea group). Sea lions and walruses, on the other hand, were thought to be related to bears [6]. It was not just that sea lions were bear-like; both sea lions and walruses were placed in the bear family line.
If this theory were correct, it would mean that life in the ocean started twice. First, land animals related to weasels went into the sea to become seals. Later, land animals related to bears went into the sea to become sea lions and walruses. But was this true? The next steps show how scientists began to doubt this idea.
2. How Did Ear Bone Studies Challenge the Old Idea of Different Ancestors?
In 1987, a scientist named Wyss re-examined the bones around the ear (the auditory region) of these animals [6]. He looked at the features that were said to support the idea that seals and weasels were close relatives. His conclusion was that these ear features did not provide strong evidence for separate ancestors.
Wyss’s work suggested that seals, sea lions, and walruses might actually come from one single common ancestor (a monophyletic group) [6]. This was a big change. It meant the old idea of two separate lines was weak.
However, this study did not "prove" they had one ancestor. It only showed that the evidence for two ancestors was shaky and that one ancestor was possible. To be sure, scientists needed more types of proof.
3. What Did the 2006 DNA Studies Reveal About Their Shared Ancestor?
In 2006, two different studies using DNA pointed to the same conclusion: these animals share one ancestor [6]. One study compared 29 species of land carnivores (like bears and weasels) using three genes. The results showed that pinnipeds (seals, sea lions, walruses) came from a single ancestor shared with the Musteloidea group (weasels, otters, raccoons), but not with bears [6].
The second study also supported the idea that pinnipeds form one single group [6]. Together with the 1987 ear bone study, this genetic data strongly suggested they are all related.
Analysis using multiple genes has also produced estimated dates for when these animals split apart. According to molecular clock estimates, pinnipeds and the Musteloidea group separated about 45 million years ago [6]. Later, sea lions and walruses (the Otarioidea) separated from true seals about 27 million years ago [6]. These are estimates based on genetic changes over time.
4. Is the Walrus Closer to Seals or to Sea Lions?
While Wyss supported the idea of one common ancestor, he had a specific view about walruses. He suggested that walruses might be closer to seals than to sea lions because they share many evolutionary clues [6]. This challenged the then-common belief that walruses and sea lions were close relatives.
However, later DNA studies corrected this detail. The 2006 molecular study found that while all pinnipeds share one ancestor, the first split happened between true seals and the group containing both sea lions and walruses [6]. This means the walrus is on the sea lion side of the family tree, not the seal side.
So, while the big idea (that they all come from one ancestor) remained strong, the specific position of the walrus was adjusted. The core conclusion of a single common ancestor has continued to be supported [6].
5. What Does the Fossil Enaliarctos Show About Animals Between Land and Sea?
Fossils help us see what animals looked like in between land and sea. One animal, *Enaliarctos*, was found in California and Oregon. It is said to have teeth and movement features that are halfway between land carnivores and modern pinnipeds [6]. Its age is estimated to be between about 30 million and 17 million years ago, depending on the source.
Even more surprising was *Puijila*, found in a crater lake sediment on Devon Island in Nunavut, Canada. This animal lived about 23 million years ago [6]. It had a long tail and limb proportions similar to land carnivores. However, its flat finger bones showed it had webbed feet [6]. Importantly, it did not have flippers.
This fossil suggests that early evolution might have included a freshwater stage, and the Arctic region may have been a center for this early change [6]. *Puijila* is not an ancestor of modern seals directly, but it is described as a fossil reflecting a point in the middle of the branching from land to sea [6].
6. How Can You Explore Seals, Sea Lions, and Walruses Further?
If you want to see this for yourself, look at books or encyclopedias that compare seals, sea lions, and walruses. Look closely at their ears, limbs, and body shapes. If you visit an aquarium or zoo, watch how the seals, sea lions, and walruses move.
Write down what is different and what is similar. You might find that even though they look different, there are hidden similarities. Old researchers looked at these same features but reached different conclusions [6].
Check the "classification" pages in modern encyclopedias to see the current scientific view. You can also look for restoration drawings of *Puijila* and *Enaliarctos* in museum books or online resources to see what these ancient animals looked like [6].