1. A whale's flipper has the same kinds of bones as a human arm and hand
The pectoral flippers of cetaceans are made of the same types of bones as a human arm. There is 1 upper arm bone, 2 forearm bones, followed by wrist bones, hand bones, and finger bones.[1] Even though the outside of the flipper looks like a single flat plate, the design of an arm and hand is inside it.
There are 5 fingers. An article from a Danish museum researcher shows a photo where the flesh between the fingers of a whale flipper was removed, revealing a 5-fingered hand exactly as it is.[2] It is similar to a mitten glove. A mitten also has 5 fingers inside, but from the outside, it looks like one bag. However, a real whale flipper is much longer and thinner than a mitten, with bones that have grown long.
In many mammals, each finger has 2 to 3 finger bones (phalanges). In cetaceans, the fingers have become longer, and in some species, the number of bones in a single finger has increased to more than 6. This is called hyperphalangy.[3] The number of fingers has not changed; only the number of bones in each finger has increased. If the inside of the flipper is an arm and hand, have the bones in other parts of the body also been modified from those of a land animal? Next, let us look at the neck bones.
2. Whales still have 7 neck bones, but they are short and compressed
Do whales and dolphins, whose necks are not visible, have fewer neck bones? According to the Ogasawara Ocean Center, the cervical vertebrae (neck bones) of cetaceans are the same 7 as in other mammals.[4] It is also said that human neck bones are 7.
The difference lies in the shape of each bone. In many cetaceans, the cervical vertebrae become short and flat compared to the body length and can hardly move. The first and second cervical vertebrae are fused together like a single bone. In some species, the third cervical vertebra is also fused.[4] One might think that stiffening the neck prevents the head from shaking side to side while swimming, but this source does not state that reason.
On the other hand, beluga whales and river dolphins are said to have less fusion and can move their necks relatively freely.[4] Even though the number of bones is the same, the degree of fusion differs by species. The neck bones changed shape while keeping their number. So, what is inside the tail fluke, which provides the power for swimming?
3. The flat part of a whale's tail fluke has no bones inside
The tail fluke provides the great power for swimming. One might think thick bones run through it, but a 2024 study on beluga embryos reports that the flat part of the tail fluke (the leaf) is supported by the tail vertebrae and strong connective tissue covered by skin.[5] There are no bones or muscles in the leaf.
Connective tissue is a strong, string-like tissue made of bundles of collagen fibers. The tail bones continue only to the base of the fluke, and this bundle of strings forms the shape of the leaf beyond that point. The tail fluke serves both as a source of forward propulsion and as a rudder for changing direction.[5]
It is important to note that "no bones in the tail fluke" applies only to the flat leaf part. The tail bones continue up to the point before entering the leaf. The pectoral flippers retain hand bones, but the tail fluke has no bones. There should be one more thing at the back of the body that land animals had. Where did the hind legs go?
4. Whales lost their hind legs, but small pelvic bones remain in the muscles
Modern cetaceans do not have hind legs. However, degenerated bones are buried in the muscles of the flank.[6] These are the remains of the pelvic bones that supported the hind legs when they lived on land.
These bones are very small compared to the body size. Yet, they still have a function. According to a university article introducing a 2014 study, the muscles that move the penis (a highly mobile reproductive organ) in males attach directly to these pelvic bones. Furthermore, species with larger testes relative to body size also have larger pelvic bones relative to their body size.[7] This is viewed as a bone that once supported legs being repurposed for a different function.
Regarding why the hind legs themselves disappeared, there is genetic research. The activity of a gene called SHH, which is involved in forming hind legs, stops in the hind limb buds of cetacean embryos. This is considered one factor in the degeneration of hind legs.[3] It is "one factor," and it is not certain that this alone explains the disappearance. What did cetaceans with hind legs look like in the past?
5. Fossils of whales with legs show how walking legs became useless for swimming
Did cetaceans with hind legs really exist? Ambulocetus, from about 50 million years ago (the date varies by source), had hind legs that could be used on both land and in water.[6]
Cetaceans are thought to have evolved from land-dwelling even-toed ungulates (relatives of cows and deer) or animals close to them, about 49 million years ago.[8] Pakicetus, one of the oldest, is said to have lived mainly on land.[6]
A more recent find is Peregocetus, excavated from the coast of southern Peru in 2011. It is about 42.6 million years old, and its hind legs were not much shorter than its front legs. Bones indicating a kneecap, small ankle bones, and small hooves remain. It is inferred that the feet had webbing and that it could both swim and walk on land.[9][10] The body length is reported as about 4 m (about 13 feet) by CNN and less than 3 m (about 10 feet) by Keele University.
In Basilosaurus, a fully aquatic cetacean from about 40 million years ago, the hind legs became very small and no longer served a swimming function.[6] The time between Peregocetus and Basilosaurus is about 2.6 million years. Since they are separate species, we cannot say that the legs of one single species changed. However, the fact that species with walking legs and species with small legs useless for swimming are found within a few million years can be read from the numbers. Bones are clues to past lifestyles. We can check the traces of this skeleton on our own bodies.
6. How to feel the bones in your own hand and neck and look for them in specimens
First, touch the back of your own hand. You can feel the ridges of bones extending from the base of your fingers toward your wrist. These bones are also inside a whale’s flipper. While moving your fingers one by one, imagine the mitten shape inside the flipper.
Next, slowly turn your neck. The number of neck bones is 7 for both humans and whales, but many whales have them fused so they hardly move, while humans move each one. Do not force a large rotation; try it within a pain-free range.
If you have the chance to see a whale or dolphin skeleton at a museum or aquarium, look for three places: the finger bones inside the flipper, the point where the tail bones end near the tail fluke, and the small pelvic bones in the flank.
When new fossils are found, this sequence of dates and leg lengths might be rewritten. If you see a news story about a fossil whale with legs, add the date and leg length to the numbers in this article line by line, and you can create your own table.