All human embryos also go through a stage in which they have tails that are later absorbed. But sometimes a person is born with an actual tail.
Similarly, dolphin embryos pass through a stage in which they have hind limbs that disappear as the embryo develops.
The recently captured dolphin developed an extra set of flippers from these limbs that are about the size of human hands.
It is unclear whether the dolphin would actually make use of its spare fins for maneuvering, Katsuki Hayashi, director of the Japanese whaling museum, told the Associated Press.
Related to Hippos?
It has long been believed that dolphins are descended from land animals that took to the water.
Initial genetic studies have provided evidence that the marine mammals are most closely related to hippopotamuses.
And early last year paleontologists from California, France, and Chad reported fossil evidence that dolphins, whales, and hippos diverged from a common ancestor that likely lived about 50 to 60 million years ago.
The four-finned dolphin, now swimming in an aquarium tank in Taiji, will require considerably more study before scientists can determine what it reveals about dolphins' evolutionary history.
"There's not much science yet," Baker said. "It's an interesting finding, and I think the interpretation that it is consistent with our expectations from evolution is true. [But] that's as far as it can go at the moment."
Japanese scientists have announced that the dolphin will be kept at the Taiji Whaling Museum for x-rays and DNA tests.
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