Wednesday22 January 2025
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The ancestor of four-legged creatures emerged from the sea 400 million years ago but quickly returned. What made it change its mind?

Researchers have "rewound time" to explore the mysteries of the evolution of the largest creatures inhabiting the ocean.
Четвероногие предки вышли из моря 400 млн лет назад, но вскоре вернулись обратно. Что же их не устроило на суше?

The Earth is home to an incredible number of species: some are well-known to scientists, while the existence of others remains shrouded in mystery. It is known that whales are the largest animals on Earth, and their lifespan may be twice as long as previously thought by scientists, but that’s not all the mysteries of whales — in a new study, researchers have revealed details about the evolution of these marine giants, as reported by IFLScience.

It is understood that the ancestor of all four-legged creatures on Earth crawled out of the ocean about 400 million years ago. Then, 350 million years later, the ancestors of modern whale species crawled back into the ocean to thrive in it. The descendants of these ancient beings, which today roam the Earth's oceans, may be among the largest and most discussed species of the ocean. Scientists have already determined that whales have an astonishing lifespan, impressive hunting techniques, and a fascinating history. They are also unique animals not only in the ocean but across the entire planet.

In a new study, scientists turned back time to reconstruct the evolutionary history of whales. Surprisingly, the closest relative of whales is the hippopotamus — they are quite different, with distinct habitats, diets, and distributions. Yet, despite their semi-aquatic lifestyle, this large terrestrial animal turned out to be extremely closely related to the largest mammal on Earth.

Researchers believe that hippos descended from a non-aquatic group known as anthracotheres about 15 million years ago. At the same time, whales evolved over 50 million years ago from creatures that lived on land. Somewhere in the distant past of evolutionary history, hippos and whales shared a common ancestor that was also terrestrial.

One of the earliest known ancestors of land whales is a creature known as Pakicetus, also referred to as the "first whale" or the first known cetacean. Scientists believe that this creature walked on four limbs and fed on terrestrial animals and fish. It was first discovered in 1983 and likely inhabited the edge of the Tethys Sea in present-day Pakistan and India. Fossil remains showed that it had an ear bone similar to modern whales with a unique structure that might have even allowed Pakicetus to hear underwater.

Subsequently, creatures evolved that ventured further into the water, adapting to the new environment. One of these is Ambulocetus, which lived on the planet about 50-48 million years ago. This species had a long tail for swimming and four limbs but could spend time both on land and in water.

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After Ambulocetus, around 40-33 million years ago, dorudontids and basilosaurids emerged. The latter were characterized by tiny hind limbs, and their nostrils were located further back — it is believed this is an intermediate stage between a snout and a blowhole. Dorudontids reached lengths of about 5 meters, had tiny hind legs, and flippers. They also spent all their time in the water.

According to cetacean researcher Dr. Ellen Kumbas, for 8 million years, the ancestors of whales transitioned from a fully terrestrial lifestyle to a fully aquatic one. In terms of evolution, this is actually incredibly rapid.

It is known that the descendants of Dorudon eventually became modern whales. Baleen whales began developing their filter feeding with flatter skulls about 34 million years ago, while others retained their teeth and became orcas, sperm whales, and dolphins. In fact, it was during this period that the split between toothed and baleen whales occurred — the animals underwent physical changes to adapt to new diets.

Genetic studies have shown that alongside the hind limbs and physical changes evident in the fossil record, cetaceans lost much more than just their legs when they returned to the ocean. Previous research also indicated that cetaceans lost the gene responsible for producing saliva. Furthermore, toothed whales also discovered the loss of many of their olfactory and taste genes. Scientists also believe that genetics will ultimately explain why whales became the largest animals on the planet.