A fish swimming against the evolutionary tide

A fish swimming against the evolutionary tide

At a time when the ancestors of land vertebrates took their first steps out of the water, one of their relatives took the opposite path — back into the sea.

About 370 million years ago, vertebrates first moved onto land. This did not happen all at once: initially these were fish that moved to live in shallow water, rivers and ponds, and used their fleshy fins to support their bodies on the bottom. Over several generations they moved, so they felt more and more comfortable on the ground. It was these adventurous fishes that, over the next tens of thousands of years, were the forerunners of the evolution of land vertebrates: first amphibians, then reptiles, mammals, and birds.

We often think of evolution as a one-way path, suggesting constant progress in the same defined direction or even a predetermined destination. According to this concept, the transition from water to land is a necessary step along the same path. Thus, we would expect to see fish that evolved to become more frog-like or salamander-like and spend more and more time out of water—and above all, we would not observe a change in the opposite direction.

But evolution It doesn’t really work that way. It has no final goal or destination: the changes animals undergo reflect the conditions in which they live. Thus, a fish that has already moved to live in shallow water is likely to return to swimming in the open sea rather than going through further adaptation to life on land. Of course, depending New researchThat seems to be what happened.

In most cases the development of the species took place from the direction of the sea towards the land, but not in the case of Kikiktania. Kikiktania and Kay (center) and Tiktallik Rosi (below) | Credit: Alex Boersma, University of Chicago

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Tiktaliq’s relative

The fossil described in the new study is a fish from the fleshy fin class, a relative of another fish, the more familiar tiktalis: an ancient fish that represents its beginnings. Transition from sea to land. Tiktalike lived about 375 million years ago and had the features expected to be found in an “intermediate fossil,” which is not yet an amphibian but already shows some of their features. His pectoral fins did not turn backward like most fishes, nor laterally like terrestrial vertebrates, but were found somewhere between the two positions. The fin itself contains finger-like structures, which the fish can spread and place on the ground by flexing the fin. In this way Tiktali was able to lift himself off the ground by supporting his body weight on his wings, perhaps crawling, but not really walking.

Tiktalli had other characteristics that placed it among fish and amphibians. It had a neck, which fish usually don’t, and the change in gill bones suggests that it relied heavily on air breathing—because Tiktali, like some fish still alive today, had lungs in addition to gills.

In 2004, the first Tiktalic fossil was discovered in the northern Canadian island of Alsemere. Four days earlier, the same researchers found another fossil, only about one and a half kilometers from where Tiktali was found. But this fossil, a collection of rocks still with hints of bones, awaited someone’s interest for years. “He was sitting in a drawer because we were focused like a laser beam on Tiktok.” He said in an interview with the New York Times Neil Shubin (Shubin) was one of the leading figures in the research on Tictalic. Chauvin is also the senior researcher who signed the current article on the same forgotten fossil.

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It was only in 2020 that researchers began to examine the fossil, with the help of which it was scanned Computer tomography (CT). They found that it had jaws and part of a pectoral fin, and that the fin was different from what they expected. Tiktalaik had a humerus that had ridges and ridges, signs that powerful muscles were attached to it. However, this fossil had a thin, boomerang-shaped humerus, and the distal part of the body was broad and flat. “It’s not a flexible limb — it’s an oar,” Chaubin said.

Thomas Stewart Examining a Fossil Credit: Thomas Stewart
The researchers scanned the discovery with the help of computerized tomography and found a different fin than they expected. Thomas Stewart Examining a Fossil Credit: Thomas Stewart, University of Chicago

Return to the water in new ways

The researchers named the ancient fish Kikiktania and Kei.Kikiktaniya Vaki) – the genus name Kikiketania, named after the locality where the fossil was found, Wakey, as it is called in the local Inuit language, the genus name of the late American biologist David Wake (Wake). Its features, particularly the shape of its fin, convinced them that this fish was not part of the evolutionary path that led to its ascent to land, but actually took the opposite path.

“While his relatives were playing by the water’s edge, learning what the land had to offer, Kikikatenia was doing something else,” wrote Thomas Stewart, who led the study. On the conversation website. “Its arm bone is completely different from other fishes we know. This suggests to my colleagues and I that Kikkicatenia moved away from the water line and re-evolved to life in the open sea.”

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Of course, Kikiketania wasn’t the last to decide that life at sea appealed more to him—he might have been the first. Chauvin noted that the ancestors of whales and dolphins returned to the water about 50 million years ago. They have not become fish again, of course: they continue to breathe air and live young like all mammals, but have adapted to life in the sea. As such, Kikicatenia still exhibit characteristics unique to their group and have not recapitulated and adopted the anatomy of their ancestors. For example, Chaubin said, fish from this group seem to have evolved a different hunting pattern, which Kikiketania likely continued in its new home. “They’re not just returning to the water, they’re doing it in new ways.”

“Evolution is not a simple, linear process,” Stewart concluded. “Although the ancestors of terrestrial vertebrates seem to have evolved inexorably toward life on land, Kikicatenia shows precisely the limitations of this one-way view. Evolution did not build a ladder toward humans. Evolution is a series of processes that grow together the tangled tree of life. New species are created and change. Branches can turn in all directions.”

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