Shubin went back to the Canadian Arctic and found more fossils of Tiktaalik which showed all four fins had evolved into limbs. This transitional fish was our ancestor who lived 375 million years ago.

Tiktaalik, a transitional fish, was originally thought to have only one pair of sturdy, limb-like fins. But new specimens show that the hind fins also functioned like limbs, helping the fish crawl in shallow water and on land.

An Ancient Fish’s Four-Wheel Drive

Based on fossils found a decade ago, scientists had assumed that the earliest known species of fish to make the momentous transition to four-legged animals, capable of living and moving about on dry land, had developed a kind of front-wheel drive. They used enhanced forward fins to crawl out of shallow waters, and only later adapted their rear fins into limbs.

But that was before subsequent discoveries turned up well-preserved hindquarters of Tiktaalik roseae, a transitional species that lived 375 million years ago. New findings, reported Monday in Proceedings of the National Academy of Sciences, challenge the theory that vertebrates did not gain four-limbed mobility until well after they had settled the land.

Five new specimens of Tiktaalik (pronounced tic-TAH-lick) fossils found in the Canadian Arctic reveal that the modification of fins into four limbs actually began as adaptations for life in shallow water, according to a research team led by Neil H. Shubin, an evolutionary biologist at the University of Chicago. The change may have enabled the fish to walk on a lake floor, paddle about and even make brief forays on land, the scientists said.

Fins to Feet

Tiktaalik, a transitional fish, was originally thought to have only one pair of sturdy, limb-like fins. But new specimens show that the hind fins also functioned like limbs, helping the fish crawl in shallow water and on land.

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The New York Times

In an interview, Dr. Shubin said that comparisons of the upper and lower anatomy of a single Tiktaalik fossil showed the hind appendage to be at least as long and complex as the forward appendage. The pelvic girdle, though still fishlike, appeared to be larger and more robust to support strong rear limbs, and the hip joint made a wide range of movements possible.

“It’s clear that the emphasis on hind appendages and pelvic-propelled locomotion is a trend that began in fish,” he said, adding that the trend was “later exaggerated” during the origin of four-legged animals known astetrapods in the Late Devonian period, 395 million to 362 million years ago.

Another author of the journal article, Edward B. Daeschler of the Academy of Natural Sciences at Drexel University in Philadelphia, said the pelvis, particularly the hip socket, was “very different from anything that we knew of in the lineage leading up to limbed vertebrates.”

Tiktaalik, which grew as long as nine feet, looked like a cross between a fish and a crocodile. Dr. Shubin, Dr. Daeschler and Farish A. Jenkins Jr. of Harvard, who died in 2012, discovered the first fossil specimens in 2004 in blocks of stone on Ellesmere Island, part of the Canadian territory of Nunavut.

After the discovery was published two years later, Jennifer A. Clack of Cambridge University, a specialist on tetrapod evolution, said Tiktaalik was “one of those things you can point to and say, ‘I told you this would exist,’ and there it is.”

Other scientists were struck by the anatomical changes already evident in the fossils.

The transition was more complex than fins’ evolving into sturdy limbs. The head and braincase were changing, a mobile neck was emerging, and a bone associated with underwater feeding and gill respiration was diminishing in size — the beginning of the bone’s adaptation for an eventual role in hearing for land animals.

In addition to gills, Tiktaalik had primitive lungs. It also had a robust rib cage, and its large forefin had shoulders, elbows and partial wrists that allowed it to support itself on the ground.

The scientists wrote in the new report that Tiktaalik “was likely a denizen of a continuum of channel, shallow water and mud flat habitats where appendage-based support, locomotion and head mobility would have been at a premium.”

All this was happening, they said, while the fish was still a fish but changing in many ways for its emerging role as a predecessor of amphibians, reptiles and dinosaurs, mammals, and eventually humans.