Fish Fins Are Not Fingers That Failed

Fish and crossed fingers

News to Know

by Dr. Elizabeth Mitchell

Fish fingers were never on the evolutionary menu.

The evolutionary “Just So Story” of how fish turned into terrestrial tetrapods is still coming up empty. Geneticists from Geneva report they have found the necessary genetic switches to turn on limb development in fish. They had hoped to understand how fins turned into fingers but ultimately determined that they didn’t. The rays in fins, they decided, don’t correspond to fingers after all. Furthermore, the researchers did not discover any leg-making or finger-making genes in their fish. The Swiss study shows that it would take more than flipping a few genetic switches to get a fish to grow fingers.

Key Evolutionary Steps to Taking Steps on Land

For evolutionists, the belief that land animals evolved from fish has always been an article of faith. Missing from the evolutionary arsenal, however, are explanations for how fins turned into weight-bearing legs and divided themselves into limbs and digits.

Despite all the hype claiming the extinct lobe-finned fish Tiktaalik was the ancestor of us all, not even the discovery that Tiktaalik had a sturdy pelvis has come close to putting legs on the fish. Neither has it provided the fish with any sort of weight-bearing connection to support its evolution into a terrestrial tetrapod. Another fishy conundrum—the origin of fingers and toes—has also eluded those searching for a transitional pathway to terrestrial locomotion.

The Quest for Fish Fingers

Tiktaalik’s discoverer Neil Shubin explored the genetic feasibility of switching on a fish’s “inner terrestrial” a few years ago. He swapped fish and mouse regulatory genes. In mouse embryos, fish genes switched on limb development. In fish embryos, mouse genes switched on fin development. But fish still grew fins and mice still grew legs.

Other scientists reported about a year ago that mouse regulatory (HOX) genes disrupted fin development in fish embryos, replacing the wrecked fins with distorted proliferating cartilage. As one researcher admitted, “Of course, we haven’t been able to grow hands.”

While many evolutionists have assumed that fin rays and fingers were homologous, ancestrally related structures, the evidence is notably lacking. Not finding any clues in the fossil record, evolutionary geneticists hope to fill this gap in the path from fins to fingers. Joost Woltering, lead author of the Swiss fish story just published in PLOS Biology, explains, “To determine where the genetics behind this subdivision into ‘hand’ and ‘arm’ came from during evolution, we decided to closely compare the genetic processes at work in both fin and limb development.”

Fingers Need More Than Switches

HOX genes are master switches that turn groups of other genes on and off and thus control many aspects of embryonic development. University of Geneva geneticist Denis Duboule’s group has found that two clusters of HOX genes in fish and mice are quite similar.

Because these genes are expressed during embryonic development of fish fins, Duboule’s group thought fish genes would be associated with finger development in the mice, but they were not. When the fish genes are inserted into mouse embryos, they are not expressed in the part of the limb that forms fingers after all.

The geneticists nevertheless consider the discovery that fish and mice have similar regulatory HOX genes an evolutionary milestone. All that would be needed for fingers to evolve on a fish, they suggest, is a re-purposing of these regulatory genes. With the right genetic information to regulate, fish fingers should appear on the evolutionary horizon.

“It’s not the end of the argument,” Duboule says. “The end of the argument is the day you can produce a fish with digits—the day where people can sit around the table and say: Hey, these are digits.”…

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