On Wednesday, a team of researchers at the University of Chicago reported that our hands share a deep evolutionary connection not only to bat wings or horse hooves, but to fish fins.
The unexpected discovery will help researchers understand how our own ancestors left the water, transforming fins into limbs that they could use to move around on land.
To the naked eye, there’s not much similarity between a human hand and the fin of, say, a goldfish. A human hand is at the end of an arm. It has bones that develop from cartilage and contain blood vessels. This type of tissue is called endochondral bone.
A goldfish grows just a tiny cluster of endochondral bones at the base of its fin. The rest of the fin is taken up by thin rays, which are made of an entirely different tissue called dermal bone. Dermal bone doesn’t start out as cartilage and doesn’t contain blood vessels.
These differences have long puzzled scientists. The fossil record shows that we share a common aquatic ancestor with ray-finned fish that lived some 430 million years ago. Four-limbed creatures with spines — known as tetrapods — had evolved by 360 million years ago and went on to colonize dry land.
For over two decades, Neil H. Shubin, an evolutionary biologist, has investigated this transition in two radically different ways.
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Dr. Nakamura inserted bits of DNA into the fish versions of the Hoxa-13 and Hoxd-13 genes. The inserted DNA garbled the sequence of the genes, so that the fish couldn’t make proteins from them.
Zebrafish with defective copies of both Hox genes grew deformed fins, the scientists found. But to their surprise, the fish failed to make fin rays. In the fish, their experiment showed, the Hox genes were controlling cells that became dermal bone rather than the endochondral bone found in our own limbs.
Dr. Shubin got a similar surprise when he saw the results of a parallel experiment run by his graduate student, Andrew R. Gehrke. Mr. Gehrke engineered zebrafish so that he could follow individual cells during the development of embryos.
In Mr. Gehrke’s altered fish, cells that switched on the Hox genes started to glow. They kept glowing throughout development, until they reached their final location in the fish’s body.
Mr. Gehrke observed that a cluster of cells started making the Hox proteins early in the development of fish fins. When the fins were fully developed, Mr. Gehrke found that the fin rays were glowing. In a similar experiment on mice, the digits and wrist bones lit up.
“Here we’re finding that the digits and the fin rays have some sort of equivalence at the level of the cells that make them,” Dr. Shubin said. “Honestly, you could have knocked me over with a feather — it ran counter to everything that I was expecting after working on this problem for decades.”
The new study was important because it revealed that the development of fins and limbs follows some of the same rules, said Matthew P. Harris, a geneticist at Harvard Medical School. In both cases, the Hox genes tell a clump of embryonic cells that they need to end up at the far end of an appendage. “The molecular address is the same,” said Dr. Harris, who was not involved in the study.
In zebrafish, the cells that get that molecular address end up making dermal bone for fin rays. In tetrapods like us, the research indicates, the same cells produce endochondral bone in our hands and feet.
