Two former Penn researchers study the evolution of fins and feet.
More than 370 million years ago, our water-dwelling ancestors crept up onto land because their fins had become bonier, developing into very primitive versions of hands and feet.
Turns out something similar had occurred in fish who stayed behind in the water, according to University of Chicago biologists.
The researchers, two of them formerly at the University of Pennsylvania, made the prehistoric finding by studying the genes of the modern paddlefish - best known for its caviar.
Different animals have many of the same genes. What makes them different, in many cases, are the molecular instructions that dictate which genes are turned on, and when.
In last week's issue of Nature, the biologists said some of the genes used to make paddlefish fins are also activated to make hands or feet in legged animals.
But paddlefish are on a different branch of the evolutionary tree - the one that leads to trout, salmon and other ray-finned fish, not the branch that leads to humans.
So the fishy ancestors of both major groups must have had these genetic instructions before they split apart more than 400 million years ago, says senior author Neil Shubin.
While this genetic program to make the precursor of a "hand" still exists in the paddlefish, in most modern fish it seems to have been lost, says lead author Marcus Davis - who, like Shubin, used to be at Penn.
"You have two directions," says Shubin, who grew up in Lower Merion: "The direction to modern, ray-finned fish, where that program has been lost, and the direction that led to us, where that program has been enhanced."
The big split was at least 25 million years before the arrival of Tiktaalik roseae, the crawling fish discovered in 2004 by Shubin and Ted Daeschler, of Philadelphia's Academy of Natural Sciences.
Next on the menu: a similar study in sharks.
- Tom Avril