for National Geographic News
The basic process for developing fingers and toes in land animals may have existed for more than 500 million years in shark genes, according to a new study.
Researchers identified genetic activity in spotted catsharks embryos that signal the creation of digits.
The discovery pushes back the date of the evolutionary "fin to limb" advance by some 135 million years.
When a gene—essentially a set of instructions—is translated into a trait, such as red hair or an arm, it is said to be expressed.
Scientists have long believed that the gene for digit development was first expressed some 365 million years ago in the earliest tetrapodsthe first vertebrates to walk on land. (Related: "Ancient Fish Fossil May Rewrite Story of Animal Evolution" [October 18, 2006].)
But the new study suggests the finger-and-toe gene was first expressed much earlier, in fish—though not to such an extent that it yielded actual digits.
"We've uncovered a surprising degree of genetic complexity in place at an early point in the evolution of appendages," study leader Martin Cohn of the University of Florida said in a statement.
The findings appear this week in the journal PLos ONE.
Limb development, which happens in the fetal stage of all limbed animals, is driven by the so-called Hox gene. (Get the basics on the human genome.)
The early stage of the Hox gene expression regulates the development of limbs down to the forearm and shin. The later phase is responsible for forming fingers and toes.
"It has long been thought that fish fins exhibit only the early wave of Hox expression..." Cohn told National Geographic News.