A study has shed light on a key genetic step in the evolution of animals' limbs from the fins of fish, scientists say.
A team of researchers identified two new genes that are important in fin development.
They report in the journal Nature that the loss of these genes could have been an "important step" in the evolutionary transformation of fins into limbs.
Marie-Andree Akimenko, from the University of Ottawa in Canada, led the research.
She and her colleagues began their study by looking at the development of zebrafish embryos. They discovered two genes that coded for proteins that were important in the structure of fins.
These proteins were components of the thread-like fibres known as "actinotrichia". These are found in fish larvae and they eventually develop into the bony fin rays of mature fish.
"We found there were no [equivalent genes] in limbs, so this suggested these may have been lost in evolution," explained Dr Akimenko.
To confirm this, they looked for - and found - the same family of genes in the genomes of elephant sharks, which are a very basal (or ancient) fish species.
This suggested that the "ancient family of genes persisted in [bony fish] and was lost when they evolved" into four-footed animals, Dr Akimenko said.
Embryo development can provide important genetic and molecular clues about evolution; many early developmental changes are believed to mirror evolutionary changes.
The scientists were able to manipulate zebrafish development, to study these changes in more detail. They inactivated the newly discovered genes in a developing zebrafish embryo. When they did this, they found that it developed shorter "truncated" fins with no bony rays.
The loss of these fin rays, the scientists say, was a key step in fin-to-limb evolution.
The team then compared the development of normal zebrafish embryos with that of mouse embryos.
"When we compared fin development and limb development, the early steps are very similar," Dr Akimenko said.
"But at one point there is a divergence, and that correlates with the beginning of the expression of these genes."
Professor Jonathan Bard, a retired developmental biologist now working with the department of physiology, anatomy and genetics at Oxford University, said the findings were only a very small part of the evolutionary story.
He said that this still did not tell us about digit formation - "how the broad, multi-ray fins of fishes became transformed into the eight digits of the hand or foot plate of the first tetrapods".
"More generally," he said, "hundreds of millions of years of separate evolution divide [bony fish] and mice."
He added: "It is an interesting paper... and it will be be interesting to see what the [researchers] do next."