We know that the Earth was once, almost entirely, covered in water and that water animals predate land animals. Little is known, however, about the process that led water animals to make their way onto land. A new paper by Joost Woltering of University of Geneva and his associates published in the January 21, 2014 edition of PLoS Biology, provides some important clues.

Woltering and colleagues looked at clusters of Hox genes, called HoxA and HoxD, that are involved in both fin and limb development.

Hox genes are present in all land and sea animals. They are responsible for the specification of embryonic cells. In other words they determine which part of the embryo will form which part of the body. HoxA and HoxD genes specify tissue structures along the proximal-distal axis, the area between torso and fingertips. Ancient fish had the HoxA and HoxD genes and the capacity to produce limbs and digits, however, these were not expressed until they moved onto land.

The researchers were interested in whether digits are homologous to distal fin structures in fish. To test this they inserted into mice embryos the genomic regions that regulate Hox gene expression in fish fins.

“As another surprise, regulatory regions from fish triggered Hox gene expression predominantly in the arm and not in the digits. Altogether, this suggests that our digits evolved during the fin to limb transition by modernizing an already existing regulatory mechanism,” explains Denis Duboule.

“A good metaphor for what has probably happened would be the process of ‘retrofitting’, as is done in engineering to equip outdated machine frames with new technology. Only, in this case, it was a primitive DNA architecture which evolved new ‘technology’ to make the fingers and toes,” says Woltering.

This area of study is called evolutionary developmental biology or “evo devo.” It combines embryonic development and evolutionary theory to study the ancestral relationship between species.

According to a press release, further investigations will look at what has changed between the DNA elements in fish and land animals. “By now we know a lot of genetic switches in mice that drive Hox expression in the digits. It is key to find out exactly how these processes work nowadays to understand what made digits appear and favor the colonization of the terrestrial environment,” concludes Denis Duboule.

Source: Eurekalert