Evolution: How Fish Evolved Limbs Millions of Years Ago

Millions of years ago, animals made the transition from water to land. Yet exactly how they managed this feat has long been a source of controversy and study. The evolution of limbs in particular has long remained a mystery. Now, scientists have looked a bit closer at this phenomenon to find out exactly how limbs first emerged.

Both fish and animals possess clusters of Hoxa and Hoxd genes, which are necessary for both fin and limb formation during embryonic development. In animals, the Hox genes are responsible for organizing the body structuers during embryonic development. During mammalian development, though, Hoxd genes depend on a "bimodal" 3-diemensional DNA structure in order to direct the development of the characteristic subdivision of the limbs into arm and paw, a division which is absent from fish fins.

"To determine where the genetics behind this subdivision into 'hand' and 'arm' came from during evolution, we decided to closely compare the genetic processes at work in both fin and limb development," said Joost Woltering, one of the researchers, in a news release.

The researchers compared the structure and behavior of these gene clusters in embryos from both mice and zebrafish. In the end, they discovered similar 3-dimensional DNA organization of the fish and mouse clusters, which indicated that the main mechanism used to pattern limbs was already present in fish. Yet when inserted into transgenic mouse embryos, the fish Hox genes were only active in the mouse arm but not in the digits. This showed that fish DNA lacks essential genetic elements for digit formation.

So what does this mean? It reveals that although the digit part of the limbs evolved as a novelty in land animals, it probably happens as an "add on" to pre-existing DNA infrastructure from fish.

"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good metaphor for what has probably happened would be the process of 'retrofitting,' as is done in engineering to equip outdated machine frames with new technology," said Denis Duboule, one of the researchers, in an news release.

The findings reveal a bit more about the process of limb evolution. More specifically, it reveals how fish managed to crawl onto land millions of years ago.

The findings are published in the journal PLOS Biology.