Recent findings from a study led by Anthony Graham’s lab have revealed that the textbook account, that suggests an equivalence between pharyngeal arch development into the gill apparatus for aquatic species and the larynx for species that live on the land is wrong.
A key step in early vertebrate evolution was the transition from life in water to life on land and we ultimately evolved from the animals that took this step. Many studies have shown that during our development we retain signs of our evolutionary past – our “inner fish”. This is noticeable in many areas of the body but perhaps most clearly in the formation of a series of bulges on the side of our embryonic head, which are called pharyngeal arches.
These arches are common to the embryos of fish, frogs, reptiles, birds and mammals, and they are thought to form the blueprint for the later anatomy. In fish, these bulges will form the skeleton and muscle of the gills while in us it will form the skeleton and muscle of the larynx, the voice box. It has long been assumed that the gill apparatus and the larynx are basically similar; that is, they both form from the pharyngeal arches and they use the same genetic programme.
Poopalasundaram et al.’s open access paper, recently published in Zoological Letters, demonstrates that this is not the case and that the animals that live fully on land: reptiles, birds and mammals, have radically altered the ancient developmental programme of fish in this region of the body. The posterior pharyngeal arches in these animals do not make skeleton and muscle and have turned off the genes that fish use to organize this development. Consequently, a key step in the movement to land, was to suppress the “inner fish” in this region and to invent a new development programme that allowed for the formation of the larynx. Thus, in our development, we can see traces of both old and new and it is these together that underpin our evolution.