Australia: The Land Where Time Began

A biography of the Australian continent 

Vertebrate origins & development - Paedomorphosis

The study of the embryonic stages of basal chordates and vertebrates can give a great deal of information of the ancestral stages of vertebrates, and developmental biologists have been able to gain much information about the relationship of specific anatomical structures to the genes in the genome, often finding instances in which genes coding for particular organs or functions are shared among organisms that differ widely with very long histories of independent development. This has allowed the testing of hypotheses of homology between organs by finding shared genes, a case in point is the findings of recent studies of amphioxus.

According to an older theory on the origin of vertebrates, the organism that gave rise to all later chordates was the sea squirt, based on studies by Walter Garstang in the 1920s, when he noted the similarities between the larval sea squirt, the adult amphioxus and the vertebrates. Garstang believed the tail of the sea squirt lava was a transient appendage, an outgrowth from the body, to aid in the dispersal of these animals, proposing an evolutionary link between the sea squirt and all higher vertebrates being achieved by the process of paedomorphosis, in which what is essentially a juvenile animal becomes sexually mature (Garstang, 1928). In his theory at some point in the distant past a sea squirt lava failed to metamorphose before becoming reproductively mature, remaining as a swimming larva, though with the capability of reproducing.

This theory has now been rejected by recent tunicate phylogenies based on molecular studies suggesting the developmental characters are unique, so not giving rise to the vertebrates.

Sources & Further reading

1. Benton, Michael J., 2005, Vertebrate Palaeontology, 3 rd ed., Blackwell Publishing.