The diverse early embryonic development of vertebrates and implications regarding their ancestry

It is well known that the embryonic development of vertebrates from different classes (e.g., fish, reptiles, mammals) pass through a “phylotypic stage” when they look similar, and this apparent homology is widely seen as evidence of their common ancestry. However, despite their morphological similarities, and contrary to evolutionary expectations, the phylotypic stages of different vertebrate classes arise in radically diverse ways. This diversity clearly counters the superficial appearance of homology of the phylotypic stage, and the plain inference is that vertebrates have not evolved from a common vertebrate ancestor. The diversity extends through all stages of early development—including cleavage and formation of the blastula, gastrulation, neurulation, and formation of the gut and extraembryonic membranes. This paper focuses on gastrulation, during which the germ layers originate and the vertebrate body-plan begins to form. Despite its key role in embryonic development, gastrulation occurs in fundamentally different ways in different classes of vertebrates. The inference against common ancestry becomes progressively stronger as more is discovered about the genetic and molecular mechanisms that implement development. It is increasingly evident that these are of such complexity that it is unrealistic to think that undirected variations (random mutations) could produce constructive changes to development, such as those required to account for a diversification of development from that of a common ancestor, especially while retaining a similar phylotypic stage.