Phylotypic stage
[2] In 1866, Ernst Haeckel proposed that each developing organism passes through the evolutionary stages of its ancestors, i.e., ontogeny recapitulates phylogeny.
[8] While this concept was originally devised using morphological comparisons of developing embryos from different species,[7] the period of maximal similarity has recently been identified using molecular evidence.
Evidence for an alternative model arose from careful comparisons of the temporal divergence in morphology of the embryos of different species.
For example, Klaus Sander noticed that the “incredible variation in larvae and adults” of insects occurs after they "develop from nearly identical rudiments in the germ band stage.
[15] The advent of next-generation sequencing enabled scientists to use molecular methods to identify the period of development that has the most conserved gene expression patterns among different species.
Domazet-Lošo and Tautz[10] analyzed the transcriptome of zebrafish (Danio rerio) over developmental time, from unfertilized eggs to adults.
[citation needed] Other recent genomic studies have supported a mid-developmental phylotypic stage in vertebrates[9] and in the plant Arabidopsis thaliana.
The pharyngula stage occurs when the four distinguishing features of vertebrates (notochord, dorsal hollow nerve cord, post-anal tail, and a series of paired branchial slits) have developed.
[10] They found that applying a log-transformation to the gene expression data changed the results to support highest conservation in early development.
[20] Despite increasing evidence supporting the hourglass model, identifying the point in development that is most conserved among species with a phylum (the phylotypic period) is a controversy in the field of developmental biology.
