During the phylotypic stage, embryos of birds, fish and even humans start to look the same — before they diverge again and become very different looking animals. The similarity was first described by 19th-century embryologist Karl Ernst von Baer, who accidentally mixed up phylotypic-stage embryos of different vertebrate species and was unable to differentiate between them.
“If you were to put a human embryo next to a fish, a toad and a mouse at that stage, the human embryo would look very much like the others,” said lead author Dr Ozren Bogdanovic, from the ARC Centre of Excellence in Plant Energy Biology at UWA. This is despite the time taken to reach the phylotypic stage varying between species, occurring 1–2 days after fertilisation in fish and toads, 9.5 days after conception in mice and four weeks after conception in humans.