Origin of body plans

[TeleMad]

Some relevant material. The author is refering to the origin of body plans during the Ediacaran and Cambian explosions. Bold added by me.

"The animal populations involved in these explosions were all marine, and it is probable that most inhabited the shallow waters of continental shelves. Their unicellular and algal food supplies were already well diversified (Lipps 1993), while established multicellular competitors, predators, and parasites were absent. There was thus a situation of considerable ecological opportunity - or empy niche space - that has never been repeated in biospheric history. ...

These early animals were all (by definition) multicellular, but consisted of relatively few cells compared with most present-day animals. Their ontogenetic trajectories were therefore much simpler than those to which most of our detailed embryological information relates. Also, since they had not been subject to a comparable history of selection for integration and canalization, their ontogenies were almost certainly more evolutionarily flexible than those of their later counterparts. ... a highly coordinated development that is resistant to change [is an evolutionarily derived state]. Mutations of genes controlling early development thus occurred, at the beginning of animal evolution, in a very different ontogenetic context to those occurring in Caenorhabditis, Drosophila, Mus, or any other present-day animal." (The Origin of Animal Body Plans: A Study in Evolutionary Developmental Biology, Wallace Arthur, Cambridge University Press, 1997, 226-227)