From Pattern to Mechanism

It is clear from the previous chapter that a spectrum of views exists on the pattern of early animal evolution and the way in which body plans originated. This spectrum can be characterized by describing its extremes.

At the ‘radical’ extremity: there was no multicellular animal life prior to 600 my ago; there was an explosion of body plans in Ediacaran times, with many becoming extinct, and a second bodyplan explosion in the early Cambrian, again followed by many extinctions; evolution in Vendian and Cambrian times was much more ‘experimental’ than it is now; and internal factors such as developmental constraint (or early lack of it) are important in evolution as well as considerations about niche space and external adaptation.

At the opposite ‘conservative’ extremity: multicellular animals have existed since more than a billion years ago; body plans have come into being very gradually since then, and their origins require no special explanation; any appearance of ‘explosiveness’ is illusory and caused by differing degrees of preservation (due to both presence/absence of hard parts and changes in the prevailing geological conditions); the Ediacaran and Cambrian faunas are composed entirely of members of present-day phyla; early evolution was just as conservative as its later counterpart; and external adaptation is the overwhelming driving force throughout evolution, with internal, developmental considerations being of limited consequence.

Of course, the spectrum of views extending between these extremes is not a simple, unidimensional one.