Further from the “clades diverge early” paper…

… noted here:

We stress that ecological and developmental explanations for early high disparity are not mutually exclusive; neither do our results allow us to distinguish between them. The hypothesis of increasing developmental constraint predicts that the increasing complexity and interdependence of ontogenetic processes with evolutionary time effectively lock down the potential for subsequent morphological innovation (14, 61–65). Such mechanisms purportedly explain why bodyplans become invariant and inflexible with time, although mechanisms by which these constraints may be lifted have been posited (66). Notable examples are the tetrapod pentadactyl limb [early tetrapods explored a range of higher digit numbers (67)], the seven cervical vertebrae of all mammals except sloths and manatees [otherwise invariant from mice to giraffes (68)] and the diagnostic head segmentation of arthropod subphyla [Cambrian genera explored numerous alternatives with relative freedom (14, 69)]. Such body patterning characters are usually controlled by Hox (homeobox) genes, which are also frequently exapted for other (often functionally and positionally unrelated) developmental roles (70). This increasing pleiotropy (more and more varied roles for the same regulatory genes) may account for the observed reduction of developmental lability. Testing this hypothesis would require detailed ontogenetic data far beyond the scope of this study.

The prevalence of early high disparity as the dominant pattern of clade evolution ranks alongside the well-known tendencies for increasing complexity (7, 8, 71, 72) and diversity (2, 8) underpinning putative macroevolutionary trends of the widest possible generality. Moreover, it seems to apply throughout the Phanerozoic, and not merely at times of global diversification (e.g., the early Paleozoic).

In short, a life system gets started with a certain early potential, runs its course, becoming more inflexible over time, and then fades out, like most human institutions. Natural selection does not create complexity; it is just how things die.