Historical Constraints and the Evolution of Development 1143
large-scale rapidity as the Cambrian explosion. Under Lewis's (1978) original model
of evolution from ancestral homonomy (multiple, identical segments) by accretion of
duplicated Hox genes to achieve differentiation of specialized parts along the body
axis, the Bauplan of the major animal phyla must originate separately and gradually,
as each added developmental component permits further differentiation. How, then,
could so many basic designs make such a coordinated first appearance in five to ten
million years, unless some genetic glitch or unknown environmental trigger initiated
a rampant episode of duplication in many lineages simultaneously, or unless the
pattern only represents an artifact of preservation, rather than an actual
macroevolutionary event?
But the first fruits of evo-devo (see next movement, pp. 1147-1178 for full
discussion) have reversed this scenario by documenting a full complement of Hox
genes in the most homonomously segmented invertebrate bilaterian phyla, thus
suggesting the opposite process of loss and divergence for the differentiation of
numerous complex and specialized patterns from initial homonomy (De Rosa et al.,
1999). The punctuational character of the Cambrian explosion seems far easier to
understand if the basic regulatory structure already existed in ancestral homonomous
taxa, and the subsequent diversification of Bauplan therefore marks the specialization
and regionalization of potentials already present, rather than a dedicated and
individualized addition for each major novelty of each new Bauplan. The Cambrian
explosion still requires a trigger (see Knoll and Carroll, 1999, for a discussion of
possible environmental mediators, including the classical idea of an achieved
threshold in atmospheric oxygen), but our understanding of the geological rapidity of
this most puzzling and portentous event in the evolution of animals will certainly be
facilitated if the developmental prerequisites already existed in an ancestral taxon.
Knoll and Carroll (1999, p. 2134) stress this point in a section of their article
entitled "Cambrian diversification: So many arthropods, so little time." They add (loc.
cit., see also Grenier et al., 1997, p. 551):
The entire onychophoran-arthropod clade possesses essentially the same set of
Hox genes that pattern the main body axis. Thus, Cambrian and recent
diversity evolved around an ancient and conserved set of Hox genes...
Increase in segment diversity is correlated with changes in the relative
domains of Hox gene expression along the main body axis... Most body plan
evolution arose in the context of very similar sets of Hox genes, and thus was
not driven by Hox gene duplication... Bilaterian body plan diversification
has occurred primarily through changes in developmental regulatory networks
rather than the genes themselves, which evolved much earlier.But returning to the very different, central subject of this section—the possibility
and meaning of evolutionary saltation at the organismic level of discontinuity across
generations—we may at least assert a case for plausibility, so that, at the very least,
this perennially contentious subject will not be dismissed a priori. First of all, we
cannot deny either the existence of such large