The Questions of Developmental Biology

"Life's splendid drama"
Until this decade, "many invertebrate biologists saw the reconstruction of relationships
among the phyla as an insoluble dilemma. . . . Indeed, as late as 1990, a comprehensive summary
concluded that the relationships between most of the higher animal groups were entirely
unresolved" (Erwin et al. 1997). However, in the 1990s, a broad consensus on the general form of
a phylogenetic tree of life began to emerge among paleontologists, molecular biologists, and
developmental geneticists (see Winnepenninckx et al. 1998; Davidson and Ruvkun 1999; Erwin
1999). This consensus (one representation of which is shown in Figure 22.1A) came about from
(1) improved methods of analyzing DNA, taking into account its variation within groups of
animals, (2) new data on conserved regulatory gene sequences such as the Hox genes, which are
usually stable within phyla but can diverge between phyla, (3) morphological evidence for the
related nature of some structures that had once been thought to be distinct, and (4) computer
programs that can sort out enormous amounts of data, not privileging any particular set of
relationships over others. The results were surprising to many scientists.
1. The animal kingdom can be divided into Porifera (sponges), Cnidaria and Ctenophora (jellyfish
and comb jellies), and the Bilateria. The Porifera lack any coherent epithelium or any symmetry.
The Cnidaria and Ctenophora are diploblastic (with two epithelial layers, lacking mesoderm) and
have radial symmetry.
2. The Bilateria are triploblastic (with true endoderm, mesoderm, and ectoderm), have bilateral
symmetry and can be divided into two groups, the deuterostomes and the protostomes. The
deuterostomes include the echinoderms and the chordates. (Echinoderm larvae are originally
bilateral.) Deuterostomes form their anuses from their blastopores. They probably arose from a
tornaria larva similar to that of the pluteus.
3. The protostomes can be divided into two groups, the Ecdysozoa (animals whose bodies are
covered by an exoskeleton, which therefore molt as they grow) and the Lophotrochozoa
(animals that have most or all of their soft tissues in contact with the environment and which
generally use cilia in feeding or locomotion). Nematodes and flatworms (which lack true
coeloms) had formerly been considered basal groups, perhaps ancestral to both the protostomes
and the deuterostomes. However, the new studies have shown that the nematodes belong to the
Ecdysozoa and the flatworms (as Wilson had predicted) to the Lophotrochozoa. Each of these
two groups is monophyletic, meaning that the phyla in each of them share a common ancestor.