The Origin of Animal Body Plans

             Douglas Erwin, James Valentine and David Jablonski

                                  Section 2

                         The Family Tree of Animals

Only 10 years ago many invertebrate biologists saw the reconstruction of
relationships among the phyla as an insoluble dilemma; two centuries of
comparative anatomy had not yielded a consensus, and there seemed little
hope of resolution. Indeed, as late as 1990 a comprehensive summary
concluded that the relationships between most of the higher animal groups
were entirely unresolved. Yet even as that summary was being written, the
introduction of molecular techniques was beginning to provide the data
necessary for a new assessment of animal affinities. The molecular data
consist of long sequences of the four nucleic acids that make up the
information encoded in DNA. Sequences from closely related species differ
only slightly because of random mutations within a gene, whereas sequences
from more distantly related species have accumulated more differences. The
evolutionary relationships among species can thus be determined by comparing
sequences of the same gene from different species. After these comparisons
are made, species are placed on an evolutionary tree, where the branch
points represent points of divergence between species or even whole animal
groups.

Choosing appropriate genes to study can be difficult, however, because
different genes evolve at different rates. If the changes in sequence are
too few, there is not enough difference among species to resolve the
branching pattern on an evolutionary tree; too many changes overwhelm or
"saturate" the DNA, so that any original similarities resulting from common
ancestry are lost. Thus, for example, a gene that changes rapidly enough to
be useful for examining the differences between two recently separated
species of mice evolves too rapidly to be appropriate for examining the
differences between the ancestors of a mouse, an earthworm and a fly, which
lived over half a billion years ago.

Even with the appropriate genes, the molecular tree of life is difficult to
interpret. For one thing, many of the phyla appear to have branched within a
relatively short period of time. Therefore, the slowly evolving genes
suitable for probing such ancient events changed relatively little between
successive divergences, and it may be difficult or impossible to resolve the
order of branching. Furthermore, with only four nucleic acids involved in
the genes, similarities can arise by chance or through biases in
substitutions of one nucleic acid for another that are unrelated to kinship
among the species. More than 100 different numerical techniques have been
developed to counter such problems, and whereas many of the divergences
among phyla remain uncertain, others seem to be well established by the new
molecular analysis.

The pattern of divergence among the phyla does not solve the larger problem,
for the branching sequence tells biologists too little about when the body
plans themselves originated. This is because at the branchpoints‚when the
lineages split and the molecules began to change independently‚each branch
had precisely the same body plan, and it may have been many millions of
years before a new body plan arose. Consider an evolutionary tree depicting
a swordfish, a fly and Marilyn Monroe. Marilyn would be placed closer to the
swordfish than to the fly. However, the divergence between Marilyn and the
swordfish, which happened over 400 million years ago, did not immediately
produce their disparate architectures. When these lineages first separated
they shared nearly all morphologic characteristics and differed in very few;
it would have taken an expert to distinguish them. Not any more. And when
did these striking architectural changes take place? To reconstruct the
events that led from the branchings to the various animal body plans we must
leave the molecules and turn to the fossil record.
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