FEATURE ARTICLE
The Origin of Animal Body Plans
Recent fossil finds and new insights into animal development are providing fresh perspectives on the riddle of the explosion of animals during the Early Cambrian
Douglas Erwin, James Valentine, David Jablonski
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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