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
Early Evolution of Hox Clusters
As a first step towards a developmental history of animal
architectures, we can begin to reconstruct the evolution of the
Hox clusters using information from developmental biology
and knowledge of the relationships between different phyla. Any gene
found in both flies and mice, for example, must have evolved prior
to the last common ancestor of the two lineages. As each key
developmental control gene is discovered, developmental biologists
quickly search for its cognate gene in distantly related groups,
effectively peering back in time to the ancestors of each lineage.
Six Hox genes are shared between mice and flies,
indicating that their common ancestor, which lived before the
Cambrian explosion, had a Hox cluster composed of at least
six genes. As living arthropods have eight Hox genes, two
of them must have originated by duplication after the divergence of
the ancestors of arthropods and vertebrates. Today these two newer
Hox genes mediate the development of segments in the middle
of the arthropod body. Hox genes mediating the development
of the midbody, in addition to other developmental features, were
also duplicated within the lineage leading to mammals. Although not
all phyla have been studied, and the quality of the data remains
variable, Hox clusters within phyla that have been well
studied are distinctive, produced by unique patterns of gene
duplication and loss.
Both mammals and arthropods have segmented body plans, and one might
reasonably conclude that their common ancestor might also have been
segmented, for each group employs the Hox gene array to
control segmentation. However, the evolutionary tree suggests that
these two groups arose from a nonsegmented ancestor in which
Hox genes probably helped to specify the production of a
series of structures repeated along the body axis, but not of
segments, just as they mediate cell differentiation along the axis
of nematode worms today. A similar situation is found with the genes
that control the development of limbs: Some regulatory genes, such
as distalless and its relatives, help generate both
arthropod and mammal legs, yet both the family tree and the fossil
record indicate that the common ancestor of these groups lacked
limbs, which evidently arose just before the Cambrian explosion and
thus after the groups diverged. Eyes provide still another example:
The regulatory gene at the top of the cascade that produces eyes in
mice (Pax-6) is so similar to that in insects that the
genes can be interchanged and still function correctly. Yet insect
and mammal eyes are both complicated structures and each quite
different. Each eye has clearly evolved independently from a very
simple common precursor.
These examples begin to give biologists a picture of how body
plans, and the genetic machinery that generates them, actually
evolved. As animals emerged, developmental control genes evolved
that regulated the architecture of their multicellular,
differentiated bodies. The fundamental job of these genes was to
mediate the production of various cell types by other genes farther
down the cascade of gene expression, and to array the cell types
within tissues and organs in the appropriate order. Even as body
plans changed and anatomical structures evolved, the basic
regulatory genetic system nevertheless remained intact. Doubtless,
as body plans became more elaborate and more cell types were
required, the gene-regulatory systems were enlarged. Still, it seems
that regulatory genetic modules were conserved during evolution and
suites of genes already present were deployed to generate novel
structures. Thus the genes that direct animal development evolve in
the same quirky, opportunistic ways as the morphologies that they produce.
Perhaps the relative abruptness with which metazoan body plans
were elaborated to produce the Cambrian explosion can be explained
by this organizational structure. A cascade of developmental
signals, perhaps organized into a complicated hierarchy of gene
expression, was able to alter the network of structural gene
expressions and interactions, rapidly producing distinctive body plans.
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