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
Ancient Developmental Systems
One of the most remarkable discoveries of the past few years is that
the major elements controlling animal development are quite similar
across a wide range of body plans. Most animals start from a single
cell, the fertilized egg or zygote. However, during development
their cells multiply and differentiate into specialized cell types
that make up muscles, nerves, glands and all the other tissues and
organs. This is an extraordinary process, given that each and every
cell in a developing embryo has exactly the same genetic information
in its DNA. Unlike their unicellular ancestors, multicellular
animals need a genetic regulatory system to specify different gene
activities in the different cell types as development unfolds to
produce an adult body plan. Many of the regulatory genes in
butterflies, giraffes and squid, for example, are similar, having
been inherited from their last common ancestor, the
protostome–deuterostome ancestor, which lived at least half a
billion years ago. Thus the striking changes in body plans have been
accompanied by relatively modest tinkering with the machinery of
early development of that long-extinct precursor.
Developmental regulation proceeds through the sequential activation
of a series of regulatory switches that in turn activate networks of
other genes. In general, the regulatory genes produce proteins that
bind to and influence the activity of other genes. The protein
products of these genes then activate still other genes, and the
cascade continues. Regulatory genes that are active early in
development help set up the body axes by determining which end of
the embryo becomes the head and which the tail, which part becomes
the back and which the belly. These early expressing genes also set
up the basic tissue types. Genes that are active later in the
cascade help block out distinctive morphological regions within the
body–differentiating, say, a head from an abdomen. Later still
in the cascade, genes mediate the growth of appendages such as
limbs, until the most refined morphological details have been
achieved. Many different classes of regulatory genes share a common
DNA sequence known as the homeobox, which predates the origin of animals.
The best-studied class of homeobox-containing genes are the
Hox genes, usually found clustered next to each other along
animal chromosomes. In complex organisms the Hox cluster
specifies the developmental fate of individual regions within the
body, and usually the genes are activated and expressed in the body
in the same order as their position in the cluster. Thus, in
arthropods, the first genes in the cluster mediate the expression of
the head and associated structures, those in the middle of the
cluster control genes that produce legs and wings on appropriate
body segments, and so on. In the phylum Nematoda, which lacks limbs
or wings, the cluster simply mediates the expression of a series of
different cell types along the body (along with other functions).
Obviously such sophisticated control systems were not needed in the
single-celled ancestors of animals, and thus their evolution is
intimately associated with the establishment and initial elaboration
of animal body plans.
The number of genes in the Hox cluster varies among animal
phyla. Sponges, the most primitive of animals, have at least one
Hox gene, whereas arthropods have eight. In mammals, the
cluster has been repeatedly duplicated to form four clusters, all
slightly different, with a total of 38 genes. It looks very much as
if the Hox clusters become larger with increasing body plan
complexity, although this cannot be the entire story. Some primitive
forms have clusters unlike the Hox cluster of higher forms.
Furthermore, mammals and arthropods both display a striking
diversity of morphologies within each of their body plans, but this
diversity is generated chiefly by genes active after the
Hox-cluster genes have done their work. Many different body
plans can be specified by similar genes early within a cascade,
whereas morphological complexity can be achieved by regulatory
evolution in many parts of the cascade. What seems clear is that
morphological evolution of body plans, such as is documented by
Neoproterozoic and Cambrian fossils, involved increases in the
complexity of both the body plans and the regulatory systems that
specified them. With each new variation, there is an alteration in
the relation between the regulatory genes and the so-called
structural genes that actually produce the proteins and eventually
the lipids and other building blocks that make up an organism.
Combining molecular views of animal phylogeny with the trace-fossil
record helps evolutionary biologists reconstruct the primitive body
plans that gave rise to the living phyla. As the important findings
of developmental biology lead to a greater understanding of gene
regulation, scientists can begin to reconstruct primitive
developmental systems and their pattern of evolution. The synthesis
of these fields, which is just beginning, will yield a much more
complete picture of the early evolution of animal life.
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