The Origin of Animal Body Plans

             Douglas Erwin, James Valentine and David Jablonski

                                  Section 4

                           The Cambrian Explosion

Although abrupt geologically, the divergence of the early animal groups was
somewhat more drawn out than paleontologists recognized even a decade ago.
Recent fieldwork in Siberia and Mongolia has demonstrated that skeletal
fossils gradually became more common and diverse in the earliest part of the
Cambrian (known as the Manykaian Stage). At the same time, trace fossils
increased in diversity and abundance, including the first trace fossils that
reflect the presence of animals with limbs. However, it is in rocks of the
later Tommotian and Atdabanian stages of the Early Cambrian, between about
530 and 525 million years ago, that fossil assemblages first include most of
the basic body plans of living animals. This is the "Cambrian explosion,"
with the first appearance of mineralized skeletons of such phyla as the
Mollusca, Brachiopoda (lamp shells), Arthropoda and Echinodermata. Trace
fossils exhibit a dramatically expanded range of animal activities,
suggesting that as skeletonized forms diversified, soft-bodied groups
expanded as well. Furthermore, soft-bodied phyla, such as Annelida,
Onycophora and Priapulida, which do not have mineralized skeletons, also
make their appearance, thanks largely to a beautifully preserved soft-bodied
fauna from the Late Early Cambrian of Chengjiang, Yunnan Province, China. A
number of forms from now-extinct phyla occur in these beds. The Middle
Cambrian Burgess Shale fauna from British Columbia, Canada preserves many
soft-bodied fossils similar to those of the Chengjiang fauna, indicating
that these forms were widespread and persisted for many millions of years.
These faunas serve to emphasize the spectacular morphologic breadth that was
achieved so early in animal history. This fossil record raises many
questions as to how new body plans evolve and just how rapidly such novel
evolutionary innovations may be produced. Answering these questions requires
information from the field of evolutionary developmental biology.

                        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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