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
Body Plans: A Neoproterozoic Prologue
Molecular data indicate that the metazoa—multicellular
animals—arose from single-celled organisms related to
choanoflagellates, a group that apparently originated about one
billion years ago; the date is not closely constrained. Just when
the first animals evolved is also uncertain, but it must have been
sometime before the oldest traces of wormlike forms appeared, about
565 million years ago. Morphological and molecular evidence agree
that the most primitive of living animal phyla are the sponges
(Phylum Porifera). Sponges have only a few cell types differentiated
to perform specialized functions, and they lack the sort of
cell-to-cell junctions that form sheets of tissues in higher forms.
Fossil sponges have recently been discovered in Neoproterozoic sediments.
The next most advanced phyla are Ctenophora (comb-jellies) and
Cnidaria (jellyfish and sea anemones), which have two thin but
well-differentiated tissue layers separated by a gelatinous
material: a protective one surrounding the body and a digestive one
lining the gut. The majority of the late Neoproterozoic soft-bodied
fossils most resemble representatives of these two phyla.
Most analyses suggest the next major branch produced a body type
with three primary tissue layers, the flatworms (Platyhelminthes),
whose inner tissue layer produces muscles and some other organs.
However, flatworms do not have a circulatory system, so oxygen must
be transported to their inner tissue layer by diffusion, and thus
they must be flat in order to keep these tissues near their surface
oxygen supplies. Flatworm guts, like those of jellyfish, contain
only one opening, so all of the contents enter and exit through the
same aperture. Although most molecular and morphologic evidence
indicates that flatworms evolved very early in the history of
animals, they are small and soft-bodied; consequently no fossil
flatworms are definitely identified in the fossil record.
The meandering trails and burrows of the Neoproterozoic were made by
organisms capable of displacing sediments to form grooves and tubes,
sometimes marked by structures that indicate pulses of creeping or
burrowing, and in some cases containing pellets that are interpreted
as fecal remains. Most of these are traces that cannot be attributed
to sponges, anemones, or even flatworms; some of those animals can
disturb sediments, but they do not produce elongate rounded burrows
or fecal pellets. To produce such traces requires an organism that
is not flat, can propel itself by generating peristaltic waves
(waves of contraction and expansion moving along the body or along
its ventral surface, as a "foot") and has a complete gut.
Thus the fossil record puts a minimum age on an important
branchpoint in metazoan evolution: The earliest known animal traces
must have been produced by lineages more advanced than flatworms.
The kind of peristaltic locomotion that must have produced the early
traces requires a "skeleton" of fluid-filled spaces inside
a muscular sheath that can be deformed into waves to displace
sediment. Two main kinds of fluid-filled spaces that could act as
such hydrostatic skeletons are found within animal bodies:
hemocoelic (blood) spaces, which develop between the tissue
layers mentioned above, and coelomic spaces, which develop
inside the third or innermost layer. In general, animals
with only blood spaces are found in lower branches of the tree than
are animals with coelomic spaces.
Flatworms lack both sorts of body spaces, but above them on the
animal tree is the most famous branchpoint of all, a division that
gave rise to a wealth of more complex animals that have one or both
types of body space. One branch, the deuterostomes, includes
echinoderms (starfish and sea urchins), chordates (from fish to
mammals) and a number of minor groups. The second branch, the
protostomes, contains most of the familiar invertebrate animals,
including arthropods (crabs and insects), annelids (earthworms),
molluscs (snails, clams and squid) and a host of other phyla known
mostly to those lucky enough to have had an in-depth course in
invertebrate zoology. It is quite likely that most or all of the
Neoproterozoic traces were made by organisms with hemocoel-based
locomotive systems.
Among living phyla, a simple body plan that could be responsible for
some of the traces is that of the phylum Priapulida, which has a
complete gut surrounded by a capacious hemocoel that is sheathed in
turn by the muscles of the body wall. Priapulids burrow in soupy
sediments at the surface of the sea floor. Other traces look as if
they were formed by creeping snail-like animals of the phylum
Mollusca, although, as snails themselves do not appear until
significantly later, the Neoproterozoic traces may have been made by
a common ancestor of molluscs and their relatives. Just such a form,
known as Kimberella, has recently been reconstructed from a
large number of body fossils from the White Sea of Russia. This is
the first solid indication of what some of the creeping animals were
like. The diversity of these traces increases throughout the late
Neoproterozoic, and they probably represent a variety of body types.
Interpreting the enigmatic body fossils of the Neoproterozoic has
proved more difficult than assessing the trace fossils. If the body
fossils could reliably be assigned to some living phyla, it would
pinpoint a minimum date for the origin of the specific body plan
involved. Unfortunately, this is not yet possible. Although the
soft-bodied fossils that appear about 565 million years ago are
animal-like, their classifications are hotly debated. In just the
past few years these fossils have been viewed as protozoans; as
lichens; as close relatives of the cnidarians; as a sister group to
cnidarians plus all other animals; as representatives of more
advanced, extinct phyla; and as representatives of a new kingdom
entirely separate from the animals. Still other specialists have
parceled the fauna out among living phyla, with some assigned to the
Cnidaria and others to the flatworms, annelids, arthropods and
echinoderms. This confusing state of affairs arose because these
body fossils do not tend to share definitive anatomical details with
modern groups, and thus the assignments must be based on vague
similarities of overall shape and form, a method that has frequently
proved misleading in other cases.
Until 1995 paleontologists had believed there was a substantial gap
between the Neoproterozoic fossils and the first Cambrian fauna.
Most estimates placed the soft-bodied Neoproterozoic fossils as
between 600 and 640 million years old, separated from the Cambrian
by a gap of several tens of millions of years. Then field work at
late Neoproterozoic sections in Namibia revealed volcanic ash beds
near the earliest body fossils and other ash beds close to the
Cambrian boundary. These beds provided the first accurate
radiometric dates and revealed that they were younger than 565
million years. Furthermore, there was no gap: Neoproterozoic fossils
continued right up to the base of the Cambrian, which has been
established as nearly 543 million years ago by analyses of rocks
from northern Siberia. Other correlation techniques have suggested
that many Neoproterozoic assemblages found elsewhere in the world
are about the same age as those from Namibia. Thus the fossil record
of the early metazoan diversifications, including the Cambrian
explosion, is only about 40 million years long, from about 565 to
525 million years ago.
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