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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.

Figure 3. Body cavities of animals...Click to Enlarge Image

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.

Figure 4. Multicellular animals arose from single-celled organisms...Click to Enlarge Image

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.

Figure 5. Simple, architecturally unsophisticated animals...Click to Enlarge Image

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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