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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

This article originally appeared in the March-April 1997 issue of American Scientist.

The barren Namibian desert in southern Africa, the dry outback of South Australia and the Winter Coast of the White Sea in northwestern Russia might seem unpromising Edens. But the rocks exposed in those far-flung areas hold the oldest record of animal life, a prelude to the evolutionary explosion of animal body plans that was to come. Preserved in those ancient marine sediments, which date from nearly 550 million years ago during an interval geologists call the Neoproterozoic, is a startling variety of frond-like fossils resembling sea pens, disc-shaped forms resembling jellyfish and a number of completely enigmatic forms that do not resemble any creatures living in modern oceans. The animals that left this ancient record were nearly all soft-bodied; hard skeletal remains are represented by only a few scrappy tubes whose biological affinities are uncertain. However, the activities of worms or slug-like animals are recorded by trails and burrows left behind as they crawled and dug about on their ancient sea floor. These ancient traces of animal activity closely resemble those produced by present-day organisms, and thus provide insights into the expanding anatomical complexity and behavioral repertoires of these early forms, another piece of the puzzle posed by the spectacular appearance of a diversified skeletonized fauna over only a few million years, beginning nearly 530 million years ago–the Cambrian explosion.

Figure 1. With its five eyes...Click to Enlarge Image

The Cambrian record of life is in sharp contrast with that of the preceding eons. The remains of single-celled organisms can be traced back to nearly the oldest sediments on the planet, about 3.5 billion years ago. And for the next 3 billion years or so, the earth was chiefly populated by single-celled organisms, although algae achieved a multicellular grade about 1 billion years ago. About 565 million years ago, the larger, multicellular animals of the Neoproterozoic appear in the fossil record, with their striking variety of form, only to be overshadowed about 35 million years later by the explosion of body plans recorded in early Cambrian rocks: Nearly all known kinds of shelled invertebrates appear then, including clams, snails and arthropods (the group including crabs and trilobites), soon joined by echinoderms and soon thereafter by chordates, the lineage that gave rise to humans and all other vertebrates. All of the basic architectures of animals were apparently established by the close of the Cambrian explosion; subsequent evolutionary changes, even those that allowed animals to move out of the sea onto land, involved only modifications of those basic body plans. About 37 distinct body architectures are recognized among present-day animals and form the basis of the taxonomic classification level of phyla.

The fossil record of the last 3.5 billion years thus shows not a gradual accumulation of biological form, but a relatively abrupt transition from body plans of single cells to those of a rich diversity of animal phyla. In geological terms, an explosion indeed. Was this explosion real, or is it an artifact of a strangely biased fossil record?

Over the past few years new fossil discoveries have greatly clarified the sequence of events up to and during the Cambrian explosion. This in turn has set the stage for integrating information from several fields that had once operated in near isolation. Modern techniques for extracting and analyzing molecular data have shed new light on the evolutionary relationships among the living animal groups whose roots extend back to, or even precede, the beginning of Cambrian time. Perhaps most extraordinary have been the discoveries in developmental biology. Molecular techniques have shown that the developmental systems that control patterns from eggs through embryos to adults, and thus determine animal architectures, are remarkably similar across a wide range of phyla. In spite of their similarities, which have persisted despite the long separation of the phyla, the systems produce very disparate body plans. Evolutionary biologists can now reconstruct basic aspects of the developmental control systems of long-extinct animals, and can attempt to track not only the diversification of animal form but also the establishment and evolution of the genetic controls that regulate it. Taken together, all these advances are permitting a new, multidisciplinary look at the early history of animals and into the mysteries of the Cambrian explosion.

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