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
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.
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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