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
The Next Questions about Body Plans
Clearly many difficult questions remain about the early radiation of
animals. Why did so many unusual morphologies appear when they did,
and not earlier or later? The trigger of the Cambrian explosion is
still uncertain, although ideas abound. If the evolutionary trees
are right and the fossil record is not deceptive, then many
different lineages must have acquired complex anatomies and hard
parts at about the same time. Whether the burst was the result of an
increase in oxygen, an ecological arms race or something else, the
elaboration of Hox complexes may have been necessary, but
it was not sufficient, to drive the evolutionary creativity of the Cambrian.
It is also difficult to explain why the innovation declined.
Possibly, the ecological barrel become full, so that major novelties
could no longer readily gain a foothold. Alternatively, the
integration of regulatory controls reached a point where major
restructuring of body plans became exceedingly difficult. These
debates are ongoing and reflect both the difficulties inherent in
resolving such complex problems and the health of a fast-moving,
intellectually ambitious set of once-disparate disciplines.
Throughout these debates, the timing of events as recorded in
the rocks has been vital in shaping our understanding of the
Cambrian explosion. New geological evidence has both spread out the
stratigraphic interval over which these new morphologies appear and
simultaneously constricted the duration of the radiation. The new
dates for the late Neoproterozoic restrict the entire radiation,
from the beginning of the radiation with the trace and body fossils
of the Ediacaran through the basal Cambrian explosion of the first
good skeletal fossils and the explosion of trace fossils, to a mere
40 million years. This is the most extraordinary pace of morphologic
innovation yet recognized in the fossil record, and there are strong
suggestions that the origin of the regulatory controls that underpin
animal development played an important role in these events. Clearly
we have much to learn about the behavior of developmental-control
genes during morphologic evolution, but just as clearly, there is
potential in this partnership of paleontology, developmental biology
and molecular systematics for profound advances in our understanding
of the origin and diversification of body plans.
Bibliography
- Akam, M., et al., eds. 1994. The Evolution
of Developmental Mechanisms. Cambridge, England: The
Company of Biologists, Ltd.
- Brasier, M. D., G. Shields, V. N. Kuleshov and E. A.
Zhegallos. 1996. Integrated chemo- and biostratigraphic
calibration of early animal evolution: Neoproterozoic-early
Cambrian of southwest Mongolia. Geological Magazine 133:445-485.
- Bowring, S. A., J. P. Grotzinger, C. E. Isachsen, A. H.
Knoll, S. M. Pelechaty and P. Kolosov. 1993. Calibrating rates
of Early Cambrian evolution. Science 261:1293-1298.
- Carroll, S. B. 1995. Homeotic genes and the evolution of
arthropods and chordates. Nature 376:479-485.
- Chen, J.-Y., and B.-D. Erdtmann. 1991. Lower Cambrian
fossil lagerstatte from Chengjiang, Yunnan, China: Insights for
reconstructing early metazoan life. In The Early Evolution
of Metazoa and the Significance of Problematic Taxa,
ed. A.M. Simonetta and S. Conway Morris. Cambridge, England:
Cambridge University Press. pp. 57-76.
- Crimes, T. P. 1994. The period of early evolutionary
failure and the dawn of evolutionary success: The record of
biotic changes across the Precambrian-Cambrian boundary. In
The Paleobiology of Trace Fossils, ed.S. K.
Donovan. London: Wiley. pp. 105-133.
- Erwin, D. H. 1993. The origin of metazoan development.
Biological Journal of the Linnean Society 50:255-274.
- Fedonkin, M. A., and B. M. Waggoner. 1996. The Vendian
fossil Kimberella: The oldest mollusk known. Geological
Society of America, Abstracts with Program. 28 (7): A-53.
- Grotzinger, J. P., S. A. Bowring, B. Saylor and A. J.
Kauffman. 1995. New biostratigraphic and geochronological
constraints on early animal evolution. Science 270:598-604.
- Kappen, C., and F. H. Ruddle. 1993. Evolution of a
regulatory gene family: HOM/Hox genes. Current
Opinion in Genetics and Development. 3:931-938.
- Knoll, A. H., A. J. Kaufman, M. A. Semikhatov, J. P.
Grotzinger and W. Adams. 1995. Sizing up the sub-Tommotian
unconformity in Siberia. Geology 23:1139-1143.
- Raff, R. A. 1996. The Shape of Life. Chicago:
University of Chicago Press.
- Valentine, J. W. 1994. Late Precambrian bilaterians:
grades and clades. Proceedings of the National Academy of
Sciences 91:6751-6757.
- Valentine, J. W., D. H. Erwin and D. Jablonski. 1996.
Developmental evolution of metazoan body plans: The fossil
evidence. Developmental Biology 173:373-381.
- Wilmer, P. 1990. Invertebrate Relationships: Patterns
in Animal Evolution. Cambridge, England: Cambridge
University Press.
» Post Comment