Tracing the Explosion to Its Roots

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Science  12 May 2000:
Vol. 288, Issue 5468, pp. 929
DOI: 10.1126/science.288.5468.929a

The term Cambrian “explosion” describes the rapid diversification (on a geological timescale) of animals to form most of the major extant phyla. The fossil record of metazoa shows a sudden expansion at around 550-530 million years ago. Molecular data, however, suggest that the roots of this diversification stretch back much further in time, perhaps to the origin of the eukaryotes some 1000 million years ago.

Increasingly, palaeontologists, molecular phylogeneticists, and developmental biologists are attempting to integrate insights from their respective disciplines to address this conundrum. Conway Morris argues that the Cambrian explosion is a genuine phenomenon, driven largely by ecological processes—filter feeding, predation, and defense. Nevertheless, he points out that much of the metazoan genome was probably in place substantially earlier, the explosion representing a redeployment of existing genetic resources. In another commentary, Peterson and Davidson explore aspects of this redeployment with reference to the developmental biology of bilaterally symmetric animals. In their view, the advent of regional specification mechanisms (which govern the development of different body parts) and the full complement of Hox gene complexes that control development had come into existence by the beginning of the Cambrian.

Chen et al. provide possible fossil evidence for early developmental stages of metazoan organisms from the Precambrian. Their microfossils, from formations in Southwest China, resemble the larvae and free-floating embryos of cnidaria (sea anemones, jellyfish, corals), lending weight to the theory of earlier evolution of developmental genetic architecture. The confirmation that these fossils are indeed biogenic awaits further study, but if confirmed they will provide evidence for a long fuse to the explosion.—AMS

Proc. Natl. Acad. Sci. U.S.A.97, 4426; 4430; 4457 (2000).

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