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Chance and necessity: the evolution of morphological complexity and diversity


The primary foundation for contemplating the possible forms of life elsewhere in the Universe is the evolutionary trends that have marked life on Earth. For its first three billion years, life on Earth was a world of microscopic forms, rarely achieving a size greater than a millimetre or a complexity beyond two or three cell types. But in the past 600 million years, the evolution of much larger and more complex organisms has transformed the biosphere. Despite their disparate forms and physiologies, the evolution and diversification of plants, animals, fungi and other macroforms has followed similar global trends. One of the most important features underlying evolutionary increases in animal and plant size, complexity and diversity has been their modular construction from reiterated parts. Although simple filamentous and spherical forms may evolve wherever cellular life exists, the evolution of motile, modular mega-organisms might not be a universal pattern.

“Drawn out of the realm of pure chance, the accident enters into that of necessity, of the most implacable certainties.” J. Monod1

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Figure 1: History of major evolutionary events from the fossil record.
Figure 2: The phylogenetic relationships of multicellular taxa.
Figure 3: Patterns of diversification.
Figure 4: Passive and active evolutionary trends.
Figure 5: The evolution of limb-type complexity in aquatic arthropods.


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I thank A. Knoll for discussions and helpful pointers; R. Losick and A. Johnson for information on microbial gene regulation; G. Budd, J. Crow, N. King and J. True for suggestions on the text; J. Carroll for preparation of the manuscript; and L. Olds for the artwork. S.B.C. is an Investigator of the Howard Hughes Medical Institute.

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Carroll, S. Chance and necessity: the evolution of morphological complexity and diversity. Nature 409, 1102–1109 (2001).

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