After the Cambrian Explosion The Evolution of Diversity

Another aspect of the Cambrian (and one that we admit to rather cavalierly omitting) is that not only did diversity and complexity of species increase

Figure 7.3 Various interpretations of the history of life and its disparity. (A) The traditional view, whereby disparity steadily increases through geological time. (B) The view presented by S.J. Gould, whereby maximum disparity occurred in the Cambrian. (C) The view that disparity increased very rapidly in the Cambrian and thereafter stayed much the same. (D) The view that disparity increased rapidly in the Cambrian and since then has generally increased, though at varying rates. (From Simon Conway Morris.)

through time, but the ecosystems in which they lived changed as well. The evolution and emergence of eukaryotic creatures, culminating in the Cambrian Explosion, was accompanied by a shift from bacterial ecosystems to assemblages far more diverse and complex. The dramatic decline of stromatolites, the layered bacterial structures that were so common until a billion years ago, may be evidence of this transformation from a prokaryote-dominated to a eukaryotic world. With the rise of animals, efficient herbivores appeared, and the passive bacterial mats that we call stromatolites when they are fossilized, served as food for the emergent herbivores.

The Cambrian was the time when the most profound of these changes occurred. Yet it was not the last period of major diversification. Paleontologist Jack Sepkoski of the University of Chicago has spent more than two decades analyzing the diversity of organisms through time. He identifies two significant episodes of diversification after the Cambrian: one in the lower Ordovician (the period immediately succeeding the Cambrian) and one at the start of the Cenozoic era, the time interval some 65 million years ago that immediately followed the great extinction that wiped out the dinosaurs and so many other species. The major unanswered question is whether the great rise in species that characterized the last 500 million years was inevitable once animals arose or was itself due to chance.

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