Major Changes Over the History of Life: Origins, Radiations and Extinctions

Ever since Darwin, origins of higher taxa have proved elusive, yet many missing links have come to light. Archaeopteryx was the first dramatic find, and the most recent is Tiktaalik. Living fossils have also come into play. The discovery of Neopilina in effect brought monoplacophorans to life and made them clearly the crown group of the Mollusca.

G.G. Simpson emphasized adaptive breakthroughs as triggers for evolutionary radiation. Continuously growing teeth of rodents are an example, as are mantle fusion and siphon formation in bivalve mollusks and origins of vascular tissue and seeds in land plants. Early on, explosive radiations seemed problematical. Darwin was troubled by the initial Cambrian radiation of animals and termed the dramatic Cretaceous radiation of the angiosperms "an abominable mystery." We now recognize that these radiations were not instantaneous, and the punctuational model of evolution alleviates problems concerning rates of diversification.

Our understanding of mass extinctions has expanded dramatically in the past four decades. Remarkably, many of these events have been recognized only recently: the terminal Ordovician, Late Devonian, mid-Carboniferous, Guadalupian, terminal Triassic, Cenomanian-Turonian, Late Eocene, and regional Plio-Pleistocene crises. There are still puzzles. For example, for the terminal Cretaceous event, the ultimate cause -- a meteorite impact -- is well established, but we have not yet identified the proximate causes (agents of death). The opposite problem persists for the terminal Ordovician event, for which it is clear that global climatic changes associated with the expansion and contraction of continental glaciers were the proximate agents of extinction but the causes of these climatic changes are still debated. Recognition of mass extinctions has recently led to the discovery that important taxa have inherited the earth following major extinctions. Dinosaurs did so after Late Triassic therapsid extinctions and angiosperms expanded ecologically, following Late Cretaceous extinctions of gymnosperms.