Establishing a framework for understanding the origin and diversification of leaves in the Palaeozoic requires information from a broad range of disciplines that include palaeontology, plant physiology, geochemistry and molecular developmental genetics. Progress in many of these fields of research has seen such a framework begin to emerge and suggests that the exceptionally long 40- to 50-Myr delay in the advent of large megaphylls was a product of environmental opportunity and genetic potential. Once the morphogenetic potential of plants was released by falling atmospheric CO2 concentrations, leaf evolution entrained global consequences not only for the regulation of CO2 and climate but also for terrestrial organisms. Plants themselves effectively policed their own evolution through their influence on the silicate-rock-weathering–CO2-climate feedback cycle.
I thank Robert Berner, Ben Fletcher and Colin Osborne for comments on the manuscript, Andrew Fleming for discussions on the evolutionary developmental biology of leaves, Carl Bowser (University of Wisconsin) for permission to reproduce his photograph in Fig. 4 and Elizabeth and Robert Berner for drawing it to my attention. I am indebted to Bill Chaloner for many stimulating discussions on the evolution of megaphylls.