Biomineralization and Evolutionary History

Abstract

The Dutch ethologist Niko Tinbergen famously distinguished between proximal and ultimate explanations in biology. Proximally, biologists seek a mechanistic understanding of how organisms function; most of this volume addresses the molecular and physiological bases of biomineralization. But while much of biology might be viewed as a particularly interesting form of chemistry, it is more than that. Biology is chemistry with a history, requiring that proximal explanations be grounded in ultimate, or evolutionary, understanding. The physiological pathways by which organisms precipitate skeletal minerals and the forms and functions of the skeletons they fashion have been shaped by natural selection through geologic time, and all have constrained continuing evolution in skeleton-forming clades. In this chapter, I outline some major patterns of skeletal evolution inferred from phylogeny and fossils (Figure 1⇓), highlighting ways that our improving mechanistic knowledge of biomineralization can help us to understand this evolutionary record (see Leadbetter and Riding 1986; Lowenstam and Weiner 1989; Carter 1990; and Simkiss and Wilbur 1989 for earlier reviews). Figure 1. A geologic time scale for the past 1000 million years, showing the principal time divisions used in Earth science and the timing of major evolutionary events discussed in this chapter. Earlier intervals of time—the Mesoproterozoic (1600–1000 million years ago) and Paleoproterozoic (2500–1600 million years ago) eras of the Proterozoic Eon and the Archean Eon (> 2500 million years ago)—are not shown. Time scale after Remane (2000). My discussion proceeds from two simple observations: 1. Skeletal biomineralization requires energy and so imposes a metabolic cost on skeleton-forming organisms. 2. Mineralized skeletons have evolved in many clades of protists, plants and animals. If both statements are true, then clearly for many but not all eukaryotic organisms the biological benefits of biomineralization must outweigh its costs. But cost-benefit ratio is not static. It will change through …