ABSTRACT
While a precise defi nition of life may remain elusive, the elements of a description of life have been widely agreed on for some time, e.g. information, replication, energy, and so on (Schrödinger 1945, Morowitz 1992, Maynard Smith and Szathmáry 1999, de Duve 2002, Martin and Russell 2003). These common aspects of life have doubtless been honed by selection throughout life’s history. Consider, for instance, one of the canonical features of living things-obtaining energy from the environment. No one would question that in the stem lineage leading to the last common ancestor of all life key innovations evolved in the basic cellular components used to convert environmental energy (e.g. food, light) into forms that were more useful to the cell. A number of shared primitive features of all modern cells, e.g. the Krebs cycle and electron transport chains, thus evolved. Perhaps less widely appreciated is that even in crown groups such as animals (see Valentine 2006 for discussion
Department of Biological Sciences, Northern Illinois University, DeKalb, Illinois 60115, USA. E-mail: neilb@niu.edu
of terminology) novel mechanisms related to energy conversion continue to evolve and to profoundly shape diversity. While little in the way of celllevel innovation has taken place in animals (but see Bryant 1991), very signifi cant steps have occurred to exploit the potential for heterotrophy inherent in multicellular Bauplans. A number of food-related adaptations have thus evolved. Some of these are structural, e.g. the animal mouth, gut, and trophic apparatus, while some required developing new ways of sensing and responding to environmental inputs, which are often food related. Many of these adaptations link old structures and pathways to new sources of environmental input and stimulus (Jacobs et al. 2007). Given the primacy of obtaining energy from the environment, strong selection continues to drive this process even in modern animals.
