physical parameters that are not sensed by some living organism. Elephants hear sounds of much longer wavelength than humans and have specialized cells in their feet to sense seismic vibration. Their vibrational sense is specific enough to distinguish vibrations with different meanings, and the elephants react differently depending on whether the signal is coming from a familiar elephant or a stranger (O’Connell-Rodwell et al., in press). Plants acquire information about day length and temperature, and in temperate climates they use this to time their budding and flowering. Organisms from bacteria to humans use light and other cues to entrain circadian, or daily, rhythms so that these internally generated rhythms are synchronized to the fluctuations in their environment (Nakajima et al., 2005).
The ability to detect different environmental signals and adjust behavior accordingly is evolutionarily ancient. Bacteria produce many small molecule signals that are used for communication both within and among species (Bassler and Losick, 2006). The realization that single-celled organisms—often perceived as “primitive” and “simple”—live in complex mixed-species communities and use a variety of chemical signals to detect community density and composition has stimulated a reexamination of our theories regarding the basic parameters of bacterial life. Theoretical approaches to understanding how organisms sense and respond are likely to be profitably employed across biological scales.
The ability to receive and process external cues also plays a critical role in multicellular development. For a single fertilized egg to develop into a highly differentiated and organized multicellular organism, individual cells must receive cues about their location and future role in the organism to migrate to the right place and differentiate into the appropriate specialized cell. As each of the organism’s cells contain the same genetic material, the interplay between external cues, the triggering of various genetic pathways, and the subsequent modification of the cell to be able to respond to different external cues (e.g., by the expression of receptors or ion channels) results in an intricate and tightly regulated cascade that reliably produces a functional multicelled organism.
Humans and other “higher animals” have complex brains that allow them to acquire information from the environment, compare this information with both memories of prior experience and internal models of the world, and then respond, often appropriately but in the case of humans too often inappropriately for the world of 2007. For example, many humans find themselves overeating, gambling, or otherwise engaging in counterproductive behaviors, as mechanisms that have evolved over millions of years for survival in simpler social and environmental circumstances are evoked by the stimuli and circumstances of today’s world. But humans are not the only organisms that may find themselves responding inappropriately in unusual environmental conditions. In the early winter of 2006-2007,