Chimeras and Consciousness: Evolution of the Sensory Self

4. Early Sensibilities

4 Early Sensibilities Kenneth H. Nealson What do bacteria sense, how do they sense, and how does their sensing affect them as individuals and as communities? Here a microbiologist asks “What is the microbial IQ?” and “How do bacteria use their metabolic gifts to move, to detect light and oxygen, and, in general, to survive?” Nealson surveys what we know about bacterial sensation and points out challenges and research opportunities related to the origins of microbial sensitivity. The ability to sense the environment and to respond to it by measurable movement is the essence of behavior. There is a general sense that the responses of life forms became more complex with time. But what do organisms sense? How and why do they sense their surroundings? How do they respond to their sensations? Without attempting to be encyclopedic , I attempt here to stimulate thought about the earliest chemistryand gene-modulated appearance of sensations in our cellular ancestors. The smallest, morphologically simplest cells, the bacteria in the broad sense, were unified under the term “prokaryote” in the early 1960s. Since then, “prokaryote” has been used to denote life composed of bacterial rather than nucleated cells. Prokaryotes do not enclose their genes, their DNA, within a pore-studded nuclear membrane. The word “prokaryote” has been challenged recently because it includes two distinct groups of microorganisms: the Bacteria (also known as eubacteria or true bacteria) and the Archaea (also called archaebacteria) (Woese et al. 1990). Part of the rationale for rejection of the term “prokaryote” is that some bacteria taxa (e.g., Gemmata obscuriglobis, a member of the group Planctomycetales) have membranous structures that surround their DNA (nucleoids). The features of true nucleated cells include many other characteristics, such as incessant intracellular movement (e.g., cytoskeletal activity mitosis, chromosome movement and that of other 46 Chapter 4 organelles, phagocytosis, and exocytosis). Also, the physical separation of translation (messenger RNA synthesis) and transcription (protein synthesis) is not found in either prokaryotes, either bacterial or archaeal. Carl Woese admonishes that we now let “prokaryote” mean only cells that are not eukaryotic, with no monophyly implied. He believes that “prokaryote can still be used as long as conveniently needed, but it will now imply nothing about relationships or structure.” I think we need either a term to replace “prokaryote”—a term that allows discussion of both archaeal and bacterial cells (which share many properties and which populate ecosystems that may be devoid of eukaryotes)—or a new acceptable term or definition. In the absence of a new word, I abide by Woese’s suggestion. I simply define prokaryotes as organisms that are not eukaryotic, because of features shared by bacteria and archaea but not eukaryotes. (See appendix A.) Features shared by the bacteria and the archaea relate nearly exclusively to physiology and ecology. Prokaryotes differ most dramatically in cell structure, details of multicellularity, and behavior. The only definitional difference without exception is the presence in all eukaryotes at all times of the double-membrane-bounded nucleus that encloses protein-studded chromosomes segregated to offspring cells by a cytoskeleton, the mitotic microtubular apparatus. Several of the properties unique to prokaryotes may be relevant to the nature of the sensory self. Prokaryotes, in general, are small. They are not capable of phagocytosis or mitosis. Thus, their metabolism is restricted to chemistry-driven lifestyles, dominated by small cells (high surface-to-volume ratios), extracellular enzymes, transport systems, and diverse metabolic abilities. Their chemical signals allow them to identify energy sources (i.e., electron donors and acceptors, light) rather than other organisms. Many of the properties that characterize prokaryotes are related to their metabolic versatility. Their modes of metabolism include chemolithotrophy, anaerobic respiration, methanogenesis, nitrogen fixation, and photosynthesis. Eukaryotes, by contrast, show limited metabolic features: nearly all have aerobic respiration, have some form of heterotrophy (in the dark, at least), have predatory and other complex behavior, and synthesize secondary metabolites. Further, animals and plants show complex tissue-level morphology and meiotic sexuality. Embryos are among the many features limited to animals and plants. Self This book is deeply concerned with defining the sensing, ultimately thinking self. An investigation of the self begins with an investigation of Early Sensibilities 47 the cell. In the structurally and organizationally complex eukaryotes, the cell is the unit of life. An organism is made of one or more cells, and often of a variety of cells in tissues. Each...