HR: 16:00h
AN: U44A-01 INVITED     [Abstracts]
TI: Biologic History and the Cardinal Rule of Life
AU: * Schopf, J W
EM: schopf@ess.ucla.edu
AF: Earth and Space Sciences, University of California, Los Angeles, Los Angeles, CA 99095 United States
AB: In broad perspective, the history of life is remarkably static -- once set, a system that has changed little over all of geological time. The basic chemistry of living systems (CHONSP, and the monomers and polymers they compose), the genetics and cellular structure of life, even the ecologic division of the biologic world into "eaters" (heterotrophs) and "eatees" (autotrophs), are innovations all dating from the Archean that have carried over to the present. Throughout Earth history, biology has followed the Cardinal Rule of Life -- avoid change, never evolve at all! Biology maintains the status quo, opportunistically responding only if conditions change. Life's credo might well be "if it ain't broken, don't fix it." Of course, biomolecules do get "broken," by mutations, but living systems have many biochemical repair mechanisms. Evolution is a result of small changes that slip through unfixed. We see the results of evolution in the fossil record only because of the vastness, the true enormity, of geological time. What events punctuated this static underpinning to produce the modern living world? Only three, each in its own way shaping the course of life's history. The earliest, photosynthesis, freed life from dependence on foodstuffs made by nonbiologic processes. The advent of the advanced form of this process, oxygenic ("green plant") photosynthesis -- also an Archean innovation -- pumped oxygen into the environment (markedly increasing energy yields), "rusted the Earth" (evidenced by banded iron-formations), and, by $\sim$2,300 Ma ago, led to establishment of an aerobic-anaerobic ecosystem like that today. Not surprisingly, given the Cardinal Rule of Life, the inventors of this innovation, microbial cyanobacteria, evolved little over billions of years. The second major innovation was sex. In the modern world, this reproductive process is exhibited only by nucleated (eukaryotic) cells, derived from non-sexual eukaryotic ancestors. Although eukaryotes date from $\sim$2,000 Ma ago, they first evolved slowly -- following the Cardinal Rule of Life -- until $\sim$1,000 Ma ago when sexual reproduction took over. This development markedly speeded the development of new species that could compete, and eventually dominate, in habitats previously owned by their non-sexual prokaryotic ancestors, as evidenced both in the fossil record and by molecular biology-based rRNA phylogenetic trees. The third innovation was cellular differentiation and multicelluarity. Although the "Cambrian Explosion" -- the great radiation of animal life during the Cambrian Period beginning $\sim$550 Ma ago -- is commonly viewed as reflecting this event, it seems more a continuum than a step-function change. Evolution speeded in the half-billion years between 1,000 Ma ago and the beginning of the Cambrian: phytoplankton gave rise to multicellular seaweeds by $\sim$850 Ma; and primitive protozoans, present as early as $\sim$950 Ma, had by $\sim$600 Ma given rise to soft-bodied multicelled animals. Soon thereafter, animals developed shelly protective armor -- marking the beginning of the Cambrian Period, and thus of the Phanerozoic Eon. The Phanerozoic history of life is familiar to all, from spore-producing to seed-producing to flowering plants, from animals without backbones to fish, land-dwelling vertebrates, then birds and mammals. Plants ("eatees") and animals ("eaters") co-evolved in sequence. Again, life followed the Cardinal Rule, changing little, then evolving rapidly, as new ecologic opportunities became available.
DE: 0400 Biogeosciences
SC: Union [U]
MN: 2004 AGU Fall Meeting