A Thumbnail Sketch of the History of Life

Abstract

While conceptually the history of life clearly exists, its reconstruction may be challenging. Gene trees are useful in this regard, but horizontal gene transfer complicates their use. Further, such trees are insightful only after the last universal common ancestor (LUCA) of life. Before LUCA, the stem group of life is entirely extinct. Vestiges of early life may be found in modern life, however. Also useful in reconstructing the history of life is the relative time of the fossil record and chronological time from radionucleotides. While the secondary simplification of living things occurs constantly, over time life became more complex and diverse—e.g., genes, genes within chromosomes, chromosomes within cells, cells within complex cells, and complex cells within multicellular organisms. The study of these major transitions has come to dominate considerations of the history of life. Each transition between levels of selection required the mediation of evolutionary conflicts so that cooperation could occur between the lower-level units, allowing the higher-level units to emerge. Evolutionary conflict follows from the nature of life. Living things typically exhibit the ability to convert energy and to replicate. Conflict typically arises when some units lose the ability to convert energy and rely on sister units to do this. In this way, a replicatory advantage is gained. Nevertheless, this advantage is frequency dependent. A group composed only of such defectors inevitably fails. Beyond these simple generalities, the history of life may be characterized by fundamental relationships between nutrient supply, metabolism, and cooperation that are only now beginning to come into focus.

The history of life is a history of different units of selection. Novel selective scenarios dominate at times of transition between units of selection. Whereas the lower self-replicating unit was previously selected by the external environment alone, following the transition it became selected by traits expressed by the higher unit. Variants expressed in the lower unit influence not only the relative replication rate of the lower unit, but also that of the higher unit. The potential clearly exists for variants to have a synergistic effect (that is, to favor the replication of both the lower and the higher unit), or for conflicts to arise. The organization of any unit will come to reflect those synergisms between selection at the higher and the lower levels which permit the new unit to exploit new environments and those mechanisms which act to limit subsequent conflicts between the two units. This explicitly hierarchical perspective on evolution predicts that the myriad complexities of ontogeny, cell biology, and molecular genetics are ultimately penetrable in the context of an interplay of synergisms and conflicts between different units of selection.

Leo Buss [1]