The interaction of a multitude of historical contingencies helps to shape the evolution of any organism. If some superhuman being could reset the evolutionary clock to zero and restart it, the outcome would be unpredictable. Microbes, marigolds, mice, mammoths, and men all owe their existence and eventual demise to a complex aleatory game played with ecological, genetic, geological, and cosmic dice. Play a fresh game, and the result would be different.

Against this background of evolutionary happenstance, it might be surprising if life should show any direction or pattern in its history. Nevertheless, it does. Larger and more complex forms evolved from simple unicellular progenitors, and the process demanded innovations to provide new ways of living (cf. Carroll 2001). Indeed, there is much support now and in the past in favour of an evolutionary tendency towards larger size, greater complexity, and richer diversity. Two schools of thought offer radically different mechanisms to explain these evolutionary trends. The first school, arguing that there is 'nowhere to go but up', favour a random and passive tendency to evolve away from the tiny size, less complex, and low diversity that characterized the first communities on the Earth. The second school subscribes to a non-random and active (or 'driven') process that tips evolution towards ever-higher levels of size and complexity. The fossil record and the phyloge-netic 'tree of life' provide the basis for working out the sequence and direction of evolution. The fossil record, biased and imperfect though it be, is now widely accepted as robust, at least for the Phanerozoic (e.g. Conway Morris 1998). Furthermore, since about 1980, research in molecular biology and genetics has enabled the investigation of new levels of detail in aspects of evolutionary change (e.g. Carroll 2002). To establish the number of times particular events occurred and the order in which important sets of traits evolved, and to identify the possible sister groups of major taxonomic groups, demands the integration of fossil systematic data (Carroll 2001). The fossil record also yields up information on the dates that different taxa first appeared, although initial appearances in the fossil record give only minimum ages of clades and many of the most challenging and controversial questions in evolutionary history concern the origin of major clades, including multicellular eukary-otes, animals, land plants, insects, and flowering plants (Carroll 2001).

Life on Earth first appeared at least 3.8 billion years ago and has afterwards followed secular trajectories of increasing size, complexity, and diversity punctuated by 'key events' and displaying an unbroken continuity of genome evolution from prokaryotes through the rich diversity of multicellular organisms (Figure 8.1). The first living things - the prokaryotes -were anaerobic, fermenting heterotrophic bacteria. The first autotrophic bacteria evolved

Eras and subdivisions Aeons of aeons

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