Directionality versus contingency

During the Precambrian, eukaryotes evolved from prokaryotes, and in the Cambrian, most animal phyla evolved. Ever more environments were colonialized during the Phanerozoic, including the land and the air. For the evolution of the vertebrates, the long-portrayed succession of the "age of fishes,'' "age of reptiles,'' and "age of mammals'' is of course a gross oversimplification. Osteichthyans and chondrichthyans were never as diverse as today! Yet although measuring morphological complexity is not straightforward, there was clearly a trend toward increasing complexity in the evolution of life from bacteria to today's biota (Carroll 2001).

There are two fundamentally different views on this complexity increase. According to one view, the rise and ultimately dominance of complex life forms were an inevitable outcome of natural selection whereby more complex organisms outcompete the more primitive ones. It also inevitably culminates with the development of intelligent animals. Such a view is championed, e.g., by Conway-Morris (1998, 2003) and is reminiscent of an Aristotelian "chain of being.''

A radically different view of the evolution of life does not see complex mammals as superior to bacteria. The increase in diversity is not neglected but explained in a different way. Bacterial cells cannot evolve toward ever smaller sizes, unicellular eukaryotes cannot evolve toward zero cells but only toward multicellularity, and marine organisms cannot evolve to colonialize a nonexisting environment, but they can evolve land-dwelling species, and among animals also flying taxa. The evolution toward greater complexity is, thus, simply a move away from an "impermeable left wall'' and an increase in variance, but in a sense we still live in a bacteria-dominated world (Gould 1996).

Perhaps the truth lies somewhere in between. There is no necessity to evolve toward intelligence, but ultimately this was made possible through successive evolutionary steps after the increasing variance crossed several thresholds. In such a view, life on earth followed a megatrajectory along the following sequence (Knoll and Bambach 2000): prokaryote diversification (including metabolic pathways, but inability for sexual reproduction), early eukaryote diversification (first consumers, multicellularity, increased size, sex), aquatic multicellularity (large size, packaging of biomass, fast movement, complex food chains), invasion of the land (huge biomass of producers, adaptation to widely fluctuating environments), and ultimately intelligence (which perhaps could also have evolved in the water). Nevertheless, if "life's tape would be replayed,'' the outcome would certainly be totally different (Gould 2001).

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