The Greening of the Earth
ONE BILLION YEARS AGO, the continents were barren expanses of rock and lava, deserts baking in the sun by day and freezing by night, rarely refreshed by rainfall and unable to retain moisture for lack of topsoil.1 In contrast, the oceans were filled with all kinds of unicellular life. Bacteria were abundant. So were unicellular eukaryotes, which already had diversified into a variety of phototrophic and heterotrophic species, many of which had developed a sexual mode of reproduction as an alternative, under special conditions, to their usual vegetative way of multiplying by division. In this watery laboratory, protists formed all sorts of associations, most of which failed to survive. A few of these associations turned out to be advantageous and developed further.
Multicellular eukaryotic forms of life probably arose initially from small clones of cells that remained associated after their production, by successive divisions, from a single parent cell. The cells were held together either by intercellular connections or by a shared external wall or shell. Roughly speaking, the former mechanism led to animals and the latter to plants and fungi. This division reflects key differences in lifestyle. The heterotrophic animals had to maintain freedom of movement in order to catch prey, even if this freedom meant greater fragility. The phototrophic plants needed only to catch sunlight (and dissolved mineral nutrients) and could afford to remain immobile, even derived an advantage from being immobilized in a favorable location. Fungi, which developed a scavenging form of heterotrophy based on the breakdown of dead organisms by means of secreted digestive enzymes, were able to forsake mobility for the advantages of a protective coating. Because of such fundamental differences, these three kingdoms followed very different evolutionary pathways.
Most easily reconstructed is the early history of plants,2 because species that may be representative of successive evolutionary stages still exist today. There is danger in this extrapolation from present to past. Extant alleged "missing links" all evolved over long periods and may in no way resemble their distant ancestors. One might even say that they could not possibly resemble their ancestors. Otherwise,