The early evolution of land plants: Aquatic ancestors and atmospheric interactions

Summary

Although there is a much greater biodiversity (in the sense of genetic distance among higher taxa) of extant marine O₂-evolvers than for their terrestrial counterparts, the terrestrial organisms account for almost twice as much primary productivity and have about six times as many known species. O₂-evolvers have existed in the sea for up to 3.45 billion years, and when the oxidation of marine inorganic reductants had occurred the sedimentation of organic C was paralleled by O₂ accumulation in the ocean and atmosphere. These prokaryotic O₂-evolvers were supplemented by marine eukaryotic O₂-evolvers from about 2.16 billion years onwards. O₂-evolvers in the ocean modified the terrestrial environment before the first evidence of terrestrial phototrophs some 1.2 billion years ago in a number of ways. They may have helped to reduce atmospheric CO₂ (to perhaps 20–30 times the present value) and were essential in generating atmospheric O₂ (to a significant fraction of today's value) and hence provided O₃ and thus a UV-screen. Their production of volatile organic compounds could have helped to supply S to inland sites but also could have delivered Cl and Br to the stratosphere and thus lowered the O₃ level. The earliest terrestrial primary producers are recognized inter alia by their stimulation of weathering by CO₂ generation upon their decomposition. The higher CO₂ levels in the Palaeoproterozoic than today might have permitted 3 or 4 times the productivity of terrestrial microbial mats when compared with that of today. The earliest vascular plants (homoiohydric (i.e. able to maintain their water potential higher than that in the atmosphere in the absence of a liquid water supply) and desiccation-intolerant) evolved some 420 million years ago. Several tens of millions of years before this there is evidence of the immediate embryophytic ancestors of vascular plants growing on land, and of primary productivity at a significant fraction of the extant value for mesophytic vascular plants. While this high productivity can, in part, be explained by the higher CO₂ (at least 10 times present values), it also requires plants which are architecturally more complex than microorganisms. However, these plants would have been poikilohydric (i.e. unable to maintain their water potential different from that in the atmosphere in the absence of liquid water) and desiccation-tolerant, and much more dependent on rainfall or dew for continued primary productivity than are vascular plants.