The Nonlinear Effects of Evolutionary Innovation Biospheric Feedbacks on Qualitative Environmental Change: From the Microbial to Metazoan World

摘要

The biomass of living organisms hosts only a small portion of the elemental abundance at the surface of the Earth, yet biology plays a defining role in the composition and stability of the biosphere by acting on sensitive geochemical feedbacks controlling global element cycles. This type of influence is evident in a class of evolutionary innovations that have a profoundly disproportionate effect on the biosphere, referred to here as evolutionary innovation biospheric feedbacks (EIBFs). A particular biological innovation need not be complex, rather its influence is amplified by its effect on geochemical feedbacks controlling elemental cycling. The lead-up to the metazoan radiation (similar to 585 million years ago) provides an example of such an EIBF. While commonly attributed to an increase in free oxygen concentration, the reason for this step increase in O-2 almost 2 billion years after the advent of oxygenic photosynthesis is traced to a seemingly unrelated evolutionary innovation resulting in a critical by-product of the first soils: secondary clay minerals. Detrital clay minerals deposited in continental margin sediments sequester organic carbon compounds and thus prevent consumption of atmospheric oxygen produced during photosynthesis. The transition from the abiotic to biotic land surface at the end of the Precambrian shifted biogeochemical cycling to this terrestrial-dominated modern mode that enabled sufficient oxygenation of the biosphere to trigger the metazoan radiation.