MICROBIAL CRUSTS AS THE EARLIEST LIFE ON LAND
There is a paradox, however. Physical purification through multi-cycling seemed to be supported by apparent absence of pre-Silurian macroscopic land vegetation, which implied much more intense past wind and braided fluvial abrasion. But how can the mature paleosols be explained without stabilizing vegetation? Very flat landscapes offer a possible rationale, but transport of fluvial quartz sand and pebbles required sufficient topographic relief to support streams with competent velocities. The solution we suggest is the Precambrian appearance of subaerial biological (microbial) crusts or mats such as characterize many present-day arid and other extreme environments. Microbial soils are complex communities of cyanobacteria, green and brown algae, microfungi, lichens, mosses, and liverworts in varying proportions. Sticky sheaths and filaments bind soil surfaces and the organisms enhance weathering biochemically. Botanists have argued that such microbial communities must have been the first land organisms, having evolved from marginal marine or lacustrine microbial mats. Precambrian evidence in the form of fossil filaments and the geochemistry of organic caps on paleosols support their appearance as early as the late Archean.