ALGAE AND THE LATE NEOPROTEROZOIC EVOLUTION OF MACROSCOPIC MULTICELLULARITY

Late in the Neoproterozoic Era, animals evolved macroscopic multicellular forms capable of preservation in the sedimentary record. This radiation required an intrinsic blueprint in the form of regulatory gene networks, as well as environments capable of supporting oxidative metabolism by subsurface tissues of thick organisms. Animals, of course, are but one of at least seven eukaryotic groups that evolved macroscopic multicellularity, and increasing fossil evidence suggests that our opening sentence could be rewritten usefully simply by substituting "algae" for "animals." Eukaryotic disparity originated early: multicellular red, green, and heterokont algae are known from rocks >750 Ma, in agreement with recent molecular estimates for protistan divergence. Nonetheless, algal diversity appears to have remained low until ca. 600 Ma. Latest Proterozoic rocks contain diverse algal compressions unknown in earlier strata and permineralized thalli more complex than anything observed in older rocks. Taphonomy potentially limits the interpretation of this record - for example, phosphates that contain anatomically preserved algae in terminal Proterozoic beds are little known from older successions. On the other hand, some thalli found in terminal Proterozoic phosphates also occur in contemporaneous, but not older, cherts. The Ediacaran fossil Beltanelliformis has been reinterpreted as a coenocytic green alga. Given its abundance in Ediacaran sandstones, Beltanelliformis should be preserved in older beds - if it existed. Also, trace fossils of linear objects tethered at one end are common on Ediacaran and Cambrian bedding surfaces. Algae likely contributed to this record, but such features remain unknown from older beds. Macroscopic photoautotrophs appear to develop by means of genetic regulatory networks whose logic approximates that of animals (although constituent genes are commonly distinct). And late Neoproterozoic increase in the oxidation state of the atmosphere and oceans may have sparked metaphyte, as well as metazoan, diversification, by altering the availability of nitrate and the trace elements needed to incorporate it into biomass. Algal evolution, thus, has a key role to play in comparative biological and paleobiological research on the early evolution of macroscopic organisms.