<title>Evolution of biologic complexity: evidence from geology, paleontology, and molecular biology</title>

Abstract

The evolution of biological complexity on Earth has taken many pathways. Prokaryotes were the only life for the first 2000 million years (My); protistan eurkaryotes evolved through symbioses with preexisting prokaryotes and radiated into a variety of environments during the next 200 My. These protist photosynthesized and probably lived in both benthic and pelagic habitants where light was available. Heterotrophic protists were surely extant then too, but they left no fossil record until just 550 Ma. The algae began to diversify in the Neoproterozoic after 1000 My of rather static evolutionary development. Between 1000 and 530 Ma, algal diversity increased and decreased in response to oceanographic and climatic changes, probably associated with plate tectonic motions and ice ages. In the last part of the Neoproterozoic soft-bodied multicellular eukaryotes appeared and diversified somewhat. Study of modern trace makers, the taphonomy of living cnidarians, and molecular phylogenies of living basal metazoans indicates that the complexity represented probably includes mostly diploblastic levels along with possible metazoans indicates that the complexity. Algal protists diversified and the morphology of their cysts became intricate, while heterotrophic protists developed skeletons, as did the Metazoa. Over 15 hypotheses have been suggested to account for this radiation, or 'explosion' of Cambrian organisms, but none is clearly acceptable now. The geochemical evidence indicates that oceanographic circulation increased, upwelling may have become common seasonally and geographically, and this may well have increased productivity and the trophic complexity of both the benthic and pelagic marine biota.