Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes

Multicellular filaments from the ca. 1200-Ma Hunting Formation (Somerset Is land, arctic Canada) are identified as bangiacean red algae on the basis of diagnostic cell-division patterns. As the oldest taxonomically resolved eu karyote on record Bangiomorpha pubescens n. gen. n. sp. provides a key datu m point for constraining protistan phylogeny. Combined with an increasingly re solved record of other Proterozoic eukaryotes, these fossils mark the o nset of a major protistan radiation near the Mesoproterozoic/Neoproterozoic boundary. Differential spore/gamete formation shows Bangiomorpha pubescens to have be en sexually reproducing, the oldest reported occurrence in the fossil recor d. Sex was critical for the subsequent success of eukaryotes, not so much f or the advantages of genetic recombination, but because ii allowed for comp lex multicellularity. The selective advantages of complex multicellularity are considered sufficient for it to have arisen immediately following the a ppearance of sexual reproduction. As such, the most reliable proxy for the first appearance of sex will be the first stratigraphic occurrence of compl ex multicellularity. Bangiomorpha pubescens is the first occurrence of complex multicellularity in the fossil record. A differentiated basal holdfast structure allowed for positive substrate attachment and thus the selective advantages of vertica l orientation; i.e., an early example of ecological tiering. More generally , eukaryotic multicellularity is the innovation that established organismal morphology as a significant factor in the evolutionary process. As complex eukaryotes modified, and created entirely novel, environments, their inher ent capacity for reciprocal morphological adaptation, gave rise to the "bio logical environment" of directional evolution and "progress." The evolution of sex, as a proximal cause of complex multicellularity, may thus account for the Mesoproterozoic/Neoproterozoic radiation of eukaryotes.