Using modern organisms to investigate molecular fossils and microfossils of eukaryotes: Examples from animals and algae.

摘要

Molecular fossils and organic walled microfossils in sedimentary rocks have the potential to illuminate both the evolution of eukaryotic organisms and the history of primary production in the oceans. To realize the potential of these geological tools, their taxonomic specificity of must be determined. In this dissertation, I apply modern biological tools to investigate molecules commonly found as ancient organic matter, focusing on two case studies: the origin of animals and the origin and diversification of the green algae. It has been suggested, that 24-isopropylcholestane, a distinctive sterane found in Neoproterozoic and Cambrian bitumens, is diagnostic for the presence of sponges and, hence, represents the earliest evidence of the Metazoa. To test this hypothesis, I characterized the sterol profile of a unicellular relative of sponges, the choanoflagellate Monosiga brevicollis. I also explored the sterol biosynthetic capacity of M. brevicollis though comparative genomics and phylogenetics, showing that it is indeed capable of sterol synthesis, and uses a biosynthetic pathway intermediate between those of metazoans and fungi.; In the second case study, I explored the evolutionary record of green algae; both molecular fossils and organic walled microfossils suggest that this group was more important in late Neoproterozoic and Paleozoic oceans than in modern oceans. I focused on two chemical markers currently used as evidence for green algae, C29 sterols and algaenan biopolymer. Statistical analyses indicate that ulvophyte and early diverging prasinophyte green algae can account for the abundances of C29 steranes pre-Mesozoic rocks. In contrast, a phylogenetic survey of algaenan production shows that it is limited to one group, the chlorophytes, which are unlikely to have sourced the abundant algaenans in ancient marine sedimentary rocks. It is likely that geological "algaenans" are produced during organic diagenesis.; I also characterized the ultrastructure and wall chemistry of phycomata produced by a newly discovered species of the prasinophyte Halosphaera. This work indicates that prasinophytes are poor candidates for the modern counterparts of many early fossils and suggests that our current understanding of microfossil formation requires significant modification. Collectively, the tools of modern biology shed new light on the geological record of evolution in the oceans.