| Summary: | While microbial life originated early in Earth history and continues to underpin most modern ecosystems, its evolutionary trajectory remains murky. This is predominantly due to the dearth of diagnostic microbial fossils and biomarkers. Conversely, metazoans (animals) emerged relatively recently and have deposited a thorough archive of their existence in the rock record. The ancient shared ancestry of these two groups presents an opportunity to leverage the rich metazoan fossil record to constrain the timing and mechanisms of microbial evolution. In this thesis, I connect these two disparate groups in three different ways: 1) identification of genes with a history of recent transfer between Bacteria and Metazoa, 2) characterization of a bacterial protein family with specificity for a metazoan substrate, in this case collagen, and 3) using animal fossil age ranges as a proxy for hypothetical microbial lineages on the early Earth. I report a number of novel transfers from Bacteria into Metazoa, Opisthokonta, and Eukarya, and use two of them to constrain the ages of the bacterial phylum Bacteroidetes and the archaeal family Methanosarcinaceae.
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