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NASA Astrobiology Institute: Advent of Multicellular Life
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Research Interests Coming at the end of one of the most intensive glaciation periods in Earth history, the explosive rise of animals 530 million years ago is one of the few major events in the history of life that combines the evolution of novel developmental regulatory circuitry in the context of unique environmental circumstances. This “Cambrian explosion” is the primary focus of my laboratory, and taking a distinctly molecular paleobiolgoical approach, we are currently interested in three interrelated questions: 1) The Tempo of Early Animal Evolution: Because the early evolutionary history of animals is largely unknown, we are using molecular clocks to date the appearance of major animal groups in absolute time. Our analyses suggest that the Ediacaran was the time in Earth history when bilaterians first evolved, as well as the time when they, along with most other animal groups (including sponges) diversified (Figure 1). Thus, the Ediacaran and into the Early Cambrian was truly an evolutionarily explosive time in Earth’s history. 2) The Mode of Early Animal Evolution: The results derived from our molecular paleobiological studies suggest that the underlying trigger behind the Cambrian explosion might be the appearance on animals themselves, specifically the eumetazoans. Our molecular phylogenetic studies strongly suggest that sponges are a paraphyletic grade, suggesting that the water-canal system is primitive for metazoans. Hence, the ancestral urmetazoans were benthic sessile micro-suspension feeders. Because the design of the water-canal system precludes sponges from feeding on most types of eukaryotes, animals living before the evolution of the gut (roughly 670 Ma) would have had no ecological impact on the eukaryotic realm, and their existence would be paleoecologically invisible. Eumetazoans, on the other hand, are characterized by two innovations; a gut and a nervous system, and together these constitute an entirely new grade of organization – macrophagous mobile metazoans – with the potential to have a singularly profound impact on contemporaneous ecology/evolution: for the first time in the history of our planet, eukaryotes could now prey on other eukaryotes generating arms races, and ultimately giving rise to Phanerozoic-type macroecology and macroevolution. We are currently focused on refining our understanding of early sponge evolution, as well as identifying genes involved in the development (and hence possibly the evolution) of the gut. 3) Metazoan Complexity and Constraint: Despite their profound affect upon Earth’s biota, one of the biggest surprises in comparative genomics is that much of the eumetazoan developmental “tool-kit” (i.e., the transcription factors and signaling systems) is found in relatively more simple taxa like sponges. What, then, accounts for the dramatic increase in morphological complexity of eumetazoans with respect to sponges? We have hypothesized that part of the answer might lie in the non-coding portion of the genome, specifically with a set of regulatory RNA genes called microRNAs. Our lab has shown that microRNAs are continually acquired and fixed in animal genomes with the most morphologically complex animals having the greatest number of microRNAs (Figure 2). A core set of microRNAs is conserved in animal with organs (protostomes and deuterostomes), but is absent in basal groups that lack organs including sponges, cnidarians, and acoel flatworms. In addition, the continuous acquisition and fixation of miRNAs in various animal groups strongly correlates both with the hierarchy of metazoan relationships and with the non-random origination of metazoan morphological innovations through geologic time, suggesting that miRNAs might at least partially govern the macroevolution of metazoan body plans.
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