kevin j. peterson

Research Interests

Coming at the end of one of the most intensive glaciation periods in Earth history (glaciers at sea level on the equator!), 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 we are currently interested in four interrelated questions:

1) Metazoan Phylogeny: Our lab is particularly interested in the early cladogenic events of animals, specifically the phylogeny of sponges. Our work (along with others) suggests that sponges are paraphyletic such that calcisponges and homoscleromorphs are more closely related to cnidarians and triploblasts (collectively called eumetazoans) than they are to demosponges (Figure 1). If true, the implications of this result are profound – we could then state with a high degree of certainty that the last common ancestor of metazoans was designed like a modern sponge, complete with a water-canal system. Not only would this tell us much about animals living ca. 650 million years ago, it would also indicate that the origin of complex animals occurred within the biological context of sponges, and suggests that the evolution of eumetazoans involved a transition from benthic, sessile microsuspension feeder to mobile macrophagous predators just after the Marinoan glaciation interval ~635 Ma ago.

2) The Tempo of Early Animal Evolution: Because the early fossil record of animals is hidden in the mist of time, we are using molecular clocks to date the appearance of major animal groups. Our analyses suggest that Bilateria arose about 580 million years ago, approximately 50 million years before the Cambrian explosion, and 25 million years before the first appearance of bilaterian fossils in the paleontological record (Figure 1). Current research is focused on combining a better understanding of the phylogenetic tree (see above), and then dating when these divergence occurred to address questions concerning the early evolutionary history of animals.

3) The Mode of Early Animal Evolution: Both developmental and paleontological data suggest that the earliest bilaterians were “direct developers,’ that is embryogenesis proceeds directly to the juvenile stage without an intervening larval stage. Nonetheless, about 70% of modern marine invertebrate species have a planktic larval stage, many of which feed in the plankton for weeks to months before they become competent to undergo metamorphosis (Figure 2). Thus, the evolution of a larval form must be a convergent feature among wide variety of metazoan groups (Figure 3). We are testing this hypothesis that the evolution of a complex life cycle evolved multiple times independently by comparing the gene regulatory circuitry underlying the development of a larval-specific structure, the apical organ, between a wide variety of eumetazoans, but current work is focused on echinoderms and gastropod molluscs(Figure 4). We have shown that there is no similarity between the gene regulatory network governing apical tuft development in the sea urchin Strongylocentrotus purpuratus (Fig 4, top) and the gastropod mollusc Haliotis rufescens (Fig 4, bottom). Thus, our current data suggest that their last common ancestor (i.e., the population of animals at 580 Ma ago) did not have a complex life cycle, and the evolution of larvae and other forms of mesozooplankton may explain, in part, the ensuing Cambrian explosion and the Ordovician biodiversification event.

4) Metazoan Complexity: One of the biggest surprises in comparative genomics is that much of the developmental “tool-kit” (i.e., the transcription factors and signaling systems) of complex animals like vertebrates, sea urchins, and insects, is found in relatively more simple taxa like cnidarians and even sponges. What, then, accounts for the dramatic increase in morphological complexity of protostomes and deuterostomes as compared with jellyfish and sponges? We believe 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 5). 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 (Fig. 5, top) and with the non-random origination of metazoan morphological innovations through geologic time. This suggests that microRNAs could have played a pivotal role in evolution of both animal body plans and morphological complexity (Fig. 5, middle), with organs being the ultimate manifestation of this innovation. And because microRNAs regulate tens to hundreds of protein-coding genes, they may also constrain the future evolution of animal taxa (Fig. 5, bottom).