Tempo and Mode of Early Animal Evolution: Estimating Metazoan Divergence Times with a Molecular Clock

Dartmouth College, Hanover NH

Investigators

Abstract

Tempo and mode of early animal evolution: estimating metazoan divergence times Kevin Peterson EAR-0207766 ABSTRACT Accurate dating of when the first bilaterally-symmetrical animals (i.e., Bilateria) arose is crucial to our understanding of early animal evolution. The fossil record suggests that bilaterians arose about 555 Ma ago, whereas most previous molecular-clock analyses estimate they arose between 800-1000 Ma ago. Because of this pronounced disparity, PI seeks to show that a molecular clock can accurately, and with precision, date deep divergences among metazoan taxa. Distance analysis of four concatenated proteins from nine invertebrate taxa gives the correct phylogenetic topology, and regression analysis using four calibration points shows that branch lengths are directly correlated with time. PI's clock estimates appear to be accurate, because a molecular estimate of the divergence between echinoderm subphyla (507 Ma) falls within the range established from the fossil record (485-520 Ma). His analyses estimate that bilaterians arose 570 Ma ago, and animals themselves arose almost 700 Ma ago. Intriguingly, these originations follow two of the most extreme environmental episodes in the history of life ("snowball earth"). The objective of this proposal is to further test these ideas by first calibrating the clock using the same methods as above, but from bivalve molluscs. Using both sets of calibration points the age of the origination of Bilateria will be more reliably ascertained. Furthermore, PI will test two hypotheses concerning the ecology of these early animals. First, did metazoans live in the plankton during snowball earth? Because of the putative global planktonic shut-down during snowball earth, food would be very scarce suggesting that metazoans could not be wholly planktonic during this geological interval. If metazoans were not planktonic, then they must have been benthic, but if they were they must have been very small in order to explain the absence of trace fossils during this interval of time. Hence, PI's second question concerns these benthic micrometazoans: are living animals like gastrotrichs, which are very similar to these purported early animals, basal bilaterians? Fulfillment of the goals of this proposal will give unique insight into the tempo and mode of animal evolution, and serve as the training ground for at least one undergraduate's intellectual foray into the Precambrian/Cambrian transition.