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EVOLUTION
Origin of the Eumetazoa: Testing ecological predictions of molecular clocks against the Proterozoic fossil record
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Department of Biological Sciences, Dartmouth College, Hanover, NH 03755; and Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, United Kingdom
Communicated by Eric H. Davidson, California Institute of Technology, Pasadena, CA, May 11, 2005 (received for review January 28, 2005)
Molecular clocks have the potential to shed light on the timing of early metazoan divergences, but differing algorithms and calibration points yield conspicuously discordant results. We argue here that competing molecular clock hypotheses should be testable in the fossil record, on the principle that fundamentally new grades of animal organization will have ecosystem-wide impacts. Using a set of seven nuclear-encoded protein sequences, we demonstrate the paraphyly of Porifera and calculate sponge/eumetazoan and cnidarian/bilaterian divergence times by using both distance [minimum evolution (ME)] and maximum likelihood (ML) molecular clocks; ME brackets the appearance of Eumetazoa between 634 and 604 Ma, whereas ML suggests it was between 867 and 748 Ma. Significantly, the ME, but not the ML, estimate is coincident with a major regime change in the Proterozoic acritarch record, including: (i) disappearance of low-diversity, evolutionarily static, pre-Ediacaran acanthomorphs; (ii) radiation of the high-diversity, short-lived Doushantuo-Pertatataka microbiota; and (iii) an order-of-magnitude increase in evolutionary turnover rate. We interpret this turnover as a consequence of the novel ecological challenges accompanying the evolution of the eumetazoan nervous system and gut. Thus, the more readily preserved microfossil record provides positive evidence for the absence of pre-Ediacaran eumetazoans and strongly supports the veracity, and therefore more general application, of the ME molecular clock.
Porifera | acritarchs | Ediacaran | coevolution
Abbreviations: DPM, Doushantuo–Pertatataka microbiota; Fm, formation; Ma, million years ago; ME, minimum evolution; ML, maximum likelihood; myr, million years.
Data deposition: The sequences reported in this paper have been deposited in the GenBank database (accession nos. DQ087437–DQ087507).
¶ Love, G. D., Grosjean, E., Fike, D. A., Grotzinger, J. P., Bowring, S. A., Condon, D., Lewis, A. N., Stalvies, C., Snape, C. E. & Summons, R. E. (2005) 22nd International Meeting on Organic Geochemistry, September 12–16, 2005, Seville, Spain.
To whom correspondence may be addressed. E-mail: kevin.j.peterson{at}dartmouth.edu or njb1005{at}esc.cam.ac.uk.
© 2005 by The National Academy of Sciences of the USA
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