Testing the Cambrian explosion hypothesis by using a molecular dating technique (original) (raw)
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Can fast early rates reconcile molecular dates with the Cambrian explosion?
Proceedings of the Royal Society B: Biological Sciences, 2000
Molecular dates consistently place the divergence of major metazoan lineages in the Precambrian, leading to the suggestion that the`Cambrian explosion' is an artefact of preservation which left earlier forms unrecorded in the fossil record. While criticisms of molecular analyses for failing to deal with variation in the rate of molecular evolution adequately have been countered by analyses which allow both siteto-site and lineage-speci¢c rate variation, no analysis to date has allowed the rates to vary temporally. If the rates of molecular evolution were much higher early in the metazoan radiation, molecular dates could consistently overestimate the divergence times of lineages. Here, we use a new method which uses multiple calibration dates and an empirically determined range of possible substitution rates to place bounds on the basal date of divergence of lineages in order to ask whether faster rates of molecular evolution early in the metazoan radiation could possibly account for the discrepancy between molecular and palaeontological date estimates. We ¢nd that allowing basal (interphylum) lineages the fastest observed substitution rate brings the minimum possible divergence date (586 million years ago) to the Vendian period, just before the ¢rst multicellular animal fossils, but excludes divergence of the major metazoan lineages in a Cambrian explosion.
Origin of the metazoan phyla: Molecular clocks confirm paleontological estimates
Proceedings of the National Academy of Sciences, 1998
The time of origin of the animal phyla is controversial. Abundant fossils from the major animal phyla are found in the Cambrian, starting 544 million years ago. Many paleontologists hold that these phyla originated in the late Neoproterozoic, during the 160 million years preceding the Cambrian fossil explosion. We have analyzed 18 protein-coding gene loci and estimated that protostomes (arthropods, annelids, and mollusks) diverged from deuterostomes (echinoderms and chordates) about 670 million years ago, and chordates from echinoderms about 600 million years ago. Both estimates are consistent with paleontological estimates. A published analysis of seven gene loci that concludes that the corresponding divergence times are 1,200 and 1,000 million years ago is shown to be flawed because it extrapolates from slow-evolving vertebrate rates to faster-evolving invertebrate rates, as well as in other ways.
Molecular Evidence for Deep Precambrian Divergences Among Metazoan Phyla
Science, 1996
A literal reading of the fossil record suggests that the animal phyla diverged in an "explosion" near the beginning of the Cambrian period. Calibrated rates of molecular sequence divergence were used to test this hypothesis. Seven independent data sets suggest that invertebrates diverged from chordates about a billion years ago, about twice as long ago as the Cambrian. Protostomes apparently
Phylochronology of early metazoans: combined evidence from molecular and fossil data
Geological Journal, 2007
Timing the evolutionary appearances of early Metazoa on Earth may rely on the combined information of fossil occurrences, molecular divergences and the geological background. Recent advances in the studies of Neoproterozoic and earliest Cambrian strata in South China and elsewhere, together with numerous works on molecular dating, provide an opportunity to draw a more precise timescale for the early metazoan evolution. This study presents a synthesis of available molecular and fossil dating results, placed in the palaeoenvironmental background, which leads to the recognition of the following time events for early metazoan cladogenesis: (1) Animal multicellularity occurred most probably during early to middle Neoproterozoic times (between 1000and1000 and 1000and660 Ma) during the geological period of gradual increase in oxygen content; (2) Eumetazoans and sponges (or sponge-like organisms) parted from each other during the Neoproterozoic glaciation interval (ca. 750-580 Ma), but both experienced adaptive radiation after the glaciations as evidenced by the Doushantuo Biota (ca. 635-551 Ma); (3) A great metazoan radiation event (MRE) occurred during the late Ediacaran and earliest Cambrian periods (ca. 550-530 Ma), interpreted here as a metazoan radiation at higher taxonomic levels, as compared with the early Cambrian radiation representing one at lower taxonomic levels (lower than subphylum or class). In order to better constrain the phylochronology of early metazoans, we present evidence for the divergences of arthropod subgroups on the basis of molecular dating results and the fossil record. It is shown that the arthropod stem group diverged from its sister taxon at between 700and700 and 700and565 Ma and major arthropod subgroups originated coevally with MRE. The MRE is probably correlatable with the d 13 C negative anomaly in the late Ediacaran Period.
The cambrian evolutionary ?explosion? recalibrated
BioEssays, 1997
The sudden appearance in the fossil record of the major animal phyla apparently records a phase of unparalleled, rapid evolution at the base of the Cambrian period, 545 Myr ago. This has become known as the Cambrian evolutionary 'explosion', and has fuelled speculation about unique evolutionary processes operating at that time. The acceptance of the palaeontological evidence as a true reflection of the evolutionary narrative has been criticised in two ways: from a reappraisal of the phylogenetic relationships of the early fossils, and from predictions of molecular divergence times, based on six appropriate metazoan genes. Phylogenetic analysis of the arthropods implies an earlier, Precambrian history for most clades, and hence an extensive period of cladogenesis unrecorded by fossils. A similar argument can be applied to molluscs, lophophorates and deuterostomes. Molecular evidence implies divergence between clades to at least 1000 Myr ago. The apparent paradox between the sudden appearance of recognisable metazoans and their extended evolutionary history might be explained by a sudden Cambrian increase in body size, which was accompanied by skeletisation. A new paradigm suggests that the 'explosion' Accepted in the record may have been decoupled from the evolutionary innovation.
Uncertainty in the Timing of Origin of Animals and the Limits of Precision in Molecular Timescales
Current Biology, 2015
The timing of divergences among metazoan lineages is integral to understanding the processes of animal evolution, placing the biological events of species divergences into the correct geological timeframe. Recent fossil discoveries and molecular clock dating studies have suggested a divergence of bilaterian phyla >100 million years before the Cambrian, when the first definite crown-bilaterian fossils occur. Most previous molecular clock dating studies, however, have suffered from limited data and biases in methodologies, and virtually all have failed to acknowledge the large uncertainties associated with the fossil record of early animals, leading to inconsistent estimates among studies. Here we use an unprecedented amount of molecular data, combined with four fossil calibration strategies (reflecting disparate and controversial interpretations of the metazoan fossil record) to obtain Bayesian estimates of metazoan divergence times. Our results indicate that the uncertain nature of ancient fossils and violations of the molecular clock impose a limit on the precision that can be achieved in estimates of ancient molecular timescales. For example, although we can assert that crown Metazoa originated during the Cryogenian (with most crown-bilaterian phyla diversifying during the Ediacaran), it is not possible with current data to pinpoint the divergence events with sufficient accuracy to test for correlations between geological and biological events in the history of animals. Although a Cryogenian origin of crown Metazoa agrees with current geological interpretations, the divergence dates of the bilaterians remain controversial. Thus, attempts to build evolutionary narratives of early animal evolution based on molecular clock timescales appear to be premature.
Molecular Biology and Evolution, 2003
Multicellular animals, or Metazoa, appear in the fossil records between 575 and 509 million years ago (MYA). At odds with paleontological evidence, molecular estimates of basal metazoan divergences have been consistently older than 700 MYA. However, those date estimates were based on the molecular clock hypothesis, which is almost always violated. To relax this hypothesis, we have implemented a Bayesian approach to describe the change of evolutionary rate over time. Analysis of 22 genes from the nuclear and the mitochondrial genomes under the molecular clock assumption produced old date estimates, similar to those from previous studies. However, by allowing rates to vary in time and by taking small species-sampling fractions into account, we obtained much younger estimates, broadly consistent with the fossil records. In particular, the date of protostome-deuterostome divergence was on average 582 6 112 MYA. These results were found to be robust to specification of the model of rate change. The clock assumption thus had a dramatic effect on date estimation. However, our results appeared sensitive to the prior model of cladogenesis, although the oldest estimates (791 6 246 MYA) were obtained under a suboptimal model. Bayes posterior estimates of evolutionary rates indicated at least one major burst of molecular evolution at the end of the Precambrian when protostomes and deuterostomes diverged. We stress the importance of assumptions about rates on date estimation and suggest that the large discrepancies between the molecular and fossil dates of metazoan divergences might partly be due to biases in molecular date estimation.
The Cambrian fossil record and the origin of the phyla
Integrative and Comparative Biology, 2003
Whilst the ''Cambrian Explosion'' continues to attract much attention from a wide range of earth and life scientists, the detailed patterns exhibited by the terminal Proterozoic-Early Cambrian biotas remain unclear, for reasons of systematics, biostratigraphy and biogeography. In particular, recent changes in absolute dating of the Cambrian have refined the period of time that the fossil record might be of most help in revealing the dynamics of the undoubted radiation taking place at this time. The famous exceptionally preserved faunas seem to be rather close temporally, and as yet reveal little about the earliest and critical period of evolution, deep in the Cambrian. Nevertheless, the most parsimonious interpretation of the Cambrian fossil record is that it represents a broadly accurate temporal picture of the origins of the bilaterian phyla. by guest on June 4, 2013 http://icb.oxfordjournals.org/ Downloaded from
Journal of Molecular Evolution, 1998
We document the phylogenetic behavior of the 18S rRNA molecule in 67 taxa from 28 metazoan phyla and assess the effects of among-site rate variation on reconstructing phylogenies of the animal kingdom. This empirical assessment was undertaken to clarify further the limits of resolution of the 18S rRNA gene as a phylogenetic marker and to address the question of whether 18S rRNA phylogenies can be used as a source of evidence to infer the reality of a Cambrian explosion. A notable degree of among-site rate variation exists between different regions of the 18S rRNA molecule, as well as within all classes of secondary structure. There is a significant negative correlation between inferred number of nucleotide substitutions and phylogenetic information, as well as with the degree of substitutional saturation within the molecule. Base compositional differences both within and between taxa exist and, in certain lineages, may be associated with long branches and phylogenetic position. Importantly, excluding sites with different degrees of nucleotide substitution significantly influences the topology and degree of resolution of maximum-parsimony phylogenies as well as neighbor-joining phylogenies (corrected and uncorrected for among-site rate variation) reconstructed at the metazoan scale. Together, these data indicate that the 18S rRNA molecule is an unsuitable candidate for reconstructing the evolutionary history of all metazoan phyla, and that the polytomies, i.e., unresolved nodes within 18S rRNA phylogenies, cannot be used as a single or reliable source of evidence to support the hypothesis of a Cambrian explosion.