Trace ayala dating
However, the absence of earlier metazoan fossils could be caused by systematic biases in preservation that left the Precambrian history of recognized phyla unrecorded (7).
Molecular data have the potential to provide an independent test of gaps in the fossil record by providing a continuous historical record of all extant taxa, which can be used to infer the history of lineages that have low fossilization potential, or the presence of lineages over periods for which the fossil record is poor.“Molecular clock” studies, which use genetic distance to infer time since separation of lineages, can be criticized on the grounds that variation in the rate of molecular evolution, could affect the accuracy of the date estimates.
If the log likelihood difference observed for the real quartet was greater than that of 95% of the simulations, we rejected that quartet as failing to fit a rate-constrained model (15).
Only quartets for which the rate- constrained model was not rejected were used to produce date estimates.
We address these criticisms by using a molecular phylogenetic technique that deals explicitly with rate heterogeneity and allows the Cambrian explosion hypothesis to be tested within a statistical framework.
The uncertainty in phylogenetic reconstruction and the stochastic nature of the molecular clock preclude precise dating (8), but when the imprecision is expressed as appropriate confidence intervals, molecular data are ideally suited to testing specific hypotheses.
The combination of a rate constancy test with high resolution of long DNA sequences (two data sets totaling over 7,000 bp), use of a wide range of taxa and fossil dates (rather than reliance on a single calibration rate), and a maximum likelihood analysis makes this method more reliable than previous molecular dating techniques (11).) nuclear-encoded 18S r RNA (1,710 bp).
Although recent discoveries of Ediacaran metazoans have extended the record of sponges and bilateral animals to 570 Ma (4, 5), the biological affinities of many Ediacaran organisms remain controversial, and the earliest palaeontological evidence of metazoan life is no more than 600 Ma (6).We address the problem of rate heterogeneity in two ways.First, having a fossil date for each pair allows us to estimate a rate for each side of the quartet (pair plus internal lineage leading to that pair, Fig. Second, a quartet is rejected if the likelihood for a free-rate model (each branch allowed to have a different rate) is significantly better than that for the rate-constrained model (each pair has its own rate, Fig. Only those quartets that passed the rate constancy test were used to generate date estimates.The data are not compatible with the Cambrian explosion hypothesis as an explanation for the origin of metazoan phyla, and provide additional support for an extended period of Precambrian metazoan diversification.The early history of the Metazoa, whether originating as part of a Cambrian “explosion” or with an extended, Precambrian “phylogenetic fuse,” remains controversial (1–3).Molecular studies have the potential to shed light on the origin of the animal phyla by providing independent estimates of the divergence times, but have been criticized for failing to account adequately for variation in rate of evolution.A method of dating divergence times from molecular data addresses the criticisms of earlier studies and provides more realistic, but wider, confidence intervals.Here, we test the Cambrian explosion hypothesis by asking whether molecular date estimates are compatible with the initial radiation of animal phyla around the time of the Precambrian-Cambrian boundary.We use a maximum likelihood implementation of the quartet method (9), which incorporates a test of rate constancy.For a given sequence alignment, pairs of taxa for which independently derived dates of origin are available are combined into quartets (Fig. An array of quartets was formed from all possible combinations of monophyletic dated pairs, then each of the quartets was subjected to a rate constancy test, based on the likelihood ratio test (10).The maximum likelihood position of the deep node for each of the quartets provides an estimate of the date of divergence between the pairs.