![]() Most analysts accept a baseline date of 3.5-3.8 Ga for the universal common ancestor, the first living thing on Earth. The reconsideration of molecular clock methods has now opened the way for a great number of studies of the dating of other parts of the UTL (see Fig. (2004) brought the date of divergence of bilaterian animals down to 573-656 Ma, and so the split of all animals would be just a little older, in line with Ayala et al.'s (1998) estimate. So, if vertebrate clocks are slower, it takes longer for a certain amount of genome change to occur than in other animals, and so any calibrations extrapolated from such dates will be much more ancient than they ought to be. In a further revision, Kevin Peterson and colleagues from Dartmouth University (2004) showed that Wray had unwittingly found a very ancient date because vertebrate molecular clocks tick more slowly than those of most other animal groups. (1998), who recalculated a date of 670 Ma for the basal radiation of animals, much more in line with the fossil record. Wray's calculations were criticized by Ayala et al. ![]() Extrapolation (fixing dates outside the range) is tougher than interpolation (fixing dates within a range between a known date and the present day): small errors on those Paleozoic dates would magnify up to huge errors on the Precambrian estimates. So, he had to extrapolate his dates from the Paleozoic fixed points back into the Precambrian. In Wray's case, mainly vertebrate dates were used, the assumed dates of branching between different groups of fishes and tetrapods in the Paleozoic. 133) suggests that genes mutate at predictable rates through geological time, so if one or more branching points in the tree can be fixed from known fossil dates, then the others may be calculated in proportion to the amount of gene difference between any pair of taxa. The molecular clock model of molecular evolution (see p. Their work is based on gene sequencing from RNA of the nucleus, and it is calibrated against geological time using some fixed points based on known fossil dates. ![]() Wray's view was confirmed by a number of other molecular analyses of basal animal groups, but also of plants, Archaea and Bacteria. 8.4) would have to be pushed back deeper into the Proterozoic and Archaean. ![]() This proposal suggested three consequences: (i) the Precambrian fossil record of animals (and presumably all other fossils) was even more deficient than had been assumed (ii) the Cambrian explosion, normally dated at 542 Ma, would shift back deep into the Proterozoic and (iii) all other splitting dates in the UTL (see Fig. In other words, the molecular time scale seemed to be double the fossil age. This estimate predated the oldest animal fossils by about 600 myr. There was a sensation in 1996 when Greg Wray of Duke University and colleagues announced new molecular evidence that animals had diversified about 1200 Ma. ![]()
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