A molecular-clock date for the origin of the animal phyla

Although the reliability of the molecular clock for determining divergence times that are not visible in the fossil record has been questioned, the amino-acid sequence differences in the α and β haemoglobins of a variety of living vertebrates do not support this view. While the molecular clock is clearly probabilistic rather than metronomic, it can be shown that the α and β haemoglobins have been evolving at a statistically equal rate since they first appeared some 450–500 million years ago. If this rate has always been constant for all globins, then the percentage sequence differences between several invertebrate and some vertebrate globins can be used to indicate that the initial radiation of the animal phyla occurred at least 900–1000 million years ago. ?Molecular evolution, Metazoa, haemoglobin.