Molecular clock calibrations and metazoan divergence dates.

It has recently been argued that living metazoans diverged over 800 million years ago, based on evidence from 22 nuclear genes for such a deep divergence between vertebrates and arthropods (Gu 1998). Two "internal" calibration points were used. However, only one fossil divergence date (the mammal-bird split) was directly used to calibrate the molecular clock. The second calibration point (the primate-rodent split) was based on molecular estimates that were ultimately also calibrated by the same mammal-bird split. However, the first tetrapods that can be assigned with confidence to either the mammal (synapsid) lineage or the bird (diapsid) lineage are approximately 288 million years old, while the first mammals that can be assigned with confidence to either the primate or the rodent lineages are 65 million years old, or 85 million years old if ferungulates are part of the primate lineage and zhelestids are accepted as ferungulate relatives. Recalibration of the protein data using these fossil dates indicates that metazoans diverged between 791 and 528 million years ago, a result broadly consistent with the palaeontological documentation of the "Cambrian explosion." The third, "external" calibration point (the metazoan-fungal divergence) was similarly problematic, since it was based on a controversial molecular study (which in turn used fossil dates including the mammal-bird split); direct use of fossils for this calibration point gives the absurd dating of 455 million years for metazoan divergences. Similar calibration problems affect another recent study (Wang et al. 1999), which proposes divergences for metazoans of 1000 million years or more: recalibrations of their clock again yields much more recent dates, some consistent with a "Cambrian explosion" scenario. Molecular clock studies have persuasively argued for the imperfection of the fossil record but have rarely acknowledged that their inferences are also directly based on this same record.