Molecular Clock Calibrations and Metazoan Divergence Dates |
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Authors: | Michael SY Lee |
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Institution: | (1) Department of Zoology, University of Queensland, St. Lucia, Brisbane, QLD 4072, Australia, AU |
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Abstract: | 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.
Received: 26 January 1999 / Accepted: 14 April 1999 |
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Keywords: | : Molecular clock — Divergence times — Calibration — Fossils — Metazoans — Cambrian explosion |
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