The age of a branching process: estimating the divergence time of primates
Simon Tavare
Dept of Mathematics, University of Southern California, Los Angeles, USA
(jointly with Charles Marshall (UCLA), Bob Martin (Zurich) and
Oliver Will (USC))
Abstract
The fossil record used to be the only source of information
on the divergence times between species. More recently
the degree of molecular divergence between species has been used to
estimate divergence times. These molecular-based divergence times often
considerably predate the earliest fossil representatives of the groups
studied. While the incompleteness of the fossil record suggests that
divergence times estimated from molecular data should be older than the
oldest fossils, without an adequate way of quantifying the incompleteness
of the fossil record the significance of the discrepancies between
molecular and fossil-based divergence times are difficult to evaluate.
Here a new maximum likelihood method, based on an estimate of the
proportion of species preserved in the fossil record and a model of the
pattern of the diversification of the group, is used to determine the most
likely time of origin of primates based on their fossil record. Unlike
other methods, the maximum likelihood estimate of the time of origin of a
group given by this approach is older than the oldest fossils of the group.
Application of the method to the fossil record of primates suggests a
Cretaceous origin of the group, despite the
lack of Cretaceous fossil primates, providing paleontological
corroboration for a Cretaceous origin of primates as predicted by molecular
data. An interesting class of inference problems for branching
processes arises in extending this analysis to a more detailed
model of species diversification and preservation.
