How old are animals? A molecular clock reopens the debate

Posted By News On May 30, 2017 - 1:43pm

When did animals evolve on earth? It's a harder question to answer than it might seem. Though fossil embryos suggest a little more than half a billion years ago, fossils are rare, difficult to interpret, and always being replaced by older fossils as paleontologists, biologists and geologists take a multi-disciplinary approach to determining where fossils are most likely to be found.

An alternative biological approach to fossils is the molecular clock, which uses genetic information. A study using a relaxed molecular clock method called RelTime dated the origin of animals at approximately 1.2 billion years ago reviving the debate. Early molecular clock studies assumed that mutation accumulated at a fixed rate across all species and inferred that our oldest ancestor might have existed around 1.5 billions of years ago, but a date that is almost three-times as old as the oldest fossil evidence of animal life was not accepted based on a computer simulation alone. Newer “relaxed” clock methods, that do not assume constancy of the mutation rate, have hoped to close the gap between molecules and fossils indicating that animals are unlikely to be older than around 850 million of years. 

'Relaxing the clock' is Bayesian analysis, a way to converge on a better answer as more new information is provided. In the beginning of a baseball season, for example, Bayesian analysis can calculate a result but the only reasonable answer will come if there is good scouting data on all players, an idea of schedules and who will get injured, etc. But at the end of the season, with months of data including from previous games, a World Series winner can be projected, but only given a number of chances, 6 times out of 10 or something. Other examples model the stochastic variation in stock prices with time.

For evolution, a molecular clock analysis uses explicit probability models to account for the uncertainty in the fossil record and in the mutation rate. So Bayesian methods relax the clock and estimate divergence times, but because RelTime is not a Bayesian method the authors of a new paper dismiss the result as an outlier, preferring something that is closer to the fossil date.  It did not relax the clock.

Dr Jesus Lozano-Fernandez, also from the University of Bristol, said, "Estimating divergence times is difficult and different relaxed molecular clock methods use different approaches to do so. However, we discovered that the RelTime algorithm failed to relax the clock along the deepest branches of the animal tree of life.”

Citation: J. Lozano-Fernandez, M. dos Reis, P. Donoghue, D. Pisani, ‘RelTime rates collapse to a strict clock when estimating the timeline of animal diversification’ Genome Biology and Evolution