by Robin Andrews
Not all clocks measure time in hours, minutes and seconds. The molecular clock is one such timepiece; it uses the random mutations in sequences of DNA to estimate when species diverged from one another. Although it is an invaluable tool for biologists and paleontologists alike for tracking evolutionary changes over time, it doesn’t always precisely match up with the dates given by the corresponding fossils. The University of Kansas has announced that they are working on a paper which shows that cosmic radiation is to blame for this offset.
Genetic mutations happen randomly, due to mistakes in cell division, or due to external influences – such as increased radiation exposure. Over vast periods of time, these changes appear to occur at a relatively constant rate. So it stands to reason that the number of differences between any two gene sequences increases over time, and it is this measurable increase that can be converted into time.
However, as with any timepiece, it needs to be calibrated first. If a species is extinct, the fossil record is analyzed, and the approximate change in the emergence of new species within its ancestral history (its “speciation rate”) is estimated. Then the rate of mutation within this lineage of gene sequences is calculated, and the two strands of evidence are matched up, providing scientists with a “clock.”
For example, if the rate of genetic mutations is 10 every million years, and you count 30 mutations in the lineage between two sets of DNA – two different species – then you can say that the two species diverged around three million years ago.
It’s not entirely accurate, however: The molecular clock’s timing of species divergence doesn’t always match up perfectly with the fossil evidence. Still, not all fossils have been discovered, and defining when one species becomes another is sometimes difficult for paleontologists to agree on. Without a “perfect” fossil record, there will always be a small discrepancy with the molecular clock. There is, perhaps, another addition factor at play here.
Image credit: Changes in genetic mutations can be used to give a measure of time. Tatiana Shepeleva/Shutterstock
Adrian Melott, professor of physics and astronomy at the University of Kansas, has been looking at the effect of cosmic radiation on the rate of genetic mutations in various bird species over time. “In general, molecular clock estimates are far older than the fossil recordindicates that the species diverged,” Melott said in a statement.
Taking the incomplete fossil record into account, he has hypothesized that periods of increased incoming cosmic radiation, perhaps from a nearby supernova, change the rate of mutations. Looking at specific radioactive elements found within the geological record shows that between 2 and 2.5 million years ago, there was a sudden uptick in incoming radiation and genetic mutations in these birds.
“Going back toward this increase of radiation, the molecular clock would speed up. Going back before the event, say 10 million years, the rate of the clock would slow down again toward a more long-term average,” Melott concluded.
Although this is one of several possibilities, Merlott thinks it should be considered when re-calibrating the molecular clock. His findings, supported by the NASA Exobiology and Evolutionary Biology Program, will be published in an upcoming paper.
