UNIVERSITY PARK, Pa. -- Molecular clocks -- based on changes in genetic material -- indicate much younger ages for a wide variety of plants found as fossils in southern Argentina than do the solid, geologic dates of those fossils, according to geoscientists who surveyed recent paleobotanical discoveries in Patagonia.
The finding suggests serious biases in molecular clocks, which are heavily used to date many kinds of living things. It also directly refutes a widely-held idea about how most Southern Hemisphere plant and animal groups attained their current distributions.
Geologists date fossils by radioisotopic analysis, which can produce absolute ages with uncertainties less than 0.05 percent. Molecular clocks apply rates of molecular change and fossil 'calibrations' to the tree of life to construct a 'timetree' that estimates when evolutionary events occurred. Substitution rates come from DNA found in multiple genes, and known, dated fossils provide the calibration anchor points. Even though the clock method's stated errors are much larger than for geologic dating, it offers the hugely appealing advantage of dating the large proportion of living organisms that have very limited, or no, fossil record.
"Paleontology and molecular clocks have a long, uneasy relationship," said Peter Wilf, a paleobotanist and professor of geoscience, Penn State. "Paleontologists want molecular clocks to work. However, for years we have seen molecular dates, mostly for very deep evolutionary events, that are much older than the corresponding fossils. This situation has been a frustrating Catch-22 because if the clocks are wrong, no fossils exist that could demonstrate they are wrong. Here, we looked at many new plant fossils from the extremely productive region of Patagonia, and we found the opposite, that the fossils are much older than the clock dates. In this case, we can definitely say that the clocks are wrong. The fossils prove it."