In a recent news article published in Science Daily, researchers made the claim that evolution of the placental mammals sped up three times faster in the 10 million years following the extinction of the dinosaurs at the end of the Cretaceous geological period. Prior to that event, conventionally dated at 66 million years ago, evolution of Eutherian mammals from which placental mammals supposedly arose was assumed to be constant. The findings of the research team from the University College of London were that early placental mammals took advantage of the dinosaur’s extinction at the end of the Cretaceous and, without these predatory pressures, rapidly evolved into new forms. Using this new extrapolation, the researchers concluded that the last common ancestor for placental mammals lived at most about 69 million years ago, compared to 89 million years, under a slow, constant rate of evolution paradigm, based on Bayesian dating methods.
What Data Was Found to Support This Hypothesis?
The researchers analyzed fossils from the Cretaceous to the present, and taking the (assumed) dates of their occurrence in the fossil record, they estimated the timing of divergences based on a tree of life, which they had updated in 2015. This new tree of life included Paleocene taxa (conventionally dated at 66–56 MY), which had been mostly or entirely excluded in previous studies.
The scientists measured bones and teeth from the fossils of 904 placental mammals, and then mapped all of the differences between the species on their new tree of life. The number of changes allowed them to reconstruct a presumed evolutionary timeline for these placental mammals after the Cretaceous extinction. Their conclusion was that “no definitive crown-placental mammal has yet been found from the Cretaceous.”
According to senior author Professor Anjali Goswami:
Our findings refute those of other studies which overlooked the fossils of placental mammals present around the last mass extinction. Using rigorous methods, we’ve successfully tracked the evolution of early placental mammals and reconstructed how it changed over time. While the rate differed between species, we see a clear and massive spike in the rates of evolution straight after the dinosaurs become extinct, suggesting our ancestors greatly benefitted from the demise of the dinosaurs. The huge impact of the dinosaur extinction on the evolution of our ancestors really shows how important this event was in shaping the modern world.