29 April 1998
Mammals Made Good Before Dinosaurs Copped It
The ancestors of modern mammals may have set up shop far earlier than previously believed - well before the mass extinction of dinosaurs 65 million years ago, say researchers at Penn State University. Based on data from the largest ever study of gene sequences, the findings repute the idea that our fuzzy progenitors could only flourish when the dinosaurs died out.
"The evolution of mammals appears to have occurred gradually by the isolation of breeding groups when the continents broke apart, not suddenly by the rapid filling of ecological niches left vacant when the dinosaurs became extinct," says Blair Hedges, associate professor of biology. Which means modern orders of mammals were alive and kicking up to a hundred million years ago.
Scrutinizing a massive collection of gene sequences stored at Genbank, the genetic databases maintained by the National Institutes of Health, the researchers were able to estimate more accurately than ever before when certain species appeared on earth. In a process likened to the ticking of a molecular clock, they measured the development of genetic mutations to trace species' histories back to their origins.
Other researchers cite the absence of fossil records supporting the theory as problematic and the challenge now is for palaeontologists to find vertebrate fossils in previously ignored geological strata. "We are saying mammals definitely were living on Earth during the Cretaceous period from seventy to a hundred million years ago, " says Hedges. "We don't yet know what they look like, but from the genes of their descendants we now know that they were there."