Early mammals languished in obscurity for eons, becoming diverse only after an asteroid smashed into Mexico about 65 million years ago, drastically cooling the climate and wiping out the dinosaurs. At least, that's been the long-standing idea of how mammals came to populate the world. But now scientists have discovered genetic evidence that could effectively quash that popular cause-and-effect hypothesis.
Much of what is known or imagined about the end of the Cretaceous period on Earth and the beginning of the Tertiary period that followed has been teased out of the geologic and fossil records. For example, a thin layer of iridium--a relatively rare element on Earth but more plentiful in asteroids--can be found all over the planet in sediment layers dead on the Cretaceous-Tertiary boundary. Also, dinosaur fossils abound prior to that 65-million-year-old layer but are nonexistent afterward. Mammal fossils reveal the opposite: They are virtual no-shows before the impact and grow more and more populous after. The problem with all of these data, however, is they are incomplete and largely circumstantial.
The same characteristics apply to the genetic history of mammals, which has been investigated for decades. The timekeeper of this history is DNA, the central molecule of life itself, which acts like a clock, because it mutates on a fairly regular schedule (ScienceNOW, 27 February).
An international group of researchers has examined the mammalian DNA clock in greater detail than ever before by looking at 66 well-sequenced genes in more than 2500 species and supplementing their findings with previously published analyses. The team was able to determine when a particular species emerged and which species are most closely related to each other. The result is what the researchers call the first "supertree" of mammalian evolution. Reporting in tomorrow's issue of Nature, the researchers say their study does show a spike in mammal diversity relatively soon after the end of the Cretaceous, but the new genetic evidence also indicates most of these early groups died out or greatly declined. The real explosion in mammalian diversity occurred 10 million to 15 million years after the dino demise--far too long for the two events to be related.
The reason for this second mammalian wave is a mystery, says evolutionary biologist and co-author Andrew Purvis of Imperial College London in the U.K. He says it's possible that, like the dinosaurs, the older mammal group so dominated the landscape that they prevented the rise of modern species until planetary conditions changed in a way to allow the second group to emerge.
That might not be the only uncertainty, however. Evolutionary biologist Lawrence Heaney of The Field Museum in Chicago, Illinois, says the diversification chronology suggested by the research requires that many lineages originated at times "vastly older" than the current fossil record can support--incomplete as it may be. The differences should raise questions about how precisely the molecular clock predicts the actual dates of origin of the mammal groups, Heaney says.