The Red Notebook

A few days ago, I wrote about an apparent disagreement between two sets of scientists over the evolution of mammals. I confessed to general confusion as to whether the findings of two different studies actually conflicted with each other. It turns out they did. New Scientist this week contained a short article which nicely summarised the differences:

New Scientist: When did placental and marsupial mammals split?

… According to the fossil record, our ancestors didn’t split into modern groups of placental and marsupial mammals until after the dinosaurs bit the dust at the end of the Cretaceous, 65 million years ago. So say John Wible of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, and colleagues, who have compared late Cretaceous fossils with modern placental groups…

That bolsters the traditional view of palaeontologists, but flies in the face of molecular studies of genetic divergence of living species, which put the origin of placentals 80 to 140 million years ago… “We’re in total discord with the molecular dates,” Wible says. He thinks genetic clocks fail to account for the post-Cretaceous burst of mammalian evolution.

Are palaeontologists missing fossils, or do bursts of evolutionary diversification throw off molecular clocks? You have to take both sides seriously, says Rich Cifelli of the Oklahoma Museum of Natural History in Norman.

I have to say, I’ve always had my doubts about the use of so-called genetic clocks to estimate dates of key evolutionary events. It stands to reason that genetic analyses should be able to give us a very good idea of the sequence in which such events happened, but using them to estimate actual dates for these events seems (to this ill-informed outsider at least) hopeful in the extreme.

The very concept of a genetic clock assumes that genetic mutations occur at a constant rate. This may or may not be the case, but to me it seems a bit too convenient. Physicists use radiocarbon dating and potassium-argon dating to give pretty good estimates of the ages of particular samples (although such techniques are not without their problems), but the biological world is far more messy than the physical one with its precise radioactive half-lives. My gut feeling is that using genetic clocks to provide actual dates for evolutionary events is giving in too much to physics-envy.

For the time-being, I’ll side with the palaeontologists, who deal with hard—albeit sparse—physical evidence.

But what the hell do I know? If I turn out to be wrong, I will happily stand corrected.

What on earth is an interested member of the general public supposed to think? I do wish those scientist types would make up their minds. Compare and contrast:

BBC (28-Mar-2007): Mammal rise ‘not linked’ to dinos

The extinction of the dinosaurs 65 million years ago had little effect on the evolution of mammals, according to a study in the journal Nature…

[A new mammal supertree construction] shows that the placental mammals had already split into [their main] sub-groups by 93 million years ago, long before the space impact and at a time when dinosaurs still ruled the planet.

Reuters (20-Jun-2007): Mammals burst on the scene after dinosaurs’ exit

… We wanted to test whether there were any Cretaceous placentals,” [John] Wible [of the Carnegie Museum of Natural History in Pittsburgh, whose research appears in the journal Nature] said in a telephone interview. “If the molecular dates are correct, we should be finding things that look like modern placentals in this time period and we are not.”

They found that none of these Cretaceous forms of early mammals are related to any living placental mammals. “They are just extinct dead ends,” he said.

Wible said his work reinforced the idea that the death of the dinosaurs created an opportunity for explosive growth of modern mammals.

“You’ve got all of these ecological niches that were occupied by the dinosaurs. They go extinct, and you’ve got wide open spaces. It’s like the Oklahoma land rush,” he said.

All joking aside, though, this is a fascinating subject. And, as far as I understand these two studies from the press reports (rather than the original papers), they don’t necessarily contradict each other.

… But I could be wrong!

Postscript: It turned out I was wrong: the findings of the two studies are in conflict. More here.