Nature reports on this new paper that shows a major conflict resolving the fossil and molecular records of mammalian evolution. It’s entitled, “Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary” and the major finding is that mammals seem to have evolved largely after this boundary based on their discovery of fossil evidence of a new mammal. This isn’t a new finding for the fossil record, but this study represents the largest fossil-based evolutionary tree to date.
However, this conflicts with the molecular record (the editorial gets the lead author’s name incorrect – it’s Beninda-Emonds – nature news coverage here) which constructs a evolutionary tree of 99% of mammals suggesting more than 40 mammals survived the Cretaceous period – ended by the mass extinction of the dinosaurs at the Cretaceous/Tertiary (K/T) boundary 65 million years ago . Their finding suggested that the end-cretaceous extinction event may not have provided the impetus for the expansion of mammalian species implied by the fossil record.
It’s an interesting debate as the molecular and fossil records tend to conflict when it comes to the dates of branching along the evolutionary tree. And, because it’s a debate, it’s just a matter of time before it’s quote-mined by the evolution denialists at Uncommon Descent. In particular, I would be concerned with passages such as these from the nature editorial.
Yawning gaps between molecular and palaeontological approaches to the dating of evolutionary landmarks have appeared ever since molecular approaches based on DNA sequences first became widely used about 15 years ago. When results differ, the molecular technique almost always pushes events further back than the fossil record. For example, some molecular studies estimate that multicellular animals arose about a billion years ago, but the fossil record goes back a mere 600 million years. Fossil plant spores date back 475 million years, but molecular dates put plants on land 700 million years ago.
And this passage:
But although Kevin Peterson of Dartmouth College in Hanover, New Hampshire, agrees that some groups’ fossil records are too patchy to be trusted, he is scathing of the notion that the first animals could have gone unfossilized for nearly half a billion years. “To say that because the record is incomplete you can have a billion-year-old bilaterian is a woeful misunderstanding of the fossil record,” he says.
Let’s see if we can get in a pre-emptive strike. How much do you want to bet they will won’t quote these paragraphs:
However, there are signs that this deep-running discord may be being resolved, at least in some cases. “Ten years ago the divide looked very sharp,” says palaeontologist Mike Benton of the University of Bristol, UK. “But there’s been a certain amount of movement both ways — palaeontological and molecular people are moving towards one another.”
That molecular dates are a little earlier is to be expected. DNA should record the moment that a lineage split, but the imperfections of the fossil record mean that the first preserved post-split organisms will date from later on. And there are some groups — such as the microscopic, soft-bodied animals that make up about half the extant animal phyla — that have no fossil record at all. “We should be willing to accept biases in the fossil record regarding certain types of animal,” says Blair Hedges of Pennsylvania State University in University Park. Hedges has been among the most vigorous advocates of earlier DNA dates for major evolutionary splits, including those that produced the animals, the land plants, the fungi and mammals.
Benton sees hope in new statistical methods that can better account for uncertainty in both the timing of calibration points and measurements of evolutionary rates. In such treatments, both fossil and molecular dates go from being fixed points with error bars to being a range of probabilities. “The flexibility is hugely liberating,” says Benton. “I see a glowing future for this.”
Using such an approach, Peterson has obtained molecular dates for the origin of the animals that match their first appearance in the fossil record (K. J. Peterson & N. J. Butterfield, Proc. Natl. Acad. Sci. USA 102, 9547-9552; 2005). He presented his latest analyses this week at a meeting at the Royal Society in London. “All of the latest estimates are putting the division of the bilateria right in the Ediacaran,” he says, referring to the geological period pre-dating the Cambrian explosion of about 540 million years ago. A group led by Emmanuel Douzery at the University of Montpelier in France has got similar results in the past few years. Hedges argues, however, that these more recent dates are the result of calibration errors.
Let’s see what happens, but don’t expect the denialists to miss the opportunity to use a scientific debate to castigate the science as a whole. Now, I’m not an evolutionary biologist, and I’ll await commentary on this paper from some of those who have expertise on the topic – I’m unsure exactly why the discovery of this new Eutherian is such convincing evidence. But I do know an opportunity for a crank to latch onto a sentence out of context when I see one – this is just a matter of time.