As I mentioned earlier, there’s a really interesting paper on mammal evolution in the latest issue of the journal Nature. The authors of the paper compiled a really fantastic sampling of molecular data that included data from about 99% of all currently known extant mammals. The data was then used to conduct an analysis that was by far the most comprehensive look at the molecular evolution of mammals ever undertaken. The researchers concluded, based on this analysis, that mammals diversified a lot earlier then was previously believed – so much so, in fact, that it seems to cast some doubt on how important the K-T mass extinction really was to mammal evolution.
The nature article is behind the subscription wall, unfortunately, but if you have access it’s a good read. (You can find the full citation at the bottom of the post.) They did some cool stuff, and got some cool results. How the results should be interpreted, on the other hand, is much more complex and will take a lot longer for scientists to work out.
Here’s the basic deal with the article:
The researchers developed a software program that searched through GenBank (a National Institutes of Health-run database that contains a virtually unimaginable number of DNA sequences). This program found sequence data for over 4000 different species of mammal. The researchers took this data, combined it with data from a number of published studies that had constructed evolutionary trees for various subsets of mammals, and used some fairly advanced computational techniques to construct a “supertree” that showed the evolutionary relationships for all the included species.
They then used molecular clock techniques to estimate when various groups of mammals first appeared. Molecular clock techniques are pretty complex, and pretty controversial, and I’m usually pretty skeptical of the conclusions obtained from molecular clock methods. I’m not going to clutter up this post with the three long jargony paragraphs I had written on the perils of clocking. Instead, I’ll just say this: it looks to me like the authors of the paper made a good effort to avoid the worst of the possible problems, and that the dates are probably reasonably good.
Here’s what they found:
As you might expect, the earliest split within living mammals occurred when the monotremes (the egg-laying mammals) split off from the rest. This study puts that split at about 166 million years ago, during the late Jurassic. The marsupials and the placentals split from each other next, about 147 years ago (just before the end of the Jurassic). After that, there appears to have been a long period – almost 50 million years – when nothing else split out from the placentals. At about 100 million years ago, there was a burst of evolutionary action, with all four of the superorders of mammals splitting off within about 2.5 million years of each other. The evolutionary action continued at a pretty fast pace after that, and all of the currently living orders of mammals had appeared by about 75 million years ago – well before the K-T impact put an end to the non-bird dinosaurs (that happened 65 million years ago).
Here’s why it’s cool:
The traditional view of the origin of mammals – the one you might have seen in a museum display or learned about in basic biology – basically said that mammals were the downtrodden proletariat of the Cretaceous, oppressed by the dinosaurean bourgeois and forced out of all of the good ecological niches. After the asteroid lined the dinosaurs up against the wall, the mammals were able to break the chains of their working class bondage and burst forth to fulfill their full ecological potential.
Under this view, the expectation was that there probably weren’t a lot of mammal lineages that made it across the K-T boundary. Based on this study, though, it looks like almost twice as many lineages crossed the boundary as was previously believed. This doesn’t demolish the traditional view completely, but it does (at least) indicate that we need to take a much closer look at it.
The traditional view also suggested that there probably wasn’t a lot of what scientists sometimes refer to as “ecological diversity” within the Cretaceous mammals. In other words, we thought that the groups that did make it through probably weren’t doing too many different things. These new findings, assuming that the dates do hold up, make that view look a lot less likely.
To explain that, I need to first explain a technical term (and the underlying concept) that gets used a lot in this sort of study: the “crown group:”
In this picture, taken from Wikipedia, the crown groups are outlined in the pinkish color. (Don’t worry about the other colors.) In simple terms, a crown group is defined as the evolutionary grouping that includes all of the living descendants of a common ancestor. That common ancestor presumably had all of the features that are unique to all members of the crown group.
That was probably a little confusing, so let’s look at an example. This study indicates that the egg-laying mammals split off from the rest of the mammals about 166 million years ago. To put it another way, that’s when the last common ancestor of all mammals – the grandma of both you and the duck-billed platypus – was wandering around the world. What did that umpteenth-great-grandma look like? Without a fossil, it’s hard to know for sure, but by comparing you to the platypus we can figure out a few things. You and the platypus have hair. Your mother and the platypus’ mother produced milk. You and the platypus are warm blooded. The last common ancestor of you and the platypus probably had all those traits, too.
Those principals can be carried over to this study, and they can be used to say something about the features that must have been present in the mammals that made it through the K-T boundary. Because this study shows more lineages making it through the boundary, and because it shows which ones, we now know that the mammals living in the Cretaceous were a lot more diverse than was previously thought. It’s reasonable to assume that the increased diversity in terms of the number of mammals around and the physical characteristics they had also implies that there was a lot more diversity in what they were doing ecologically.
What’s the bottom line:
If the conclusions of this paper hold up – and I’m fairly certain that they will be tested by other researchers using other methods – it does indicate one thing clearly: the evolutionary history of mammals is more complex than we thought. It happened over a longer period of time, and the K-T mass extinction was not the driving force (at least for modern mammals) that we previously thought.
All of this is far from certain, of course. I’m sure that other researchers are going to do their best to take this paper apart and find any problems with it. It will probably be years before everything is settled – but it’s going to be interesting to watch how this shakes out.
Bininda-Emonds ORP, Cardillo M, Jones KE, MacPhee RDE, Beck RMD, Grenyer R, Price SA, Vos RA, Gittleman JL, Purvis A (2007) The delayed rise of present-day mammals. Nature 446:507-511.