The Questionable Authority

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.

Why?

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:”

i-24ac98c432e6478bd7e462e8d5c3cc78-Stemgroups-tm.jpg

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.

References

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.

Comments

  1. #1 fnxtr
    March 30, 2007

    In retrospect, doesn’t early diversification make sense? The mammmals would have been under selection pressure not only from each other (especially if their niches were limited) but also from the presumedly ‘ruling class’ dinosaurs… if not for the same resources, then trying hard not to wind up as food.

  2. #2 marty
    March 30, 2007

    about 147 years ago
    Psst… dontcha mean ‘million’ there? :D

  3. #3 wolfwalker
    March 30, 2007

    The question of evolutionary diversification has always interested me. So I’ll be waiting for further information on this, with great interest.

    The suggested burst of mammalian diversification at 100MYA correlates roughly with a major burst of diversification in dinosaurs. Basal ceratopsids, hadrosaurs, tyrannosaurids all first developed around that time, among others. That suggests an environmental cause.

    I’m not really surprised at the lack of a K-T extinction in mammalian lineages. I’ve been highly suspicious of the conventional explanation for the K-T mass extinction ever since I learned that for lower herps — amphibians, lizards, snakes, crocodilians — it was barely a mass extinction at all. More herp families survived K-T than didn’t.

  4. #4 nico
    March 30, 2007

    I read a few articles about this (NYT, etc) and it’s not quite clear what’s so novel about this compared to what I understood from Dawkins’ book, the Ancestor’s tale. In it mammal orders were already separated before the K-T event, although they might not necessarily have displayed the current wide range of adaptations. Basically those mammals kinda all looked like shrews, even if their DNA were widely different. This didn’t preclude them from quickly diversifying when new niches opened. Or did I miss something?

  5. #5 ruidh
    March 30, 2007

    I don’t have a subscription, so I can’t read the original article. But, I’m skeptical. Let’s assume that the mammals underwent a period of rapid evolution after the dinosaurs went extinct. Then, when we went back and looked at the numbers of genetic mutations and added an (incorrect!) assumption of a constant rate of mutation, then all of these branches would look like they split earlier in time then they really did.

  6. #6 J-Dog
    March 30, 2007

    Any information on when the YEC’s and IDers split off from the snakes?

  7. #7 RPM
    March 30, 2007

    ruidh — you’re thinking of functional nucleotides. The majority of sites in the analysis would have had nothing to do with any phenotypic evolution. And the mutation rate wouldn’t have changed; it’s the substitution rate the changes during rapid evolution of DNA sequences. But that substitution rate would have only changed at sites responsible for phenotypic evolution. The clock like nature of DNA (and protein) evolution across a genome or at many loci is remarkably robust to things like generation time, let alone natural selection which would have a very focused effect.

  8. #8 Dan Gaston
    March 30, 2007

    I had a few issues with the paper. There are several issues concerning supertrees, and they are a highly contentious and controversial methodology right now. My main issue is that they appear to be working with DNA sequences as opposed to protein sequences, and here we are talking about some relatively deep evolutionary splits. The much more robust methods based on protein sequences I think would be of more use in this situation.

    Of course the issue of calibrating molecular clocks is difficult. Apparently this supertree was also compiled using pre-existing supertrees.

  9. #9 Greg Laden
    March 30, 2007

    I like the fact that you qualify the importance of the paper in relation to modern mammals.

    It has been known for some time that the major groups of mammals diversified prior to the KT boundary. Indeed, maybe this has been known for longer than the pinning down of the KT event in all it’s details, which you’ll remember is a fairly recent discovery… (1980s)

    But in the paper they make the point that “mammalian diversificastion” happens in two stages, one early (over 90 million) one later (ca 54 million and later) leaving the KT boundary high and dry.

    But this does not obviate the fact that the fossil record still shows a major diversification of mammals after the KT event. The real questions that arise from this new (and wonderful) paper are about the interaction between the molecular record and the fossil record. The paper is not a test of the post KT boundary diversification hypothesis.

    See: http://gregladen.com/wordpress/?p=594

    SE

  10. #10 Torbjörn Larsson
    March 30, 2007

    The blogs are active on this now, so I’m glad that this blog could add new information. This is interesting even for me as a layman.

    Any information on when the YEC’s and IDers split off from the snakes?

    According to Michael on The Panda’s Thumb, at the time of Ad-Hominids.

    I’m not so sure however, since we have plenty of examples of chthonic individuals from quote-mines… Uh, you said “snakes”, I was thinking worms. Do we know they have a spine – or are you suggesting subsequent loss?

  11. #11 Torbjörn Larsson
    March 30, 2007

    Blockquoted myself as I finished edit in the comment box – probably my first pseudo-meta-comment. :-)

  12. #12 Christophe Thill
    April 2, 2007

    “Any information on when the YEC’s and IDers split off from the snakes?”

    They did???

  13. #13 Terminus Est
    April 6, 2007

    I don’t think the whole K-T bangup job needs to be junked just yet vis a vis mammalian evolution…just tinkered with. It appears to me that what this research has found was that the big takeover by the mammals was more a placid, semi-passive event. The people were there, in all their diversity when the dinos got rubbed out. From that point, rather than getting an explosion of evolutionary diversity, the already extant mammals simply stepped in to enjoy the then empty niches and all the benefits that accrue. No more sharing. The K-T extinction event simply replaced one set of predominant creatures with a new set with no explosion of evolutionary diversification being necessary.

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