Revisiting the Rise of Mammals

[FlaSoxxJim]

This is an important study. It provides very strong evidence against the conventional wisdom that the dinosaur mass extinction at the K-T Boundary immediately set the stage for an explosive mammalian diversification event.

 

In looking at the DNA from fully 99% of known extant mammal species, researchers concluded that the big mammal diversity explosion did not begin until 10-15 million years after the dinosaurs vanished.

 

The mass extinction that wiped out dinosaurs and other life 65 million years ago apparently did not, contrary to conventional wisdom, immediately clear the way for the rise of today’s mammals.

 

In fact, the ancestral branches of most mammals, including primates, rodents and hoofed animals, emerged long before the global extinction and survived it more or less intact. But it was not until at least 10 million to 15 million years afterward that the lineages of living mammals began to flourish in number and diversity.

 

Some mammals did benefit from the extinction, but these were not closely related to extant lineages and most of them soon died off.

 

These are the surprising conclusions of a comprehensive study of molecular and fossil data on 4,510 of the 4,554 mammal species known to exist today. The researchers are to report the findings in Thursday’s issue of the journal Nature, and they said this is the first virtually complete species-level study of existing mammals.

 

And now just a bit of soapboxing. . .

 

In threads here and in discussions in general, evolutionists regularly get blasted for being as dogmatic and blindly faithful and certain of what they believe in as the religious anti-evolutionists. If that were in fact true, then a solid piece of research like this that potentially lays waste to a major dogmatic evolutionary tenet (i.e., mass extinction sets the stage for adaptive radiation) should be dismissed and decried and reviled as heretical in some way.

 

But it won't be. It will be scrutinized and replicated and poked and prodded, but if it stands up to that scrutiny then it is our understanding of the biological past that will be changed. Our understanding of organic evolution will itself continue to evolve, as it has for nearly 150 years.

[Balance]

A very interesting article and a very good post.

[vandy125]

In threads here and in discussions in general, evolutionists regularly get blasted for being as dogmatic and blindly faithful and certain of what they believe in as the religious anti-evolutionists. If that were in fact true, then a solid piece of research like this that potentially lays waste to a major dogmatic evolutionary tenet (i.e., mass extinction sets the stage for adaptive radiation) should be dismissed and decried and reviled as heretical in some way.

 

But it won't be. It will be scrutinized and replicated and poked and prodded, but if it stands up to that scrutiny then it is our understanding of the biological past that will be changed. Our understanding of organic evolution will itself continue to evolve, as it has for nearly 150 years.

 

That is very interesting, but I wanted to provide a counter-balance to your soapbox. This is not a tenet that would be attacked and be blasted for changing because it does not threaten the evolution theory. It changes an aspect or view of that theory just as (in the case of Christianity) has been done for 2,000 years (after all, how many years ago did people believe that the universe revolved around the earth). The basic view that science can look back and through trial and error discover and describe natural origins is not under attack. The scientific method nor its assumptions are not under attack with this discovery.

 

Now back off my soap box. I really do find this stuff interesting, and I actually do believe in evolution. Thanks for the post

Edited March 29, 2007 by vandy125

[FlaSoxxJim]

That is very interesting, but I wanted to provide a counter-balance to your soapbox. This is not a tenet that would be attacked and be blasted for changing because it does not threaten the evolution theory. It changes an aspect or view of that theory just as (in the case of Christianity) has been done for 2,000 years (after all, how many years ago did people believe that the universe revolved around the earth). The basic view that science can look back and through trial and error discover and describe natural origins is not under attack. The scientific method nor its assumptions are not under attack with this discovery.

 

Now back off my soap box. I really do find this stuff interesting, and I actually do believe in evolution. Thanks for the post

 

Bravo, you pretty much got to the heart of it. There is no 'cult of evolution' as so many detractors would wich to pay it. And the only 'faith' involved is faith in the scientific method and that the process will continue to lead to a more refined approximation of 'the truth' in all valid fields of scientific inquiry.

 

Heck, it can be a real bummer when sensible hypotheses don't stand up to scrutiny and contrary bodies of evidence. Ontogeny recapitulating phylogeny and all that. . . so elegant and easy to buy into - except of course that it was a load of hogwash. Maybe the rise of mammals on the heels of the demise of the dinos is similarly heading to the ash heap of discarded hypotheses, which will make the story less tidy maybe, but mostly it means there's a lot still to investigate as to what environmental factors really did align to allow the great diversification of mammals.

 

Heck (again), we really don't have all the answers as to what led to the Cambrian diversity explosion so much earlier on. And maybe the accepted dogma on adaptive diversification of various primarily non-sessile marine phyla in the wake of the Permian extinction event needs to be revisited. Maybe half of everything I was ever taught in my evolution classes will be revised or tossed outright in our lifetime.

 

And it is acceptance of those very real possibilities - that we have many of the details wrong but that the process of inquiry will get us incrementally closer to 'the truth' - is what makes all of evolution's detractors sound silly when they attempt to paint evolution as some sort of religion or cult unto itself.

[CanOfCorn]

You lost me at "Mammals."

[southsider2k5]

Isn't 10-15 million years pretty much instanteous when it comes to the planets timeline?

[Balta1701]

Isn't 10-15 million years pretty much instanteous when it comes to the planets timeline?

In terms of geologic events, for the most part yes, it is pretty darn quick.

 

However, in terms of biology, it's a hell of a long time. 2 million years ago humanity was basically a bunch of big apes. Especially right after a crisis like the Cretaceous extinction event which opens up a ton of niches for new species to figure out ways to fill, there can be an enormous amount of biological innovation over that timescale.

[FlaSoxxJim]

Isn't 10-15 million years pretty much instanteous when it comes to the planets timeline?

 

I certainly do think that the vacant niche space left in the wake of the dino demise was still an important ingredient in the eventual explosive mammalian radiation event even if it happened 10-15 million years later, yes. But we're only talking 65 mybp for the K-T extinction, so 10-15 million years is a substantial chunk of that time span. The new question becomes, why the lag between the demise of one dominant taxa and the rise of the other?

 

Although my undergraduate degree carries an emphasis in evolution, my graduate work and professional experience is not in that field so I'm very much an armchair evolutionary biologist. But, I do have a hunch that the timing of mammalian diversification events will turn out to have a lot to do with that fact that the Tertiary (which started when the dinosaurs died out) is the period in which angiosperm plants (particularly monocot grasses) became the dominant vegetation on the planet. Off the top of my head, I'd say that is the most significant ecosystem re-engineering event of the time.

[Balta1701]

Although my undergraduate degree carries an emphasis in evolution, my graduate work and professional experience is not in that field so I'm very much an armchair evolutionary biologist. But, I do have a hunch that the timing of mammalian diversification events will turn out to have a lot to do with that fact that the Tertiary (which started when the dinosaurs died out) is the period in which angiosperm plants (particularly monocot grasses) became the dominant vegetation on the planet. Off the top of my head, I'd say that is the most significant ecosystem re-engineering event of the time.

The press reports sort of gloss over this, but this is one of the key issues discussed in the paper. The paper gives a couple of different possibilities.

 

1. There was some continued inhibition of mammalian development by the remaining elements of the cretaceous fauna. Not every single non-mammalian species died out at that impact event, and there were potentially other species which could have continued competing with mammals until conditions changed more. This I think sort of fits in with your bit there, and I'm going to argue against it in a second.

 

2. There was inhibition of the development of modern mammalian lines because other mammalian lines were better suited to the conditions that predominated on Earth after the K-T boundary events. I think that as a geologist, this is the one I would favor, because there is significant evidence in the fossil record of rapid mammalian divergence appearing shortly after the K-T boundary. The key element that this paper would present is that the species which made up this radiation did not in fact wind up being parts of the lines that survived to the modern period.

 

The way to think about this is that there were mammals that were around before and after the impact...but the conditions after the impact favored certain groups relative to others. But these groups which wound up being well-adapted to the post-Cretaceous conditions wound up not being well adapted to the conditions that prevailed about 10 million years later and died off.

 

Conveniently enough, the time period they note as the start of the radiation of the modern mammalian fauna happens to coincide pretty well with what we know as the Eocene thermal maximum, a period where the earth seems to have been at its warmest since the mid-Cretaceous (10 degree C or so temperature increases along with very large spikes in atmospheric CO2 concentration, to the levels that humanity will need 200-300 years at current rates to reach). There was then a fairly rapid cooling of the earth over the next 10 million years combined with multiple pulses of extinction before the Oligocene begins. It's possible that these climate shifts selected against the species that were favored at the end of the Cretaceous and more strongly for the species that had been suppressed by the fauna that did succeed.

[FlaSoxxJim]

That is a helpful summary. I have the paper in hand but it will be at least the weekend before I get to it to digest it all.

 

The second point you bring up is one I had wondered about - whether the extant lines represented by the current research missed out on earlier diversification that resulted in lineages that disappeared later in the Tertiary. If so, then the dogma of the rapid rise of mammals after the K-T event may not be totally abandoned. Which is good because I don't want to have to toss out all my kids' dinosaur fact books.

[Balta1701]

That is a helpful summary. I have the paper in hand but it will be at least the weekend before I get to it to digest it all.

 

The second point you bring up is one I had wondered about - whether the extant lines represented by the current research missed out on earlier diversification that resulted in lineages that disappeared later in the Tertiary. If so, then the dogma of the rapid rise of mammals after the K-T event may not be totally abandoned. Which is good because I don't want to have to toss out all my kids' dinosaur fact books.

As far as I can tell, this study is entirely done by constructing evolutionary trees based on DNA from modern lineages. It matches the fossil record in several important ways, in that it shows the initial divergences of mammals in the mid-Cretaceous and shows the post-Eocene radiation, but it doesn't give any information about what happened in lines that don't make it to the present.

[southsider2k5]

but it doesn't give any information about what happened in lines that don't make it to the present.

 

Psst... they go extinct. But you didn't hear that from me.

[FlaSoxxJim]

That's what I wondered, and the way I interpreted your post as well. All those pesky lines that don't make it are always the trick.

[CanOfCorn]

In terms of geologic events, for the most part yes, it is pretty darn quick.

 

However, in terms of biology, it's a hell of a long time. 2 million years ago humanity was basically a bunch of big apes. Especially right after a crisis like the Cretaceous extinction event which opens up a ton of niches for new species to figure out ways to fill, there can be an enormous amount of biological innovation over that timescale.

I see not much has changed.