Evolution of Mammals

 
A paper in this week's issue of Nature presents a nice summary of recent work on mammalian evolution. Bininda-Emonds et al. (2007) have combined a lot of data from various studies in order to construct a supertree of mammalian evolution. The study incorporates fossil data with molecular sequence data to arrive at estimates of divergence times for 4,510 species of mammal out of a total of 4,554 extant species (99% complete).

This is a study of macroevolution. The authors are addressing questions about the mode, tempo, and pattern of speciation over a period of more than 150 million years. The main questions are when did mammals diversify and did it have anything to do with the mass extinction event at the Cretaceous/Tertiary (K/T) boundary. This is the event that resulted from an asteroid impact 65 million years ago.

The results are presented in the form of a large phylogenetic tree showing the major groups of mammals. The first split in the mammalian tree occurred 166 million years (My) ago when monotremes such as platypus and echidnas (black) split off from the other mammals. Marsupials such as opposums, kangaroos, and koalas (orange) separated from placental mammals 148 My ago.

Within the placental mammals, all of the extant orders appeared by 75 My ago. This includes the clades labelled on the outside of the circle plus other. For a compete list and a description of the species, see the NCBI Taxonomy website [Eutheria].


All of these orders were established at least 10 Myr before the mass extinction event (dashed circle on the circular tree). This is one of the main conclusions of the meta-analysis. The most significant diversification of mammals takes place well before the extinction of non-avian dinosaurs.

The other conclusion is that subsequent radiations at the level of families were not significant until after 50 Myr ago. This period of diversification lasted until about 10 Myr ago. There is no evidence to suggest that the radiations within each order were synchronous, ruling out global climate change as a mechanism.

Furthermore, the data clearly shows no connection between the mass extinction event at the K/T boundary (65 Myr ago) and subsequent radiations of mammalian groups. This effectively rules puts an end to the long held belief that mammals diversified after the devastation in order to fill up the niches left by dinosaurs. This is not the first paper to refute that belief but it may be the final nail in the coffin.

This summary serves as a warning to those who continue to associate evolution with environmental change. At this level of analysis there does not seem to be a connection between rates of speciation and climate change. This is most obvious with respect to the asteroid impact of 65 My ago. While it led to mass extinction, it did not lead to increases in the rate of evolution of the survivors. The branching pattern of cladogenesis in the figure is hardly affected by the cataclysm.

Similarly, there are no other speciation events that correlate with known climate change over the past 150 million years, including recent ice ages. There is growing recognition among evolutionary biologists that rates of speciation cannot be attributed to large-scale environmental change. (The data has not prevented speculation. Many reports on this paper attempt to manufacture some correlation between global environmental change and speciation. The old idea of a link between them is too entrenched to give up so easily.)

There's an interesting sidebar to this story. The paper clearly states the two main conclusions,

... the pivotal macroevolutionary events for extant mammalian lineages occur either well before the boundary (significant decrease in diversification rate at approximately 85 Myr ago, after establishment and initial radiations of the placental superorders and major orders at approximately 93 Myr ago) or well afterwards, from the Early Eocene onwards (when net diversification began to accelerate)....

Therefore, the demise of the non-avian dinosaurs, and the K/T mass extinction event in general, do not seem to have had a substantial direct impact on the evolutionary dynamics of the extant mammalian lineages.

However, in the title of the paper, The delayed rise of present-day mammals, the authors focus attention on the second conclusion at the expense of the first. Some of the press releases picked up on this emphasis, leading to the false impression that mammalian evolution is more recent than scientists thought [Did the Dino Die-Off Make Room for Mammals?] while others got it right [Mammals not such late developers, after all].

The point about early diversification is emphasized in the Nature News & Views commentary that's published with the article in the March 29th issue. David Penny and Matthew J. Phillips begin with a summary of the evidence for early evolution,

On page 507 of this issue, Bininda-Emonds and co-authors1 present an evolutionary tree of more than 4,500 mammals, and conclude that more than 40 lineages of modern mammals have survived from the Cretaceous, some 100 million to 85 million years (Myr) ago, to the present. This is paralleled by Brown and colleagues' analyses for birds, just published in Biology Letters: they claim that more than 40 avian lineages have likewise survived from before the extinctions at the Cretaceous/Tertiary (K/T) boundary 65 Myr ago. These numbers of surviving lineages push back the evolutionary history of many mammals and birds much further than earlier estimates based on smaller data sets. But strong claims need strong evidence to support them.

Later on they re-emphasize this point,

But the most challenging aspect of the phylogeny is the inference that more than 40 lineages of living mammals (and of birds, as described by Brown et al. 2007 ) survived from the Cretaceous to the present.

There are some quibbles about the data. Personally, I think the estimates for early divergence are too recent rather than too late . It all depends on the first fixed data point which is the separation of monotremes. This date (166 Myr ago) is a minimum estimate and there's evidence for an older date. The popular report on the Nature website [Disappearing dinos didn't clear the way for us] mentions this possibility. Mark Springer of the University of California, Riverside (USA) is interviewed and the article states,

"This is a reasonable first approximation," he [Springer] says. "Some of the dates and relationships are probably right on, and some are probably going to move around."

For example, says Springer, the team estimates that the deepest split in the mammals' family tree, between the egg-laying monotremes (such as the duck-billed platypus) and the rest happened 166 million years ago. But some molecular analyses suggest it happened more than 200 million years ago; Springer thinks this earlier date is probably closer to the truth. If that fundamental point changes, he notes, other things will have to shift too. "That date influences everything else through the tree," he says.

I suspect he's right and all the dates will move back in time. One wonders whether the late radiation at 50 My will then shift closer to the K/T boundary.

It's clear that more work needs to be done but the significance of this paper is that it assembles a lot of evidence into one place and publicizes a debate that's been smoldering among evolutonary biologists for over adecade.

Bininda-Emonds, O.R.P., Cardillo, M., Jones, K.E., MacPhee, R.D.E., Beck, R.M.D., Grenyer, R., Price, S.A., Vos, R.A., Gittleman, J.L., and Purvis, A. (2007) The delayed rise of present-day mammals. Nature 446: 507-512. [PDF]

Penny, D. and Phillips, M.J. (2007) Evolutionary biology: Mass survivals. Nature News & Views, Nature 446: 501-502. [PDF]