June 07, 2007

Observing "Macroevolution": the Molecular Evolution of Key Mammalian Traits

One of the important changes that occurred as reptiles evolved into mammals was a series of alterations to facial structure, particularly those that led from the reptilian jaw to the mammalian jaw and inner ear bones. It's a classic example of transitional forms that goes:

Summing up what's going on here, note that some of the bones that were originally part of the reptilian jaw (the quadrate, angular and articular) eventually separated from the jaw itself and became the tiny bones of the mammalian inner ear (hammer, anvil and stirrup). It's not something we human mammals think about on a daily basis, but it's an important change. It allows we furry critters to chew and hear at the same time. Reptiles don't chew their food much, and they can't hear very well when they're working it down their throats. That's a distinct disadvantage in survival; if you can't hear what's coming while you're trying to eat, then competitors attracted to the same food that you were can sneak up o you. If you make your living eating relatively large prey, those competitors are likely to be other carnivores, and if they manage to tip-toe up behind you, you may very well also wind up on the menu. Perhaps this is why almost all modern reptiles eat prey that's relatively much smaller than they are and don't tend to hunt large animals or scavenge food that would involve much prolonged use of their jaws (i.e., tearing off chunks of meat from a carcass). That's a niche that was occupied by reptiles before mammalian competition came along, and now is left largely to mammals and a few big birds.

I bring all of this up because a very interesting study is being published that's relevant to this transition and provides some seemingly solid clues as to how it happened at the molecular level:
Evolutionary Relevance Of Retinoic Acid-induced Craniofacial Malformations

...They start from the observation that intake of high doses of vitamin A or of its derivative, retinoic acid (RA), during early pregnancy is known to increase dramatically the risk of severe craniofacial malformations to the developing foetus...

The authors perform a detailed anatomical analysis of the craniofacial defects induced by treating mouse embryos with RA at precise developmental times within a short temporal interval corresponding to the 3rd to 5th week of human foetal development. The defects induced are profoundly different depending upon the exact moment at which RA is administered.

The authors show that these defects are associated with the progressive down-regulation of molecular signals involved in instructing neural crest cells to generate the various structures of the face through the activation of Dlx patterning genes.

The most intriguing observation of this study is that the morphological characters obtained when treating embryos at slightly different developmental times resemble closely anatomical features found in other species. For example the authors describe the progressive transformation of ossicles of the middle ear into components of the jaw apparatus.

Furthermore, the treated skulls display combinations of characters present in non-mammalian species arranged in a temporal sequence strongly reminiscent of the evolutionary sequence of gnathostomal jaws. They conclude with the hypothesis that modulation of a common regulatory signaling mechanism could be at the origin of changes in jaw structures between species...

The paper that this release reports on is cited:
Citation: Vieux-Rochas M, Coen L, Sato T, Kurihara Y, Gitton Y, et al (2007) Molecular Dynamics of Retinoic Acid-Induced Craniofacial Malformations: Implications for the Origin of Gnathostome Jaws. PLoS ONE 2(6): e510.
It can be read in its entirety, free of charge, at the Public Library of Science.

Cutting through the jargon and formality, though, this is some very cool stuff if you're an evolutionary biology fan. Essentially, what's happened here is that these researchers have discovered that by using a chemical, one can go into the genome of a developing organism and interrupt the expression of a series of proteins that, in a normal fetus, causes the development of a mammalian jaw and inner ear system instead of a reptilian one. This not only provides us with knowledge of the sequence of genes that normally proceeds as this happens in mammals (including humans, of course), but it also allows us to effectively observe transitional forms in the laboratory. In other words, by interrupting the process at one point or another, we can observe for ourselves how a particular bunch of reptiles evolved into a bunch of reptiles with traits that were in-between reptiles and mammals and, finally, into mammals themselves. This series of transitions can then, at least hypothetically (until someone gets around to doing it — and in all likelihood someone will), be matched to the transitional forms containing these same traits in the fossil record. If that can be done successfully, we can then make predictions about transitional forms that haven't been seen as fossils yet, but that should have once lived based on the results observed in the laboratory. In other words, this is some choice science we're looking at right here.

Of course, it's almost impossible to completely extricate the logic of the scientific method as applied to evolutionary biology from the ongoing "culture wars," as exemplified by various creationists and intelligent design (creationism lite) proponents. One of their key arguments runs along the lines of "Well gee, we're not denying that microevolution occurs, but nobody has seen the transitional forms that would be evidence for macroevolution." Leaving aside that this is nonsense, as we certainly have a number of therapsid fossils at our disposal, such an argument would now beg the question. If we can recreate any number of transitional forms that start with purely reptilian traits and end with purely mammalian traits, where is the objection to this mysterious thing called "macroevolution?" To put it more finely, if we can see a number of small changes ("microevolution") and literally watch them adding up to a tremendous and profound divergence ("macroevolution"), how is one justified in claiming that one is possible but somehow so limited that it cannot add up to the other?

And the answer is, based on all the evidence from every appropriate discipline and every bit of research, now including this key new finding, that one wouldn't be so justified. That objection is increasingly squarely within the domains of either ignorance or delusion. We can watch it happening; the last bastion of the objection, then, is that evil spirits are tricking us. If one holds to the argument, one is advocating for shamans, not science.

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