Theory, Prediction and Converging Lines of Evidence, Part 3
Note: One of the challenges for discussing evolution within evangelical Christian circles is that there is widespread confusion about how evolution actually works. In this (intermittent) series, I discuss aspects of evolution that are commonly misunderstood in the Christian community.
In the last two posts in this series we have described a number of lines of evidence for whale evolution to illustrate two things: the incredible predictive power of evolutionary theory, and how a wide array of evidence from multiple disciplines converges to support it. In this post, we continue to explore how whale evolution is supported by converging lines of evidence, briefly comment on how multiple lines of evidence also support human evolution, and discuss how Christian anti-evolutionary approaches offer scientists less, not more, predictive power.
Whale evolution: tying a few remaining threads together
In our first post on whales, we noted that an evolutionary understanding of their origins from terrestrial ancestors raised certain questions:
Instantly this prediction raises a host of uncomfortable questions: where did their hind limbs go? How did they acquire a blowhole on the top of their heads when other mammals have two nostrils on the front of their faces? How did they transition to giving birth in the water? What happened to the teeth of the baleen whales? What happened to the hair characteristic of mammals? and so on. In some ways, evolutionary thinking about whales creates more difficulties than it appears to solve.
And yet, these difficulties are the stuff of science. If indeed our “educated guess” of terrestrial, tetrapod ancestry for whales is correct, the evidence will show that these transitions, challenging though they may seem, did indeed occur on the road to becoming “truly cetacean”.
Of this list, we’ve tackled all but a few: the transition from nostrils to blowholes, and the transition to birth in a water environment. The first has some good lines of evidence that speak to it, while the second remains more of an open question. As with studies on hind limb loss, the combined approaches of embryology (studying the development of modern whales from fertilized eggs) and paleontology (looking for relatives of modern whales in the fossil record) provide some clues as to how these features came to be in modern whales.
From nostrils to blowhole, and other details
Numerous whale species in the fossil record support the notion that the nostril-to-blowhole transition arose slowly. Earlier whales in the fossil record have nostrils at the tip of their snouts, and several later species are known with nostrils progressively further back on the skull. Some of these species have been discussed here previously and provide some “snapshots” of the process over large spans of time.
In addition to these observations, embryology yields still more clues. During early embryogenesis, modern cetaceans look like a typical mammalian embryo: four limb buds (as we have discussed previously) with distinct digits visible, a tapering tail that extends past the anal opening (the characteristic mammalian “post-anal tail”), and two nares (the technical term for nostrils) on the front of the face. Indeed, one would be hard-pressed to distinguish whale embryos from other mammals at this point, even humans, unless one knew about small differences to look for. Later in development, however, things are markedly more whale-like: the digits are no longer discernable in what are now clearly flippers, the tail is no longer slender and tapered but is developing projections that will become flukes, and the nostrils have migrated to the top of the head to form a blowhole. Fascinating images of cetacean developmental stages that illustrate these transitions in dolphins can be seen at the Digital Library of Dolphin Development and are well worth exploring. Taken together, paleontology and embryology provide independent lines of evidence that these features arose over time as gradual modifications to a more typical mammalian body plan.
Dolphin embryos show hindlimb buds in early stages of development.
Giving birth: from land to water?
One area of whale evolution that remains less well understood is the transition from birth on land (the ancestral state, since whale ancestors are terrestrial) to birth in the water. One key difference between cetaceans and land-dwelling mammals is giving birth tail first, instead of head first. The discovery of one early whale, Maicetus, has provided evidence that early amphibious whales retained land-based birthing, though this interpretation has be challenged by other experts in the field. The evidence comes from one fossil specimen that appears to have a near-term fetus positioned for head-first birth inside it. While the evidence is suggestive and intriguing, other interpretations have been put forward (such as the possibility that the second, smaller skeleton was actually a recent meal, and not a fetus). Future work, with additional specimens, will hopefully shed more light on this issue.
Et tu, Homo?
While these posts have used whale evolution as an example to illustrate the predictive power of evolution and the converging lines of evidence that support it, human evolution stands on an equally strong footing. From genome comparisons, to shared pseudogenes, to embryology (did you know you once had a post-anal tail, for example?) to the abundant fossil record of transitional forms that blur the distinctions between humans and other primates, these and other converging lines of evidence continue to support human evolution - and cause consternation for anti-evolutionary approaches.
The scope of the evidence and the challenge for Christian anti-evolutionism
As we have seen, the strength of evolution as a scientific theory does not rest in any one piece of evidence, but rather in the numerous pieces from multiple disciplines that fit together in a cohesive way, mutually reinforcing one another. One aspect of Christian anti-evolutionary materials that I find frustrating is that the broad sweep of evidence for evolution is avoided in favor of focusing in on specific, isolated details in an attempt to refute them individually. This approach fosters the misleading impression that evolution, as a theory, stands or falls on the interpretation of small experimental details. In reality, evolution as a theory is supported by a vast array of data from many independent fields, and any attempt to refute evolution will fail scientifically unless it addresses that vast array. As such, Christian anti-evolutionary approaches do not offer a significant scientific challenge to evolution. Rather, they merely create an impression of evolution that does not do justice to its true strength.
If Christian groups wish to supplant evolutionary theory with a different approach they feel is more consonant with their theistic convictions, the challenge is to offer an explanatory framework that is more useful to scientists than evolutionary theory. To date, all such models offer scientists less that is scientifically useful, not more. The reason Christian anti-evolutionary approaches are absent from the mainstream scientific literature is not because scientists are theologically or philosophically biased against them, but rather because they offer little in the way of useful tools for making accurate predictions about the natural world. Scientists are pragmatists, by and large: they go with what works. Evolution, as a theory, has worked, and worked exceptionally well, for over 150 years – a fact that even scholarly young-earth creationists concede. Any contender for its place as an explanatory framework in biology will have to work even better, while also accounting for everything evolution already explains so well. That’s a tall order for any model, and one that, to date, anti-evolutionary approaches have not come close to fulfilling.
In the next post in this series, we’ll turn our attention to one such anti-evolutionary argument - the Intelligent Design concept of irreducible complexity - and examine some recent evidence for how irreducibly complex structures arise through evolutionary processes.
For further reading:
Gingerich PD, ul-Haq M, von Koenigswald W, Sanders WJ, Smith BH, et al. (2009) New Protocetid Whale from the Middle Eocene of Pakistan: Birth on Land, Precocial Development, and Sexual Dimorphism. PLoS ONE 4(2): e4366. Available online.
Bajpai S, Thewissen, JGM, and Sahni, A (2010) The origin and early evolution of whales: macroevolution documented on the Indian Subcontinent. J. Biosci 34 (5); 673 -686.