Vestiges as Evidence for Evolution
Asked to contribute to the recent “How Stupid Not to Have Thought of That!” series here at the Science League of America, Warren D. Allmon took a different tack, choosing to write about a theme in Darwin’s work that is too often overlooked: the importance of “vestiges” as evidence for evolution. Allmon is the Director of the Paleontological Research Institution in Ithaca, New York, and the Hunter R. Rawlings III Professor of Paleontology in the Department of Earth and Atmospheric Sciences at Cornell University.
Among Darwin’s many “great ideas” that seem obvious in retrospect, one of the most important gets a lot less attention than it deserves. In the Origin, Darwin wrote that, of all the categories of evidence for descent with modification, it was “comparative anatomy”— specifically those features of organisms that seem to have no apparent function or adaptive value, or to have functions very different from those they were apparently originally built to perform—that was most convincing. No one had ever thought of this before. Patterns of similarities among species, Darwin said,
seem to me to proclaim so plainly, that the innumerable species, genera, and families of organic beings, with which this world is peopled, have all descended, each within its own class or group, from common parents, and have all been modified in the course of descent, that I should without hesitation adopt this view, even if it were unsupported by other facts or arguments. (1859:457–458)
It was one of Darwin’s central insights that such similarities are numerous, and best explained by descent with modification. Darwin saw that if organisms have had a history of change, then that history should—in fact must—leave traces. “Organs or parts in this strange condition,” he wrote, “bearing the stamp of inutility, are extremely common throughout nature” (1859:450). The existence of such structures, he added, “organs in a rudimentary, imperfect, and useless condition, or quite aborted, far from presenting a strange difficulty, as they assuredly do on the ordinary doctrine of creation, might even have been anticipated, and can be accounted for by the laws of inheritance” (1859:456).
In the Origin, Darwin referred to these features as “vestiges” or “rudimentary, atrophied, or aborted organs.” In modern terms these are usually referred to as a subset of “homologies,” which in turn are discussed by modern evolutionary biologists mainly in the context of reconstructing evolutionary history—the tree of life. Only in the obligatory summaries of “evidence for evolution” in textbooks, however, is the point made that homologies in general, and vestiges in particular, are important evidence that this history has happened at all. Seldom is Darwin’s insight emphasized, that it is the ubiquity of vestiges—features that appear to have no function, or which currently serve a function in a suboptimal or compromised way, or a function different from that which they previously served—that constitutes the main reason that evolutionists see evolution virtually everywhere, and why we are so stupefied by claims that there is insufficient evidence for descent with modification.
Outside of such textbook summaries, most “evidence for evolution” encountered by students and the general public consists of examples of adaptation. The problem with this, of course, is that adaptations can also be attributed to an intelligent creator, in the long tradition of natural theology. Such non-evolutionary views, however, have much more difficulty explaining vestiges. As Darwin wrote in the Origin: “On the ordinary view of the independent creation of each being, we can only say that so it is—that it has so pleased the Creator to construct each animal and plant” (1859:435).
The relative scarcity of detailed discussions of the logical basis for homologies in general, and vestiges in particular, as powerful and universal evidence for descent with modification is unfortunate, because it handicaps those evolutionary educators who wish to encourage students and others to learn about evolution by going out into “nature,” as opposed or in addition to less open-ended exercises. Darwin notwithstanding, homology as evidence for evolution is not an intuitive idea. Use of the present static condition of something to infer how it changed in the past is an unfamiliar way of thinking to many people. Fixed features are difficult to accept as evidence of evolution, because they do not convey the impression that anything is evolving. This is a widespread problem with all evolutionary science, which largely uses static data as evidence of a dynamic process, and is likely to be one explanation for the common objection from creationists that we cannot “see evolution happen.”
It has always puzzled me that creationists don’t object to applying this method to human history, where it has long been part of standard methods and philosophy. Historians of humanity have long realized that the combination of continuity and change that is inherent in the very nature of history itself guarantees that there will be indications preserved of both. As the philosopher John Herman Randall put it:
… all objects of historical analysis have had a development and … this development can be rendered significant, or understood, by tracing the spatio-temporal continuity of its structure … Every process at work in histories is continuous; and the histories themselves have an aspect of continuity… but they must also have an aspect of discontinuity, of novelty, of new factors coming into operation, or else they would not be histories at all. Thus a falling stone has no “history,” though its motion is continuous and its velocity cumulative. For its acceleration remains constant… A history would be no history at all if everything in it persisted… (Randall 1958:65–66,67)
With a few changes in vocabulary, this could be Darwin talking about organic evolution.
Importantly, the logic of this argument is not—as frequently claimed by some evolutionists—that vestiges are evidence for evolution because an intelligent designer or creator would not have made organisms that way. Such a claim assumes knowledge of what such a designer wanted to achieve. As Darwin himself cautioned: “Have we any right to assume that the Creator works by intellectual powers like those of man?” (1859:188).
Darwin’s insight was that history requires the persistence of traces of previous states. These vestiges are evidence for evolution because common ancestry and descent with modification would be expected to produce them, in abundance and in such a pattern that implies a hierarchical, branching tree and change through time. If species had been independently created as we see them, there would be no reason to expect characters to be arranged in such a pattern. We do not need to speculate about the unknowable motivations of a divine creator to make predictions about what we would expect to see if descent with modification is true.