Lists of vestigial biological structures in biology textbooks are so short that some young-Earth creationist authors claim that scientists have lost confidence in the existence of vestigial structures and can no longer identify any verifiable ones. We tested these hypotheses with a method that is easily adapted to biology classes. We used online search engines to find examples of 21st-century articles in primary scientific literature in which biological structures are identified as vestigial. Our results falsify these creationist hypotheses and show that scientists currently identify many structures as vestigial in animals, plants, and single-celled organisms. Examples include not only organs but also cells, organelles, and parts of molecules. Having students repeat this study will give them experience with hypothesis testing, introduce them to primary scientific articles, and further their education on vestigial structures.
Many organisms possess biological structures that are recognizable as degenerate versions of their homologs in related organisms and that do not perform the functions that those homologs perform. For example, degenerate eyes in blind cave fishes and cave salamanders are useless for vision (Eigenmann, 1900), and degenerate limbs in numerous lizard species are useless for locomotion (Moch & Senter, 2011). Such degenerate structures are called “vestigial structures” because they are vestiges (remnants) of ancestral structures. Biologists recognize vestigial structures as evidence for biological evolution (Starr & Taggart, 2004; Reece et al., 2011). For example, blind cave fishes and salamanders arguably have eyes only because they inherited them from sighted ancestors.
Until recently the human and ape appendix has been considered a vestigial organ, a remnant of a much larger ancestral cecum. A cecum is a side branch of the large intestine that houses bacteria that break down cellulose, enhancing the digestion of plant matter in herbivorous mammals (Kardong, 2011). However, an anatomical study of primates showed that the appendix of humans and apes is not a remnant of a cecum but is instead an evolutionarily new structure with no homolog in lower primates (Scott, 1980). It appears to function as a protective reservoir for beneficial bacteria that inhabit the colon, a microbial “Noah’s ark” from which beneficial bacteria can repopulate the colon if a disease decimates them (Bollinger et al., 2007).
The recognition of the appendix as vestigial ceased not because it has a function but because it is a newly evolved structure instead of a vestige of an ancestral structure. A structure does not have to be useless or functionless to be a vestige. Even so, scientists generally hesitate to use the term “vestigial” for a structure unless it has lost its most salient previous function. For example, the degenerate pelves of whales currently function as anchors for reproductive structures but are considered vestigial because they have lost their previous function as anchors for hindlimbs that are used in locomotion (Simões-Lopes & Gutstein, 2004). Likewise, the degenerate ink glands of certain marine snails store algal pigments but are considered vestigial because they have lost their previous function as organs of ink production (Prince & Johnson, 2006).
Anti-evolution authors in the young-Earth creationist (YEC) camp have long insisted that all structures previously identified as vestigial are actually misidentified as such (e.g., Morris, 1974; Koop & Schaeffer, 1987; Bergman & Howe, 1990; Bergman, 2000; Menton, 2010). According to the YEC argument, no truly vestigial biological structures exist. Rather, in each case, the structure is functional but its function was unknown when it was labeled as vestigial. Such authors fail to understand that a structure can have a function and yet be a vestige. Nevertheless, some of these YEC authors have noticed something that is worth noticing: Lists of vestigial structures in biology textbooks have dwindled through the decades. These authors use this as evidence that scientists have lost confidence in the existence of vestigial structures or that scientists cannot find more examples of valid vestigial structures (Koop & Schaeffer, 1987; Bergman & Howe, 1990; Bergman, 2000). As one YEC author puts it, “vestigial organs…have now been thoroughly discredited” (Bergman, 2010, p. 63).
Indeed, lists of vestigial biological structures in current biology textbooks are usually quite short, with only one to three examples (e.g., Starr & Taggart, 2004; Reece et al., 2011). This is the case even in textbooks for evolution classes (e.g., Ridley, 2004; Kardong, 2008), one of which does not mention vestigial structures at all (Volpe & Rosenbaum, 2000). It is therefore worth testing the YEC hypotheses that biologists have lost confidence in the existence of vestigial structures and that more examples than those in short textbook lists cannot be found. Both hypotheses make the same prediction: that a review of recent primary scientific literature will find only a small number of examples (or none) of biological structures that are identified as vestigial. This is because scientists primarily communicate via primary literature (technical journals, etc.), not textbooks. Here, we report a test of these YEC hypotheses.
The test described below is one that can be employed as an assignment in a biology class to serve three purposes that are important for science students. First, it involves students in hypothesis testing, which gives them experience with scientific method. Second, it introduces students to primary scientific literature, so that they can see firsthand the ultimate sources of the information that ends up in textbooks and in secondhand reports in popular science magazines. Third, it expands their education on vestigial structures beyond the meager information found in textbooks. All three goals were in fact attained when this test was performed in a class taught by one of us (Senter), in which the rest of us were graduate students.
We used the search terms “vestigial” and “vestige” to search online databases of primary scientific articles such as JSTOR (http://www.jstor.org) and Science Direct (http://www.sciencedirect.com) for examples of articles in which biological structures are explicitly identified as vestigial. We counted such identifications only if the following five criteria were met: (1) The authors’ wording indicates that they themselves consider the structure vestigial and are not merely citing previous opinions on vestigiality. (2) The authors use the word “vestigial” or “vestige,” not just a synonym (e.g., “rudimentary” or “reduced”). (3) The authors are not describing a rare developmental anomaly. (4) The organism with the vestigial structure is extant. (5) The vestigial structures are not just mentioned in passing but are important to the main focus of the article. To avoid the appearance of “stacking the deck,” we did not use any articles for which any of us was an author.
We used only articles published in the 21st century, to ensure that the identification of a structure as vestigial is recent enough to be considered current. We did not use articles from the year 2000, because that is actually the last year of the 20th century.
In 21st-century articles from primary scientific journals, we found enough examples of biological structures that scientists identify as vestigial to place 64 entries in Table 1. Several of these entries include multiple species or supraspecific taxa. This falsifies the YEC hypotheses that scientists have lost confidence in the existence of vestigial biological structures and that scientists cannot find more than a few examples of vestigial biological structures in nature.
To make our results more useful to others, we have included information on function in Table 1. A few vestigial structures are explicitly recognized as entirely useless in primary scientific literature (Table 1), but most are not.
It is probable that we have missed numerous examples of biological structures that scientists currently consider vestigial. This is because the online search engines cannot find every single scientific article published in the 21st century, because we examined no primary scientific literature from sources other than journal articles, and because we used only English-language articles. Table 1, therefore, should not be considered a complete list, and the absence of a structure therein does not necessarily mean that scientists do not currently consider it vestigial. Furthermore, we did not include the numerous examples of vestigial structures recognized in fossil taxa (e.g., Senter, 2010). These facts, in addition to the fact that Table 1 contains a plethora of examples despite its incompleteness, show that biological structures that scientists currently consider vestigial are common, not rare or nonexistent.
As Table 1 shows, some body parts are particularly prone to vestigiality in certain taxa or in organisms in certain ecological niches. For example, vestigial reproductive structures are common in plants. Vestigial limbs are common in lizards. Vestigial eyes are common in burrowing vertebrates. Vestigial mitochondria are common in microbes that inhabit anoxic environments.
Our results show that scientists recognize vestigiality at numerous levels of biological organization in addition to the organ level. In some cases, a major bodily region is vestigial (e.g., the abdomen of a barnacle; Blin et al., 2003). Structures smaller than organs can also be vestigial. Vestigial organelles have been identified in unicellular organisms (e.g., vestigial mitochondria in several species [Regoes et al., 2005] and vestigial chloroplasts in others [Sekiguchi et al., 2002]).Even parts of molecules can be considered vestigial. Researchers have recently identified vestigial genes in whales (McGowen et al., 2008) and a vestigial region in antibody molecules of wobbegong sharks (Streltsov et al., 2004).
It is rare for biology textbooks to mention vestigial structures other than organs and to list more than three examples. We therefore hope that our compilation in Table 1 will be useful to educators who wish to supplement meager textbook information with further examples. We also recommend that longer lists of vestigial structures be added to biology textbooks, to counter the YEC hypotheses that are falsified here.
Our study is easily adapted to biology classes as an assignment. If students are assigned to find a certain number of publications on vestigial structures in primary scientific literature, they need only be taught how to enter the term “vestigial” or “vestige” in an online search engine and to recognize primary scientific articles (e.g., by the presence of an abstract). The experience and knowledge gained during such an exercise would be a valuable addition to a student’s biological education.
- © 2015 by National Association of Biology Teachers. All rights reserved. Request permission to photocopy or reproduce article content at the University of California Press’s Rights and Permissions Web site at http://www.ucpressjournals.com/reprintinfo.asp.