Human beings, like all other species on this planet, have a history. We came into existence through a process of slow, grinding trial-and-error, occurring over geological time via the sieve of differential survival. And like all species, our bodies and our genes reflect and bear witness to that history. Far from being perfect, one-time creations, we still bear the scars of the evolutionary process that made us.
This post will discuss some of the lines of evidence which hint at humanity’s past. I won’t repeat that well-known example of an evolutionary vestige, the human appendix. Instead, I’m going to focus on a few other examples that aren’t as widely discussed.
• Toes. It’s only because we’re used to having toes that we don’t usually consider how strange they are. Why do our feet have these stubby, non-functional digits on the ends? They can’t grip nearly as well as fingers, and we don’t need them to balance or to walk. (Why not just have a fused front of the foot?) By contrast, anyone who observes other primate species can see that they have, not two hands and two feet, but four hands, all of which are good for grasping. As human beings gained the ability to stand and walk upright, our feet lost their grasping function, but the digits themselves, though now shrunken and largely useless, remain.
• Lanugo. This little-known developmental phenomenon is an important clue to our mammalian past. Lanugo is a coat of fine, downy hair that fetuses grow while in the womb, covering the entire body except for the soles of the feet and the palms of the hands. Typically, lanugo is shed by the seventh or eighth month of pregnancy, although premature infants may retain it for several weeks after birth. The question is why we grow it at all, and the theory of evolution can easily explain this as a vestigial characteristic retained from our furry ancestors.
• Goosebumps. Fitting neatly together with lanugo is the vestigial human trait called the pilomotor reflex. When a person is cold or frightened, tiny muscles at the base of each hair contract, causing body hair to stand on end. In animals with thicker fur, this is a useful reflex: erect hairs trap air to create a layer of insulation, and they also make the animal appear larger and more intimidating. In humans, however, it is pointless. Like lanugo, goosebumps are a giveaway clue indicating that relatively hairless human beings are descended from furry progenitors.
• Hiccups. Yes, hiccups are a sign of humanity’s evolutionary past. In fact, unlike goosebumps or lanugo, which merely point to our shared history with hairier mammals, hiccups point all the way back to the time when humanity’s ancestors were amphibians. According to this article by Neil Shubin (HT: The Panda’s Thumb), the hiccup reflex is controlled by an area of the brain that we share with tadpoles. The hiccup consists of a sharp inhalation followed by a closing of the glottis (the valve at the top of the windpipe). In tadpoles, which have this same reflex, the inhalation draws water into the mouth, where the gills can process the oxygen it contains, but closes the glottis so the water does not enter the lungs. For tadpoles, it’s a vital breathing reflex; in humans, it’s a hiccup. And the same measures that often arrest hiccups in human beings (inhaling carbon dioxide, stretching the chest wall by taking a deep breath) also stop the gill-breathing reflex in tadpoles.
In fact, all human embryos grow tails while in the womb, and normally they are reabsorbed before birth. The true human tail is the result when this does not happen. Usually they are surgically removed, although they are benign. For an evolutionary scientist, the reason why we grow them is obvious: we are descended from an ancestor species which had them. For creationists, who claim that human beings were created complete and separate as we currently are, it must be difficult to explain why we have so many vestigial structures that link us to other species of mammals.
• The fused chromosome 2. It’s long been known that human beings have 23 pairs of chromosomes, while the other great apes such as gorillas and chimpanzees have 24. It is all but impossible that the lineage that led to humans could have completely lost all this genetic material and still produced a viable organism. Where, then, did the extra chromosomes go?
Chromosomes are tipped with distinctive segments of DNA called telomeres and have another special segment called a centromere in the middle. Lo and behold, human chromosome 2 has a telomere on one end, then an inactivated centromere, then a segment of telomeres in the middle, then another centromere, then a final telomere – the structure we would expect to find if two chromosomes had fused into one. When we compare this chromosome to the two appropriate ape chromosomes, we find a compelling match, indicating that this chromosomal fusion occurred at some point after the human lineage split from our ape relatives.
• The vitamin C pseudogene. Unlike most mammals, human beings can’t synthesize their own vitamin C; we must ingest it as part of our diet, or else we get the disease of scurvy. Under the hypothesis of special creation, humans were created this way from the beginning, so we wouldn’t expect evidence that we once had this ability but have since lost it. However, according to evolution, we are descended from other mammals, and since most mammals can make their own vitamin C, we’d expect that human ancestors did have this ability at some point as well. If this is so, our genes may preserve evidence of it.
Sure enough, human beings have a version of the vitamin C synthesis gene, but ours is “broken”, disabled by mutations. Our primate relatives, who also lack this ability, also have broken versions of the gene. Just as evolutionary theory would predict, the same disabling mutations that exist in the human gene can be found in the genes of chimpanzees, orangutans, and macaques – compelling evidence that we are all descended from a primate common ancestor who incurred this mutation at some point in the past. (It’s likely that this mutation wasn’t selected against because all primate diets are rich in fruit, providing abundant vitamin C.)
Taken together, the scars of evolution provide abundant evidence of humanity’s history. Like all species on this planet, we are not unique special creations. We are one end result of a long process of mutation sieved through selection, a countless series of adaptive compromises and tradeoffs. Our very bodies testify to the natural forces that have shaped us through the vast expanses of time.