The common ancestry of all species on earth is evidenced by, among many other things, the persistence of no longer useful bodily features — called vestiges — and the occasional appearance of features that have reverted to some ancestral form — called atavisms. The most common examples from the animal kingdom are the vestigial eyes of species who no longer need them, like the blind cave crayfish; the useless wings of kiwis; and the rare whales and dolphins born with rudimentary hind limbs that betray their history as land mammals. Humans, of course, are no different in this regard, and carry the history of their relation to the rest of nature in the codes of their genes and the flesh of their bodies.
The Vestigial Appendix
Properly speaking, a vestigial feature is one that is retained by most or all members of a species, but no longer performs the function for which it originally evolved (though this does not preclude the feature being later co-opted for some other function). The best-known example of this phenomenon in humans is the appendix, an organ that nearly everyone is born with.
The original function of the appendix is not clearly understood; in mammals whose main diet consists of leaves it seems to play a role in helping to break down tough leaf fibers into more digestible sugars, and some recent studies have also theorized that it may be some minimally functioning part of the human immune system.
However, the fact that many people have had the organ completely removed with no detrimental effect is a strong indicator that the appendix is no longer useful to humans in any meaningful way. On the contrary, this vestigial feature can often become infected and rupture, sometimes fatally, leaving humans with what amounts to a ticking time bomb in their abdomens.
Goosebumps and Ear Wiggles
Whenever a human being gets chilly or frightened, he or she has the potential to demonstrate another one of our vestigial traits: goosebumps. These tiny bumps on the flesh are caused by the arrector pili, the muscles at the base of each hair on the body. Though goosebumps serve no useful function in humans, in their hairier ancestors the muscles would have raised a thick coat of fur in order to keep warm or to appear larger to oncoming enemies.
Likewise, the fact that some humans can wiggle their ears is also due to shared ancestry with mammalian ancestors like cats, who can move their ears around independently in order to determine where a sound is coming from.
Fur and Human Tails
As distinct from vestigial features, atavisms are occasional reversions to ancestral type, such as the relatively common appearance in horses of extra toes that do not touch the ground; this is considered a reversion because the one-toed modern horse is descended from a five-toed ancestor. There are also the occasional atavisms manifested in the lab, such as hens born with teeth.
Human atavisms are actually somewhat common, and one of the most common of all is babies born with tails. All humans still retain the genetic recipe for making tails, and indeed all human fetuses briefly possess a tail early in their embryonic development. In a few cases, this tail does not become absorbed back into the body, and instead develops much the way it would in a monkey. Sometimes the tails are boneless, but other times they more resemble normal mammalian tails, with vertebrae, hair, and nerves.
Similarly, all human fetuses grow a thick coat of fur around six months after conception. This coat is known as lanugo, and normally falls out about a month before birth, only to be replaced with the expected coat of sparse human hair. But in rare cases, this lanugo does not fall out, and the baby is born with an abundance of thick mammalian fur (babies born prematurely often display this phenomenon as well) which is usually shed shortly thereafter. Atavisms like tails and lanugo are rare but expected reminders of humans' shared evolutionary history with apes and monkeys.
Source:
Coyne, Jerry A. (2009). Why Evolution Is True. Penguin Books. ISBN 9780143116646.
Jenny Ashford - Jenny Ashford is a writer and graphic artist from central Florida. Her main area of interest in her Suite 101 articles is science, with a ...
Join the Conversation