ATAVISMS & VESTIGIAL STRUCTURES - COMPELLING EVIDENCE OF MACROEVOLUTION

Our macroevolution deniers have given the fossil record a wide berth, refusing to touch it with a 10-foot pole! So much for that debate. Let's move on to vestigial structures and atavisms, two strong lines of evidence that also scream "macroevolution." Both vestigial structures and atavisms entail an evolutionary past.

A vestigial structure is a structure in a plant or animal that has lost most or all of its original function in the course of evolution. The rudimentary structure is still present in the great majority of individuals and, in some cases, may even have been modified by evolution to perform a useful service! ("Vestigial" doesn't mean "useless.") Here's an example of a vestigial structure: Some species of cave fish have non-functional eyes which are degenerate and covered with skin. They no longer perform their original function but are still hanging around as vestigial structures.

"There is even a blind cave crayfish that still has eyestalks, but no eyes atop them!" (Jerry A. Coyne, "Why Evolution is True", p.60.)

The evolutionary explanation is very simple. No magic mumble jumble. Eyes are useless in the pitch-black darkness of deep caves. Thus, those fish that inherit a bad combination of genes (for their eyes), or a mutated eye gene that doesn't work anymore, are not at risk. Evolution doesn't eliminate fish with poor vision in an environment where vision is useless. So, the evolutionary defects randomly pile up generation after generation until that fish eye no longer works even in daylight. How far this degeneration goes probably depends on how long that species has been living in its dark environment. Some cave fish have no eyes at all!

"To paraphrase the quote from the geneticist Theodosius Dobzhansky that begins this chapter, vestigial traits make sense only in the light of evolution. Sometimes useful, but often not, they're exactly what we'd expect to find if natural selection gradually eliminated useless features or refashioned them into new, more adaptive ones. Tiny, nonfunctional wings, a dangerous appendix, eyes that can't see, and silly ear muscles simply don't make sense if you think that species were specially created." (Jerry A. Coyne, "Why Evolution is True.")

Dr. Coyne has been a professor at the University of Chicago in the Department of Ecology and Evolution for the past 20 years (the cover bio was written in 2009) where he specializes in evolutionary genetics and the origin of new species.

An atavism is a structure in a plant or animal that is normally not present, a kind of throwback in evolutionary time. Atavisms appear in plants and animals from time to time. Example: Some whales occasionally develop the rudimentary structure of hind legs which protrude from their sides. Paleontologists now have the fossils whereby whales can be traced back to four-legged river animals, the ultimate source of this atavism.

The evolutionary explanation is very simple. An evolutionary change that eliminates some structure in an animal (or plant) does not necessarily eliminate the genes for it. A mutation (for example) often disable the mechanism that expresses that gene, leaving the gene intact within the DNA. Generations later, in a few individuals, an unusual combination of genes or new mutations may provide the means to reactivate those ancestral genes. Atavisms only show up now and then because evolution "got rid of" the old structure for a good reason, and its reappearance would require an unusual combination of enabling factors. Nor would it be selected for by evolution unless it fit into a new role. Discontinued genes might still "live" in the DNA, but they are subject to random mutations that would no longer be removed by natural selection. Over geologic time those genes would tend to become degenerate with respect to their original role.

A good example of the above was provided by chickens. As you know, chickens and modern birds don't have teeth. Scarcer than hen's teeth went the old saying. However, the fossil record shows that early birds often had sharp teeth. To make a long story short, scientists were able to supply the means (in a laboratory experiment) to activate those ancient chicken genes for teeth! The experiment was done by E. J. Kollar and C. Fisher in 1980 at the University of Connecticut. The teeth were rudimentary because the genes had, no doubt, collected damaging mutations over the ages. But they were still teeth!

An evolutionary explanation of vestigial structures and atavisms is so natural that it's kind of like falling off of a floating log. I can't hardly wait to see how macroevolution deniers will explain these features! Of course, judging by how fast they ran away from the fossil record, we probably shouldn't expect too much. Do expect total, time-consuming fixations on irrelevant or insignificant details at every turn. Do expect multiple jumps into other topics. Do expect long-established conclusions (used as supporting evidence for macroevolution) to be questioned over and over as though they were debatable. That's my prediction. Place your bets ladies and gentlemen!

Below, are some concrete examples to focus on, taken from "biologydictionary.net, Wikipedia, and Dr. Jerry A. Coyne ("Why Evolution is True"). An excellent book that also hits these subjects along with a solid survey of the fossil record is Dr. Donald Prothero's "Evolution: What the Fossils Say and Why It Matters."

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1) Studies of fruit flies, when various genes were knocked out, produced a wide variety of vestigial structures. In this case the time interval was really, really short! But it does show that vestigial structures can arise naturally from mutated genes.

2) Boas and pythons have vestigial pelvis remnants that protrude as two, small pelvic spurs! "These spurs are sometimes used in copulation, but are not essential, as no colubroid snake (the vast majority of species) possesses these remnants." (Wikipedia - vestigiality). Snakes and other limbless vertebrates occasionally express atavisms in the form of rudimentary hind legs. We also find that snakes have one good lung while the other is shrunken or missing. We thus have a vestigial glimpse of a time when a lizard-like, snake ancestor had two good lungs and four legs.

3) Many whales carry vestigial remnants of hind legs, expressed as a collection of bones that, although detached from the main skeleton, are placed where hind limbs would be expected. In rare cases, for some species of whales, an atavism is expressed whereby degenerate hind legs protrude where legs would have been expected in a land mammal.

…but about one whale in five hundred [of those species that retain vestigial pelvises] is actually born with a rear leg that protrudes outside the body wall. These limbs show all degrees of refinement, with many of them clearly containing the major leg bones of terrestrial mammals--the femur, tibia, and fibula. Some even have feet and toes! (Coyne, p.64)

As Coyne noted, some of that DNA for making legs is still there, though degraded, and there is only one place it could have come from! Any ideas? Fossil discoveries in modern times have now connected the whales to a four-legged river animal, something like a hippo.

4) A classic example of a human vestigial structure is the tailbone or coccyx, a compressed, fused collection of vertebrate. Why would vertebrate, useful for a flexible spine, be squeezed together and fused? Doesn't that speak of a past where something flexible was attached? Our primate ancestors once lived in trees where a long, prehensile tail would have been extremely useful. Working tails are great for trees but not so hot for apes on the ground. It's not surprising that Gorillas, Chimpanzees, and other great apes have lost that tail. Since we trace our evolution through the apes, we don't have a tail either.

A human atavism is sometimes expressed whereby a rudimentary tail comes with the baby! Obviously, we still retain tail-making genes even though they have been degraded by mutations over millions of years.

And they aren't just simple flaps of skin, but can have hair, muscles, blood vessels, and nerves. Some can even wiggle! … What could this mean, other than that we still carry a developmental program for making tails? Indeed, recent genetic work has shown that we carry exactly the same genes that make tails in animals like mice, but these genes are normally deactivated in human fetuses. (Coyne, p.66)

So, we don't get a nice monkey tail! Hmmm, that would make sitting in a nice, stuffed chair a bit awkward!

5) Another classic example of a human vestigial structure is manifested when it gets cold and we get "goosebumps." Small, vestigial muscles at the base of our hair follicles still pull the hairs upright when we get cold, bunching up in contractions (making the goosebumps) as they do so. In our furry ancestors that would make for a fluffier and thicker fur coat that trapped more insulating air, a perfect response to cold weather. Birds fluff their feathers in cold weather for the same reason--to keep warm.

An additional explanation is that many animals, when threatened, puff up their hair to make them look bigger and more formidable. In both cases a fur coat is involved.

6) Flightless birds, such as ostriches and emus, obviously have vestigial wings. Various anatomical structures are present that would make no sense if flight were not in the cards at one time. In the case of the ostrich, its vestigial wings help the bird keep its balance when running about. Note that a vestigial structure, useless for the original purpose, can be adapted to serve other needs. Either way it is compelling evidence for macroevolution.

The vestigial wings of the kiwi, a flightless bird of New Zealand, are so small that they are buried in the feathers of the bird. That it is a vestigial relic of no particular use is the best conclusion anyone can come up with.

7) There are muscles connected to human ears. Most of us can't move our ears diddly-squat, showing that those muscles are not needed. However, you don't have to look very far to see that they are useful for many mammals who point their ears at a suspicious sound for better hearing. Flapping ears also scares away some of the dangerous flies. In those human ear muscles we have a slam-dunk case for a vestigial structure. It takes us way back in time to mammals of a very different type.

8) We humans also have vestigial molecular structures! Example: A gene involved in the making of vitamin C in most other mammals is a useless vestigial structure in us! Think how nice it would be if your body made all the vitamin C that you needed! "A documented mutation deactivated the gene in an ancestor of the modern infraorder of monkeys and apes, and it now remains in their genomes, including the human genome, as a vestigial sequence called a pseudogene." (https://en.wikipedia.org/wiki/vestigiality \cite_note-33)

And the evolutionary prediction that we will find pseudogenes has been fulfilled--amply. Virtually every species harbors dead genes, many of them still active in its relatives. This implies that those genes were also active in a common ancestor, and were killed off in some descendants but not in others. (Coyne, p.67)

Coyne notes that we humans carry more than 2000 pseudogenes, each one pointing back to animals and times when the gene was useful.

9) Except for a few egg-laying mammals, all mammals bear their young live. Thus, they have no need for egg laying. But these mammals still "…carry three genes that, in reptiles and birds, produce the nutritious protein vitellogenin, which fills the yolk sac. But in virtually all mammals these genes are dead, totally inactivated by mutations." (Coyne, p.71). Now, why would we have dead genes for making egg yokes? Egg yokes are necessary for the development of a growing embryo in the laid egg. Mammalian embryos get their sustenance directly from their mother.

"What's more, mammals like ourselves still produce a yolk sac--but one that is vestigial and yolkless, a large, fluid-filled balloon attached to the fetal gut…" (Coyne, p.71-72)

Odd, don't you think? All that vestigial, egg-laying gear points back to a time when our ancestors were egg-laying animals!

10) Dolphins are another interesting showcase for dead, vestigial genes (pseudogenes). Land animals detect all-important odors in the air through a whole suit of olfactory receptor (OR) genes. (Linda Buck and Richard Axel got a Nobel Prize in 2004 for explaining how the olfactory receptors work.)

Detecting odors underwater is another ball game, so dolphins have a completely different set of genes for doing that job. What is interesting is that dolphins also carry the OR genes of land mammals--except that 80% of them are now inactive! Now, why in the world would an ocean-going animal be packing a whole bunch of inactive genes for smelling airborne odors? Tough one! Hint: Maybe they evolved from land animals as indicated by their anatomical details and their DNA.

Most of the old OR genes no longer worked in the ocean and were eventually deactivated by accumulating mutations. Meanwhile, a new system of genes evolved to do the job. Smelling underwater was still important and evolution would favor such adaptations. Macroevolution gives us a natural, magic-free explanation of this very strange situation! (See Coyne, p.69-70 for some interesting additional details.)