In my article on vestigial features, I had promised to omit the animal kingdom, in the exp

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From: Loren I. Petrich In my article on vestigial features, I had promised to omit the animal kingdom, in the expectation that others would have superabundant animal-kingdom examples. That expectation only partially fulfilled, I will now give some animal-kingdom examples. I hope it is good FAQ material :-) The wings of flightless birds. For most flightless birds, the wings are non-functional, aside from possible display functions. The only major exceptions are diving birds, like penguins, whose "wings" serve as control surfaces. In some cases, the wings are _very_ small, as for kiwis. The effect is to reduce the number of usable limbs from 4 to 2, which can hardly be called an improvement. Bird-teeth genes. All the living birds, and all the known Cenozoic fossil birds, are toothless. Most Mesozoic birds and dinosaurs possessed teeth (any toothless Mesozoic birds?). A recent experiment in growing chicken-embryo jaw tissue next to some mouse/rat jaw tissue in a mouse's eye revealed that teeth formed. And the teeth did not look like any rodent teeth, but were peg-shaped with a conical top, just like the fossil bird teeth. The ability to grow teeth was thus preserved for over 65 million years, perhaps as a side effect of certain growth-control genes specifying more essential things. Extra toes of ungulates. Various hoofed mammals typically have toe bones in addition to those that bear the hooves. This is readily evident on the feet of artiodactyls (cows, deer, pigs, etc.). For equids, two splints are sometimes present alongside the main toe bone. Also, domestic horses are sometimes born with three-toed feet. Relatively recent fossil equids, however, often had three-toed feet, indicating that the one-toed feet of the extant equids is a development of the last couple million years, but that the animals still have the ability to produce three toes per foot. Solid-color equids having genes for making stripes. The living equids are the domestic horse, its wild progenitors, the donkeys, and the zebras and quaggas. Matings of different breeds of solid-color equids (horses and donkeys) sometimes produce offspring with zebra-like stripes. It is as if the genes for making stripes, which are expressed in zebras, are switched off in the solid-color equids, only to re-emerge in certain circumstances. Flies growing legs instead of antennae on their heads, and mosquitoes with legs for mouthparts. These "homeotic mutations" suggest that these appendages were originally legs, but that they were specialized to different functions. Removing or disabling genetic instructions which roughly translate into "A limb on this segment is to become an antenna" and "a limb on this segment is to become a mouthpart" leaves the limb following a default instruction that goes something like "a limb on this segment is to become a leg" (it's not even _that_ simple, because insect legs on different segments are often specialized differently). There is another mutation that causes fly larvae to start growing legs on the abdominal segments; this mutation is lethal, but if it was not, then an adult fly would emerge from the pupa with lots of extra legs down its body. The results of these limb-growth-control mutations are consistent with the hypothesis that the original arthropod had essentially identical, unspecialized limbs, which were specialized to different functions, or even suppressed, among its descendants. These limbs would have been specified in cookie-cutter fashion, and the various specializations and suppressions would have resulted from later add-ons to the growth instructions. Interestingly, trilobites and the Burgess Shale arthropods show relatively little evidence of limb specialization/suppression, so the earliest fossils are consistent with the overlaid cookie-cutter hypothesis. Crab tails. Under their broad, flattened bodies can be found small tails. These are clearly a leftover from when their ancestors had long, thin bodies, as lobsters still do. Ancestral wing configurations reappearing. Flies sometimes grow a second pair of wings instead of halteres (balancing organs); most other living insects have two pairs of wings. Cockroaches sometimes grow a third pair of wings, like some fossil insects. Fetal teeth missing from adults. Baleen whale fetuses have teeth and fetal calves have upper front teeth; adult (and probably newborn) baleen whales are toothless (the baleen is not teeth), and cows lack upper front teeth. These teeth never erupt and are resorbed as the fetus grows. Snakes with vestigial limbs. Boa constrictors have small vestigial hind legs; these may aid in copulating. However, most other species of snakes lack this feature, and seem to do fine without them. Cetacean hipbones. Some whales have hipbones deep inside their bodies, attached to no limbs. One possible purpose is to serve as an attachment point for muscles that move the penis, however. Mammal tails, at least in many cases. These are much reduced from the reptilian ancestral form, and when they serve a function, it is usually for whisking away flies (as for horses) or for signaling (consider dogs wagging their tails). New World monkeys, however, use them as an extra limb, and kangaroos have big tails for balancing, so mammal tails sometimes do have important new functions, however. There are some with very tiny tails, like elephants, and some which lack them, such as bears and apes/humans. The ancestral ape was probably capable of brachiating (moving around in trees suspended from tree limbs that one is holding), which gibbons and siamangs still do today. This would have made a tail a nuisance, thus leading to its suppression (the same thing may have happened to the ancestor of the frogs and toads). The disappearance (or only near-disappearance?) of bear tails is less easily explainable, however. But even there, evidence of tails is sometimes present, as in human embryos having tails for awhile. A side effect of a brachiating ancestry may be our ability to point our arms straight upward (in the direction of the head), an ability not as critical for our species as it is for gibbons and siamangs. Flounder eyes. On sea floors, there live these fish that lie on their sides. They have two eyes -- on one side of their heads. But they start off life with eyes on both sides of their heads, and one eye moves to the other side. Why two eyes instead of one? And why originally on both sides of the head? Original embryonic eye positions. In human and dog embryos, as in most other vertebrate embryos, the eyes are originally on the sides of the head. However, the eyes move forward as human and dog embryos grow, to make possible binocular vision. One human birth defect is for this process to be incomplete, making the eyes too far apart. Among the vast majority of the animals with backbones, the eyes are at the sides of the head; the main exceptions I know of are the bats, the primates, the carnivores, the owls, and possibly some of the more cerebrally endowed small carnivorous dinosaurs. In their family trees, they are surrounded with eyes-on-the-side animals, suggesting that binocular vision evolved several times. Giraffe neck lengths. Baby giraffes start out with necks whose relative length is similar to those of other ungulates; it is as they grow that they acquire the relatively long necks that the species is noted for. Human toes. Our feet have toes, one of which is big and slightly separated from the others. For walking, there is no special need of having a split front end of the foot; it should not be surprising that the toes are small. But they are there, and in most primate species they are much more prominent. In some species at least, the big toe points outward, just like a thumb. Interestingly, in some early hominid species, the toe bones were relatively longer than in our species. Wisdom teeth. Our jaws are a bit small for these late-erupting teeth; some people have them, while others do not. Outsized hind legs of some four-legged dinosaurs. _Stegosaurus_, especially, had hind legs much bigger than its front legs. This is probably a byproduct of being descended from a two-legged ancestor that went back to walking on all fours. Many of the dinosaurs walked on their hind limbs only, with the front limbs remining at various levels of development. In _Tyrannosaurus_, they are _very_ small, though still there, which has led to the suggestion that they are vestigial. The earliest dinosaurs known, like _Herrerasaurus_, were like this. Transitional cases? Possibly! _Iguanodon_ or some other such dinosaur apparently walked on two legs when juvenile, and on all fours when adult (and a lot heavier). [My memory runs out at this point...] Good sources for some of this material: Charles Darwin's _Origin of Species_ and Stephen Jay Gould's essays, notably _Hen's Teeth and Horse's Toes_. In addition, studies of embryonic development often reveal an abundance of vestigial features, some examples of which are given here. On the molecular level again.... An abundace of "pseudogenes" have been discovered, which are not prefaced with a "start" codon, but which have a resemblance to known genes that is too improbable to be coincidence. These are most likely the results of gene duplications and mutations that turned the "start" codon into something else. Thus the DNA-to-RNA transcription system does not "know" that here is a gene to be expressed. /Loren


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