- Homologies—Related organisms that share similarities that are derived from common ancestors. Comparative anatomical studies of cells and embryological development reveal characteristics that most probably derive from more ancient forms. Examples include plants with modified or adapted leaves such as the pitcher plant with leaves that form pitchers which catch insects, the Venus Flytrap with leaves that are modified into jaws that catch insects, the Poinsettia with its bright red leaves that superficially resemble flower petals to bring insects to its pollen, and the cactus with leaves that have evolved into spines.
Another area in which homologies are found is in the forelimb of tetrapods. Creatures as diverse as frogs, birds, rabbits, lizards, and humans all have apparently different forelimbs adapted to different modes of living. However, all of these tetrapods share the same set of bones including the humerus, radius, and ulna. All of these same bones are seen in fossils of the extinct transitional animal, Eusthenopteron. All of those disparate animals have a common ancestry.
While it is common to castigate belief in evolution because it suggests that man descended from monkeys, the more accurate concept is that Homo sapiens descended through multiple adaptations–most of which became extinct–from the great apes or more recently from a common ancestor to chimpanzees, bonobos, and humans. Humans, chimpanzees, and great apes have homologous chests that are broader than they are deep, with shoulder blades flat in back which permits the ability to suspend themselves by using the upper arms, indicative of a common ancestor that was able to do so. Monkeys and other quadripeds with their different modes of locomotion have narrow, deep chests with shoulder blades situated at the sides.
As dissimilar as they appear, baleen whales and humming-birds have tetrapod skeletons inherited from a common ancestor. Although some bones have been lost during the lengthy and complicated process of adaptation, nearly every bone in their limbs corresponds to an equivalent bone in the other—humerus, radius, ulna, phalanges, ribs, hind limb parts, and skull, and these homologies showing convincing relatedness could only have come from a process of evolution.
Developmental biology has revealed homologies such as currently living snakes having hind limb buds in developmental embryological stages that are comparable to fossil ancestral snakes, like the Cretaceous snake, Pachyrhachis problematicus, that had hind limbs that included an ilium, pubis, femur, fibula, calcaneum, and astragalus.
More generally, all living things have fundamental molecular and cellular similarities, best explained by common ancestry. At the molecular level DNA changes, on average, by point mutation accumulation of 1-2% per 5 million years, and multiple remarkably different appearing species, share genetic homologies based on those regular changes. For example, time scale evaluations reveal differences of alpha and beta chains of hemoglobin which differ by 65% in any vertebrate; and when traced backwards through the fossil record, the two chains are found to be descended from a single common ancestral gene that existed 600 million years ago. Gorillas and men share almost identical alpha chains excepting only one amino acid.
Roundworms share 25% of their genes with humans which are only slightly different from each other. DNA and RNA coding is a shared four base code that constitutes the pattern of reproduction and function of all living things. Transferal of genetic material from one living cell to another results in the recipient following the new genetic instructions as if they were its own. On the cellular level, all organisms are made of cells; all cells have membranes filled with water containing genetic material, proteins, lipids, carbohydrates, salts and a myriad of other substances similar from species to species. Most cells use sugar for fuel and produce proteins as building blocks and information messengers. Animal and plant cells have only three components that are unique to one or the other.
Vestigial structures were described previously in blogpost 8. We revisit the subject here because of it being a clear evidence of evolution, and the process which results in the retention of a vestigial structure is a fairly readily apparent piece of evidence.
4. Vestigial structures—Are evidence of the homologies described above and are a clear indication of shared evolutionary genes. Vestigial structures have lost most or all of their original function present in earlier animals through evolution—disuse or altered use. These vestigialities may present as anatomical structures, behaviors, and biochemical pathways. The characters can all be traced to the genes that code for such characters in the animal under study, including humans, and match with the genetic codes of more ancient animals. The process of vestigiality may extend so far as to match genes that have no function at all–so-called junk genes–which are genetic and evolutionary relics.
One thing to be clear about is that vestigiality is not the same as exaption in which a structure originally used for one purpose is modified to perform a new one. The wings of penguins, although small and apparently degenerative, are in fact modified for underwater loco-motion as opposed to flying. It is not a necessary part of vestigiality that the structure be entirely useless. The human vermiform appendix is a remnant of the large cecum of herbivore ancestors which evolved to breakdown cellulose. In humans, the appendix apparently serves a limited purpose of acting as a reservoir for symbiotic microbes that contribute to digestion.
According to later versions of Robert Wiedersheim’s list of 86 human organs that are vestigial, there may be as many as 180 known examples. In addition to the appendix, such examples include the coccyx, a remnant of a lost tail which is regularly seen in embryos and which rarely actually forms in a newborn infant–23 instances recorded in the medical literature since 1884–the plica semilunaris on the inside corner of the eye is a remnant of the nictitating membrane of lower animals–such as birds, reptiles, fish, monotremes, and marsupials–muscles in the ear and by the coccyx and other parts of the body which no longer perform a function despite their physical presence–humans and other primates compensate by their ability to turn their heads in the horizontal plane, an ability not common among monkeys–male nipples, tiny non-functioning sperm ducts that lie behind the ovaries of women, the formation of “goose bumps” under stress which is a vestigial reflex used in pre-human ancestors to raise their hair in times of stress to frighten away predators and to enhance warmth, wisdom teeth, and the transitory infantile grasping reflex wherein a baby can support its own weight by hanging from a rod harking back to the ancestral primate infant clinging to its mother’s considerable hair.
There are vestigial molecular structures in humans which are no longer in use. One such molecule which is indicative of common ancestry with other species is L-gulonogama-lactone oxidase, or GULO produced by a gene that is functional in most mammals to produce an enzyme that can make L-ascorbic acid (vitamin C). A mutation inactivated the gene in a common ancestor of primates, and it now remains dormant in the human genome so that biosynthesis is missing in man, and only a vestigial gene sequence remains, i.e., it is a pseudo-gene, a molecular vestigial compound.
Other animals have vestigial organs: whales, dolphins, and snakes have vestigial feet buried deep in their backs that are sometimes externally evident in adults. The wings of emus, ostriches and other flightless birds are vestigial remnants of their flying ancestors’ wings. Boas and pythons have vestigial pelvis remnants which are visible externally as two small anal spurs on each side of the cloaca. In most snakes, the left lung is greatly diminished in size or absent altogether. Amphisbaenians, which independently evolved limblessness, also retain vestiges of the pelvis including the pelvic girdle and have lost their right lungs. Crabs have small tails tucked between their rear legs that are no longer in use. The functional version of such tails is found in lobsters, the close crustacean relative of crabs. Flightless beetles have wings sealed under wing covers that never open. Some moths (e.g. the Gypsy moth) have flightless females which nevertheless possess small wings. In other moths, females utilize the wings to fly. Sightless moles have functionless vestigial eye structures. Horses and some other animals stand on a single toe but have vestigial toes evident in x-rays of their hooves. These toes rarely become evident on the horses’ hooves. Plants also have vestigial parts. Dandelions and other asexually reproducing plants produce unneeded flower petals which in more primitive flowering plants that reproduce sexually, the petals are crucial for attracting insects to affect pollination.
The existence of vestigial structures is attributable to changes in the environment and behavioral patterns of animals that produce selection pressure. As the function of the trait is no longer important or even beneficial for survival or for enhanced procreative ability, or even proves to be detrimental, it is likely to be phased out or removed entirely, but there may remain remnant gene sequences, and degenerative or rudimentary structural evidence of the evolutionary connection with ancestors for whom the structure or function had value. In some instances, the vestigial gene, structure, molecular change, or behavior may persist because its total removal or destruction by a mutation would result in serious adverse effects for the animal and the species. continued…