A hearty Sunday welcome to all of you! This is the sixth article in my series on evolution. This will be a short entry because I am hard at work this week and didn't have a lot of time to research/write. However, the material is quite interesting, so I hope you enjoy it. Incidentally, some of you may have missed last week's installment because of the server maintenance. Last week's article continues the discussion of mitochondrial DNA and Neandertals. It is here:
http://www.jehovahs-witness.com/6/88271/1.ashx
Previous articles:
Retroviral sequences: http://www.jehovahs-witness.com/6/86797/1.ashx
Cytochrome c: http://www.jehovahs-witness.com/6/87238/1.ashx
What evolution is not: The role of randomness: http://www.jehovahs-witness.com/6/87711/1.ashx
Mitochondrial DNA, part 1: http://www.jehovahs-witness.com/6/87781/1.ashx
This week's primary source:
http://www.talkorigins.org/faqs/comdesc/section2.html
This week we will be briefly discussing a phenomenon called atavisms. An atavism is the re-activation of a dormant feature that does not normally exist in an organism. For example, living whales are sometimes found with hind legs tucked away inside their bodies.
Hindlimbs have been found in baleen whales (Sleptsov 1939), humpback whales (Andrews 1921) and in many specimens of sperm whales (Abel 1908; Berzin 1972, p. 66; Nemoto 1963; Ogawa and Kamiya 1957; Zembskii and Berzin 1961). Most of these examples are of whales with femurs, tibia, and fibulae; however, some even include feet with complete digits.
(All quotes are from the primary source cited above.)
Atavisims are expected by evolutionary theory when they are directly traceable to an ancestor on the phylogenic tree. For example, whales are mammals and the standard phylogenic tree shows them coming from legged, terrestrial ancestors. Since their ancestors had legs, and since the instructions for legs are carried in DNA, it makes sense that whales would still carry the DNA for such legs, now dormant and unused.
Keep in mind that the prediction for atavisms is a specific one. The prediction is not merely for any abnormal feature that is not normally found in an organism. It is for a feature that was present in an ancestor according to the phylogenic tree. So, for example, if we ever found a fish with femurs, tibia, and fibulae, it would be totally unexpected. If we found mutant reptiles with nipples, it would unexpected because nipples are a mammalian feature that developed after mammals split from reptiles.
One very curious atavism is the occasional occurance in humans of true tails. True tails are not to be confused with pseudo-tails, which are lesions are protusions that happen to occur where a tail might be expected in a different animal. A true human tail is a complex organ sometimes even complete with articulating vertebrae.
The true human tail is characterized by a complex arrangement of adipose and connective tissue, central bundles of longitudinally arranged striated muscle in the core, blood vessels, nerve fibres, nerve ganglion cells, and specialized pressure sensing nerve organs (Vater-Pacini corpuscles). It is covered by normal skin, replete with hair follicles, sweat glands, and sebaceous glands (Dao and Netsky 1984; Dubrow et al. 1988; Spiegelmann et al. 1985). True human tails range in length from about one inch to over 5 inches long (on a newborn baby), and they can move via voluntary striped muscle contractions in response to various emotional states (Baruchin et al. 1983; Dao and Netsky 1984; Harrison 1901; Keith 1921; Lundberg et al. 1962).
Although human tails usually lack skeletal structures (some medical articles have claimed that true tails never have vertebrae), several human tails have also been found with cartilage and up to five, well-developed, articulating vertebrae (see Figure 2.2.2; Bar-Maor et al. 1980; Dao and Netsky 1984; Fara 1977; Sugumata et al. 1988). However, caudal vertebrae are not a necessary component of mammalian tails. Contrary to what is frequently reported in the medical literature, there is at least one known example of a primate tail that lacks vertebrae, as found in the rudimentary two-inch-long tail of Macaca sylvanus (the "Barbary ape") (Hill 1974, p. 616; Hooten 1947, p. 23).
True human tails are rarely inherited, though several familial cases are known (Dao and Netsky 1984; Ikpeze and Onuigbo 1999; Touraine 1955). In one case the tail has been inherited through at least three generations of females (Standfast 1992).
Here is a picture of a true human tail, showing how it can be extended and contracted:
(Source: http://www.vh.org/adult/provider/anatomy/AnatomicVariants/SkeletalSystem/Images/19.html)
Here is a picture of a six-year-old girl with an atavistic tail:
(From the primary source for the article.)
Again, evolution predicts the atavistic tail as a dormant piece of DNA being incorrectly activated.
Perhaps more interesting than the atavistic tails is the case of the missing vitamin C gene. Unlike most mammals, humans lack the ability to synthesize vitamin C. Instead, we must take it in from external sources; if we do not, we get scurvy. Humans share this sad lack with primates and guinea pigs. However, since all other mammals can synthesize vitamin C, researchers predicted that humans, primates, and guinea pigs should have a vestigial gene that corresponds to the genes in other mammals that can synthesize vitamin C. (Vestiges are related to atavisism, and unfortunately, I simply ran out of time to write about them. However, the primary source contains a very good discussion.)
Here are the results:
Recently, the L-gulano-?-lactone oxidase gene, the gene required for Vitamin C synthesis, was found in humans and guinea pigs (Nishikimi et al. 1992; Nishikimi et al. 1994). It exists as a pseudogene, present but incapable of functioning (see prediction 4.4 for more about pseudogenes). In fact, since this was originally written the vitamin C pseudogene has been found in other primates, exactly as predicted by evolutionary theory. We now have the DNA sequences for this broken gene in chimpanzees, orangutans, and macaques (Ohta and Nishikimi 1999). And, as predicted, the malfunctioning human and chimpanzee pseudogenes are the most similar, followed by the human and orangutan genes, followed by the human and macaque genes, precisely as predicted by evolutionary theory. Furthermore, all of these genes have accumulated mutations at the exact rate predicted (the background rate of mutation for neutral DNA regions like pseudogenes) (Ohta and Nishikimi 1999).
The malfunctioning gene was expected to be most similar between chimps and humans because other evidence (notably endogenous retroviral sequences and cytochrome c sequences) already strongly indicated that chimps were the most closely related primates to humans. The fact that that this prediction was so beautifully supported, and that the results dovetailed so seamlessly with what evolutionary theory predicted shows how strong the case really is.
I realize this has been a very brief summary. To those with the time, I would heartily recommend reading the TalkOrigins page from which I drew this info, cited at the top. It contains a multitude of similar fascinating evidence.
Happy Sunday to you!
SNG