barger@ils.nwu.edu (Jorn Barger) (11/03/90)
In article <1371@cluster.cs.su.oz.au> louie@cluster.cs.su.oz.au (Luis Esteban) writes: > I have received a number of solutions to why we have > eyebrows. I have some questions concerning them. > The following is a composition of the explanations and questions. [stuff deleted] I've been waiting for years for someone to ask me this (why do humans have eyebrows). It seems transparently obvious to me that they are primarily _signal-flags_, bars of contrasting color that amplify our emotional expressions, making them readable more easily at a greater distance. Why did humans lose their fur? This seems surely due to sexual selection, not natural selection. But why this baldness became sexually attractive I have no clue. In general, sexual selection is not considered often enough as an evolutionary force. I think that the punctuated equilibrium paradox makes perfect sense if viewed as a case of sexual selection overpowering natural selection until the environmental stresses finally become too great. ................................................................... "The man who pretends to be a modest enquirer into the truth of a self-evident thing is a knave." William Blake ...................................................................
mitchell@thesis1.hsch.utexas.edu (Philip Mitchel) (11/04/90)
In article <2431@anaxagoras.ils.nwu.edu> barger@ils.nwu.edu (Jorn Barger) writes: >Why did humans lose their fur? This seems surely due to sexual selection, >not natural selection. But why this baldness became sexually attractive I >have no clue. I'm curious... why is it that fur loss is surely due to sexual, not natural selection? How much of a body's resources would it take to keep that much hair growing? If that much can be saved by wearing the furs of other animals (or altering other behaviors to maintain the core temperature without the use of natural fur), mightn't that give the hairless an advantage over the hairy? I'm not proposing this as The Answer to the question, it has just seemed a possible explanation. It would (to my mind) also explain why we keep hair on our heads (the gain in heat conservation outweighing the metabolic loss, since the brain is such a greedy consumer of everything). What say the learned among us? -- Phil Mitchell mitchell@thesis1.hsch.utexas.edu "No one is to be blamed for any damned fool thing I say, either."
jack@molbio.med.miami.edu (Jack Kramer) (11/04/90)
In article <4278@lib.tmc.edu> mitchell@thesis1.hsch.utexas.edu (Philip Mitchel) writes: > >I'm curious... why is it that fur loss is surely due to sexual, not natural >selection? How much of a body's resources would it take to keep that >-- >Phil Mitchell mitchell@thesis1.hsch.utexas.edu > I'm curious. I thought there were only natural and artificial selection processes. Why isn't sexual selection a natural process when it occurs in natural populations in the wild that are not under the control of human breeders?
kuento@kuhub.cc.ukans.edu (11/05/90)
In article <4278@lib.tmc.edu>, mitchell@thesis1.hsch.utexas.edu (Philip Mitchel) writes: > In article <2431@anaxagoras.ils.nwu.edu> barger@ils.nwu.edu (Jorn Barger) writes: >>Why did humans lose their fur? This seems surely due to sexual selection, >>not natural selection. But why this baldness became sexually attractive I >>have no clue. > > I'm curious... why is it that fur loss is surely due to sexual, not natural > selection? How much of a body's resources would it take to keep that > much hair growing? If that much can be saved by wearing the furs of > other animals (or altering other behaviors to maintain the core > temperature without the use of natural fur), mightn't that give the > hairless an advantage over the hairy? I'm not proposing this as The > Answer to the question, it has just seemed a possible explanation. It > would (to my mind) also explain why we keep hair on our heads (the gain > in heat conservation outweighing the metabolic loss, since the brain is > such a greedy consumer of everything). What say the learned among us? > -- > Phil Mitchell mitchell@thesis1.hsch.utexas.edu The lack of hair on humans is one of those cases where one of the most likely hypotheses for the trait have nothing at all to do with selection. The idea is quite simple - via a change in the regulatory genes, humans arose as neotenic variants of their hairy ancestors. This gives us a higher forehead, larger relative cranial volume, a change in limb proportions *and* a loss of a great deal of body hair all in one mutational event - so selection is not a *causal* agent at all. Once the neotenic mutants exist, selection can then modify the traits, of course, but the bulk of the transition is one step. There are numerous other examples of neoteny in nature, perhaps two of the most obvious being the "gilled" salamanders (axolotls, mudpuppies), and a vast array of "puppy-like" dog breeds. Neoteny is also typified by very small genetic divergence between groups with large differences in morphology - precisely as we see between ourselves and the great apes. I seem to recall an article about all this by Stephen Jay Gould not too long ago. It's always made sense to me... ---------------------------------------------------------------- Doug Yanega (Snow Museum, Univ. of KS, Lawrence, KS 66045) My card: 0 The Fool "UT!" Bitnet: Beeman@ukanvm "This is my theory, such as it is....which is mine. AAH-HEM!"
teexmmo@ioe.lon.ac.uk (Matthew Moore) (11/05/90)
Jorn Barger > writes: > >I've been waiting for years for someone to ask me this (why do humans have >eyebrows). It seems transparently obvious to me that they are primarily >_signal-flags_, bars of contrasting color that amplify our emotional >expressions, making them readable more easily at a greater distance. > >Why did humans lose their fur? This seems surely due to sexual selection, >not natural selection. But why this baldness became sexually attractive I >have no clue. > It may be "transparently obvious" to you, but this does not make it the only explanation in the literature. The Hardy-Morgan theory of human evolution (aka the aquatic theory) considers eyebrows to be sunshades. What do you do if the sun gets in your eyes? - You frown, or lower your eyebrows. Loss of fur is a common adaptation to aquatic life (or semi aquatic life), consider the elephant, pig, hippoo, cetaceans (whales and dolphins). Indeed, Hardy first formed his theory when, on his return from a whaling trip, he was struck by the similarity between whale blubber and the human layer of subcutaneous fat. (Subcutaeneous fat being an aquatic adaptation against heat loss).
kell@cs.albany.edu (Brian A. Kell) (11/06/90)
In article <1990Nov5.123912.15186@ioe.lon.ac.uk>, teexmmo@ioe.lon.ac.uk (Matthew Moore) writes: > The Hardy-Morgan theory of human evolution (aka the aquatic theory) > considers eyebrows to be sunshades. What do you do if the sun gets in > your eyes? - You frown, or lower your eyebrows. ... and maybe shade your eyes with your hand? My theory (although I'm no expert!) has always been that eyebrows make really good rain gutters. If you ever stand/walk/hike/jog/etc. in the rain, you may notice that the raindrops hitting your forehead are neatly diverted to the side by your eyebrows, allowing clearer vision than if the water were dribbling into your eyes. This seems to have an obvious selective advantage for hunting and/or evading predators in the days before umbrellas and windshield wipers. ---------------------------------------------------------------------------- Brian Kell SUNY/Albany Departments of Computer Science and Biological Sciences brian@isadora.albany.edu kell@cs.albany.edu bk7295@albnyvms.bitnet
chappell@galton.uchicago.edu (Chappell) (11/06/90)
In article <26540.27343d31@kuhub.cc.ukans.edu> kuento@kuhub.cc.ukans.edu writes: >... There are numerous other examples of neoteny in nature, perhaps two of the >most obvious being the "gilled" salamanders (axolotls, mudpuppies), >and a vast array of "puppy-like" dog breeds. Neoteny is also typified >by very small genetic divergence between groups with large differences >in morphology - precisely as we see between ourselves and the great >apes. I seem to recall an article about all this by Stephen Jay Gould >not too long ago. It's always made sense to me... >---------------------------------------------------------------- >Doug Yanega (Snow Museum, Univ. of KS, Lawrence, KS 66045) >My card: 0 The Fool "UT!" Bitnet: Beeman@ukanvm >"This is my theory, such as it is....which is mine. AAH-HEM!" The giant axolotls of Mexico, at least, owe their neotenic condition to iodine deficiency. Feed them iodine, and they will lose their gills and otherwise become land-dwellers (although I am not denying that this "deficiency" may have evolutionary implications). Re. "puppy-like" dog breeds, Konrad Lorenz (the duck-mama) has a chapter on this in his interesting book, "King Solomon's Ring". Rick Chappell.
ronald@uhunix1.uhcc.Hawaii.Edu (Ronald A. Amundson) (11/06/90)
In article <1990Nov4.012021.27368@umigw.miami.edu> jack@molbio.med.miami.edu (Jack Kramer) writes: > >I'm curious. I thought there were only natural and artificial selection >processes. Why isn't sexual selection a natural process when it occurs >in natural populations in the wild that are not under the control of >human breeders? Not that I approve of all of this Daily News style just-so-story- mongering about "evolutionary explanations" of eyebrows, but it might reduce FURTHER confusion to mention that "natural selection" is the _name_ of an evolutionary process. The fact that some other process isn't _named_ "natural" doesn't mean that it's "unnatural." Sexual selection was distinguished from natural selection by Darwin, although some people (including S.J. Gould, but I think he's mistaken) now classify it as a form of natural selection. Sexual selection adapts a lineage of organisms to the environment of their intra-species reproductive environment; natural selection (in D's original sense) adapts a lineage of organisms (within a species) to their outside-the-species environment. Politicians who are not Democrats may still believe in democracy. Summer's Eve douches can be used in the winter. Big Boy hamburgers are not made out of .... [Your correspondent wanders off towards the horizon, babbling softly to himself ......]
sarima@tdatirv.UUCP (Stanley Friesen) (11/08/90)
In article <1990Nov5.123912.15186@ioe.lon.ac.uk> teexmmo@ioe.lon.ac.uk (Matthew Moore) writes: >The Hardy-Morgan theory of human evolution (aka the aquatic theory) >Loss of fur is a common adaptation to aquatic life (or semi aquatic >life), consider the elephant, pig, hippoo, cetaceans (whales and dolphins). Except that of these examples only the hippos and cetaceans are (semi-)aquatic. Elephants and pigs are as terrestrial as any mammal (that is *all* mammals spend *some* time in/around water). There is another set of circumstances that is leads to loss of hair - namely large size in a relatively equable climate. The examples include: elephant, rhinoceros, &c. Since humans are among the top 10% of the mammals as far as size is concerned, and since we apparently evolved in the tropics, our hairless state could well be due to that. As far as the eyebrows are concerned, the idea about sun visors may well be correct - the bright, steady sun is a major problem in the tropics. -- --------------- uunet!tdatirv!sarima (Stanley Friesen)
hank@well.sf.ca.us (Hank Roberts) (11/08/90)
>sunshades >rain gutters >signals I recall reading somewhere that people without eyebrows are at higher risk of cataracts from the ultraviolet in sunlight.
sparks@corpane.UUCP (John Sparks) (11/09/90)
Did you ever notice that Whoopi Goldberg doesn't have any eyebrows? [Maybe we have eyebrows so we don't look like Whoopi? ] -- John Sparks |D.I.S.K. Public Access Unix System| Multi-User Games, Email sparks@corpane.UUCP |PH: (502) 968-DISK 24Hrs/2400BPS | Usenet, Chatting, =-=-=-=-=-=-=-=-=-=-|7 line Multi-User system. | Downloads & more. A door is what a dog is perpetually on the wrong side of----Ogden Nash
msissom@digi.lonestar.org (Marc Sissom) (11/10/90)
> >Loss of fur is a common adaptation to aquatic life (or semi aquatic >life), consider the elephant, pig, hippoo, cetaceans (whales and dolphins). >Indeed, Hardy first formed his theory when, on his return from a >whaling trip, he was struck by the similarity between whale blubber >and the human layer of subcutaneous fat. >(Subcutaeneous fat being an aquatic adaptation against heat loss). Some more support for this 'aquatic period' of human development is the fact that while all other primates are almost completely furred; humans are the inverse, almost completely 'fur-less'. The complement to this is that no other primates(as far as I know) have any background, or history of an aquatic or amphibious lifestyle. Nor do they have any affinity for activity in the water which humans certainly do. Also none exibits this subcutaneous layer of fat. On the other side, is the evidence that no other primates are bipedal, nor do they have any features to implement or enhance the development of aerobic stamina. This supports the argument that the relatively hairless skin with perspiration and cappillary dialation can be a very effective radiator for a body that is running around Africa all day. A breeze across the skin greatly enhances the value of perspiration - the draft caused by walking/running or by the wind. However, with the loss of the insulating cover of fir, the skin(even with the blood flow restricted) can now be a tremendous drain on the body heat in other relatively cool climates. The fat layer can make up somewhat for this loss of an insulating layer in cool conditions. Note that the fat does not cut off the emission of heat when the body is hot because the cappillaries are on the outside of the fat layer and provide a channel for the flow of the excess heat.