Evolution 101

Sunday, August 27, 2006

Is Evolution Racist?

Let's start with some listener email...

Christopher asked if the imperfection of the fossil record is a serious problem for evolutionary theory. In other words, since these evidences are not capable of reproduction in a laboratory setting, don’t they fall outside the aegis of actual science? Not really. You see, the scientific process is one in which observations of the natural world are necessary for the investigatory process. Experiments, especially those conducted in a laboratory setting, are used to remove as many variables as possible. But this does not mean that observations made outside the laboratory are worthless. Paleontology, which is the branch of scientific inquiry that studies prehistoric animals by examining their fossil evidence, is unable to perform variable-controlled experiments in the classical sense, but that doesn’t matter. The scientific method isn’t just observation-hypothesis-experiment-conclusion, like you probably learned in school. The purpose of the experimental component is to generate data. If your observations are about living animals, such as mice, well, then you probably can set up an experiment in the laboratory that will provide you data about that animal. But if your observations are about long-extinct animals, then the only source of data lies in the fossils that you can discover. This is why paleontologists spend so much time out in the field, whereas molecular biologists spend so much time on the laboratory bench. We’re both in search of data, but because of the differences in our focus of study, we have to find that data in different ways.

Daniel asked if much of the information about Darwin that appears in science textbooks and popular literature, such as his being hired as the naturalist on the Beagle, and his observing the finches on the Galapagos are actually apocryphal. Well, technically, Darwin was not hired as the naturalist on the Beagle. After graduation from seminary, Darwin had intended to visit the tropics with a friend in order to indulge his interest as a naturalist, but these plans fell through when his friend died. He found a berth on the Beagle because of the recommendation of his mentor, the Reverend John Henslow, but his position was not paid and it was not as a naturalist. He was the gentleman’s companion of the captain of the Beagle, and just indulged his interest in naturalism in a purely amateur capacity. This was not a typical arrangement, however, the previous captain of the Beagle had committed suicide on its preceding voyage. The new captain, FitzRoy, was worried about the loneliness of life as a captain, and requested that a companion be found for him. He had suggested finding a naturalist, since they frequently were members of voyages as passengers, in the interest of furthering their discoveries. Darwin happened to be one of the ones suggested, and the only one who agreed to them arrangement. The captain had someone interesting to talk to during dinner, and Darwin got to explore South America, the Galapogos, and Australia. Win-freaking-win. While visiting the Galapogos, it is true that Darwin didn’t really pay much attention to the finches there- he was more interested in the different species of mockingbirds on the island. However, once he had returned to England and began to formulate his theory, he realized that the finches there were an important piece of evidence, and got more information on them including better-labeled specimens from others who were on the Beagle. But the finches themselves exist, and have been studied in depth since Darwin, confirming his theory.

Gary asked about chromosomes- what makes a chromosome, how can species with different chromosome numbers interbreed? Well, if you can envision your genome, that is- the entire collection of genetic information in each cell of your body- as a library, then a chromosome would be one book in that library. We number them by size- the largest chromosome is number 1, the second-largest is number 2, and so on. Humans have 22 regular chromosomes, or autosomes, and two sex chromosomes, X and Y. In order for sexual reproduction, we carry two copies of each chromosome, with a total number of 46, except for those cells that are used in reproduction, spermatocytes or oocytes, which only have 23. Hybridization between species with different chromosome numbers is only possible for species which are closely related enough to have very similar chromosome numbers. For example, horses have 64 chromosomes, and donkeys have 62. The hybrid of the two species, the mule, has as a result 63. The reason for this, if you recall what I said about the sex cells, is that the horse contributes 32 chromosomes, and the donkey only contributes 31. 32 and 31 is 63. Since this is an odd number of chromosomes, any attempt to form sex cells in a mule will fail, because there has to be an even pairing of chromosomes for successful meiosis, and the mule will always have one extra. Other hybrids with an even number of chromosomes may be fertile, but it’s usually the female that is, according to Haldane’s Rule. This rule comes from the evolutionary biologist J.B.S. Haldane, who observed that the heterogametic sex (usually the male) is likely to be sterile or rare in a hybrid cross. The reason for this is that, certain genes which are necessary for fertility or viability will be found on the sex chromosome of one species but not another, and so when the two are mixed, the correct configurations of genes are not present. This is less of a problem for females, since they carry two copies of their sex chromosome, and thus have a built-in backup.

Jay asked about the scientific refutation of creationism. He noted that in the final installment of my series on the molecular evidence for evolution, I pointed out that the creationist response amounts to an argument from ignorance, or a “God of the Gaps” approach. However, since the creationist position itself is not a scientific claim, he wondered how I as a scientist could refute it. Well, Jay, that’s an accurate observation, and I’m in total agreement with you. Creationism is a theological position, not a scientific position, and the only basis which I use to interact with it is on those grounds. The only thing I’m interested in doing is refuting those creationists that claim either that creationism is science, or that evolutionary theory is not science. In regards to the molecular evidence, I want to make it very clear that the creationist response does not have scientific merit, and that’s it.

Steven asked about a connection between evolutionary theory and racism. This is an important question, and I want to spend the rest of the time for this podcast on the subject, particularly because a new program has been produced by Coral Ridge Ministries, called “Darwin’s Deadly Legacy.” This program is hosted by Dr. James Kennedy (a theologian, not a scientist) and features Michael Behe as the only scientist, specifically for his views on irreducible complexity, which I’ve gone over in this podcast already. The rest of the experts interviewed are those who are already famous for their rejection of evolution, such as Ann Coulter, Ken Ham, Jonathan Wells, and others.

I want to avoid any theological criticism of this program, but I’ll just point out that it seems to me that this kind of attack on evolution only seems to come from those with a theological bent against it, which I’ve mentioned before.

But what about the question at hand? Is evolution racist? Well, quite frankly, no. Racism is the position that certain races are “better” than others. This is a moral and proscriptive position, whereas evolution is a scientific and descriptive position. Evolutionary theory doesn’t make any kind of claim concerning which species are “good” or “bad.” It simply predicts that, as I’ve said many times, gene frequencies will change within a population over time. Science is a wonderful tool for explaining reality, and it can be used to inform our moral values, but it cannot generate them for us. To claim that one can do so is to invoke the naturalistic fallacy- that is, to claim that because something is natural, it is right to do. Or in other words, to transition from an “is” to an “ought.” Any person who uses scientific facts to derive their moral position in this way is thus violating logic.

That being said, there have been many instances throughout history of people committing this fallacy in regards to evolution. Firstly, it’s important to bring up the point that racism existed long before Darwin was even born. It may seem somewhat strange to realize, but racism was really more like the default position for everyone throughout the world. It just so happens, due to the circumstances of history, that Europeans have, at least in the past several hundred years, been in a unique position of power to institute their racism to a scale which was previously not possible. The rise of colonialism meant that European power extended all over the globe, whereas before each group of people were confined, more or less, to their own small geographical patch of earth.

Darwin himself would be considered racist by today’s standards, but then again, so would pretty much everyone in his society. In fact, by his own society’s standards, Darwin was less racist than most, because he believed that all humans were members of the same species, whereas many others believed that the different races were actually different species. Of course, science now demonstrates clearly that racial differences are very minor in terms of overall genetics- there is more total genetic variability among members of a particular “race” than there are between two average members of different races. The examination of gene flow among the races by comparing genetic sequences shows that there has been an incredible amount of mixing all throughout history- the pattern of descent looks less like a simple tree-branching pattern, and more like a back-and-forth ivy vine.

A pretty good analogy for the concept of race can be seen in the different breeds of domesticated animals. Humans have amplified certain traits through artificial selection to generate different breeds of dogs, for example. But is a rottweilier a “better” dog than a cocker spaniel? Is a Siamese cat “better” than a Manx? Is an Arabian horse “better” than a Thoroughbred? It makes no sense to talk this way, just as it makes no sense to talk about “races” of humans as “better” than others, especially scientifically.

But there have been people in history who have made such claims, despite the lack of scientific justification. Interestingly, the beginning of this in modern history begins not with Darwin, but precedes him in an essay written by Joseph de Gobineau titled, “On the Inequality of the Human Races.” In this essay, he divided humanity into three main races, claiming the “Aryan” race as the most powerful. This idea influenced later racist theories. Later, when evolution was gaining acceptance, it was incorporated into these racist theories to posit that some races were “more evolved” than others. This idea is obviously incorrect, and I’ve talked before on this podcast about why the idea of certain species being “more evolved” is not supported by evolutionary theory at all.

This combination of evolution with pre-existing racist social theories came to be known as “social Darwinism,” although it’s not something that was advocated for by Darwin himself, or supported by his scientific theory. As applied, social Darwinism gave rise to the practice of “eugenics,” which is a directed and artificial selective process analogous to selective breeding in animals. Not surprisingly, those in power decreed that those groups which were not in political favor were “unsuitable” genetically, and had to be removed from the breeding population. Forced sterilizations were common all over the world, actually, during this time, including here in America. It was only after the practices of eugenics by the Nazis were publicized that public support for it dried up.

Eugenics actually runs counter to evolution, as you should be able to realize by now. Evolutionary theory shows that the genetic makeup of any given population is based on the selective pressures of its environment. This is a process that is in constant flux, but one thing is certain- every organism alive today is the ultimate descendent of a very long line of winners. You, and I, and everyone listening to this podcast are the product of an ancestry of only those people who were able to successfully survive and procreate. The results of evolution then speak for themselves. As long as you survive long enough to reproduce, evolution considers you a success, no matter what color your skin may be.

But what if evolution really was racist? What if Darwin was a racist? What if Hitler really did believe he was acting in accord with evolution? This has no bearing on the truth of evolutionary theory. Those people like Dr. Kennedy who attack evolution as racist are committing a different logical fallacy- the genetic fallacy. People who commit this fallacy make the argument that the truth of an idea is based on the source of that idea. This is a well-known logical fallacy, and is usually pretty obvious because Hitler is commonly used to condemn many other things beside evolution. However, if everything Hitler advocated was a bad thing, we have to take everything else he believed in as wrong. For example, in addition to being in favor of eugenics, he supported capital punishment, gun control, and vegetarianism. Among the things he opposed were atheism, capitalism, homosexuality, and pornography. Quite a grab-bag.

All right, so let’s review. Evolution is claimed, primarily by its creationist detractors, to be racist. However, as a scientific theory, evolution makes no proscriptive moral statement. In addition, the historical promotion of racism predates evolution, and those individuals who tried to combine racism with science were doing so in defiance of what science teaches. And finally, those who attempt to condemn evolution for the evils committed by individuals throughout history are committing the genetic fallacy. So no, evolution is not racist- but I have to wonder at those people who seek to characterize it as such- isn’t there any good scientific criticism they can use? I guess not.

Sunday, August 13, 2006

What is Information Theory?

I received some feedback from David regarding the discussion of human nudity from last week that I thought was relevant. In addition to the explanations which I gave regarding the presence of pubic hair, he mentioned that it also functions to retain the body odors produced by the apocrine glands located in and around our genitals. These odors are relevant to sexual signaling even today, it has been reported that men can tell by sense of smell when women are the most fertile. So an additional selective pressure to retain pubic hair even as humans lost hair over the rest of our bodies, was to preserve this sex-related scent, since the presence of hair prevents secretions from evaporating quickly.

Frank writes to ask:
Can human inventions, such as the car, influence evolution? For example, if enough deer get hit by cars, and there is a tiny percentage that run away from headlights, (instead of staring into them), is it possible that some day very few deer would ever be hit by cars?

It certainly is possible for humans to influence evolutionary development, since evolution is dependent on the selective pressures of a populations’ environment, and humans are a part of the environment. This is especially evident when you look at the development of domesticated animals and plants- almost always vastly different from their closest-related wild relatives. But domestication is a kind of an accelerated evolutionary adaptation, since intentional breeding patterns are set up in just about every generation. For organisms that are still outside of direct human control, like deer, changes would be much longer in development, if at all. In the example you gave, I doubt that there would be much of a trend away from being hit by cars, since the genetic causes of that behavior are very complex. In addition, you want to look at the specific behavior in question. If there is enough of an advantage to that behavior to offset any disadvantage, then it’s unlikely that you would see a dramatic reduction. Here, the tendency of deer to cross large swaths of territory in search of food and shelter necessitates their crossing of roads. It’s conceivable that if there was a large enough disparity in the number of deer hit and killed versus those deer that successfully crossed roads, there might be enough environmental pressure to select for the best crossers, but the deer population is large enough and the percentage killed this way is small enough that I doubt this would happen.

Jason writes in to tell us,
“Apparently, there has been some research of homosexuality being potentially created by Neanderthals, who then engaged with homo erectus in sex and spread this gay disease to current man. Honestly, I've read conflicted interest on whether or not Neanderthals could have sex or even breed with homo erectus. But in the event this was true, could the changes even last this long, or affected enough people to make it a modern day problem? Fox News recently had an article of the US Government arguing how homosexuality is a mental condition, a "disease" that can be cured someday.”

First of all, homosexuality is not an infectious disease. You can’t “catch” homosexuality. Homo neanderthalensis was a prehistoric hominid, but was not a direct ancestor of modern humans. It is possible that some sexual contact between our ancestors and Neanderthals took place, but if it was it was infrequent enough not to make any impact on our genetics. Certainly homosexual contact between the two would have had no impact on our genetics whatsoever. I am aware of the classification by the United States Department of Defense citing homosexuality as an example of a mental disorder, but this seems to be just a holdover from decades ago, when this was actually the majority view. Whatever their reason for doing so, I can guarantee this has nothing to do with Neanderthal gay sex.

Brad asks:
“is the wide range of intelligence in the human species similar to what you would find in other higher mammals? I realize that intelligence levels often are reflected in the social aspects of a person...are some monkeys clearly smarter than others...do some monkeys "live" in higher-class places?”

Well, although this is a really interesting question, I don’t think that there’s much in the way of “higher-class” accommodations in the jungle. But in recent years there has been growing support for something called the Machiavellian Intelligence Hypothesis, which suggests that intelligence evolved in our ancestors as a way to better adapt to the complexities of the social order. That is, in order to keep track of social relationships and how to make the most advantage of them, essentially “playing politics,” higher intelligence was selected for. In primates of nearly all species, excluding non-social species like the orangutan, we would expect the most “intelligent” members of their population to also be the most socially cunning, and would likely be the ones with the most social power. This phenomenon is also seen quite often in the human species, as embodied by the phrase, “It’s not what you know, it’s who you know.”

Jordan asks:
“A friend of mine is wondering about mimics (in biology). I explained the basic natural selection process to him - how, if a certain physical characteristic proved reproductively beneficial that it would become more frequent, etc. But he's stuck on the "what are the odds" question, and I'm not sure how to get past that. In his example, there's an island with poisonous snakes on it that are black with yellow stripes. There's also a worm that has developed the same coloration in order to avoid becoming prey. How does this happen?”

Just think about the selective forces at work here. A classic example of mimicry is the Monarch and Viceroy butterflies. The monarch caterpillar feeds on milkweed and thus the later butterfly is bitter-tasting. The viceroy does not feed on milkweed, and thus tastes quite normal. However, both butterflies appear very similar in terms of coloration and marking. Consider this: predators experience the bitter taste of the monarch, remember its coloration, and avoid killing further monarch butterflies. Since the avoidance is based on that particular visual imagery, any other butterflies that happen to look similar to it (like the viceroy) would also be avoided by the predator, since they evoke the same visual imagery. Any members of the viceroy population that happen to look significantly different from the monarch would not evoke this avoidance, and would be eaten at the same rate as other non-bitter butterflies. Thus, the only viceroys that remain to breed are those that look more like monarchs. Over time, this mimicry was amplified, to the point where it is today. This is classic natural selection in action- and a phenomenon that is predicted by evolutionary theory.

And finally, some criticism from Susan:
“Hey, I just thought that I would say that I really thought that I would like this podcast, I believe in evolution and am very well read in evolution. But after listening too quite a few podcasts, I couldn't stand it anymore, I had to delete it. You see I believe in evolution, but I am also a Christian and couldn't take the bashing every minute in the podcast, I loved the information, but I couldn't take the slamming anymore. You see, I find no problem with believing in evolution and believing in God. I know that there are creationists that may be very outspoken with their beliefs or rather disbelief in evolution, but to say that everyone that believes in God does not believe in evolution and is uneducated in the area of evolution is a very ignorant statement. Also the conclusion that was made of the correlation between creationists and intelligent design is not true, I do not believe in intelligent design because you cannot rationalize God, because God is not science, God is a belief, so to try and put God in boundaries of science would not be true. Listening to the podcast did not teach me anything about evolution, but did teach me about how someone who is so smart in one area can be so ignorant and uninformed on another. Take for example Francis Collins, the director of the Human Genome Project, he is a Christian, but also is a Christian. You slam creationists, believing that creationism is all wrong, but what you don't mention is that evolution also has things that have been changed and holes that are still present in evolutionary theory. For a well educated person, you are very narrow minded.”

Well Susan, I’m sorry that you feel I’m bashing you, but I have never said that “everyone that believes in God does not believe in evolution.” In fact, I’ve tried to make it very clear that there are many Christians that do! Ken Miller, as I’ve mentioned before on this podcast, is a Catholic and evolutionary biologist who has written the excellent book “Finding Darwin’s God,” which is an excellent resource for anyone seeking to learn more about evolution, and especially for those who want to resolve evolution and the Christian faith. If you didn’t learn anything about evolution from this podcast, you might want to check out this book, written by a fellow Christian who believes that creationism is wrong and evolutionary theory is the only scientific explanation.

Today I want to get back to one of the questions that was posted on the old site. Grelnixar asked,
“What exactly are “information” in a pure biological sense? When speaking to creationists I often encounter analogies to digital information (which always require a designer(s)) but how accurate are these analogies? Can you give an example of observed positive genetic-information increase.”

This is an excellent question, and it’s getting at one of the common objections to evolutionary theory, which goes something like this: For evolution to take place, the genome of a species must become more complex. An increase in complexity requires an increase in information, and according to information theory, random mutations cannot increase information. Therefore, evolutionary theory cannot be true. This objection is frequently made by those who are advocates of “intelligent design,” and particularly one William Dembski, who considers himself one of the front-line experts of information theory as it relates to evolution.

Any time you see one of these objections which refer to “information,” genetic or not, especially in reference to this “information” either increasing or decreasing, there’s a good bit of underlying assumption behind it, which is usually unknown to the objector. This underlying assumption is that information theory directly interacts with evolutionary theory. The short answer is that information theory is relevant to evolutionary theory, but not in the way that is intended by the objection. Unfortunately, that’s the best answer that my expertise can provide, because information theory is well beyond my training and understanding. To get to the long answer, I’ve asked a good friend of mine and mathematical expert to explain what “information theory” is in the first place, and how it relates to evolution. I’ll yield the floor to him.

Sunday, August 06, 2006

Why Are Humans Naked?

I’d like to thank those of you who have donated to Evolution 101- I’ve received several donations over the past couple weeks, and while they are not necessary, they are appreciated. If you’re so inclined, you can find buttons both at my website and at the Freethought Media website to do so.

Now on to listener questions! I do appreciate all your questions, but this podcast is getting just too popular for me to answer them all. Keep sending them in, though, and I’ll do my best to answer as many as I can, both in direct replies and also here.

The first question is from Danny, who asks: “A friend was offering some counters to evolution (though he admits there aren't any other scientific theories to replace evolution). One specific case he mentioned was the development of the human eye (or even eyes in general). He seemed to be making an argument of irreducible complexity for vision, which I talked about a little bit. However it did make me curious, how did vision evolve?”

First of all, Danny, if you remember from the podcast on irreducible complexity, this kind of argument is an argument from personal incredulity. That is to say, since your friend cannot conceive of a method by which the eye could have evolved, such a development was impossible. This is logically fallacious- just because your friend isn’t aware of such a method or doesn’t posess sufficient imagination, it does not follow that evolution of the eye was impossible.

This problem was actually singled out by Charles Darwin himself, who is often quoted by evolution deniers as saying that the evolution of the eye is “absurd to the highest degree.” This is another example of a dirty rhetorical trick- quoting out of context. Darwin did say this, but out of sheer scientific integrity, and as I’ve mentioned before, acting as his own worst critic. Darwin concedes that for such a complex organ to evolve seems counterintuitive, but much in science is counterintuitive. What most evolution deniers don’t mention is that after saying this, Darwin goes on to lay out a perfectly reasonable method for the evolution of the eye as predicted by his theory. First, there would be photosensitive cells, followed by clusters of pigmented cells, then an innervated cell cluster covered by a translucent membrane, then the formation of a small depression, followed by a deeper depression, then lens-like skin covering the depression, and finally the development of muscles allowing this lens to move. There are organisms in existence today which are known to have each one of these structures as a viable method to detect light. In addition, certain genes which are found in organisms that are known to be essential for the formation of a lens are also found in organisms which do not have lenses, suggesting that these non-lens genes have been co-opted from structures lacking a lens. Unfortunately, eyes are structures that do not readily fossilize, so we cannot compare this evidence to the fossil record, but this is certainly a plausible evolutionary explanation.

Jeffrey asks: “If we never treat a virus like HIV would the body eventually become immune to it through generations? Or are some things too lethal to become immune to? In other words, would it be quicker to let the human body build up its own resistance to it by letting HIV infected people reproduce over and over with other HIV infected people until they had an offspring that was immune to HIV and then synthesize the chemical resistance made from that offspring for the masses, instead of introducing drugs into the human body that try to resist HIV but can never hold it off fully or cure it? Making HIV and AIDS adapt and become that much stronger...”

This is a tricky question, in that it has obvious ethical repercussions. But let me address the most obvious mistake first- failure to treat an individual for HIV would not make the body immune to it over generations. This is what is known as Lamarckian evolution, the inheritance of acquired characteristics. This is not evolutionary theory as we understand it today- Darwinian evolution posits the mechanism of natural selection as the adaptive mechanism. How this would work is, if you had a population of humans, infected them all with HIV, only the ones which were already more resistant to it would survive, and the rest would die. After many successive generations of this process, eventually those individuals which had the highest genetic resistance to HIV would contribute the most DNA to the population genome, and eventually the bulk of the human population would be, more or less, “immune” to HIV.

This would not necessarily be a “chemical” resistance, but more likely a genetic resistance, as we can see with those individuals who are already resistance to HIV, by virtue of the fact that they lack expression of the chemokines receptor 5, which is a co-receptor for the HIV virus. Current drug development efforts are being made to exploit this fact, but the natural resistance is in fact genetic, not chemical.

There are often criticisms of the currently available HIV treatment drugs, since they often have unpleasant side effects and can potentially be unsuccessful. But I would point out that even the genetic resistance that I mentioned isn’t perfect. The co-receptor deficiency may work for one strain of the virus, but it’s possible that the virus could mutate and use a different co-receptor to enter host cells, in which case the intitial mutation is worthless. Evolution is almost like a competition between organisms, each competing to replicate its genes more than the other, and when the relationship between those organisms is as parasitical as a virus to a host, that competition is deadly-serious. The virus cannot exist without a host, and the host cannot exist with the virus. There will always be mutation and adaption on both sides- that’s just the way evolution works.

Sean asks: “Why do people have pubic hair and underarm hair?”

The more interesting question is not, “why do people have hair in these weird places,” but “why don’t they have hair anywhere else?” Or in other words, “why are we naked?”

The standard answer to Sean’s question is that pubic hair and underarm hair are visual sexual cues- hair begins to grow in these locations during puberty (hence, pubic hair), signaling to others in the population that individuals with pubic hair are sexually mature and ready to procreate. Now, modern humans have adopted the use of clothing, and so the impact of this particular visual cue is less relevant today. But that’s an interesting development on it’s own- the only reason we wear clothes is because we’re naked- so why are we naked?

All other primates are well-covered with a thick complement of hair. In actuality, we have just as much hair as the others in terms of hair follicles- look closely at your skin and you’ll see them, thin and tiny, but definitely there. But our hair tends to be a lot more thin and tiny than the hair on, say, a chimpanzee. So much so that on a rough inspection, the zoologist Desmond Morris has no qualms in classifying humans as “the naked ape.”

The mammalian clade is distinguished by its hair, and by far most mammals have hair aplenty. Hair can be extremely useful- it warms and insulates those mammals which have to deal with cold temperatures, and it shields from the sun those mammals which live closer to the equator. Only a few groups of mammals have given up their hair- burrowing mammals, like the aardvark, or the naked mole rat, and aquatic animals, like the cetaceans or the hippopotamus. In both those cases it’s clear that hair would be more trouble than it is worth- trapping dirt in the former case and slowing down swimming speed in the latter. Even competitive swimmer knows to shave off their hair before getting in the pool.

But how did this happen? Unfortunately, hair and skin does not readily fossilize, and so there is very little in the fossil record that exists to help us answer this question, but there are some very likely hypotheses that take into account what is known about our evolutionary origins. The best one, in my opinion, is that which suggests that neoteny is the reason for our nudity. Neoteny is the retention of juvenile characteristics into adulthood. One classic example of this phenomenon is seen in the salamander Axolotl, which unlike other salamanders, does not metamorphosis into a terrestrial form and stays aquatic, retaining the external gills that would be lost during this process. Interestingly, the axolotl can be induced into metamorphosis if given the proper hormones, or if their environment is properly manipulated. If this happens, they lose their gills, and turn from pink into dark mottled terrestrial form similar to a Tiger salamander, to which they are likely related.

As it happens, there are several aspects of juvenile chimpanzees which are also found in humans, supporting the idea that humans are a neotenic species of great ape. At birth, a chimpanzee is almost completely hairless except for on the top of their head. This would explain our nudity, but it also explains a number of other human aspects- especially our ability to learn. Young chimpanzees have an incredible capacity to learn that is turned off upon entering maturity- but in humans, this capacity continues throughout adulthood. Proportionally, the chimpanzee head is much larger in relation to the rest of its body as a juvenile, similar to the proportions of the human head to the human body. It’s in fact quite possible that neoteny may have been selected for the intellectual benefits, and nudity simply followed as part of that process.

But it is reasonable to think that if our nudity is a result of being neotenic apes, there could have been some compelling selective reason for this change. There are many explanations for this to have been the case. One is that, as you remember from the previous podcast on human origins, our ancestors distinguished themselves from their fellow apes by the ability to hunt. Upon abandoning a nomadic existence, their homes would have been infested with insects and parasites, which would have been a problem for those with hair. However, many other hairy organisms deal with skin and hair parasites without much problem, so this seems unlikely to have been of much importance. Another suggestion is that the transition to hunting would have exposed our ancestors to blood and guts and other detritus in the butchering process, which would have been a liability if they had a hairy coat to become matted and sticky with the meaty muck. For example, vultures have lost the feathers around their head and neck due to the fact that they’re always sticking them is sticky and nasty places. But certainly if our ancestors had the intelligence to develop the tools necessary to bring down prey, they also would have been able to use those tools to skin and process their prey without making a disgusting mess. Others make the suggestion that, upon the discovery and management of fire, the need for warm insulation at night was no longer a selective pressure to maintain a full body of hair.

There is another interesting idea called the “Aquatic Ape” theory, which suggests that during human evolution, our ancestors left the trees which they were accustomed to and moved to an aquatic environment of some sort, either by the ocean or near some marshland. Other mammals which have returned to an aquatic existence have also lost their hair- cetaceans, as I mentioned before, and hippos. This theory also explains several other aspects of human physiology that differ from the other apes. For example, humans tend to be fairly agile in the water even at a very young age, unlike chimpanzees, which are very poor swimmers and easily drown. It may explain why our bodies are more streamlined than other apes, and even why we have vertical posture, presumably from having to hold our bodies upright in deep water. In addition, the hairs on our backs are angled diagonally toward the center of our spine, which is different from other apes and is seemingly perfectly adapted to the flow of water across the back. This also explains a particularly notorious difference between humans and other apes- thick deposits of subcutaneous fat. No other apes have this characteristic, but it is argued that fat deposits are particularly useful for other marine mammals, in that they aid in flotation. However interesting this theory may be, there is unfortunately no good fossil evidence to support it, although there has not been much investigation of human ancestor fossils in areas which would have been aquatic in the past, but it remains a minority view.

Another beneficial aspect of nudity was the impact on body cooling. As hunters, our ancestors had to exert themselves to an extent which their evolutionary heritage had not prepared them as efficiently as it had for other hunters like lions or wolves. In order to pursue their prey, our ancestors would have experienced severe overheating, to the extent to which the lack of body hair would have been a great benefit. This loss of protection during the day could have been offset by the gain of protection from the cold afforded by the subcutaneous layer of fat that I mentioned before.

Getting back to what I initially answered about the existence of pubic hair, nudity may have been selected for as a sexual signal. Male humans are distinctively hairier than females, and it’s entirely possible that the lack of hair on a female would have been a attractive signal for males. Carried out over many generations, this would have resulted in an overall lack of body hair in both sexes, with more pronounced nudity continually seen on the female. This would also be consistent with the retention of pubic hair, as I mentioned before that this hair is a distinctive sexual cue.

So, we see that the reason for human nudity is most likely because of neoteny, which is the retention of juvenile traits throughout adulthood. Like young chimpanzees, we lack thick hair all over our bodies except for our heads. This may also have been tied in with our brain development. The selective reasons for this change could be from a number of explanations, including parasite infestation, the domestication of fire, an aquatic existence, the demands of a hunting lifestyle, and sexual cues.