Wikipedia:Reference desk/Archives/Science/2014 August 14

= August 14 =

Graphene as armor
Can graphene (in theory) be used as armor? And if so, will stacking graphene sheets on on top of each other, improve its defensive capabilities? For example, a centimeter thick graphene slab is tougher than a milimeter graphene slab? ScienceApe (talk) 00:27, 14 August 2014 (UTC)


 * You do know that stacked graphene is exactly graphite? So, no, it does not make for practical armour. Plasmic Physics (talk) 01:23, 14 August 2014 (UTC)
 * Graphene is a molecular structure. This is connected but not identical to the fabric/material structure. Its like with Kevlar. Nomatter science has been trying for decades now, to make a light, pure kevlar armor, professional Bulletproof vests still rely on ceramic plates instead. Nomatter the promises of new materials like Kevlar, Graphene or even superconductors, its a completely different challenge to implement these into working products. --Kharon (talk) 01:46, 14 August 2014 (UTC)
 * What exactly is nomatter? 108.170.113.22 (talk) 16:44, 14 August 2014 (UTC)
 * "no matter" is an adverb, see no_matter. It's not a noun, referring to an exotic substance. CS Miller (talk) 17:45, 14 August 2014 (UTC)
 * I had a feeling that stacked graphene might be regular old graphite, but it doesn't make sense to me. Stacking anything else makes it stronger. Stacking tissue paper will eventually make it bulletproof, why does stacking graphene make it weaker? ScienceApe (talk) 00:21, 15 August 2014 (UTC)
 * [Banned user]]
 * That's nanoscience for you - the investigation and application of scientific principles which only become significant at the nanoscale. For an analogy, microscience is used to explain why liquids form beads in small quantities - surface tension. That does not mean that surface tension does not exist for larger volumes, but simply that it becomes so utterly irrelevant. Thus the idea of lamination as is proposed does not extrapolate linearly to the molecular scale. Plasmic Physics (talk) 01:17, 15 August 2014 (UTC)


 * That's not what plasmic physics said. He said that stacked graphene is graphite, same stuff in pencils, 1mm slab of graphite isn't bullet proof. You're saying that if graphene is stacked properly it should be bullet proof at 1mm. So which is it? ScienceApe (talk) 02:09, 15 August 2014 (UTC)
 * I'm having trouble imagining any substance one millimeter thick which would stop a bullet. ←Baseball Bugs What's up, Doc? carrots→ 02:31, 15 August 2014 (UTC)


 * I'll clarify what he said. Natural graphite is polycrystalline, meaning that it has many crystal boundaries along which a crack could propagate. A hypothetically monocrystalline graphite block will not have any such internal boundaries, so would in theory be stronger than natural graphite. However, that difference, in all likelihood, will not be enough. Plasmic Physics (talk) 03:29, 15 August 2014 (UTC)
 * Monocrystalline is just a fancy way of saying one giant crystal? What do you mean by your last remark? My understanding of graphene is that it should be bullet proof even if it's only one atom thick. I heard that it should be able to resist the weight of an elephant concentrated on the tip of a pencil. Sounds bullet proof to me. ScienceApe (talk) 04:11, 15 August 2014 (UTC)


 * Yes, that is what monocrystalline means. I mean that being monocrystalline it won't make graphite any more practical for use as armour. I have no idea where you heard that a single sheet of graphene has such unbelievable strength, but it seems highly unlikely to be true. Plasmic Physics (talk) 05:41, 15 August 2014 (UTC)


 * It seems to be from this press release. ScienceApe, if you're serious about science, you shouldn't get your science news from university press releases, or anyone who reprints them uncritically. But anyway, the original quote is "It would take an elephant, balanced on a pencil, to break through a sheet of graphene the thickness of Saran Wrap." -- BenRG (talk) 08:13, 15 August 2014 (UTC)
 * So are you saying that's a lie? An elephant, balanced on a pencil can break through a sheet of graphene the thickness of saran wrap? 69.121.131.137 (talk) 11:04, 15 August 2014 (UTC)
 * Isn't that not in doubt? Even the PR seems to agree with that alhough it's not clear if it's referring to the side of the pencil, the tip or the end of the pencil. Anyway a better question than yours would be something like whether an elephant balanced on a permanent marker can break through a sheet of graphene the thickness of saran wrap. Or whether a white rhino balanced on a pencil can break through a sheet of graphene the thickness of saran wrap. The PR appears to suggest no but as BenRG said PRs are not exactly a good way to learn about science. Nil Einne (talk) 13:58, 15 August 2014 (UTC)
 * Never mind the adroit pachyderms - can it stand up to a bullet fired at close range from, say, a 44 Magnum? ←Baseball Bugs What's up, Doc? carrots→ 14:17, 15 August 2014 (UTC)
 * But you're not stating whether it's true or not, you're basically just saying you don't know. 108.170.113.22 (talk) 16:06, 15 August 2014 (UTC)
 * No I'm saying you need to look at the actual research not the press release to get any idea of what the authors are really saying (including what their evidence is). I would say this even for the claims "an elephant balanced on a pencil can break through a sheet of graphene the thickness of saran wrap" even though as I said, the PR itself says it can and no one particularly disputes it probably could. Nil Einne (talk) 17:20, 15 August 2014 (UTC)
 * So then it should be bullet proof and then some even at saran wrap thickness yes? ScienceApe (talk) 02:23, 16 August 2014 (UTC)
 * I have no idea. As we keep saying, you have to either read the paper, or at least a far better summation of it than a PR. The only thing which seems likely is that an elephant balanced on a pencil can break through a sheet of graphene the thickness of saran wrap since even the PR agrees it could. (It's also possible that the PR is underestimating the strength although I don't think people find that very likely. And we still don't know what they even mean by the claim. Still let's not worry too much about all that.) Whether you actually need an elephant balanced on a pencil, that you'd need to read the paper or whatever to see what evidence there is for this claim, and perhaps do any necessary calculations to determine possible bullet proof-ness. Nil Einne (talk) 14:48, 17 August 2014 (UTC)


 * Do you know what the difference between theory and reality? In theory, nothing.  How fast graphene can dissipate energy and how it reacts to dislocations will be reality.  Stacking is probably possible to create both sheets and tubes.  Is suspect other atomic structures between the sheets will help control spacing and strength and degeneration.  --DHeyward (talk)
 * Do I know the difference between theory and reality? Yes I do. But if it should deflect bullets in theory, it should also deflect them in reality. Reality is only a problem with actually making the armor. ScienceApe (talk) 17:40, 16 August 2014 (UTC)
 * Sorry, I didn't mean it to be a question. It's a paraphrase from a famous quote.  --DHeyward (talk) 19:30, 16 August 2014 (UTC)
 * Sounds like a platitude to me. ScienceApe (talk) 03:58, 17 August 2014 (UTC)

animal brainwaves (Delta waves)
Kyranf (talk) 02:45, 14 August 2014 (UTC)I'm trying to find out whether both humans and animals have Delta brainwaves. The Wikipedia article "Delta waves" mentions that all mammals have delta waves, but there's no citation for this assertion. I just want to be able to confirm that it's true.

Thanks!
 * This may lead you interesting places. -- Jayron  32  03:40, 14 August 2014 (UTC)


 * Yes, all mammals have slow (delta) waves during sleep. See, a recent review of sleep electrophysiology in mammals, birds, and reptiles. Looie496 (talk) 13:23, 14 August 2014 (UTC)

End of power-of-2 cell division in human embryos
During human embryogenesis, at what point do the rates of cell division diverge to the point where the total number of cells no longer tends to be a power of 2? Does this happen before, during or after compaction? Neon Merlin  09:50, 14 August 2014 (UTC)
 * As I understand it, this only continues up to the morula stage, which contains 16 undifferentiated cells. After that the cells differentiate, and each type develops in its own way.  See Blastocyst for more information. Looie496 (talk) 13:38, 14 August 2014 (UTC)

Plateaus in sports science
Do professional athletes hit plateaus? What are the main causes of plateau in sports and exercise science and how do they deal with it? — Preceding unsigned comment added by 94.14.219.149 (talk) 10:21, 14 August 2014 (UTC)
 * Check out Plateau effect, for one. Also, should we assume your distinguishing that kind of plateau from the age when an athlete not only can't do any better but starts to decline? ←Baseball Bugs What's up, Doc? carrots→ 12:32, 14 August 2014 (UTC)

Invisible ink
To help fight off summer boredom, I've made up a series of puzzles for my daughter to solve (Caesar ciphers, rebuses, that kind of thing). As part of that, I'd like to leave her some messages in invisible ink. Our article is pretty good, but I'd like some help narrowing down what kind of "ink" I should use. It should be: I'd appreciate the views of someone who's actually used or tested a method. Please don't just quote the article; I need to pick some "bests" from the list, not just see the list again. Thanks! Matt Deres (talk) 11:34, 14 August 2014 (UTC)
 * 1) available in a standard home or grocery store.
 * 2) as invisible as possible - no wrinkling of the sheet she could "cheat" with.
 * 3) not gross (we're a pretty relaxed family, but I don't think we're at a point where I could give my daughter a message written in semen. Call me old fashioned.)
 * 4) ideally, I could make the message up a day or so before hand, but otherwise it doesn't have to last very long.
 * 5) I'd like at least one that's visible to UV light. Ideally, I'd like a second one that requires some other method that a 12-year-old could work with.
 * The article Invisible ink gives many examples. You might consider writing in milk (goes brown when heated) or laundry detergent (brightens under UV light). From a child's chemistry set I have also used Cobalt chloride, which turns blue when heated and becomes invisible again after a while. 84.209.89.214 (talk) 12:21, 14 August 2014 (UTC)
 * For some reason, lemon juice was the classic "invisible ink" when I was a kid. UV markers are available from larger stationery stores.--Shantavira|feed me 14:05, 14 August 2014 (UTC)

How did Prince Harry end up with red hair?
I was watching a video of Prince Harry today, when this question occurred to me. As far as I can tell, his Dad has black hair and his Mom was blonde/dirty blonde. How did he end up with red hair? Thanks. Joseph A. Spadaro (talk) 15:59, 14 August 2014 (UTC)


 * Red_hair is caused by a recessive allele. This allele can be carried by dark haired people or blond people, as well as red-haired people. Thus, it is possible to have a red-haired offspring when neither parent has red hair. Same is (basically) true for e.g. blue eyes. This is a simplified explanation, as both eye color and hair color are Polygenic_traits. Our modern understanding of genetics has gone far past Mendelian_inheritance, much like physics has gone far past Newtonian mechanics. However, the Mendelian perspective is a good place to start, and the simple concepts there are good enough for many casual applications. SemanticMantis (talk) 16:22, 14 August 2014 (UTC)


 * Thanks. So is this what you are basically saying?  The Dad has a recessive gene for red hair (even though he has black hair).  The Mom had a recessive gene for red hair (even though she had blonde hair).  Those two recessive genes came together in Harry, giving him red hair.  Is that the basic/simplified explanation?   Thanks.    Joseph A. Spadaro (talk) 16:31, 14 August 2014 (UTC)
 * Yes, that is a fair summary of the basics, to my knowledge, as refreshed by skimming the red hair article. We do have a few regular contributors who know much more genetics than me, but if you're satisfied with the highschool/early college level description, that's pretty much it. SemanticMantis (talk) 16:39, 14 August 2014 (UTC)


 * Thanks. Yes, I just wanted to the basic "Cliff Notes" version.  Thanks.    Joseph A. Spadaro (talk) 19:05, 14 August 2014 (UTC)

There is of course the theory that red-haired James Hewitt may be the father of Prince Harry. 190.199.220.44 (talk) 20:05, 14 August 2014 (UTC)
 * Beware of potential BLP violations. But if the parenthood of an apparent heir to a throne were successfully challenged, that would take them out of the line, right? ←Baseball Bugs What's up, Doc? carrots→ 20:27, 14 August 2014 (UTC)
 * Richard III tried that, but still found it expedient to dispose of Edward V in a more conventional manner. The rumours about James Hewitt are (a) well-established (b) strenuously denied (c) appropriately referenced in our article about him. Tevildo (talk) 20:55, 14 August 2014 (UTC)
 * I doubt Richard III had a DNA lab. The question is, under what circumstances, if any, might a potential monarch face a challenge demanding a DNA test? ←Baseball Bugs What's up, Doc? carrots→ 21:43, 14 August 2014 (UTC)


 * Good question. My guess would be from the "next person in line", who claims that he is the rightful heir.  No?   Joseph A. Spadaro (talk) 01:07, 15 August 2014 (UTC)


 * Ask Ned Stark how well that went when he started asking those kinds of questions... -- Jayron  32  01:21, 15 August 2014 (UTC)
 * And specifically about princes' hair color... Surtsicna (talk) 21:05, 17 August 2014 (UTC)
 * The Tudors and James I had red hair, and I imagine many other red-haired ancestors could be found. TFD (talk) 01:22, 15 August 2014 (UTC)
 * At least Will seems to be developing his father and grandfather's hair patterns (or lack thereof). ←Baseball Bugs What's up, Doc? carrots→ 01:24, 15 August 2014 (UTC)


 * If you look at the Spencer family, there's a lot of red haired males, see her brothers family here. SPACKlick (talk) 10:24, 15 August 2014 (UTC)

stress hormones
I don't fully understand the role of stress hormones. First, I know that they trigger the flight-or-fight response, which is obviously beneficial to survival. But from what I understand, if there is something external such as a death of someone else, a divorce, a bankruptcy, etc, the stress hormones affect the brain and can cause anxiety and depression. Why is that? Bubba73 You talkin' to me? 20:25, 14 August 2014 (UTC)


 * For starters, check out the myriad things that cortisol does. "Why" is a very hard question in this case. Hormones serve several purposes, and evolution is often counter-intuitive. SemanticMantis (talk) 20:35, 14 August 2014 (UTC)
 * IIRC, long-term activation of the stress system has delirious effects across the body, including sleep disorders (which brings its own problems), and on the immune system. The stress response has evolved only for a short-term "get out of jail" situation, not for the long-term. CS Miller (talk) 20:52, 14 August 2014 (UTC)


 * The saying is that nothing in biology makes sense, except in light of evolution. But it is hard for me to see the evolutionary advantage of anxiety and depression.  Bubba73 You talkin' to me? 02:34, 15 August 2014 (UTC)


 * Funny that you ask this question. I just read an article about this a day or two ago.  (I will see if I can find it.  But I cannot recall where exactly I saw it.)  In any event, the headline or question asked was something like: depression plays an important role in evolution, or something like that.  The gist of the article was: a person might be heading in the "wrong direction" (for example, a job that they hate, a relationship that is unhealthy, etc.).  Depression kicks in as the body's way of telling you "this is a bad situation and you need to get out of this bad situation".  Thus, if depression never kicked in, the person would remain in a bad, unhealthy situation.  Of course, the article explained it much better than I just did.  But that was the gist of it (or, at least, what I got out of it).  First going into the article, I skeptically asked myself: "what possible role or benefit to survival would depression possibly have as an evolutionary device?".  And, it actually made a lot of sense, after having read the explanation in the article.  I will see if I can find it.  Thanks.  Joseph A. Spadaro (talk) 04:00, 15 August 2014 (UTC)


 * I haven't been able to find the article (yet). But, Wikipedia has an article entitled "Evolutionary approaches to depression".   Joseph A. Spadaro (talk) 04:17, 15 August 2014 (UTC)


 * Thank you for that. But sometimes people stay depressed for years without doing anything about it.  Bubba73 You talkin' to me? 23:54, 15 August 2014 (UTC)


 * Well, yes. Each individual is different.  And people in the throes of a depression don't necessarily make the most reasoned and rational decisions.  Nor are they necessarily equipped to do so (i.e., they don't even know that they are depressed, so why would they seek treatment?).  The theory is about why evolution would expose humans to the phenomenon of depression and what advantage would it create.  And, again, it's simply a theory.  To me, it makes sense.  Thanks.    Joseph A. Spadaro (talk) 04:19, 18 August 2014 (UTC)


 * Things in biology aren't "put there" for you to "understand!" That would be the hallmark of an intelligently designed system.  Instead, biological systems evolve in any random direction and anything that survives proliferates.  When there is strong selection pressure, certain things survive better.  When there isn't, things evolve randomly.
 * Even this succinct description might be "under-thinking" the problem. Suppose hypothetically that you need DNA that codes for a particular nerve cell protein that you have to produce in order to manufacture neurochemicals that allow your brain to function.  There is strong selection pressure to favor that DNA: carrying genes that make proteins that make your brain work will increase the probability that you can function cognitively and hunt and gather and mate.  Now, suppose a side-effect exists - because protein folding is a giant mess of molecules trying to find their lowest energy configuration!  Suppose it is the case that this DNA binds slightly better to a million pieces of dead codons - the infamous junk DNA - but only if the molecule contains more codons, farther down the chain that code for a totally unrelated protein.  Maybe that protein produces a different neurotransmitter that has no beneficial higher-tier function.  So, there's a selection pressure to favor the undesirable coding sequence, because it (hypothetically) makes a more stable molecule of DNA. This is sort of a weird priority inversion - and a complex system with n interacting components will have billions of such cases.
 * This means that even when we pose evolution as a series of selection-pressures that aim towards the general survival of the species, in actuality we have a chaotic system with very unpredictable evolution. Sometimes, "bad" designs evolve forward because they somehow interact with the whole system to improve survival. Sometimes, bad designs evolve forward because it's totally random. It's only in the really easy textbook-example cases that we can see selection pressures so strong that we can easily identify their role in the evolution of the species. Mostly, we just can't describe all the interactions in sufficient detail to really provide scientific, fact-based explanations that concretely justify every protein or chemical the biological dice roll for us. Nimur (talk) 04:20, 15 August 2014 (UTC)


 * I should probably insert this somewhere above, but it's hard to see exactly where. The main point is that the stress response is highly functional if it is only activated for a short time -- it mobilizes the body to respond more strongly to threats, and activates tissue repair mechanisms.  The damage comes when the system is chronically switched on for an extended period of time.  Even then lots of the problems it causes go away pretty quickly after it is switched off. Looie496 (talk) 12:03, 15 August 2014 (UTC)


 * The key is that traits are not necessarily adaptive traits Also, keep mind that e.g. cortisol was a pretty old development. I know it is used by biologists to study stress levels in fish, lizards, amphibians, birds and mammals (I think all vertebrates use it, but I can't find an easy cite for that just search /cortisol stress [taxa]/ into google scholar for example research). The point is, it worked well enough to keep scared fish away from predators, and we've been using it ever since. Even depression and anxiety may have some benefits in some contexts, but the important bit is that e.g. depression only has to be less detrimental than all the positives associated with flight-or-flight responses. As you can imagine, the flight response is very beneficial for keeping an organism alive long enough to reproduce. SemanticMantis (talk) 15:13, 15 August 2014 (UTC)


 * It's important to remember that evolution isn't like "intelligent design" - there are sometimes effects that it can't produce. No animal has ever evolved wheels - or tracks like a tank - even though they clearly have advantages.  Once you have legs, it's only really possible to gradually get no legs or better legs.  Since no legs is pretty much a show-stopper for most animals (although snakes and whales seem to do OK), there is no way to convert legs into wheels - so we'd first have to lose the legs and THEN evolve wheels.


 * If the chemicals that produce fight-or-flight responses also produce depression in higher mammals, it may simply not be possible to 'fix' that in gentle stages.  Having two competing mechanisms to produce "fight or flight" (one of which induces depression and the other which does not) is unlikely to be an evolutionary advantage because the animal still gets depressed and making the additional, redundant fight/flight hormone unnecessarily would be an evolutionary disadvantage.   On the other hand first evolving to LOSE the ability to generate cortisol - and then evolving a new way to produce those effects without depression - is unlikely to happen because the intermediate stage is an animal with no fight/flight response - which is clearly a disadvantage.   The only reasonable way for organisms to evolve to avoid the depression is either to generate a modified version of cortisol that doesn't have the depression side-effect, or to generate yet another hormone that turns off the cortisol when it's not needed.   This kind of thing is generally the way evolution drives us...but it results in a horribly complicated set of mechanisms with all sorts of new side-effects and opportunities for disastrous malfunctions.


 * My "go-to" example here is Sickle-cell disease.  People who come from countries where malaria is common have evolved a gene that produces a way to deal with it...HOORAY!...malaria-proof humans!  Unfortunately, the gene's effects are only beneficial if you get the gene from just ONE of your parents.   If you happen to get two copies of the gene, you get horribly deformed red blood cells, and without treatment, you'll soon die.   The mutation survived in humans because (at the time and place where it evolved) the advantage of having a degree of malaria resistance in the population easily makes it worth the risk of producing an occasional early death.   But evolution failed to come up with a gene for malaria-resistance that didn't also entail a bunch of infant mortality.


 * Evolution is an exceedingly clumsy way to design features of a complex organism - but it's all we've got until we're technologically able (...and morally OK with...) rewriting our own genome. Then we'll truly have "intelligent design" and can iron out all of these weird kinks.


 * SteveBaker (talk) 16:21, 15 August 2014 (UTC)
 * It's also important to note that unless a trait provides a significant reproductive disadvantage, it won't go away. High cortisol levels may lead to early death, but it doesn't really lead to early death before one can pass on their genes.  If highly stressed men are dying of stroke and heart attacks at 50 instead of 90, evolutionarily speaking, it doesn't matter.  They've already passed on their genes.  So there's no evolutionary pressure to eliminate the stress-inducing effects of cortisol and all that; because it doesn't stop getting passed on.  There WAS in the past an evolutionary pressure to give us stress and cortisol, because people who got stress ran away from lions better, and thus didn't get eaten at 12 years old.  But now that we're not chased by lions, there's no way to stop the genes from still getting passed on, so they stick around... -- Jayron  32  16:40, 15 August 2014 (UTC)
 * As a minor clarification/nitpick, men having already passed on their genes doesn't rule out there being selective pressure since there may still be an effect on the success or otherwise of their offspring. Notably humans tend to have significant parental involvement in child raising for long periods of time and often even involvement in to adulthood or to a second generational level. So a father dying at 50 may still have less evolutionary successful offspring on average even though they may rarely have any more offspring. The effect tends to be smaller, so even in a simple system may take a long time to disappear and in a not so simple system, may easily never really disappear, particularly since there may still be some advantages even if we're not usually encountering lions. Nil Einne (talk) 17:33, 15 August 2014 (UTC)


 * As I understand this, humans on average would be healthier if there had been a lot more lions around for the last 50,000 years. Is that correct?  Thanks, CBHA (talk) 21:25, 15 August 2014 (UTC)


 * Define healthier. Being maimed by a large predator is usually not good for one's health.  -- Jayron  32  05:32, 16 August 2014 (UTC)
 * I think CBHA is confused between positive and negative selection pressures. If lions eat all the weak and slow people, it directly follows that there are fewer weak and slow people.  However, this does not automatically imply that there will be more strong and fast humans.  It certainly does not imply that there will be any humans who are stronger and faster than the original population.  Each of these three outcomes are distinct: they might play out together, and they might not, because of the randomness of genetic mutation amongst the population, and the very very complicated intra-population dynamics that result from competition to fill an ecological niche, and so on.
 * Stephen Jay Gould wrote an article, linked from our encyclopedia article, that I have found relevant to this discussion: Darwin's Untimely Burial, which elaborates on the distinction between tautological evolution (i.e., "those who survive are the fittest to survive"), and qualitative evolution (i.e. "those whose traits are qualitatively best shall survive"). As always, you should expect Science to provide no answers, but only more intelligently-formed questions...  Nimur (talk) 16:29, 16 August 2014 (UTC)
 * Gould seems to have missed the fact that "populations evolve because of natural selection" is a nontrivial, testable hypothesis even if "natural selection" simply means that individuals that don't live to reproductive age don't reproduce. It's obvious that only the survivors reproduce, but it wasn't obvious to anyone before Darwin and Wallace that that could be a key part of the explanation of biological evolution. That was Bethell's mistake, but Gould didn't catch it. Instead he bought into the obviously creationist idea that "fit" has to be defined in some way that doesn't mention survival. Gould tried to define it as "superior design in changed environments". That isn't a definition. Things can only be "superior" relative to some goodness/utility function, and "the environment" isn't a utility function. What was Gould's utility function? He didn't consciously have one or he never would have made this "argument", but it appears that he was subconsciously going by some measure of the chance of surviving and reproducing in the environment, which brings you back to the original wording, which is fine, because there was never anything wrong with it.
 * Also—"you should expect Science to provide no answers"? Not only does science provide answers, it provides answers backed by overwhelming evidence, to questions that philosophers had bloviated uselessly about for millennia. Where do people get the idea that science can't answer questions? From Gould again? The guy's reason went out the window where anything quasi-religious was concerned. Which seems to be a common problem. -- BenRG (talk) 05:51, 17 August 2014 (UTC)


 * The problem is that you're thinking about "healthiness" as a one-dimensional thing BAD===>BETTER===>GOOD. But that's not how it is.  If there had been more predation on humans by lions, we'd undoubtedly evolve to survive better in a world full of lions...but in the modern world, where there is a marked absence of lions roaming the streets...the resulting genetic changes will likely make us no better suited - and probably worse suited - to the world we find ourselves in.  For example, we've evolved to prefer sugary, fatty foods - which was a GREAT evolutionary effect when food was in short supply and we had to forage and hunt for high-calorie foods in preference to lower calorie foods.  But in a modern world, where those things are causing obesity and all manner of strongly deleterious effects that come from over-eating, it's clear that what was better for us in the plains of Africa is a dramatically worse outcome when we live in a modern, food-rich world.  Better for surviving lion predation might very well result in worse for passing high school exams and making it into college or something.  It's just not a simple thing to predict. SteveBaker (talk) 17:02, 17 August 2014 (UTC)

Lack of exercise plays an important role here. Treatment for depression will typically also include exercise. Our recent ancestors used to get a lot more exercise than even the typical fit person who visits the gym a few times per week gets; most people fall way short of even getting this minimum amount of exercise. So, it may well be the case that what triggers a prolonged period of depression and anxiety in typical people today would only cause a mild case of depression lasting only a few days in our recent ancestors. Count Iblis (talk) 03:27, 16 August 2014 (UTC)

I think that depression is an adaptive response to predation. Even subjectively, I think people are depressed because they feel preyed upon. Even mussels, exposed to low environmental levels of fluoxetine (an antidepressant), become more active ... and more vulnerable to predators. I picture pre-humans out on the savannah moping about losing Uncle Jack to the lioness, staring out at the abandoned carcass in the distance, and one of them saying "Nah, let the buzzards have it." Wnt (talk) 11:15, 17 August 2014 (UTC)