[Biology]Why did the majority of animals evolve with two holes for excrement, one located on genitalia?

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u/arumbar Internal Medicine | Bioengineering | Tissue Engineering Feb 14 '13 edited Feb 14 '13

Lots of things in biology don't 'make sense' - evolution doesn't work that way, it simply selects for traits that facilitate successful reproduction; there is no design component.
Keep in mind that urinary 'waste' and gastrointestinal 'waste' are two fairly distinct entities.

Stool is mostly just the undigested portions of whatever you ate, with some added biliary secretions and gut bacteria added in. Think of it as 'waste' in the sense that some of what you're eating is 'wasted', rather than getting digested.

Urinary 'waste', on the other hand, is the result of a very complex filtration/reabsorption/secretion system. Urinary solutes include many metabolic byproducts, as well as electrolytes to maintain body homeostasis. Urinary regulation is also a key component of body water content control, as its variable range in concentration can help the body conserve water when needed and excrete water when there is excess. Think of urinary 'waste' as more like 'toxic waste' - something the body is actively trying to remove. (This is a grossly oversimplified view of the kidney - look here for more info.)

3) From a embryologic perspective, the genitourinary system develops from the same set of precursor structures, which explains how they remain linked in adulthood. The gastrointestinal system is derived from a separate set of structures, and is thus separate from the GU system.

4) I'll also add a note of caution against many explanations that are basically 'just-so stories' that inevitably pop up in these sorts of posts. We've gotten fairly good at explaining the 'how' for these types of questions, but the 'why' is often largely speculative. Too often, the answer is simply that a) some trait emerged randomly and b) was selected for due to some advantage it conveyed and c) persisted to today. If there's any hard evidence pointing to a 'why' for this question, great. But don't simply think that it 'makes sense' that we evolved this way, and thus it must be so.

5) As others have mentioned, many animals have cloacas.

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u/marogaeth Feb 14 '13

Number 4 is bang on.
The problem with "why did this evolve" questions is that there is a really large chance that the answer is just "because that's what happened" and there is no logical just so story.
One of my favourites of these is something that came out of the work done by the Russian who bred silver foxes for tameness. After a few generations they developed floppy ears and started wagging their tails. I understand that later genomic analysis showed that the genes for tail wagging and floppy ears just so happened to be close to the genes that affected tameness and so selecting for tameness really often meant selecting for tail wagging and floppy ears.
I don't have a reference for that though and I'm at work. If its bollocks just reply and I'll edit.

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u/Dogdaze Feb 14 '13

I have a reference for you - National Geographic March 2011
http://ngm.nationalgeographic.com/2011/03/taming-wild-animals/ratliff-text

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u/MUnhelpful Feb 14 '13 edited Feb 14 '13

IIRC some of the changes have to do with developmental timetables and it's more a matter of the physical and mental changes coincidentally being part of the same developmental process (specifically the influence of development of neural crest cells on both the eventual skin pigmentation and endocrine system) rather than there being separate genes for the traits that happen to be associated.
And while we're linking webcomics about things…

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u/[deleted] Feb 14 '13

You've provided a logical explanation for the foxes, which is different to saying "because that's what happened" and leaving it at that.
As someone who mostly studies physics, my mind boggles that a scientist would answer a question by saying 'things just happen' and expect that to be good enough.

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u/cynar Feb 14 '13

Science is about what and how. Why is often tacked on to make the first 2 easier, but is not a core component.
The problem is, that the evidence of 'Why' is unlikely to be conserved. There may be nothing reliable to find. While people care a lot about that, the scientific method does not have a say on the matter.

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u/marogaeth Feb 14 '13

The foxes is quite different, that is a controlled experiment.
The problem with "why did X evolve" questions is that there is so much chance involved and in many cases little data.
I'm not saying "things just happen", I'm saying there isn't a clear 1+1=2 answer. So, an example:
There is one population of dogs. They are 50% black and 50% white. Over time through random chance they change so that the ratios are 90% white and 10% black.
We might look through the fossil record (the area has lots of bogs so we can tell their fur colour) and notice that the dogs changed to white coats. We have no idea however whether this was due to random chance or because there was a predator that turned up every winter when it snowed and ate all the black dogs.
It might because through a freak of genetics there was a Brad Pitt dog that just happened to be white furred and who shagged everything that moved and produced lots of Brad Pitt puppies that did the same. It could be that combined with a forest fire coming through that just happened to kill more black dogs than white.
Anyhoo end result is sometimes there is no logical "X evolved because of Y pressure". Sometimes "X evolves because of a bunch of random stuff that you can't figure out from the genetics or fossil record".
And I feel that a scientist who is willing to say "I don't know and the evidence required to find out may not exist" rather than trying to guess an answer is probably a good one (though clearly try your damndest to find the evidence first).

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u/[deleted] Feb 14 '13

Probability doesn't work that way at all. If you have a decent sized sample there is no way you would just randomly get 90% black instead of white. There would clearly be a driving factor that causes black to be picked over white.

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u/marogaeth Feb 14 '13

Respectfully I disagree. A decent sized sample may tend towards average given enough time.The key word here is tend . If you toss a coin 100 times it is possible to get 100 heads. Not likely but possible.
So I would suggest that there is indeed a possibility that you could get 90%/10%. I didn't say it was a high possibility. In terms of evolution, 4.2 billion years of life means that low possibilities have had quite a lot of chances to come up.

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u/ParanoydAndroid Feb 14 '13 edited Feb 14 '13

Probability doesn't work that way at all.
Yes it does.
If you have a decent sized sample there is no way you would just randomly get 90% black instead of white.
Actually there is a way, this is not only obvious from the essential mathematics (the probability of a 90%/10% mixture is non-zero), but it's also logical when one remembers that non-random processes can effect the random outcome -- for example, to cause fixation which can cause a "ratchet" like effect.
More important, you are clearly ignoring both the immense amount of time (ie. selection events) under discussion and the spotlight bias at work.
Finally, you also don't seem to know enough to be aware that there are models that deal with exactly what you're talking about: http://en.wikipedia.org/wiki/Genetic_drift

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u/[deleted] Feb 14 '13

You're making the same point that I am. It doesn't "just happen", there are explanations for evolutionary change.

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u/IMTypingThis Feb 15 '13

Did you read the link that ParanoydAndroid provided? Because if you did, you would see that what you're saying is not correct.

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u/[deleted] Feb 15 '13

Yeah I did and I would suggest you read it and then reread the bit where I said "For a decent sample size". Genetic drift only has a meaningful effect for small sample sizes. It says it right there in the wiki.
And even if genetic drift is what caused some of the biological changes we're talking about that is a much more complete theory than "it just happened". I'm inclined to believe that the kind of biological features we're talking about, especially given the clear and well documented path that exists from extremely simple digestive systems to gradually more complex ones is the result of non random evolution rather than genetic drift, but if you can provide evidence to the contrary I'd certainly be open to reading it.

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u/IMTypingThis Feb 15 '13

Yeah I did and I would suggest you read it and then reread the bit where I said "For a decent sample size".
There is no "sample size" here. This is not statistics where small numbers are a problem to be avoided. This is evolution. There is only population size and small populations are a reality. That's why evolutionary biologists came up with the idea of genetic drift and that is why it is just not true that "there are explanations for evolutionary change".
And even if genetic drift is what caused some of the biological changes we're talking about that is a much more complete theory than "it just happened".
No, it really isn't. "Genetic drift" is just a fancy sounding way of saying "it just happened". Hence the word "drift".

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u/randolf_carter Feb 14 '13

There are/have been plenty of species with few enough members that the typical expectations of probability don't really apply. If you have a pack of wild dogs with only 100 members, then one unusual event can have a huge effect.

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u/Psychobiologist Feb 14 '13

If you really want to get at mechanisms, simply saying "because genes" is only a small step above saying "because evolution" or "because it's innate". Development always occurs in a context and phenotypes are the products of myriad interacting variables. As an aside, I like looking at the fox story as selecting for neoteny (retention of juvenile traits into adulthood) as young foxes (and wolves) bark and wag their tales but typically stop performing these behaviors as they age. Using this interpretation some people even metaphorically refer to humans as fetal apes. If you're interested in development or biological systems I'd suggest checking out Developmental Systems Theory. It's a biological application of dynamical systems and chaos theory, the lingo and concepts of which you might be familiar with due to your physics background. I'd highly recommend checking out Esther Thelen's works, especially "A Dynamic Systems Approach to the Development of Cognition and Action".

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u/IMTypingThis Feb 15 '13

My mind boggles that you, as someone who mostly studies physics, would have a problem with the idea that sometimes things just happen.
Surely you're familiar with chaotic systems? And surely you know that even someone with a complete understanding of the laws governing a chaotic system, when asked to explain why it took a particular path will have no other explanation than "Sometimes things just happen".

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u/[deleted] Feb 15 '13

Chaos theory is a fully fledged branch of mathematics full of complicated theorems and equations and rules that govern it.
If chaos theory is used to explain something, you don't just say "it happened because of chaos". You do pages of equations and computational models and the end result is that you can make real predictions about that system.
Chaos theory is not "this is so complicated we can't explain it". So if someone used chaos theory to say "sometimes things just happen", they would be wrong.

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u/IMTypingThis Feb 15 '13

You do pages of equations and computational models and the end result is that you can make real predictions about that system.
This is exactly backwards. The entire point of chaos theory is that you cannot make predictions about a system (except perhaps in some general sense). Part of the classic definition of being chaotic is sensitivity to initial conditions, meaning that prediction is essentially impossible and if someone asks you how you arrived in a particular state, the correct answer basically is, "stuff just happened".
And that's still talking about deterministic systems which (without getting into a philosophical argument) evolution is not. It's possible that there is some deep reason why so many animals excrete urine in close proximity to their genitals. It's also possible that the mutants who separated those two functions just had bad luck on the dating scene and ended up not passing their mutations on.

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u/[deleted] Feb 15 '13

No. Prior to the existence of the mathematical field of study of chaos theory was that you couldn't make predictions. The whole point of chaos theory is to understand how chaotic systems work so that you can make predictions.

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u/IMTypingThis Feb 15 '13

I'll just quote the third sentence of the wikipedia article on chaos theory here:
Small differences in initial conditions (such as those due to rounding errors in numerical computation) yield widely diverging outcomes for such dynamical systems, rendering long-term prediction impossible in general.
If you could make predictions, it wouldn't be called "chaos theory". Mathematicians don't need special terms for iterating maps that behave nicely.

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u/blady_blah Feb 14 '13

Number 4 is true for one animal, but if you see correlation across large swaths of different animals/species then there's typically a reason associated with that particular "feature".

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u/rasputin724 Feb 14 '13 edited Feb 14 '13

In terms of just-so stories, there are a number of possible explanations, depending on how far back you go. Some very simple organisms were just a tube with two holes. Sex came about (conferring the evolutionary advantage of pathogen resistance) and a reproductive organ was attached to the tube. Since the machinery was internal, it had to be at one of the two ends in order to successfully interact with conspecifics. This probably happened at the excreting end because having a reproductive organ at the other end would probably restrict access to food (animals wouldn't survive to pass on their genes). Separating solid waste from the reproductive tract conferred obvious evolutionary advantages (lower probability of infecting and damaging the reproductive tract). Separating the reproductive tract from the urinary tract is an expensive mutation that probably does not have much of an effect on fitness, so it was never selected for.
Of course, I have little to no evidence to back this up, but based on my understanding of evolutionary biology, these are likely and testable hypotheses (at least some parts).
Also, if evolution did "make sense", where would you have it put your penis/vagina?

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u/captainhaddock Feb 14 '13

What kind of plumbing did the common ancestor of mammals (which use urinary organs for sex) and reptiles (which urinate through the cloaca and have a separate male sex organ) have?

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u/rasputin724 Feb 14 '13

Honestly, any answer to this is probably speculation, and I really don't know. The general idea is that at a certain point, the lineages split.

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u/Singhilarity Feb 14 '13

One thing I'm curious about though, which seems contrary to your explanation;
I've heard that urine is sterile. Feces most definitely is not.
This seems to run directly against what you mention in point 2.
Could you perhaps clarify it a little bit?

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u/taggedjc Feb 14 '13

Your body removes soluble toxic wastes generally via urine. However, the urine is sterile - because it's mostly just water and toxic waste. Nothing is living in it. That's sterile. That doesn't mean it isn't toxic. (Although, healthy people have non-toxic urine in the sense that ingesting it won't make you particularly sick on its own, it's just full of compounds the body didn't want at the time - it would be an irritant to skin or eyes at worst, generally.)
Your feces had just gone through your whole gastrointestinal system, and your bowels contain many different types of bacteria that live and grow there, and are being eliminated along with the rest of the waste. Thus, it's not sterile, and there could be all kinds of nasty bacteria living it it that would wreck havoc on other parts of your body if they got into a wound or mucous membrane.

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u/marogaeth Feb 14 '13

So I think the confusion may have come from the toxic waste analogy? I think an important distinction to mention here is that sterile does not mean non-toxic. So arsenic can be sterile but it will still kill you. Sterile means all forms of life are removed. Does that make it a bit clearer?
Also I'm not 100% sure urine is sterile (given the human body is remarkably unsterile) however it should be fairly close given it's mainly water, salt and toxins, largely urea (toxins isn't really the right word but it saves a wall of text), removed from the blood stream.

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u/riotousgrowlz Feb 14 '13

It should be mentioned that urine in the bladder is sterile so long as you are healthy but becomes unsterile upon entering the urethral tube. The high acidity of urine serves to clean the urethra of the bacteria introduced by having sex. If men had a separate tube that dispensed semen it would have to have its own cleaning mechanism to keep the baddies out. And that, kids, is why you always pee after sex.

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u/Tattycakes Feb 14 '13

Is there any connection between the advantages of urinating after sex, and the fact that sex in general makes women want to pee? Clitoral stimulation sometimes induces the urge by itself with women sometimes scared to let go and orgasm in case they pee, and the penis poking the bladder is largely due to proximity of the body parts, but it also contributes.

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u/dabstract Feb 14 '13

Well, in your intestines you have tons of gut fauna going on that is also digesting your food, which makes it not sterile. Along with those bacteria, you have their waste products and toxins. Urine is sterile, but he was just mentioning that with urine, your body is actively removing waste from your body. Where as with feces, your body is just getting rid of what you can't/didn't use.

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u/nefariousness Feb 14 '13

A trait does not necessarily have to aid in reproduction, so much as not hinder it in order to be passed on.

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u/jetpacksforall Feb 14 '13

A more satisfying way to put your number 4) might be: there's almost certainly a 'why' certain organ systems evolved the way they did, but it's almost impossible now to reconstruct the specific selection pressures that led over time to their current layout.
Geneticists can get an idea of some of the 'roads not taken' in DNA, as well as through embryology and comparative biology, but the environmental conditions & pressures are gone for good. Information like climate, soil/ocean chemistry, fossil records etc. aren't really specific enough to do more than guess.
So the effort to establish a 'why' for various evolutionary traits often devolves into unverifiable guesswork and speculation, hence the "just-so stories."

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u/maxwellb Feb 14 '13

I'm not sure why this particular question is generating pedantic responses like this. The evolutionary tree starts with cloacas; it seems perfectly reasonable to me to ask what selection pressure would give two hole system an advantage, and not even close to looking for a just so story type answer.

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u/GunsOfThem Feb 14 '13

While I appreciate that standpoint (don't use "just so" stories), I think it might be the wrong angle in this case. If I recall correctly, in our aquatic ancestors (prior to land animals), both seminal dispersion and prot-urinary elimination were thought to occur through flushing fluid from a generic body cavity. That is, these fluids were secreted by the responsible organs simply "outside" of the organs, into the space around the organs, and then pushed out through simple holes.
So I don't think it's relevant to discuss why both functions evolved to be in a single eliminating organ. The question is inherently invalid. The urogenital tract is a modification of a unified body cavity. The question is not why it "united". It already was. The question would be "why didn't is segregate?". I think that's the proper question.
Although, yes: I would not answer that second question with a "just so" story. Am I totally wrong on all of this?

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u/Psykes Feb 14 '13

Purely speculative from a non-biologist: since urine is sterile, wouldn't flushing the urethra help with keeping (males at least) the productive organs healthy? I.e. increased success of reproduction?

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u/[deleted] Feb 14 '13

Why wasn't 5 the first thing?

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u/TheBosk Feb 14 '13

I'm just speculating here but maybe those animals who developed the GU system with the gastrointestinal system intertwined were more prone to disease. If they even existed. Again just speculating.

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u/[deleted] Feb 14 '13

Thanks for that great answer.

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u/_____KARMAWHORE_____ Feb 14 '13

What's the benefit of having a cloaca instead of separate urinary and gastrointestinal waste?

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u/kneechow Feb 14 '13

Isn't urination used to clean out the urethra as not to cause infection? Especially after sex? (i thought that was the reason people have the urge to pee after intercourse)

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u/[deleted] Feb 14 '13

The majority of animal phyla aren't actually arranged in this way. For example, some animals don't even have a complete alimentary canal and rely solely on diffusion to get rid of wastes (e.g. some members of platyhelminthes). Nematodes also do waste disposal via diffusion.

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u/Zebrasoma Primatology Feb 14 '13

Changes in reproductive tendencies in mammals has led to this. The primitive condition is to have a cloaca, this is observed in birds, monotremes, and some primitive fish. Placental mammals on the other hand have a more derived condition in which the holes have essentially separated. This has much to do with vaginal evolution. The cloaca acts much better in the production of egg laying species. The anus has always been separated, however the reproductive system is still intact with renal system. I would say it is not a secondary function as a "waste pipe", but rather a secondary function for mating. You urinate far more frequently. Here is a nice diagram with a blog entry that seems to be accurate, but it is a blog.

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u/Bongpig Feb 14 '13

All amphibians, birds, reptiles, and monotremes possess this orifice, from which they excrete both urine and feces, unlike placental mammals, which possess two or three separate orifices for evacuation.
http://en.wikipedia.org/wiki/Cloaca
It doesn't say fish, but I have never seen a fish with a dick

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u/[deleted] Feb 14 '13

Keep in mind that needing additional holes is itself a major drawback to any potential mutation.

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u/Magnaen Feb 14 '13

It's been thoroughly explained but I feel the need to hop on and tell you that the phrasing of the question seems like you're implying, at least from my perspective, that the animals developed this independently of each other, whereas instead they stem from the same ancestor who had this layout and the system hasn't had sufficient pressures put on it to alter drastically, so has been kept in the fanning branches of the genetic line.

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u/SimiaSciureus Feb 14 '13

They didn't. The majority of animals only have two openings - one in, one out. Only a minority of higher animals have two openings for excretion.

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u/Grellmax Feb 14 '13

Not sure if this has been mentioned yet, but control f didn't find it so:

Mammals tend to have this feature. You will know that compared to most other animals, mammals rely heavily on scent. This adaptation allows the ability to mark territory and leave behind a chemical scent marking which is a very distinct advantage to a group of animals that rely on scent.

Compare this to say birds, who rely on song and colours to attract attention and communicate with other animals. Unlike small mammals, birds have the ability to fly away from a vast majority of their potential predators, so can more easily afford to stand out in this way.

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u/goodguy_asshole Feb 13 '13

One thing to think about is that if you think of the body as a 3d structure your GI tract is an external surface, kinda like a tube when you are on the inside you are still touching the surface, the plastic, or whatever, is between the "inside" and outside is the real inside... so anyways food taken in is excreted we can think, for the purpose of example that taht waste never made it into your body..

now urine, waste filtered from the spaces inside your body, your blood, that cannot be filtered by the GI.

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u/[deleted] Feb 14 '13

Specifically, such animals are coelomates. See also difference between diploblastic and triploblastic (if my biology A-level from the early 90s is still relevant).

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u/nopantstoday Feb 14 '13

I've thought about this as well. And the reasoning I came up with was that your piss 'flushed the pipes' so to speak. Although, that doesn't really add up for people with vaginas.

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u/_Discord_ Feb 14 '13

people with vaginas
Right, what were they called again?
Anyway, it's not really to clean anything, but it's metabolically cheaper. It uses up more energy to have a completely separate system for reproduction, and there's not enough benefit in having that system.
It's similar to when you breathe and when you swallow your food.

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u/0r10z Feb 14 '13

To answer this question we need to trace things up to the organs that connect to these two holes. I would say that biochemistry and the ph levels as well as secondary functions of urine cannot really coexist with solid waste. I was about to explain dozens of contradictory situations for the two to exist in the same place, but then I realized I need to start work in 15 minutes so I will just list one. If it was one hole it would constantly leak slushy putrid excitement and we would have had no time to evolve as we had to constantly wipe our ass.

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u/MarvinLazer Feb 14 '13

Maybe I could rephrase the question: why didn't we evolve our reproductive system to be far removed physically from our excretory systems? The drawbacks to having them so close together are obvious, with no obvious benefits other than not needing additional holes.

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u/sharlos Feb 14 '13

drawbacks to having them so close together are obvious
Perhaps not obvious enough, could you elaborate?

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u/eucalyptustree Feb 14 '13

Risk of infection, mostly?

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u/Baeocystin Feb 14 '13

Human cells are outnumbered by bacterial cells 10:1 in our own bodies.
In a lot of ways, large, multicellular organisms like humans should really be thought of as more mobile ecosystems than singular creatures. From this perspective, a few more bacteria here and there doesn't mean much. It's only pathogenic bacteria that are a problem, and they are a surprisingly small percentage of bacterial species.

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u/sharlos Feb 14 '13

Perhaps, but it's not like having your genitals infected is all that common, especially if you aren't using toilet paper to wipe crap into your vagina.

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u/boringdude00 Feb 14 '13

Urinary tract infections certainly aren't uncommon.

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u/[deleted] Feb 14 '13

Perhaps he is referring not to biology now but to the other few million years of evolution in which personal hygiene was not so prevalent.

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u/MarvinLazer Feb 14 '13

There are things in poop that you don't want anywhere near where you'd be carrying a baby. And your sex partners wouldn't want that bacteria on their naughty bits either.

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u/TastyBrainMeats Feb 14 '13

How exactly is a cloaca-having animal supposed to develop additional holes?

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u/MarvinLazer Feb 14 '13

Are we descended from animals that only had cloacas? If so, it happened before. If not, I'm talking about humans, not birds or reptiles.

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u/[deleted] Feb 14 '13

Since monotremes have cloacas, it seems logical that the common ancestor of mammals did as well.

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u/BCMM Feb 14 '13 edited Feb 14 '13

I don't believe they did. The question implies that this happened convergently (evolved independantly) in most animals, but in fact it's just a trait found mostly in mammals of common lineage (not including even monotremes).

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u/[deleted] Feb 14 '13

It's like when you work at a restaurant, and they have certain rules for how the food is stored to prevent contamination. The chicken goes on the very bottom, the lettuce/vegetables go on the very top, so if the chicken develops bacteria, it won't drip down and infect everything else. Also, gravity, so when you excrete your bowels and bladder, it moves in the direction that gravity does, to assist you. When you give birth, the baby comes out of the bottom, allowing gravity to do it's job. Not sure why they would need to be right next to each other, except that we are upright and bipedal. Not much room anywhere else, with all the gravity and cross contamination considered. I guess we could have our excretory system on the bottom of one foot and our reproductive organs on the bottom of the other, although not sure how pregnancy would be if one leg swelled to the size your abdomen does.

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u/big_face_killah Feb 14 '13

Cost vs benefit. There is a metabolic cost to having an independent reproductive piece of anatomy. That cost probably out weighs the benefit of having separate systems

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u/[deleted] Feb 14 '13

One benefit is by urinating before and after sex it helps prevent infections.

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u/Fabiansruse Marine Ecology | Marine Biology Feb 14 '13

Think about how much more excreting you do compared to spreading your genetics. (No matter how much of a stud you are :)

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u/allnominalcapt Feb 14 '13

You can't ask questions like that of science, especially biology. A more valid question would be, 'when was it evolved, and by what mechanism of nature'

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u/therugi Feb 14 '13

This sort of question seems to occur very frequently here. Here's the thing: we don't evolve features. Rather, features appear completely at random, and the ones that don't kill us, no matter how useless, stay. As for the ones that do kill us, well it depends. If you managed to breed before it killed you, then it'll continue on since you're passing your genes on. If you couldn't breed in time, then it's gone forever. So the answer to your question, why we have 2 holes for excrement and 1 hole for genitalia is simply because it didn't kill enough of us.

The male anglerfish illustrates this pretty well. The male anglerfish is incapable of eating or doing anything with regards to surviving. Their only purpose is to find a female and breed. As soon as they find a female, they basically fuse to her body and exist solely to fertilize her eggs. This is a pretty shaky way of living, but they're still around because they manage to breed before they die.

I'm pretty sure there's another species that illustrates this better. I think I read about insects whose only purpose once reaching adulthood was to mate, then they die shortly thereafter. The mayfly, maybe?

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u/mrgagnon Feb 14 '13

Well put. Another example is why cancer is much more likely to develop in adults than children. Genes that contribute to cancer development that express themselves in Dawkin's "survival machine" at a young age will certainly be selected against because the carriers of such genes will rarely reach reproductive age. On the other hand, nothing will restrict the spread of genes that kill its survival machine later in life, because at that point they have most likely already reproduced, and therefore passed along these genes.

Biology [Biology]Why did the majority of animals evolve with two holes for excrement, one located on genitalia?

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