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We eat food for getting energy to our body parts, and we excrete the wastes through urine, feces, and perspiration.

Why has nature combined the urinary tract with the genital system (urogenital system) in humans?

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    $\begingroup$ they really don't. Only overlap, i guess, is in urethra $\endgroup$ – aaaaaa Jun 15 '15 at 12:12
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    $\begingroup$ I think this is a question open for much debate, but part of the answer may be that sharing functions reduces anatomical complexity. In other words, adding functions to existing structures is one of the essences of evolution; complexity through parsimony. $\endgroup$ – AliceD Jun 15 '15 at 13:31
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    $\begingroup$ @AliceD thanks for editing the question the right way and properly. $\endgroup$ – sagar Jun 15 '15 at 13:35
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    $\begingroup$ Actually it is there in all mammals (in fact most vertebrates), not just humans afaik. It is better to not limit the question to just humans. Related post. $\endgroup$ – WYSIWYG Jun 15 '15 at 13:44
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    $\begingroup$ @sagar "nature" doesn't make mistakes, because there is no conscious thought involved. Evolution produces changes in DNA that accumulate over time and lead to new phenotypes. Those phenotypes either give a reproductive advantage, a disadvantage, or are neutral. These either allow a population to survive/flourish in a particular ecological niche, or not. Generally, traits that negatively impact a population's likelihood of survival are selected against, and tend to disappear over time. Since this combination you are interested in is widely spread across species, it likely confers an advantage. $\endgroup$ – MattDMo Jun 15 '15 at 23:51
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Developmentally, the urinary and genital systems (typically you will hear them referred to as "urogenital system") are derived from the same embryonic tissue, the intermediate mesoderm. The embryonic kidneys are drained by the mesonephric duct in both females and males. This embryonic tissue also gives rise to the ovaries and testes. The mesonephric duct degenerates in females, and becomes the ductus deferens in males. A second tube, the paramesonephric duct develops in both, but degenerates in males. The female paramesonephric duct becomes the uterine (Fallopian) tube.

Embryonically, the ureter, which drains the adult kidney is connected to the mesonephric duct. This connection is maintained in males, so that both urine and semen share a tube (urethra) for some of their exit pathway. In females, the paramesonephric duct and urethra are separate, so there are two openings.

In males, sphincter muscles and autonomic control prevent the simultaneous expulsion of urine and semen.

Urogenital development

This system has worked acceptably well in mammals for at least about 200 million years (and in humans for the last 4 million or so), so I think it's not correct to call it a mistake. It's not how you would consciously design such a system, but evolution doesn't have a conscious designer.

Here are some slides that cover the embryology of the human urogenital system.

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  • $\begingroup$ Excellent answer, well done! $\endgroup$ – MattDMo Jun 15 '15 at 23:55
  • $\begingroup$ Evolution has no conscious designer?? What about the great architect? 😉 $\endgroup$ – Raoul Jun 16 '15 at 16:48
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I would just modify the excellent previous by pointing out that the embryology describes a local maxima of fitness whose barrier to change is higher than any selective pressure. The re-use of the "logic" of the intermediate mesoderm was either initially a bifurcation from one or the other, or, was of enough selective advantage to have the two developmental processes and the resultant physical systems folded together.

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  • $\begingroup$ Can you please make your answer clearer? I don't understand what you try to say. $\endgroup$ – Chris Jun 17 '15 at 18:44
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    $\begingroup$ 1) Any relatively stable developmental process has some selective advantage, but is not necessarily the best - like if you could only walk upward, and you had to pick a hill to climb. You might not end up on the highest peak. 2) Embryonic processes are often "hijacked" for other purposes, through mutation and selection 3) It's possible that these two functions were at one time separate, separate "hills", but again through mutation and selection, merged embryonically. $\endgroup$ – Abram Jun 17 '15 at 21:41
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    $\begingroup$ @Abram You can just edit your answer to include more information. $\endgroup$ – kmm Jun 17 '15 at 22:42
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kmm gives what is likely the best possible answer, as it is argued from causality rather than teleology. As my ninth grade biology teacher used to say, "there is no 'why' in biology, only 'how'".

On the other hand, it is fun to speculate! Here's my two cents in the form of a Bad ad hoc Hypothesis:

Holes going into the body are an expensive and risky prospect at best for any animal. In order to maintain homeostasis they need to keep any holes constantly supplied with fresh mucus and a ton of antibodies. Total orifice count tends to be kept to a highly conserved minimum along evolutionary lineages. Thus, the reason why urinary and genital systems are merged along a single urogential tract is so that they can both efficiently share the same hole. As evidence for this view, consider the reptile: they manage to use their holes even more efficiently than we do, mating and excreting both liquid and solid waste out of their single cloaca.

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    $\begingroup$ I think the orifice count reduction that led to eating and talking at the same time out of the same hole was a mistake. $\endgroup$ – Abram Jun 17 '15 at 21:47

protected by Chris Jul 13 '15 at 18:46

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