To whom it may concern: I have a very simple question.  How did the penis and vagina evolve?

It seems to me that they would have to evolve at  EXACTLY  the same time, which is of course 100% impossible~!

How does the evolutionary biologist answer this question? But please don't talk about these kind of things. >>>

Sexual reproduction took off in a big way because, in a nutshell, greater variety among your offspring means a greater chance of some surviving in a changing environment. A brood of clones (offspring produced asexually) can be wiped out by a single disease, or change in temperature, or whatever, but a more varied batch is more likely to have some survivors.


What I want to know is how a man and a woman could evolve at the same time. And I don't want a long scientific answer. I want an answer that any lay person could understand  EASILY~!

Sincerely, Robert G. 

eHi Robert,

It's difficult to answer this since you seem very much to want a soundbite response to a complex question. I'll answer in bullet points as best I can without rambling.

1) Your question implies a misunderstanding of evolution. Neither penises nor vaginas appeared spontaneously in humans (or any other lineage) - either at the same time or independently.

2) Where present (e.g. in humans) both structures are rthe outcome of a long evolutionary process from an ancestral state in which the urino-genital tract was undifferentiated between sexes (in terms of external morphology). This - for instance of vents on most living fish species - they look the same in males and females.

3) Do not presume that a male penis has no function without a vagina. Bed bugs, for instance, inseminate females by stabbing their penis through her abdomen wall and injecting sperm.

4) More broadly the concensus in the field of genital evolution is that male and female genital morphology coevolves. Changes in (average) penis morphology impose natural selection on vagina morphology (and vice versa).

5) Some of the coevolutionary dynamic has likely been driven by male-male competition, some by conflict of interests between males and females, and some by selection for "lock and key" type mechanisms that may, for instance, reduce the costs to females of being mated by males from a different species.

6) The details of mechanisms suggested under point 5 are complex. I cannot reduce the theory, mathematical models or empirical evidence for these evolutionary mechanisms to a soundbite any more than a physicist can explain general relativity in one sentence. If you are interested in learning more then the link below may be a good starting point. In fact the primary focus of the article is actually about the field of genital evolution being too focussed on the penis and not giving enough attention to female genital morphology. Nonethless, alongside this message it gives some simple background on the state of the field.

http://www.plosbiology.org/article/info … io.1001851