Re: why do we have two testes and not just one?

David, The trivial answer is that human DNA codes for two testicles instead of 
one.  The real answer is I don’t know.  If one testicle is removed the other one 
can, in a relatively short period of time (up to two to three months), increase 
the hormone production and spermatogenesis to almost exactly the same levels that 
the two gonads used to produce together.  Thus the answer is not that it takes 
two to get the job done.  The concept of having a spare also doesn’t hold very 
much weight in my mind.  We men tend to protect our “posterity” quite vigorously 
from physical injury and the number of males that have disease processes where 
one testis must be removed but the other remains is probably very small.  
Although I could not find any specific numbers on this topic with a quick scan of 
the web, there probably are number available.  If you are interested in finding 
out these numbers you might try to contact some urologists in your area.  They 
might be able to head you in the right direction to find out that piece of 
information.  At any rate I doubt if there was any selective pressure toward 
development of two testes rather than one due to better reproductive success for 
men with two verses men with only one.  Let me know if you find a logical answer 
backed up with real data.

Moderator's Note:
Men have two testis for the same reason that women have two ovaries, for
the same reason that all vertebrates have two gonads: the gonads develop
from embryonic kidney tissues lying on either side of the spinal cord.  In
order to produce a vertebrate with only one gonad would require some major
restructuring of the urogenital system and caudal circulation.  Any
evolutionary advantage of having one gonad instead of two would have to
outweigh the disadvantage of loosing a kidney and half the bladder, while
altering the related circulation.