Lot’s of other mammals do. Why are primates excluded? What happened to our bone?
Actually, most primates do, even chimpanzees (although a chimp’s is tiny, only 6 mm.)
Do humans have one?
No. And offhand, I don’t know why we should lack one (although it’s quite small in the Great Apes).
Could it have anything to do with the difference in size between our pensises and theirs?
I never knew this. Are there any fossil "penis bones"from early hominds that have been found? IE, do we know when the bone went missing in our evolutionary past?
what purpose does this bone provide anyway?
It makes “big-boned” into a complement.
Do you really want one?
As someone once noted (Allan Sherman, I think), what if you broke it? Who would sign your cast? (National Lampoon once ran a fake X-ray of a broken penis bone. The very thought makes me hunch over.)
Incidently, the “penis bone” is called a baculum (or sometimes os penis).
Instant erections, I think. I think I saw something about this on the discovery channel a while back.
Or to quote the site linked to by Darwin’s Finch above:
Also to quote from that site:
Just in case anyone wondered.
True item: The penis of of one animal (the raccoon, I think) has a tiny hole in it, so the American Indians used it as a sewing needle – no need to try and drill that annoying eye.
That happens anyway! 
Alaskan natives refer to the penis bone of the walrus as an oosik. I have one sitting on my bookcase in the living room. Envision a bone-white Louisville Slugger, in miniature. Well, not that miniature…it’s a good 18" longs and weighs a couple of pounds! :eek:
Given the extremely fragmentary nature of almost all early hominids (and of apes as well) it would be very surprising to find one preserved. Therefore when it went missing would be almost impossible to determine.
This site discusses “What is the Significance of the Baculum in Mammals?”
I wish I could remember where I heard this, but I seem to recall reading that it has to do with our upright position and the fact that we keep our penises outside of our body. Boneless just seems to work better in our case. Sorry - I don’t remember the details.
I guess that means that a person with a baccalaureate is a bonehead.
(…fighting urge to make joke at expense of Scott Bakula…)