Regulatory DNA changes have made a huge impact on the evolution of human-specific traits.  A study in the latest issue of Nature covers not just the usual stuff, like what has been added in evolution to make us distinctly 'human', but rather what was lost.

We're obviously different from animals and the researchers set out to find some molecular occurrences that are present in chimpanzees and other mammals but not in people - they found 583, which they call hCONDELs, 510 which were validated, mostly in nonfunctional DNA.  One instance sure to catch attention is deletion of a penile spine enhancer from the human androgen receptor (AR) gene, a  change correlated with a change in human anatomy -  namely loss of penile spines. 

Even more interesting for junk science infomercials and spam emails regarding genitals in the future, they tested the AR expression pattern and found that the chimpanzee sequence drives 'significant' reporter gene expression when tested in human foreskin fibroblasts, which means that the pathways are still present in humans.

So why did this non-coding region of DNA get deleted?   They say it is because humans became couples and adapted monogamous reproduction and morphological characteristics followed.   So your penis has no spine because of pairbonding and increased paternal care.  Thanks, wife!

Citation: Cory Y. McLean, Philip L. Reno, Alex A. Pollen, Abraham I. Bassan, Terence D. Capellini, Catherine Guenther, Vahan B. Indjeian, Xinhong Lim, Douglas B. Menke, Bruce T. Schaar, Aaron M. Wenger, Gill Bejerano,&David M. Kingsley, 'Human-specific loss of regulatory DNA and the
evolution of human-specific traits',  Nature March 10, 2011 V471 p.219