It used to be thought that the alcohol if used moderately or lightly was not harmful.

But new studies are turning up to show DNA damage from any alcohol:

http://psychcentral.com/news/2014/01...dna/64002.html

With the nervous system, you may not feel much damage initially because there is a cushion built into the system. As that cushion erodes daily from environmental assaults, eventually people begin to feel the consequences over a long period of time, and cannot pinpoint the real cause because it is so slow.

As a species humans have only been living past 40 yrs for only 100 yrs now. So damage would have been mostly unnoticed because death would have occurred before nerve damage became apparent. Our life span has doubled in a very short time evolutionarily speaking.

Alcohol is metabolized slowly after being swallowed. It is the byproducts of metabolism that cause the damage also in their own way, and the discomfort the next day. Aldehydes are the main by product and they are not good for our tissues.
So it is both issues, the byproducts, and the alcohol itself that are factors to consider. And for those who drink beer or wine or dark spirits, there are chemicals called congeners in them which cloud the issue. Also sulfites in beer and wine cause symptoms in some people. So it is pretty complicated and not just the alcohol.