On average, humans of European descent have between 2-4% Neanderthal DNA that has been passed down over the last (estimated) 65,000 years which is when it is thought that these two species interbred. Researchers have established a correlation between certain types and combinations of this DNA with smoking and mood. When these types and strengths of DNA are present, an individual has a statistically greater chance of smoking and/or having problems with mood.
http://science.sciencemag.org
http://science.sciencemag.org
