Well, we're made up of DNA. Scientists unanimously agree that DNA is responsible for all of our characteristics. If you search the internet, you should be able to find plenty of papers and evidence which document the effects of specific genes on personality. Recently, I watched a lecture about a specific gene nicknamed the "Warrior gene" - Scientists found that people who carried this gene were more likely to become serial killers.

I would argue that the presence of this "warrior gene" would be strongly correlated with greed. Police officers will tell you that the serial killer mindset is about low empathy combined with a desire for power over other people - That sounds like a pretty good recipe for greed too.

I wouldn't be surprised if there were other genes that played a part in this personality trait we call greed. Having a materialist view of the world, I think it's pretty safe to say that everything which differentiates us from non-living objects can be attributed to chemical reactions which trace back to DNA itself.

The cultural/environmental aspects of greed are merely feedback loops that are rooted in the genetics of the members of any given group/culture.