Are addictions a byproduct of evolution?

Addictions are a byproduct of chemical warfare.

Plants evolved chemicals like caffeine, opioids, nicotine, cocaine, and theobromine because they are chemical poisons that protect the plant from being eaten by pests. They work by disrupting signaling in the brain, usually by binding to receptors for neurotransmitters in the brain.

They’re extremely potent neurotoxins that kill insects. A plant can’t run away from predators, so it uses chemical warfare instead. It secretes these chemicals that disrupt brain signaling, insects try to eat the plant, insects die.

Animals like humans are much bigger than insects. Doses of these chemicals that disrupt an insect’s brain signaling to the point where insects die, disrupt our brain signaling in ways that feel pleasant to us.

Many of these chemical poisons are effective at killing insects because they mimic natural neurotransmitters but they are more effective than normal neurotransmitters. They grab hold of receptors and don't let go, or they persist longer than natural neurotransmitters, or both.

Your brain tries to maintain a normal baseline. If you take a chemical that disrupts brain signaling, it tries to work around the disruption to get things back to normal.

Let’s say you take a chemical that activates opioid receptors, like morphine. Your opioid signaling system starts sending out a flood of signals. You perceive this as pleasure.

Your brain says “hang on, the opioid signaling system is going crazy. I’m going to turn it down to get back to normal.” So it changes the number of opioid receptors.

You take the morphine again, and you don’t feel that incredible pleasure, because you have more opioid receptors in your brain for the morphine to activate. This is how tolerance works.

So you take more. Now you feel that rush of pleasure again.

But your brain says “hang on, something still isn’t right? The opioid signaling system is still going bonkers. I better dial it down some more.” So it changes the number of receptors even further.

Now you stop taking the drug. Your brain has turned the opioid signaling system way way down, since you were blasting it with a chemical that sends it into hyperspace. Now you’ve stopped taking that chemical that was sending it into orbit, which means now it’s underactivated. You perceive that under-activation as intense pain. This is what withdrawal is.