Why do we grab small short-term gains to our long-term disadvantage?

From an evolutionary standpoint, our near obsession with the short-term at the expense of long makes perfect sense - at least it did 100,000 years ago. Our stone age ancestors lived at a time when there was a reasonable chance of dying during the course of any given day (back then hearing the phrase "Honey I'm home" really meant something). Your decisions didn't make it to your frontal cortex where risks and benefits are balanced. It seems clear, and with good reason, humans in that older era began to equate delayed benefits with significant degree of risk since there was such a reasonable chance that future benefits would never be realized. So, for the average stone-ager the decision to go with the smaller, surer, instant reward was hardly irrational. To do anything else, now that would have been crazy. Of course our problem is that what was Mensa brilliant back in the old days doesn't work quite as well today.