Herewith, a simplified history of the first three billion years of life on Earth:
The first living organisms got their energy by fermentation. They took
sugar molecules from their environment -- brewed up by plain old non-biological chemistry -- and broke them apart, rearranging the atoms into smaller molecules of carbon dioxide and alcohol, releasing some of the energy stored in the sugar.
Two problems: The earliest organisms were essentially hunter-gatherers, living off the land (or rather sea), and life was increasing faster than the abiotic production of sugar. The food supply was running out. Moreover, alcohol is a poison: The first toxic waste crisis.
Necessity is the mother of invention. Just in the nick, so to speak, life "invented" photosynthesis, a kind of primitive agriculture -- using sunlight to synthesize carbon dioxide and water into sugar, thus securing the food supply.
More problems. To photosynthesize you had to be in the sun, which meant exposure to ultraviolet light, which can kill. And a byproduct of photosynthesis is oxygen, the second toxic waste crisis; life was in danger of burning up by spontaneous combustion.
However, as the level of oxygen in the atmosphere rose, an ozone umbrella formed in the upper atmosphere, which helped with the UV problem. And -- necessity again -- life learned to use oxygen to burn sugar -- respiration -- breaking it down more completely and releasing far more energy than fermentation. Respiration solved the alcohol problem, and kept oxygen in the atmosphere at a safe level.
And with all that extra energy, it wasn't long before life invented sex.
There, now that was simple, wasn't it?
Not much has changed. It's spring break here in the Bahamas, and thousands of American college students are pretty much recapitulating three billion years of life history. Alcohol, sun and sex. Hangovers, sunburn, and broken hearts.