Organisms typically can't depend on the continuous availability of nutrients and other biologically important information. They therefore must develop assimilation, storage, and retrieval systems that allow them to take in whatever bonanzas come their way, and then gradually use the stored surplus during periods of scarcity.

For example, the widespread root and cellular storage systems of a tree allow it to absorb a sudden heavy rainfall in a climate that perhaps won't get another rainfall for weeks. The active ingredients in some medications are similarly released over the course of the day. We can also store food outside our body, such as the backpack provisions we carry on a hike, or the produce we preserve during summer gardening.

Learning and memory are functionally similar in that they allow us to perceive and store all the sensory and conceptual elements of an emotionally significant experience, and then later selectively retrieve the elements we need. The difference between nutrient and remembered information is that we use nutrients only once, but we can repeatedly recall a memory, and use it to solve various future problems.

And that can have a downside. Spoiled food can cause gastric distress, but it's soon excreted. Conversely, a single traumatic experience can be totally absorbed into our memory and then be negatively replayed for years, as in posttraumatic stress syndrome.