New research indicates that we spend nearly half of the 16 hours we are typically awake each day letting our minds wander - often thinking about events in the past or expected events in the future at the expense of the amount of focus we allocate to the immediate present. A significant level of attention deficit disorder is arguably designed into our wetware.
The functional difficulties of those suffering from ADD suggest that when the percent of processing capacity available to focus on the immediate present drops significantly below the "normal" level, this tendency can become manifestly counter-productive. Too much is definitely not a good thing. Redirecting 50% of our potential capabilities to daydreaming, fantasizing, contemplating, etc. would seem like a pretty cavalier expenditure considering the difficulties so many humans have dealing with the world around them using just the remaining 50%.
Some portion of our daydreaming can be justified as a form of system maintenance. It's now thought that at least part of the purpose of dreams is to reinforce synaptic connections associated with new information we've learned, past events we daydreamed about, and things we worried about during the day. This improves our ability to remember significant past events and new information, and/or prepares our minds to react more quickly in response to what has been causing us anxiety.
On the surface this would still seem to be a highly inefficient allocation of resources.
We've arguably had to evolve brains that are essentially twice as powerful as needed to accomplish the basic species imperative of staying alive long enough to procreate the next generation, just because our brains have this tendency to spend half their processing time daydreaming about novel mating rituals, or worrying about offending the in-laws during the upcoming migration. There must have been substantial evolutionary advantages to overcome the obvious negative aspects of this built-in inefficiency.
Becoming a generalist species greatly increased our need for processing capacity. The flexibility to adapt to nearly any environment after birth takes a lot more processing power than instinctual adaptation to a specific environment. As generalists we're born largely unadapted to any particular environment. We have an extended period of dependency while we learn from our parents, siblings, and peers how to function in our local environment. Our generalist brains take years to self-program themselves before they can provide the equivalent of the instinctual responses other species possess at birth.
Conscious decision making based on learned knowledge requires a great deal more processing power than the firing of a "hard wired" instinctual reaction. It seems reasonable to assume that we at least in part evolved greater capabilities because in an emergency, having a bit of "reserve" brain power that could be suddenly pulled off its busywork tasks and applied to the immediate situation, tended to improve the odds of the individual surviving long enough to contribute his genes to the next generation.
My brain is excessively prone to distraction, but I can attest from personal experience that stepping out of an airplane into the void at 10,000 ft. can be a powerful focusing aid. I'm fairly certain that in spite of my ADD, I managed to get all 100% of my brain's potential focused on the immediate task at hand for at least a brief period of time, more than once during my years as a sport skydiver.
Most of the minutes of a skydiver's lifetime are arguable significantly less intense and mentally demanding than the seconds he actually spends in free-fall. For a hunter, the kill at the end is only a brief but highly demanding moment in the long slow process of the hunt. However, in terms of survival potential, evolution is likely to favor the individual who can best deal with the most demanding moments, even if that results in surplus capabilities for the rest of the time. Having evolved brains with the capacity to handle the peak demand situations that most influenced our evolution in the natural world, much of day-to-day life in our largely artificial semi-civilized modern world can be handled by less than the typical human brain's maximum potential.
A major complication in this scheme is that cells tend to atrophy if not used, and this is especially true of brain cells. In order for a reserve of brain power to be available during those brief moments when survival was at stake, there had to be a way to keep the extra brain cells active and alive when not needed.
The brain functions that would become our imaginations likely started out as the equivalent of a load resistor - nonessential tasks that could absorb modest amounts of surplus brain activity when it wasn't needed for immediate survival. However, evolving larger brains with ever greater maximum thinking potential had sufficient advantages that we continued to develop ever more surplus capacity.
Some individuals were able to put this additional capacity into a kind of idle "sleep mode", or at least diverted into relatively harmless make-work processing. Others found it difficult to throttle down their mental processes, with the result that they had excess processing power constantly seeking ways to keep itself occupied. If it wasn't provided with something to keep it busy, it would find something on its own.
It seems reasonable to me that our tendency to let our minds wander is likely a consequence of having evolved an imagination. The parts of our brains involved in imagination were the last parts to evolve, and are unique to modern man. We developed imaginations during the same period we were evolving larger more powerful brains.
Somewhere along the way the magnitude of surplus capacity being shunted into the "thought drain" grew to the point where it took on a purpose of its own. The most logical pattern of organization to mimic would be the core operational mind it was created to supplement. It began self-organizing into a form of alternative mind influenced by and influencing in turn the parts of our brains still actively involved in managing our real world actions and perceptions.
The core operational mind had to have priority connections to the body's senses, so the alternative mind had to make do with abstracted alternatives. Our evolving imagination had to generate its neural activity from the bits and pieces it could scavenge from the core mind, mixed with the individual's accumulating memories. However, freed from the do-or-die reality-check of direct interaction with the real world, this evolving alternative mind could recombine memories and sensory perceptions in novel ways, which occasionally led to imagining novel solutions to survival challenges in the real world. Having an active imagination appears to have provided a survival advantage, which in turn encouraged the evolution of ever more active and intrusive imaginations.
By the time our evolutionary branch arrived at modern man, the "thought drain" had become our uniquely human imaginations that now manage to distract our attention away from the real world nearly half of our waking hours.
Our ability to imagine things beyond our immediate physical reality enabled us to eliminate most of our natural predators and competitors. We imagined ways to build and use complex tools, and to domesticate our food sources. Our imaginations have helped us advance far beyond the achievements of other hominid species. We've been imagining for decades what it will be like to become the first space faring species to venture beyond the confines of planet Earth - even though our technology is not yet capable of delivering the future we can imagine. But it's only because we can imagine colonizing space that we've been inspired to develop the technology that will someday allow us to accomplish our dreams.
Maybe a bit of daydreaming is a good thing...