Why Do We Think?
Intelligent thought (cleverness, creativity, insight) is not common in the animal world. Intelligence without thought, and thought without intelligence, are more common, but the combination is rare. That may seem odd, given that intelligent thought is such a dandy adaptation (it can get you to the moon and back)—why isn’t it as common as the eye? But this must be an anthropocentric prejudice, especially given the high metabolic price of thinking intelligently; it is evidently not a popular way to equip an organism for the battle for survival. More likely, it arose in a peculiar and possibly unrepeatable manner in our not-too-distant ancestors; before Homo sapiens probably but not at the time of the dinosaurs. There must have been a pressing need for it, and intense natural selection for the trait, so the question is what occasioned such a need. What could have catapulted this rare and eccentric trait into existence?
I am going to make a speculative but informed proposal: it was our ancestors’ descent from the trees that was the triggering event, along with its resultant challenges. That’s when intelligent thought began (for us anyway, maybe not for all intelligent thinkers). To be specific, it was the loss of our arboreal home that necessitated a certain cognitive enhancement. The trees made a good home for us (i.e., our ancestors): they afforded shelter from the elements (sun, rain) and protection from predators, and they had a built-in larder (nuts, berries, insects). Many animals make their home in trees, and with good reason: trees are nature’s natural place of residence, a fine place to raise a family. We hominids liked them too and lived in them for millennia, but there came a time when they were no longer habitable by us, for reasons unknown (probably climate change). We were driven down to the ground, stripped of our natural home—our castle, our luxurious penthouse. It was then that we needed to build a new home. Instead of living in a pre-made home provided by mother nature we had to construct our own home, equally comfortable and safe, and with an attached larder. We had to become builders, not just lucky squatters. We had to find suitable raw materials and construct a workable residence that a family could live in (those feline predators were a priority). Or else we would be homeless and not long for this world. The stage was thus set for a momentous evolutionary saltation: the jump from unskilled tree dwellers to skilled terrestrial house builders. The hypothesis, then, is that intelligent thought as we know it evolved because of the survival demands placed on these newly arrived and ill-equipped ground dwellers. To build a house capable of providing what a tree provides in the way of living accommodation requires mastery of intelligent thought. The OED defines “build” as “construct by putting parts or materials together”; and this requires means-end reasoning and the idea of an artifact with functional parts (floor, roof, door, etc.). We are now extremely familiar with houses and house building, but our arboreal forebears had no idea about such things (though some primitive construction may have been attempted up there among the branches). It took a considerable degree of ingenuity to come up with the idea of an abode put together down there; heaven knows what those first houses looked like—fallen trees maybe. No wonder many took to living in caves, those cold dank dark inhospitable places. Anyone who could build a decent dwelling near the source of food would have a distinct evolutionary advantage in the fight for survival. Consequently, intelligent thought came to be prized and selected for, no matter the metabolic downside. Sexual selection would ensure that male house builders had the reproductive edge over clumsy layabouts and daydreamers. The ability to construct a functioning home from the available raw materials was thus the first manifestation of what would later be known as intelligent thought (smarts, knowhow). Once established this ability could give rise to other forms of thought, but initially it was construction workers and primitive architects who ruled the roost (quite literally).
Home construction in those early days would have involved simple tools and the dexterous use of the hands, both closely implicated in the development of large-brained intelligence. Those animals that stayed in the trees were spared this upheaval in mode of living, as were ground dwellers whose homestead had never been taken away from them. But humans had to engage in a new form of behavior that required suitable cognitive machinery—what we call intelligent thought (not knowing much about what this is exactly). The adaptation supervened on a unique set of circumstances and happened quite by chance; intelligent thought was by no means inevitable. Once it was in place, however, further artifacts could be conceived and constructed: furniture, weapons, boats, bridges, villages, churches, etc. Thus it was that men became workmen, manual laborers, artisans, carpenters, masons, roofers, etc. By contrast, life in the trees was work-free, because you didn’t need to build anything—nature supplied it ready-made. Now a lot of time had to be spent in patient and painstaking construction with a long-term goal in mind. Once we had learned to think intelligently (not just daydream and hang lazily around) we were off and running, deploying our newfound cognitive skill. We became a different type of species, a creature of Reason, a Thinker, a Cartesian prototype. But it all went back to the need to build our own homes now that the trees had become uninhabitable. We think because we needed to build; our first thoughts were builder’s thoughts. Other animals don’t have the same kind of motivation, because their housing arrangements are relatively stable; we had to strike out in a completely new direction. It was either think or perish, reason or be homeless. Thinking became a biological imperative, a condition of survival.
It is easy to see how deeply the notion of building is built into our conceptual scheme (this sentence is an example of it). We speak of the “building blocks” of reality; we “build arguments”; philosophers are “system builders”; a lawyer “builds a case”; a scientist “builds a theory”; a charlatan “builds a cult”; God “built the universe”; success “builds confidence”. We are said to “construct a defense” or “construct a narrative”. We think of molecules as “constructed out of atoms” and sentences as “constructed from words”. We “build” alliances and fires, families and followings. The concept of building a house thus generalizes to that of building less concrete and practical entities. We are now surrounded by things that have been built, engineered, erected; our world is a built world. All of this results from intelligent thinking, which originated (according to the hypothesis) in building primitive dwelling structures (lean-tos, wigwams, huts, shacks, tents, igloos). Many animals build nothing (horses, tigers) simply because they have no need to; we did. This ability to build dwellings enabled us to travel to many different kinds of habitat, since we had no need to rely on naturally formed homes; we could use thought to build something from the available materials. We have covered the planet with our buildings, thus implementing the results of our distinctive form of intelligence. So, the building gene has remained in our genome, because constructive thought is a useful adaptation, despite its metabolic costs. Thought didn’t come from nowhere for no reason; it is a response to a specific evolutionary challenge, a survival problem. Just as we speak because we travel, so we think because we build; at any rate, that’s how these traits got started. If the hypothesis is correct, what set human thought in motion was our departure (or ejection) from the arboreal life-style, with all that that entailed. We had to learn to live without the comfort of trees.
 I discuss arboreal descent in Prehension (2015). This paper is intended to complement that work.
 See Prehension. The co-evolution of the human hand and brain is a commonplace of physical anthropology. House building is surely part of this story.
 Whenever I watch nature documentaries about tree-dwelling animals (e.g., bonobos) I am always struck by how happy they look, up there on their lofty perches, climbing, brachiating, generally having a good time. In the garden of Eden, we surely lived in trees not just among them. Trees have deep psychic resonance for us: we view them with reverence and affection; they provide shelter and solace.