When it comes to our brains, we have it all wrong.
A little background: a study published recently in Current Biology and summarized in a HuffPost article seeks to understand why more animals do not have larger brains. In my field of neurobiology this has been a long-standing mystery: since big brains are assumed to be beneficial, why are they so rare in the animal kingdom? As the recent article explains, the most commonly proposed explanation is the "expensive tissue hypothesis." The idea is that the high metabolic cost of maintaining a complex central nervous system exceeds the benefits of flexible and adaptive behavior that comes with a big brain. In the case of humans the brain constitutes two percent of our body weight but consumes 20 percent of the energy. The Current Biology article indeed shows that as guppies grow larger brains their guts shrink in size, making them more metabolically inefficient compared to their smaller-brained brethren. This has serious consequences: The smarter fish had almost 20 percent fewer little guppies to call their own.
None of this is terribly surprising; evolution is an endless struggle between cost and benefit. A large brain is an energy hog. In addition, the bigger the brain the more important its protection becomes, requiring even more energy for thick skulls, protective tissue, specialized blood vessels, and host of other adaptations. Somewhere along the line this high cost has to be paid, as humans clearly show: fewer offspring, long periods of infant nurturing, and the demand for a constant source of calories to feed the central energy consumer.
Asking the Wrong Question
Sure, big brains extract a high cost. That line of reasoning is convincing, and I am confident this tells part of the story. But here is where I break from many of my colleagues: the high metabolic cost of a big brain is largely irrelevant to the big picture.
I propose that big brains are rare in nature not because they are an expensive tissue to maintain, but because the consequences of complex thought are not adaptive. Being smart is a dumb survival strategy. The study in Current Biology starts from the premise that big brains are beneficial; I believe that assumption to be fundamentally wrong. The question should not be why big brains are rare in nature but why they can persist at all; the answer is that they do not. Our little blip of existence for 100,000 or so years proves nothing other than a bad trend toward self-destruction.
As are all creatures, humans are a genetic experiment resulting from selective pressure, random mutations, and pure chance that our ancestors avoided extinction from catastrophic events, such as meteorite impacts. Our ancestors made it far enough to yield us, but the prospects for our future survival are not particularly bright. Extinction is the biological norm; so far at least the pattern of evolution for humans is no different from the rest of the earth's fauna.
Human Hubris
The unquestioned assumption that big brains are good comes from a deep-seated human arrogance that our species is special. We are taught that evolution reaches a pinnacle with our species, and that our intelligence sets us apart. Religion tells us only we are made in god's image. This perspective is deeply wrong. By understanding why, we can gain perspective on how having a big brain is not all it's cracked up to be.
Without a doubt, human beings possess a level of intelligence, self-consciousness and self-awareness greater by degree than is found in any other animal. But as with almost all aspects of comparative biology, intelligence, self-consciousness and self-awareness are elements of a continuum, rather than phenomena with sharp boundaries between species. Intelligence and self-awareness do not belong exclusively in the domain of humankind. No universal measure of intelligence can be meaningful, because animals have diverse adaptations that define the context of intelligence, making interspecies comparisons suspect. A cat under water would not look too intelligent, but a porpoise might. On the other hand, you would be severely challenged to teach a porpoise to climb a tree. You may well be able to solve math problems, but your dog will learn more quickly and more effectively than you ever could to sniff out the drugs in your colleague's suitcase, and to notify you of the contraband. An animal's intelligence, or more precisely, its ability to manifest its intelligence, is tightly correlated with its natural environment, and its evolutionary adaptations.
No matter how much we want to think ourselves special, intelligence is found by degrees across the animal kingdom, and not in some nice neat linear correlation with some other trait like the development of mammary glands. Being smart seems to be a trait unique to human beings only when we artificially designate our particular suite of characteristics as the definition of intelligence, proving that circular logic is not too intelligent.
Too Much of a Good Thing
Other animals, like elephants and dolphins, have big brains. Some bird species have proven to be extraordinarily intelligent. So how then can I claim that having a big brain is maladaptive? By having too much of a good thing: in the case of humans the endless evolutionary struggle between cost and benefit may well have tilted toward too great a cost in growing a big head. Some intelligence is good; too much is destructive. Somewhere along the continuum of intelligence in the animal kingdom the balance tilts: on one side the costs and benefits of intelligence balance favorably; on the other, the consequences of intelligence become too great to be outweighed by the many advantages.
A Bad Outcome
Every species exploits the environment to the maximum extent possible, until competition, predation, resource depletion, disease or other constraints limit growth and expansion. Social animals, from insects to mammals, find equilibrium between cooperation and competition. Human survival strategies are no different from those pursued by other species, except that we have a huge technological advantage (a consequence of our big brains). In struggling to survive, humans have successfully co-opted a significant percentage of the planet's available resources. We have waged war at a scale only possible in a species with a brain big enough to contemplate such actions. Our reliance on technology to exploit resources, and each other, has had global effects over a short time period, unlike other species similarly striving to survive. Having a big brain has extracted disastrous costs. As a result, our efforts to survive and prosper may have the paradoxical consequence of causing our extinction, either directly through the use of weapons of mass destruction, or through the degradation of the resources on which we depend.
Short History
In spite of our hubris, humans are nothing but a short-lived biological aberration, with no legitimate claim to superiority. As a minor branch on a vast evolutionary bush, modern humans have been roaming the earth for no more than a few hundred thousand years of the earth's 4.5 billion-year history. Ours has been a brief presence, with too little time to demonstrate if the evolution of large brains is a successful strategy for long-term survival of the species. Our self-anointed position to exalted status has blinded us to the reality that our big brains might not be our savior but the potential source of our demise. We claim we are special, but there is a loss of credibility when you choose yourself for an award.
If evolution had a pinnacle, bacteria would rest on top. While it hurts our ego, we live in the Age of Bugs, not the Age of Humans. These single-celled germs are the most successful of all life forms, and have been dividing away for more than 3 billion years. Bacteria have been found to live in virtually every conceivable environment at extremes of pressure, temperature, salinity, radiation, alkalinity and acidity. A spoonful of good quality soil may contain ten trillion bacteria representing more than ten thousand different species. More than 1 million bacteria are found in 1 milliliter of seawater, and these constitute most of the ocean's biomass. Our self-promotion to the image of god is simply embarrassing in the face of this biological reality on the ground. When the human species is a distant memory, bacteria will be dividing merrily away, oblivious to the odd bipedal mammal that once roamed the earth for such a brief moment in time.
And so we come full circle to the question about brain size. In life's history on Earth, only a few animals have developed large brains, and only one species the largest. In elephants and dolphins we are probably witnessing the upper limit of adaptive growth of big brains, the point where cost and benefit roughly balance; beyond that we are likely observing the odd reality that brains too complex lead to behaviors resulting in extinction. The answer to the mystery of the paucity of large brains is found in behavior, not metabolism.
Of course on Earth we have just a sample size of one; so my idea about big brain rarity is difficult to test. A few million years from now the answer will be clear, even if we might not be around to appreciate it. Or perhaps sooner than that we'll discover an abundance of life elsewhere and we'll have a great sample for comparison. In the meantime we can surmise that any brain smart enough to develop a weapon capable of destroying itself, or depleting its sustaining resources, is not too smart.