The Descent of Man: Why One Scientist Thinks The Human Race May Be Getting Dumber

November 20, 2012
Image Credit: panco / Shutterstock

Jedidiah Becker for redOrbit.com — Your Universe Online

Take a glance at the arc of human civilization. As just a few notable achievements, you might start with the discovery of agriculture before moving on to survey the architectural marvels of the ancient world, the revolution of Gutenberg´s printing press and finally landing on the modern ubiquity of rapidly evolving computer technology. This view tends to give a sense that the human intellect may have a nearly limitless potential to master nature — Hannah Arendt´s Homo faber, “man the creator.” And that may well be the case. But going a step further, the furiously paced advances taking place in nearly every branch of science also incline most of us to suspect that we, as a species, just keep getting brighter with each new addition to our vast library of accumulated knowledge and technology.

However, a professor of pathology and developmental biology at Stanford University’s Crabtree Laboratory believes there´s cause to suspect that humanity´s intellectual prowess may actually be eroding, and at an astonishing rate. In a recent paper titled “Our Fragile Intellect,” Professor Gerald Crabtree opens his discussion with an odd statement: “I would be willing to wager that if an average citizen from Athens of 1000 BC were to appear suddenly among us, he or she would be among the brightest and most intellectually alive of our colleagues and companions. We would be surprised by our time-visitor´s memory, broad range of ideas and clear-sighted view of important issues. I would also guess that he or she would be among the most emotionally stable of our friends and colleagues.”

And Crabtree clarifies that this speculative superiority doesn´t just apply to the historically revered Greeks. In fact, he says, all of our ur-ancestors of roughly 3,000 to 6,000 years ago were likely smarter and more emotionally stable than us, whether they were in ancient Africa, Asia, India or the Americas. And the reason for their superiority, he explains, is not a matter of knowledge or culture, per se, but rather one of genetics.

In a provocative thesis that has already ruffled the feathers of the scientific community, Crabtree crafts a compelling argument using a back-of-the-envelope statistical analysis of genetic mutation frequencies, a dash of tried-and-true evolutionary theory and a pinch of anthropological speculation. The result is a persuasive case for why we might all be getting dumber.

At the bottom of his theory are two core ideas: The first — supported by modern neurobiology and genetics — is that the biological basis for human intelligence is made up of a strikingly large and surprisingly fragile constellation of genetic players. The second hinges on the idea that this intricate and unstable conglomerate of genes was forged together in the furnace of intense evolutionary pressures — pressures that we Homo sapiens have, ironically, largely managed to mitigate thanks to our unparalleled powers of creative intelligence.


One of the hallmarks of Mother Nature is that she invariably picks winners. And though their victories may be short-lived, every species that ever occupied a branch in the tree of life managed to do so because it was able to outfox its foes. In evolution´s Coliseum, only the robust, the adaptable and the lucky survive the perpetual competition with enemies, environment and fate. Thus it might seem counterintuitive that something as vital to our evolutionary survival as intelligence would rest upon a flimsy foundation. After all, our early human ancestors couldn´t beat a chimp in an arm wrestling match or outrun a lion on the open plain, but they could fashion and accurately launch a spear at a target from a distance, organize and execute complex hunting strategies, and tame the mysterious power of the flame — all abilities that required an advanced, adaptable and abstract kind of intelligence. And it was largely this quality rather than the brute strength and speed of the beasts that allowed our ancestors to survive and thrive in the harshest environments that nature’s creative kitchen could cook up.

So if our unique intelligence was the linchpin of our evolutionary fate, how and why was it left to dangle from such seemingly thin thread?

The Genetic Complexity of Intelligence

The first step in understanding our cognitive fragility, Crabtree explains, is to recognize the extreme genetic complexity of this thing we call intelligence. In the first half century or so following Watson and Crick´s discovery of DNA as life´s heritable playbook, there was a strong tendency among scientists towards a kind of genetic reductionism — the habit of thinking about any given characteristic of an organism as being the product of a single chunk of DNA; the notion that there exists a more or less one-to-one relationship between a gene and a given biological trait. While there are a few traits for which this kind of logic does hold true — such as for the single genes that give us a widow´s peak or the ability to roll our tongues — most complex biological traits require the combined action of numerous genetic influences.

Although advances in research have largely disabused modern scientists of this overly simplified view of the relationship between genetics and biological traits, the ghost of reductionism still haunts the halls of academic institutions and research laboratories throughout the world. But when it comes to thinking about human intelligence, the notion that we derive our richly complex cognitive abilities from just one or a handful of genes isn´t simply incorrect — it´s likely incorrect by several orders of magnitude. So how many ℠intelligence genes´ does the average human need for his quotidian routines? “How many genes are required to carry out our everyday tasks, read a book, care for a loved one, conceive of a just law or compose a song?” Crabtree asks. The answer, it turns out, is probably in the thousands.

Using several techniques to approximate the number of genes required for “full intellectual and emotional function” — the most effective of which he believes to be the analysis and extrapolation of the number of so-called ℠X-linked intellectual deficiency genes´ that reside (as the name indicates) on the X chromosome — Crabtree puts the number squarely in the range of 2,000 to 5,000 genes, all of which are necessary to keep our brain´s cognitive cogs optimally turning. That´s a lot of genes to devote to one biological process — somewhere between one-tenth and one-fourth of the total number of genes in our entire genome, to put it in perspective. But given the complexity and importance of intelligence in our survival, this high number may not seem all that surprising.

And yet the question remains as to why our intelligence should be particularly vulnerable simply because it requires a large number of genes to function properly. After all, isn´t the robustness of the human organism at least partly due to the fact that evolution has equipped most of our biological systems with overlapping redundancies, back-up programs and fail-safe mechanisms so that the whole machine doesn´t break down when one of our biological widgets goes kaput? A corporation with thousands of employees typically doesn´t experience so much as a hiccup if one worker is sick for a week. But a small business with just five people on the payroll could quickly tank if one team member isn´t there to fulfill his duties. For many of our body´s processes, this logic holds true: More genetic components translates into an increased robustness. In the case of intelligence, however, Crabtree says there are two specific reasons why this multiplicity of genetic players may prove to be more of a liability than an asset.

For starters, he explains, the large number of genes that are required for full cognitive function creates a proverbial flock of sitting ducks for random genetic mutations. “The larger the number of genes required, the more susceptible we are as a species to random genetic events that reduce our intellectual and emotional fitness.” Thanks to our finely tuned biological machinery, the frequency of mutations in our genome is actually astoundingly low. In yeast cells, for instance, random mutations only occur at a rate of about 3.8 x 10-10 to 8.4 x 10-9 per-base-pair per generation (that´s in the range of one mistake per couple trillion or so nucleotides). When it comes to duplicating and passing on life´s blueprint, evolution didn´t leave much room for mistakes, and millions and millions of years of merciless natural selection have ensured that the replication of our DNA takes place with mind-boggling accuracy.

However, while genetic mutations are statistically rare, every now and again one slips in through the backdoor. And the simple fact that there are so many genes associated with intelligence significantly increases the likelihood that one or more of these will be affected by a random mutation in the same way that buying additional lottery tickets increases your chances of winning (except in this case, you´re increasing your chances of losing “¦ Think of it as more of a Hunger Games sort of situation). If there were just one little gene for intelligence hiding amongst the other 20,000 or so in the human genome, then the probability that that single gene would get hit by a mutation would be quite low. However, the fact that there likely 2,000 to 5,000 of these genes increases that probability by several orders of magnitude.

Intelligence Genes Work as a Chain, Not a Network

So, the fact that our intelligence requires so many genes to work properly actually increases our exposure to deleterious mutations. Bummer. But in his thesis, Crabtree points to yet another reason to suspect that our intelligence genes might be particularly fragile. In addition to the problem presented by the sheer number of these genes, he says that there´s also a systemic weakness related to how they work together to produce our cognitive abilities.

As just mentioned, many of our body´s systems have multiple built-in fail-safe mechanisms that serve to increase the robustness of the whole organism. Many vital biological systems tend to work as a sort of network in which different components have overlapping, parallel or even duplicate roles, ensuring that the overall function of the system doesn´t depend on any one part.

With intelligence, however, it seems that the picture may again be a bit different. Crabtree explains that the mutation of any single intelligence gene can significantly compromise the integrity of the whole intelligence apparatus. And for this reason, he says, “these genes do not operate as a robust network, but rather as links on a chain in which failure of any one of the links gives rise to deficiency.” Thus when it comes to intelligence, a single weak link in the genetic chain can (and probably does) lead to the impaired function of the entire system.


By now the more observant reader may have spotted a small kink in Professor Crabtree´s thesis: If the unique intelligence of Homo sapiens is such an extremely brittle biological phenomenon that it appears destined for genetic degeneration, then how in the world did our ancestors ever acquire it to start with?

Even putting aside the thesis about our cognitive fragility and decline, the rise of intelligence in humans is still one of the most intriguing riddles in modern anthropology and evolutionary biology. Crabtree buttresses his theory of declining intelligence by pointing to several things that we do know about the emergence of human intelligence and what they might tell us about its rise and possible descent.

Somewhere between 50,000 and 500,000 years ago our prehistoric African ancestors began to experience a rapid enlargement of both the volume of their skulls as well as the size of their frontal cortex — a part of the brain involved in complex problem solving, decision making, social interaction and other higher-level cognitive tasks. And this development wasn´t a mere coincidence. It was part of a new kind of evolutionary survival strategy — a shift from relying predominantly on traits like strength and speed to one that depended on characteristics like cunning, prediction, abstract thought and a more sophisticated, intuitive grasp of the laws of physics.

This strategic adjustment of survival strategies was not an easy one, says Crabtree, and the casualties were probably extremely high. “In the transition to surviving by thinking, most people (our non-ancestors) probably died simply due to errors of judgment or a lack of an intuitive, non-verbal comprehension of things such as the aerodynamics and gyroscopic stabilization of a spear while hunting a large dangerous animal,” he explains in his paper. “This optimization [for survival by intelligence] probably occurred in a world where every individual was exposed to nature´s raw selective mechanisms on a daily basis.”

Thus for early pre-human species attempting to make the switch to survival by thinking instead of survival by strength and speed, the pressures of natural selection were intense, and there was very little room for error. If you couldn´t quite predict the trajectory of your spear within a thin margin of error, or weren´t able to recalibrate a complex hunting strategy in a split second while charging through an open glade, chances are you weren´t going to survive for long and you certainly weren´t going to leave behind many offspring to propagate your genes.

Roving the African plains in small dispersed packs that would not develop language until much later, survival as an early hunter-gatherer was not for the dull-witted. Life for our early ancestors who made the strategic switch to surviving by their wits was, in Crabtree´s words, “more intellectually demanding than we would commonly think.” In fact, he says, “life as a hunter-gather [sic] required at least as much abstract thought as operating successfully in our present society.” If you ask most people, they tend to view skills like the ability to write literature, operate complex machinery or design computer software as far more demanding than those needed to carry out tasks like crafting a primitive weapon out of stone and wood, or tracking a herd of animals across the plain. Yet as Crabtree points out, most of the abilities that we consider to be intellectually ℠sophisticated´ are actually nothing more than byproducts of the core intellectual traits that our ancestors developed to survive in a hostile world — traits which Mother Nature selected for us with unforgiving austerity. In this view of intelligence, the ability to write a symphony or perform higher mathematics are merely the “collateral effects” of the fundamental survival abilities that we attained in evolution´s refining fire.

Somewhere between 50,000 and 500,000 years ago, our 2,000 to 5,000 intelligence genes were being fine-tuned by life´s master engineer, natural selection, to perform tasks of unprecedented intellectual complexity. All the skills that we´ve subsequently developed since the dawn of civilization are thus, in a sense, an epilogue to the story of the rise of human intelligence. Driving home this point, Crabtree reemphasizes that “it seems that if one is a good architect, mathematician or banker, these skills were an offshoot of the evolutionary perfection of skills leading to our ancestor´s survival as nonverbal, dispersed hunter-gathers [sic].”


It is therefore interesting — and more than a little ironic — to note that if Crabtree´s thesis is correct, some of these “offshoots” of our intelligence could potentially contain the seed of our intellectual destruction. To get a grasp on this twist of evolutionary fate, it´s necessary to reiterate the fact that the stunningly complex and fragile constellation of genes that gave us our intelligence was only possible because Mother Nature was such a harsh and unforgiving tutor. The development of our brittle intellectual abilities actually required that natural selection cut us no slack. Any branches of pre-human prototypes that couldn´t quite get the right combination of those 2,000 to 5,000 ℠smart genes´ were simply pruned off the tree of life — and the score or more of extinct hominine species suggests that these casualties were extremely high.

By at least 50,000 years ago, however, evolution´s perpetual tinkering had led to the development of a species that was capable of interacting with its environment at a level of sophistication and adaptability unprecedented in the history of life. In fact, so sophisticated was this new tool, the human brain, that it rapidly began branching out to perform new types of tasks that nature had not even intended in its original design. No sooner did the brain of Homo sapiens possess the abstract problem-solving skills needed to fashion weapons and coordinate hunts that it began to extend these abilities to other tasks like constructing a symbolic language for higher-level communication with its peers, taming animals for long-term use rather than immediate consumption, and unraveling the mysteries of how to grow and harvest seeding plants. In evolutionary time, the development of all of the features of human civilization appeared just a wink after we had learned to think.

According to Crabtree´s theory, this ability to tame nature and therefore mitigate the harshness of our environment may paradoxically be the greatest blessing and curse that our unique intelligence has given us as a species. While the details of his argument are constructed around technical calculations of mutation frequencies and abstruse analyses of genetic dysfunction, the basic idea is fairly clear: As the amenities of civilization have made our lives easier, they have simultaneously weakened the genetic foundations of the intelligence which made that civilization possible in the first place. Our triumphant ancestors in northern Africa, he explains, “did not have organized agriculture that permitted life at high density in cities and societies. Thus, the selective pressures that gave us our capacity for abstract thought and human mental characteristics operated among hunter gathers [sic] living in dispersed bands nothing like our present day high-density, supportive societies.”

If the pressure of intense natural selection in the wild was necessary to bring together and maintain all those intelligence genes, then the civilizations that we have formed since the advent of human intelligence have significantly weakened those pressures. As we mastered the art of agriculture and urban living, life become softer, and the intense pressure to maintain intelligence went a little slack. “Community life would,” guesses Crabtree, “tend to reduce the selective pressure placed on every individual, every day of their life.” And as we became more urban, traits other than intellectual prowess grew relatively more significant. For instance, while the selective pressure for intelligence loosened up, other qualities like resistance to infectious diseases became increasingly vital to survival among larger, more stationary groups of individuals.

The result of softened selective pressures for intelligence, Crabtree postulates, is that more and more harmful mutations to our intelligence genes have probably managed to slip into our genome in the past few thousand years. Out in the prehistoric wild, a mutation that caused even a minor dulling of the intellect probably would have been enough to spell certain death for an ancestor who carried it. However, in the cozy berth of a large agrarian society, life became increasingly forgiving of these genetic mishaps, and rather passing away into oblivion these genetic mutations have been passed on to the next generation. And thus, per Crabtree´s thesis, the intellectual foundation of our intelligence has probably been eroding since the dawn of civilization

Using a string of dizzying calculations rooted in current data about the frequency of various genetic mutations, Crabtree has even made an estimate of the number of deleterious mutations that the human intellect may have suffered in the last few millennia: “Within 3000 years or about 120 generations, we have all very likely sustained two or more mutations harmful to our intellectual or emotion stability.” And as if that weren´t bad enough news, he also says there´s a good chance that our intelligence is eroding exponentially as we accumulate these mutations.


So is it all speculation or is there empirical substance to this gloomy thesis? Are we getting dumber or aren´t we?

Crabtree concludes his paper by suggesting that his hypothesis could be empirically tested with a relatively basic set of experiments that involve sequencing the genomes of individuals whose last common ancestors spanned the period from present day to about 5,000 years ago. This would allow researchers to estimate the speed with which mutations have been accumulating as well as whether and to what degree the selective pressures for these intelligence genes have diminished.

For his part, Crabtree says, he certainly hopes that his theory proves to be incorrect. After all, life in civilization is probably immeasurably more pleasant than the harsh existence that our first ancestors experienced in daily combat with the undiluted forces of nature. Community life has made living easier for everyone, says Crabtree: “Indeed, that´s why I prefer to live in such a society.”

But even if his thesis turns out to be spot on and we are in fact losing our intellectual edge, Crabtree suggests that we need not get too worried about it. While the deterioration of our intelligence may be happening quickly on an evolutionary timescale, the astounding pace at which the fields of genetics, nanotechnology and biotech are advancing means that we will probably have a way to address and rectify this and many other genetic problems in just a few generations, if not sooner.

“One does not need to imagine a day when we might no longer be able to comprehend the problem or the means to do anything about the slow decay in the genes underlying our intellectual fitness. Nor do we need to have visions of the world´s population docilely watching reruns on televisions that they can no longer understand or build.” The reason, explains Crabtree, is that our technological prowess is outpacing our genetic decline.

“It is exceedingly unlikely that one hundred or two hundred years will make any difference at the rate of change that might be occurring. “¦ The sciences have come so far in the past hundred years that we can safely predict that the accelerating rate of knowledge accumulation within our intellectually robust society will lead to the solution of this potentially very difficult problem by socially and morally acceptable means.”

In other words, civilization´s collective intellectual legacy may actually be able to save us from itself.

Source: Jedidiah Becker for redOrbit.com – Your Universe Online

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