“Greetings, my friend! We are all interested in the future, for that is where you and I are going to spend the rest of our lives! And remember, my friend, future events such as these will affect you in the future.”
The Amazing Criswell, Plan 9 from Outer Space (1959)
A television producer once took me to lunch to ask me a very important question: “When will humans evolve the ability to fly?” A new superhero series was coming to television, with mutations in the characters’ DNA that were gifting them uncanny comic-book powers of flight, telekinesis, time travel, mind control, and so on, rather like Marvel’s mutant X-Men, but definitely not Marvel’s mutant X-Men. The producers were interested in a science program that might sit alongside the drama, about the real science of incredible human abilities, and the real-world possibilities of the evolution of these types of superpowers.
My answer was quicksilver fast: We already have. I love comics, and read them to this day, and have spent (arguably, wasted) much of my free time over the last thirty years considering the possible realities of superpowers. But the answer doesn’t rely on strange tales or amazing fantasy. So, I made a great grandstanding speech about how we have evolved massive creative brains, capable of planning and predicting the future, of invention and creativity, and this had helped us extract ourselves from many of the historical shackles of natural selection. We have externalized the stomach with the invention of cooking, so we don’t have to digest a whole range of chewy molecules, because they are already partly broken down by our unique control of elemental fire. We have bypassed many aspects of a life of nomadic sustenance, as well as hunting and gathering, by settling and domesticating all manner of beasts of the field and plants of the ground. This also has changed our culture, technology, and even our genes (as discussed in Chapter 2). We have radically eliminated diseases that scythed down ancient populations with casual indifference—plagues, malarias, cancers, pestilence. Smallpox once killed hundreds of thousands every year. Since the 1980s, as a result of vaccination, there have been no cases of smallpox. Polio looks set to follow soon as a disease only of interest to historians. These sorts of evolutionary pressures have been radically altered as a result of invention and science and the technology that has come about through our own evolutionary trajectory.
“How long before we fly? We do it all the time.” I waxed lyrical. We invented airplanes and helicopters, and rockets to explore space, and even hoverboards and jetpacks are not far off. We’ve walked on the Moon, and soon, a son or daughter of this planet will walk on another, just as Kal-El, the Son of Krypton, came to Earth. “We are superhuman already.”
He looked pleased, and impressed. And then said, “So you think we’ll evolve how to fly within the next couple of centuries?”
I finished my pizza, thanked him, and left. Limbs do indeed come and go in evolution. The basic body plan of all animals is broadly similar, which betrays a common evolutionary origin. The genes that indicate that “a leg should go here” are broadly the same in all species with legs, meaning they have deep roots. These, called Hox genes, specify what parts of a body will go where, and it is in their mutation, duplication, and multiplication that the variation in bodies in almost all creatures resides. Insects generally have six legs, and spiders eight, but millipedes and such creatures that crawleth have loads. But they have simply arisen from the duplication of the genes that issue the standard six in the majority of small arthropods.
Bat wings and bird wings are functionally the same, but have come via different evolutionary routes. We call this “convergence.” A hundred million years ago, the forearms of some dinosaurs had shifted their form to allow gliding, and their bones became lighter and hollow. Dinosaurs had feathers already long before they had flight, including the mighty Tyrannosaurus rex and even bigger, scarier, yet perversely fluffier predators, but over millions of years they would become part of the process of uplift and propulsion through the air. Mammals had split from dinosaurs epochs earlier, and the weeny critters that would form the basis of 1,240 bat species weren’t airborne for tens of millions of years yet to come, and have never had feathers. Structurally they are similar but different; they’re both adaptations of the forelimbs, meaning that their common root is not in flight but in the origin of four limbs. As forelimbs they are homologous: Equivalent bones are present, stretched and contorted into their distinct morphologies. In providing the uplift, flap, and glide of flight, the wings of birds and bats are—in the technical language of evolutionary biology—not homologous but analogous, as indeed they are with insect and airplane wings. Unlike the functional similarity with insects, though, both birds and bat wings have similar bones, as indeed both do with dolphin fins and horse’s hooves, again showing the four-legged root of these appendages. But the winged portions of their forelimbs are different, showing independent evolutionary origins. So to see evolution of our own forelimbs into wings would see the relinquishment of our hands and arms or the growth of an entirely new set of limbs. There isn’t a great deal of evolutionary pressure for either. More importantly, there isn’t a gene “for” wings, and we would have to undergo a grotesque, energetically demanding transmogrification in utero for arms to become wings, and that couldn’t and wouldn’t happen.
Bats and birds have acquired their flight over thousands of generations of slight incremental change conferred by subtle shifts in many genes being selected as they provide an advantage to the owner. Because limbs are so ancient in the grand scheme of evolution, I suppose, if we gave in to some fantastical thinking, that an unborn might harbor a genetic anomaly that would stimulate growth of an extra pair of limb buds. That child would have to successfully reproduce and pass down the anomaly, and over generational time the genes that prompted it would have to spread through the population, each time being slightly selected to be bigger and more winglike. It would, I guess, take dozens, or hundreds of generations. Wings are merely vehicles for providing the best chance of propelling some genes into the future generations. In birds, as soon as wings stop providing that propulsion they are gone, as we see in the flightless emus, kakapos, and kiwis. And insects have acquired and shed wings seemingly willy-nilly in their evolutions as they confer advantage or not for the genes they carry. They are to evade predators, or to gain access to food, or to show off to the females—all pretty standard reasons for having any evolved trait. The timescales involved are also unimaginable to us. So while, hypothetically, morphologically just conceivably possible, the chances of this coming to pass are about as likely as a boy bitten by a radioactive spider gaining the ability to produce webs from his wrists.*
But aside from the physical unlikeliness of the sudden acquisition of wings, and the huge metabolic demands of growing limbs powerful enough to let loose the shackles of gravity unaided, the real reason we will never evolve flight is the one I gave to the producer. We fly all the time. We have no need to fly unaided. There is no ecological niche that could be filled by aerial people. If by some incomprehensibly reality-defying mutation a child was born with the nascent power of flight, their advantage over us earthbound dawdlers would be negligible, and their freakishness would probably render them an unlikely sexual partner.
This isn’t how evolution works. It’s an ultra-simplified dilution of evolution drawn from comic books, science fiction, and creationists. We see in all the wondrous adaptations of us and all species a fit for purpose. It’s so easy to apply that adaptationist observation that all things are there to suit the things they work best for. We don’t have prominent noses so that we can balance spectacles on them, as Voltaire’s Dr. Pangloss suggested, but as a result of history, time, chance, and sex. Noses and indeed smelling predate humans by hundreds of millions of years, so we carry that ancestral burden; over eons our noses have changed shape via natural variation according to what use they provide, the overall changing shape of our faces, and what we find attractive. The morphology of our noses is indeed underwritten by genes, and it is the changing frequency in a population of different alleles that is the measure of evolution. Nose shape is perhaps a bad example, because I believe people with big and small noses reproduce with equal success (at least, as far as I know; studies have not been done on nose size and reproductive success). The subtler questions of alleles that reduce or enhance reproductive success, or the health that underlies that, are really what’s at stake in the future evolution of our species. “Are we still evolving?” is a question that geneticists get asked a lot. Here is the answer: yes.
Our genomes are where evolution takes place. Our DNA changes over time, every generation. Most of these changes are subtle, many trivial. Some are teasingly interesting. We humans are trichromatic—we see in three colors. In the back of our eyes we have photoreceptors, highly specialized cells whose purpose is to literally capture the photons of light that flood through our pupils. There are two classes commonly known as rods and cones: The rods are attuned to pick up movement and low lighting conditions, and they sit in the periphery of the retina, which is why we see indistinct but moving things out of the corners of our eyes. The cones are central, which is why your sharpest color vision is right in front of your eye. If you wave something in a hand far outstretched to your side and look straight ahead, you can see it move, but not what color it is. Then there are three types of cone, each further attuned to a specific wavelength of light, which determines what colors we see. Broadly, they are short, medium, and long wave, but roughly correspond to blue, green, and red, though they overlap in their range, and are subtly variable between people. The difference between each of these cones is down to a single protein called an opsin. The photon passes through your clear cornea and the nucleus-free cells of the lens, through the jelly aqueous, then vitreous humors, through three layers of brain cells, nerves, and blood vessels, and into the very back of the eye where the opsins sit bound into the pointy tips of the cones.* There, the photons are captured by the opsin molecules, which physically jiggle their shape in response, and that molecular shrug triggers an electrical impulse, which shoots out of the other end of the photoreceptor and through the several layers of nerve cells, which collectively bundle their nerve fibers into the optic nerve, into the visual cortex of the brain, and this is how you see.
Many mammals have only two cone opsins, and so see color with less acuity than us. Most apes have three, as do the Old World monkeys that are indigenous to Africa and Asia. Cats have many more rods and so see in the dark much better than us, but not color. Certain species in the family of the mantis shrimp have at least sixteen opsins, fine-tuned to see red, blue, and green, as well as polarized light, ultraviolet, and a host of light unseen by us that we can only dream about.
The mutations that gave rise to three colors in us (and the many in the shrimp), were not initially the single letter changes that make up most genomic change, but large duplications of whole sections of DNA, and subsequent typos. Colors are determined by the wavelength of the light we see, and the gene for the Shortwave opsin is on chromosome 7, whereas the Medium and Long are on the X. This is why men are more prone to color blindness than women: A faulty opsin on one X can be compensated for by a woman’s second; men have no such insurance. The duplication of one opsin on the X to two at some point in our primate evolution allowed one of them to mutate freely without a loss of function, and thus we were free to acquire a new color sensitivity. That all happened tens of millions of years ago, long before humans, but something similar might be happening now in us—in fact, some of half of us. Some women might be tetrachromatic. They, through another random chance duplication, have acquired a fourth opsin on one of their X chromosomes. Around one in eight women are estimated to have this extra gene variant, but whether that bestows tetrachromacy is not yet known. The ones who do have this power see colors where we see monotones. It’s a new area of research, and the condition appears to be rare, and poorly accounted for. A few women have been studied, and they seem to see clear differences in colors that are merely shades to normal trichromats. When examining red-green color blindness, the Ishihara test presents a circle containing circles in different hues. Hidden in plain sight (to those with typical vision) is a number, but due to the design of the shades that pick out the number, it is invisible to color-blind people. The tetrachromat tests also rest on the ability to discriminate distinct hues of green where we only see olive.
The theories behind why we evolved three-color vision are wide and varied. Many of them suppose that the ability to discriminate the redness of berries in a busy green forest canopy would be of great advantage to our foraging simian ancestors swinging in the trees.
The advantage of the ability to discriminate four colors is a mystery. While many animals have more than our three, tetrachromacy in humans is likely to be recent and random—chance plus time—but not a mutation that has been negatively selected as it is unlikely to cause any phenotypic problem. It simply is—another example of our infinite variation. It’s not likely to spread far and wide, but who knows? Ask me again in 5,000 years.
This is just one example of a mutation that has emerged that has a distinctive effect. Most do nothing of such interesting significance. DNA replication is imperfect, and we have plenty of spellcheckers in our cells. But mutations arise, mostly single changes, occasionally chunkier sections of DNA. Without them, no evolution would occur, for there would be no variation on which selection could act. From an evolutionary point of view, perfection is boring and impractical, and infidelity is essential, at least when it comes to the code. The process of DNA replication has to be imperfect. You have acquired through nothing other than chance at least 100 mutations that are unique to you. If you have children, you may well pass them on to your children, and they will acquire plenty of their own. As long as humans keep having sex and that sex results in more humans, then we are evolving. We can avoid these evolutionary changes no more easily than we can change the weather.
Of course, we have profoundly changed the weather. We farmed the land for 10,000 years and hunted animals to extinction. Our presence on Earth has altered its terrain along with the fauna and flora. Climates changed over epochs and we adapted. Ice ages came and went, and, nowadays, they may not return as a result of our own action. Through modern farming practices and through less than three centuries of continuous industrial revolution we have made the earth considerably warmer. During that time our lives have become unrecognizably healthier. Life expectancy is hypervariable across the world, indeed across Britain, and even decreases measurably by Underground station as you travel from affluent central London out east into the relative poverty of the suburbs. But across the board, life expectancy has increased, on average and in real terms. We have fewer children than at any time in history, and more of them survive. In most cultures, we tend to marry whomever we wish. All these things might point toward us being unshackled from the binds of natural selection. We are free to choose with whom we partner (including, in more and more cultures, members of the same sex), when and if we have children.
In the recent past, though, our farming changed us: in the food we eat, in the genes we carry to process that food, in the milk that we drink, in the northern climes that we moved to. We’ve cleared forests, which brought still water and mosquitoes to breed in it, and we’ve evolved in response to that too, with the protection of sickle trait for carriers, and sickle cell anemia for homozygotes.
The new world of genomics has provided a colossal data set from which we can effectively compare all humans to see the rate of evolutionary change in our DNA, not just looking at individual genes that have popped up and given us new powers or new traits, but across our entire DNA. We use reference genomes, and panels and databases of variants, common and rare. In 2013, Josh Akey and his colleagues from the University of Washington in Seattle compared the genomes of 6,500 people, just looking at individual variations in SNPs. They scanned 15,000 genes, and found 1.15 million SNPs. They applied six different tests to gauge when each of these alleles arose in history, including factoring in that our numbers as a species have increased more than a thousandfold since we started farming. Of these differences, three quarters had arisen in the last 5,000 years. Are we still evolving? The answer again is an undeniable yes: We’re a species not of mutants but of mutations.
Approximately 164,688 of these single changes in our DNA are probably not good news. They’re changes to the DNA that alter proteins subtly, but probably making the proteins less efficient or worse, dysfunctional. According to Akey’s data, 86 percent of these also arose in the last 5,000 years. We are indeed evolving, and we are acquiring new genetic problems.
This is perhaps not surprising given our global creep from Africa. When that happened, some 100,000 years ago, only a few thousand individuals stepped forward to be the progenitors of the rest of the world. They went forth and multiplied. Five thousand years ago there were 5 million or so of us. By 2025 we estimate a global population of 9 billion. They spread in all directions, into Europe, into Asia, across the Bering land bridge, and down all the way through the Americas, east to China and south to India and Oceania.
Those migrants left their genetic traces wherever they went. We’ve seen how farming cultures changed our genomes, we’ve seen how the presence of diseases did the same. Culture has shaped our changing genes. Today, if you go for elective surgery in Hyderabad in India, one of the first questions you will be asked is “Are you a Vaishya?”
Vaishya are one of the four principal castes in India, known as a “merchant caste,” in comparison to the Brahmin religious caste or the Untouchables. That question of social identification sits at the top of the consent form in Hyderabad hospitals. Some assume that the question demonstrates latent social prejudice, which is endemic in India. In fact, it is a smart question rooted in the evolution of the Indian genome. In the 1980s, surgeons began to notice that some patients remained unconscious under anesthetic for several hours longer than they should. Typical general anesthetics are a cocktail of drugs that do specific things to beckon the sandman. By a process of elimination, the doctors identified the cause: A short-lived muscle relaxant called succinylcholine used to suppress the musculature of the lungs so that doctors can feed in a breathing tube without them closing shut in an attempt to prevent invasion. In certain cases the effect lasted for hours, rather than the typical minutes. There were no serious health repercussions, just a prolonged period of artificial ventilation and a weirdly long sleep after a minor operation. On closer inspection, they discovered that this only occurred in Vaishyas. By looking into their genomes,* we learned of a single change—a random switching of a single letter of the gene encoding the enzyme butyrylcholinesterase (BCHE), which normally helps degrade molecules in the blood similar to the anesthetic.
Upon further inspection it turns out that Persian Jews and Inuits are also prone to the same pseudocholinesterase deficiency via a different mutation. According to Indian geneticists in Hyderabad, the mutation had occurred in an unknown person around a millennium ago, and it was preserved within that group of people for evermore by endogamy. The Vaishyas are more than 20 million in number, so it seems unlikely that large-scale inbreeding could have maintained the variant within such an expansive group—even caste is not so watertight as to prevent such genetic leakage. Others have speculated that this particular allele might have conferred some incremental advantage in an unknown condition, like heterozygosity for sickle cell is protective against malaria. Vaishyas and Inuits have particularly fatty diets, ghee and blubber being the sources, respectively; as a result both populations are frequently obese, and maybe this variant has some role in the metabolism of these heavy saturated fats. Traces of selection in and around the butyrylcholinesterase gene haven’t shown any signs of selection so far, nor is there evidence that it has surfed along with neighboring genes that have provided an advantage to the owner. Nevertheless, this version may have simply had no effect for centuries, just a typical SNP in an inconsequential allele. With the advent of modern medicine, this natural variation suddenly became of interest. Now, their anesthetics are tailored to their genomes, evolution molded by the culture of a people. That allele will simply persist now. It has evolved, but may well have been neutral for all but a few years of its existence. Though now it has an effect significant for medicine, it’s not lethal nor does it reduce reproductive fitness, and so is not subject to the pressures of selection.
Caste is a strange evolutionary experiment. It’s a social hierarchy of immense complexity that primarily prevents marriage between individuals from different social groups. Despite modern attempts to ease caste boundaries, and though in urban centers the rules are relaxing, many if not most marriages in India are still arranged. The “would like to meet” and “good sense of humor” biographies in personals ads in newspapers are still filtered by caste. This means that genes and genomes have been—and continue to be—kept largely within discrete groups of people for generations. In countries like the UK, where an historically blurred social structure evolved, intermarriage between social groups is much more common, and interbreeding between the traditional upper and lower classes has occurred with the vigor of Lady Chatterley and her low-born lover. The houses of the elite rise and fall. In India, the social restrictions on caste are more strictly endogamous. For a time, there was a suspicion that the caste system was locked down in India during British colonial rule—that the British enforced and encouraged a preexisting, more informal caste system as a means of social control. With genomics we can make an estimate of the point where a social convention impacted upon the Indian genome. Studies on the genome in 2013 revealed that alleles of caste began to become stratified via endogamy at least 1,900 years ago, a date different from and far more precise than that suggested by the blurred lines of history. DNA says that caste predates colonial India by centuries.
The evolution of the genome is unequivocal. We shape it with culture and with technology. We have shaped it via our voluntary isolation from our African roots and subsequent explosive multiplication. It continues to change every generation. The question is not, “Are we still evolving?” It is, “Are we still under the spell of natural selection?”
That is a harder question to answer. Much of its inherent difficulty lies in the word natural. Nothing of our lives could be considered natural in any meaningful sense. We have altered every conceivable aspect of the world in which we have lived for tens of thousands of years, and in controlling (or at least attempting to manipulate) that environment we’ve profoundly changed the grip with which evolution would have tenaciously selected the differences between us. Food and sex are the two key resources that propel our genes into the future, but we invented almost all our own foods via that ancient form of genetic engineering better known as farming, and nowadays we eat what we want. We sleep with who we can, and almost never for the production of small humans. Sanitation, housing, medicine, and wealth have inoculated and insulated us from the pressures that historically would’ve cut through populations. Most women now live to an age when they can reproduce, and most do. On the whole, we have as many children as we wish to have, not as many as possible to maximize the chances of our genes surviving the assault of existence and time. In the nineteenth century, women in the UK averaged 5.5 children, but by the end of the First World War it was down to 2.4.
These are two key factors in assessing whether our ongoing evolution is undergoing a form of selection: the death of babies and the number of babies women have. Scandinavia is the territory from which we have the most data: In Sweden in the late eighteenth century the child mortality rate hovered around one in three. Today it’s three in a thousand. This difference is part of the changing face of humankind’s relationship with evolution. This release that we have invented from the harshness of nature means that evolution with selection is likely to have radically slowed, though not stopped. The number of children who die is a driver of evolutionary change, as their genes will not continue, and the frequency of alleles that survive are much more likely to propagate down the generations and into populations. By reducing that number via vastly improved medicine and public health, and contraception, the purchase of selection on humans is reduced.
The potential balance comes from choice. The period during which women can bear children has increased, as has lifespan, but the time during which they do has changed and continues to do so. As long as there is variability in the number of children that women bear, and as long as there is variability in the number of babies that survive themselves, then there is the potential for selection to act. These are questions only recently asked, and data that has not been collected for long. Infant mortality is highly variable through history, and though it is generally falling, is still highly variable all over the world. The death rate of children in rich countries has decreased radically, and the data over the last century is precise. According to the United Nations, the infant mortality rate today for many wealthy countries (Singapore, Japan, all of Scandinavia, much of Europe) hovers around two or three deaths per 1,000 births, defined by death of children before their first birthday. The UK is 4.19 and the USA 5.97. The bottom ten are all in Africa, between 70 and 90 deaths per 1,000. In 1950, Scandinavia was still top, but the number of deaths was around 20 per 1,000 for Sweden, Norway, and Iceland. Only six of the bottom ten were in Africa. But their numbers were around one in four (250 per 1,000).
All this inequity makes painting a global picture of our species’ evolution near impossible. We are too widespread and inequality reigns supreme. Collecting the numbers on infant mortality plays an essential part, but when what we are really interested in are the changing frequencies in bits of DNA, there’s just not enough data. Only in the last few decades have we had the foresight (and funding) to do scientifically what family trees have done for thousands of years—trace people through time, generation after generation.
A long-standing study of the population of the pretty Massachusetts town of Framingham has been running since 1948, initially just collecting data relating to heart health. But tracking more than 14,000 people over time is a seam of such potential riches that over a thousand papers have been published from doctors and scientists observing the people of a town for fifty years. What they see is a fluctuation in the number of children that women bear, and this relates to culture. In the 1950s and ’60s, when smoking, exercise, and fatty foods were of less concern to us, the Framingham women had, on average, fewer children than in the 1990s. For evolution to proceed, the traits need to be heritable. While smoking and high cholesterol levels have heritable components, this is not the potential engine for this possible evolution. It is simply that by living healthier lives, the women of Framingham were having more children, and thus passing on more genes into the future. They started, on average, having children earlier and finished, on average, later, meaning that they could squeeze out another baby, on average. They also found that, on average, slightly plumper and shorter women had more children. If the trend were to continue, unfettered by any change in the local cultural environment, then by 2400, the women of Framingham would be around an inch shorter and two pounds heavier, on average. That sort of slow demographic shift is typical for evolution more broadly, but not exactly revolutionary. It could also be wiped out by a sudden shift in environmental factors—a change in school meal policy, or something as seemingly trivial as a factory closing and a migration out of town. Traits never evolve in isolation from the environment or each other.
The Finns have scrutinized their own evolution most recently in the most depth. In 2015, scientists compiled data from 15 generations and 300 years of genealogical records—around 10,000 people. Three centuries covers a shift from an agricultural and fishing culture to an industrialized nation, and all the cultural mores that come with those revolutions. The number of Finns in the eighteenth century was merely 450,000, recovering from being a ransacked battleground for the victorious Russians to the east and the Swedes to the west. The Russians came and went, and came again, and the Swedish did the same. Sporadic peace and prosperity resulted in the population doubling by the nineteenth century, and by 2010 it was more than 5 million. Their birth rate had dropped from 5 in the mid-nineteenth century, to 1.6 per family today, but the survival rate more than compensated. Two thirds of Finnish babies survived in the 1860s, but by the time of the Second World War, that number was greater than 94 percent.
Elisabeth Bolund and her colleagues pored over church records and reconstructed the most comprehensive family tree in Finnish history. They disentangled meaningful data on lifespan, number of children, and age of the mother at the time of her first and last child, and by looking at how these numbers panned out in families over time, they could extract what proportion was due to genes, and what to the circumstances in which they were operating. Bolund found that between 4 and 18 percent of the variation in the birth data could be attributed to DNA. But they also showed that its influence has increased over time. It may be that with increased access to healthcare and sanitation, the effect of the environmental inequity is reduced, and so the impact of genes rises, at least in the trends of births and pregnancy. This, in principle, gives more purchase to Darwinian selection.
Whether this effect is felt elsewhere or worldwide is unknown. But the question of current human evolution relies heavily on two factors. The first is the question, “Is variation heritable?” To this the answer is yes. We see it in specific genes, and we see it accounted for in complex behaviors for which the specifics of DNA are not necessarily known. The second question is, “Does the survival of babies vary?” The answer to that is also very measurably yes.
The timescales in these studies are minuscule, and the changes small. The chasms between infant mortality rates around the globe means that these evolutions in affluent populations like Finland and Framingham cannot be extrapolated nor generalized, and while we see slightly similar results in small studies from the Gambia and elsewhere, the data is far from comprehensive. A few generations is a timescale in the evolutionary shallows, and we can only dip our toes in the observable data. But we are evolving, our genomes are changing and though the pressures of selection have been radically altered, humanity’s grip on modernity has not removed them altogether. We are not creatures fixed in time, we remain a species begotten not created. As long as there is difference, we will always be a species in transition.
The history of everyone who has ever lived is buried in DNA in us or in the ground, but it’s tough making predictions, especially about the future. If this all sounds a bit inconclusive as this book draws to its end, that, happily, is the nature of science.
Ignorance more frequently begets confidence than does knowledge . . .
says Darwin in The Descent of Man, his study of the evolution of humans from our hairier ancestors. It’s no secret that arguing with creationists is a waste of time, for they see things differently from most. They know that what they think is true, and in science we must assume we are wrong. We know that we doubt everything we know is correct. When it becomes harder and harder to find things wrong with your ideas and experiments, then it’s probably a good sign you’re on the right track. That is science’s eternal trump card. Christians sometimes say that they perpetually doubt their own faith, but it’s a different type of doubt from the scientist’s. Christians appear to doubt under the assumption that their assumptions will be affirmed. Scientists doubt with the explicit plan that their results will be overturned. Most Christians I know also think Darwinian evolution is the best description of how things are the way they are. Nevertheless, creationism does exist as a fringe, yet vocal, branch of Christianity, frothing with risible fallacies. There is one point worth addressing in the impotent arsenal of zombie arguments* presented by creationist dolts that has some relevance here. They claim endlessly that there are no transitional fossils, that there are no examples of one species changing into another, that a transitional eye is of zero use, and our vision is perfect as it is, and could not have come to be in incremental stages.
There are none so blind as those who will not see. In fact, the fossil record is replete with transitional forms. Any particular characteristic that one chooses to name has varied forms embedded in stone. Not only do we see eyes in myriad forms in fossils that show subtle, slight shifts toward our own versions, we see every stage in living creatures too, from the photoreceptive patch in the simple, single-celled Euglena, all the way to us and the many creatures with sight far superior to our own. In fossils, the picture is far from complete, because fossils are rare and unlikely. But they are most definitely there, and we can with the most desultory glance see dozens of small moves from one to another—an arm to a wing, a photoreceptive cell to an eye, a fin to a foot, an oxygen-sucking spiracle to a lung. All these incremental changes over epochs are subject to selection, and all are encoded in DNA.
The truth is that as long as you reproduce, from a genomic point of view, you are transitional. Your DNA is a perfect transition between your parents’ and your children’s. If we had DNA from everyone who ever lived, and indeed every creature that ever lived, we could draw an impossibly colossal pedigree that charts every transition from cell to cell, parent to child, from species to species—every single colored pixel on an inconceivably gargantuan pageant of a color chart. We can’t do that, unfortunately, so we use what genomic data we have, and compare living species and some dead to calculate the evolutionary distances traveled, and fit this new information in with the rest of the jigsaw: fossils, geology, statistics, mathematics.
It is in the accumulation of genetic change over time that new species are formed. Though the boundaries are smeary, as exemplified by the repeatedly violated boundary between Homo sapiens and Homo neanderthalensis (as argued in Chapter 1), over time, and with the unique pressures experienced by individual creatures, enough changes will be acquired and rooted in a population that they speciate. Their genes become so different that they are incompatible—the mechanics of sex, either physically or biochemically (or often both), are different enough that they are no longer capable of reproducing. This version of evolution is a real, testable fact of biology, seen in living species and in the dead. But conclusively, we have observed this in plenty of species in real time, since Darwin. The apple maggot fly is becoming two separate species as a result of generations feeding on different trees that fruit at different times. Since the introduction of apples in the United States in the early 1850s, some began feeding on apples instead of the hawthorn fruit. Now the two preferences have diverged to the extent that though they live in close proximity, apple feeders will not mate with hawthorn eaters. The European blackcap is a migrating bird, and most of them winter in Spain, but a few do so in the UK. Since the 1960s, when garden bird feeding became more popular, some blackcaps come to the UK, which is closer to their summer residence in Germany. They arrive before the Spanish migrants, and so get first dibs on mating, and now they are beginning to look like different species of bird.
All these birds and insects and we ourselves are transitional as we shift subtly and often cryptically over generational time. The creationists say there is real microevolution—the changes within a species—but there is no macroevolution—changes from one species to another. Biologists don’t make the distinction. They are the same process over a long enough timescale, with the right conditions present. Unlike the blackcap, we, for now, are firmly within the microevolutionary mode. There is no prospect of us speciating; we’re too similar, and too widespread, and we interbreed too much and too slowly. But over a long enough period, all species become something else living, or become dead. This is the continuous fact of life on Earth.
Darwin’s theory of evolution by natural selection is just that—a theory. Outside science, there is some understandable confusion about what that word means. In science, a theory is the best description we have. Unlike the common usage, it’s not a guess, or a hunch, or a hypothesis. It’s the most complete subjective picture of the living world that we have. It’s not truth, because that is the realm of math, religion, and philosophy. In science we simply lean in toward truth, every step inching us closer to the way things actually are, rather than how we perceive them to be, or would like them to be.
Darwinian evolution is a theory without peer. It doesn’t have to compete with other theories, because it’s the only game in town. There is no other scientific description of life on Earth that is supported by what we observe and what we test. Charles Darwin formulated his idea 50 years before genes, 100 before the double helix, and 150 before the human genome was read. But they all say the same thing. Life is a chemical reaction. Life is derived from what came before. Life is imperfect copying. Life is the accumulation and refinement of information embedded in DNA. Natural selection explains how, once it had started, life evolved on Earth. We busy ourselves refining the theory, and working out the details with a scrutiny and precision that has been enabled and invigorated by reading genome after genome, and crunching those numbers until comprehensible patterns emerge. We are the data.
Are humans still subject to an evolution by the forces of selection? Yes we are, though its grasp on us is weakened and slowed compared to every other species in our 4-billion-year family tree. We are animals: We are special. Are we still evolving? The answer is unequivocal. Evolution is change plus time. We’ve seen it in our deep and recent past, sometimes demonstrably the result of positive natural selection, and often just an unopposed drift through time. An unchanging species is already extinct. As long as we keep making new ones, human beings most beautiful and most wonderful have been, and are being, evolved.