The existence of transposable elements may seem like a real failure of evolution; after all, shouldn’t natural selection eliminate harmful genetic material like this? But the thing to keep in mind is that evolution often operates not just at the level of the individual but also at the level of the gene or even a small stretch of noncoding DNA. Yes, random mutations may be disadvantageous for an individual and yet still persist through the power of their own self-copying. This was Richard Dawkins’s big insight described in his book The Selfish Gene. If a small bit of DNA such as Alu can act to promote its own duplication and proliferation, it will be favored by natural selection regardless of whether it harms the animal host unless it hurts the animal host so much that the host dies before it can reproduce, which of course does sometimes happen. But in Alu’s case, this little piece of genetic code has proven itself so proficient at reproducing that it can more than withstand the occasional host death due to the element’s overdoing it.
If we add up all the various Alu sequences—more than a million copies spread throughout your DNA—this one particular molecular parasite makes up more than 10 percent of the total human genome. And that’s just Alu. If you add up all the TE insertions, it comes to about 45 percent of the human genome. Nearly half of human DNA is made of autonomously replicating, highly repetitive, dangerously jumping, pure genetic nonsense that the body dutifully copies and maintains in each one of its billions of cells.
Coda: Color Me Lucky
As you’ll see again and again in this book, certain flaws are built into nature. They are not bugs in the system; they are features (so to speak). Thus, the fact that each of us is carrying and reproducing one million useless Alu sequences in our DNA is certainly an oddity, but at this point, it is also an inherent quality of our bodies. And like some other flukes that have become features, Alu has resulted in some very rare and totally unexpected benefits.
The way that Alu has helped us is in its tendency to create mutations, those almost always damaging but occasionally helpful changes to DNA. By jumping around the genome, Alu raises the mutation rate of the organisms that have it, and it can even occasionally cause chromosomes to break in half. While that sounds awful, because mutations and damage to chromosomes are almost always bad for the individuals in which they occur, it can actually be beneficial over the long term. This is because a lineage of animals with high mutation rates will be more adaptable and thus more genetically malleable over long periods (assuming they don’t go extinct because of all those mutations).
Although it’s cold comfort to the individuals that suffer and die from harmful mutations caused by Alu, the rare emergence of a helpful mutation can change the course of evolution in dramatic ways. We have to take a very long view to appreciate this, but rare beneficial mutations provide the raw material with which natural selection produces new adaptations. The most famous example of this is the mutation that led to our species’ excellent color vision.
Around thirty million years ago, a random Alu insertion occurred in an ancestor of all Old World monkeys and apes (including humans) that would allow a subsequent improvement in the ability to see a rich variety of colors. In our retinas are structures called cones that specialize in the detection of specific wavelengths of light—in other words, colors. These cones have proteins called opsins that respond to different colors, and, prior to thirty million years ago, our ancestors had two versions of the opsin proteins, each responding to a different color. Then a happy genetic accident occurred due to something we call gene duplication.
Simply put, an Alu element, doing its normal business of crashing around the genome, popped into a chromosome very near one of the opsin genes. It copied itself and popped out, but it had inadvertently copied the opsin gene, complete and intact, and it brought the copy along for the ride. When this newly copied Alu element popped back in the genome somewhere else, it brought the copy of the opsin gene with it. And voilà, this lucky monkey went from having two versions of the opsin gene to having three. This is called gene duplication.
Gene duplication is normal behavior for Alu—that’s why we have so many duplicates of it—but it’s nearly miraculous that the tagalong opsin gene was perfectly copied and reinserted in the process. At first, the extra gene would have been identical to the one it was copied from. However, once this species had three opsins instead of two, the three genes were free to mutate and evolve separately. Following some refinement through mutation and natural selection, these ancient monkeys had three types of color-sensing cones in their retinas instead of just the original two. All descendants of these monkeys, including us, have three different types of cones, a trait called trichromacy.
Trichromacy is a vaunted attribute among animals because having three cones instead of just two allows the retina to see a broader spectrum of colors. Apes and Old World monkeys can see and appreciate a much richer color palette than can dogs, cats, and our more distant cousins the New World monkeys. This enhanced color detection served our ancestors very well in their rainforest habitats. Because their GULO gene had been broken millions of years before, finding fruit was very important to these monkeys and apes, and having much enhanced color vision is a big help in the hunt for ripe fruit in a dense forest. And here’s the kicker: We owe our superior eyesight to a mutation caused by a roaming Alu element.
The duplication of the opsin gene and the resulting trichromacy occurred through a series of extremely improbable events, but that’s evolution for you. Crazy stuff happens. Most of it is bad—but when it’s good, it’s really good.
4
Homo sterilis
Why humans, unlike other animals, can’t easily tell when females are ovulating and thus when the time is right to conceive a child; why human sperm cells cannot turn left; why, of all the primates, humans have the lowest fertility rate and the highest mortality rate for infants and mothers; why our enormous skulls force us to be born way before we are ready; and more
One of the preconditions of evolution, perhaps the most important, is that a species must be able to reproduce—a lot.
This is because life in nature is a constant struggle. In all species except us (thanks to modern medicine), most individuals that are born will not live to sire offspring of their own. This was one of Darwin’s key insights. He noticed that all organisms seem to reproduce constantly and in great numbers and yet their populations remained pretty much the same size. This meant that life was a challenge that most individuals failed.
The only way a species has any chance to survive and compete is by making a lot of babies. Some make fewer babies than others but care for them better, while others make tons of offspring but don’t care for them at all. But for all species, prolific reproduction is a key goal of an individual’s life, if not the key goal. We all have an inborn drive to make more of ourselves. It’s the only way a species survives.
Of course, living creatures, even humans, don’t really think about reproduction in those goal-driven terms. We want our offspring to survive because of a deep-seated, instinctive parental urge, not because of a conscious desire to preserve our genes for posterity. But the fact remains: we are hardwired to want to pass on our genes.
There is only one way that living things can secure their genetic legacies. They must be sure that at least one or two of their offspring will survive, thrive, and have offspring of their own. It is practically guaranteed that many offspring will die; if a predator or a rival doesn’t get them, an infectious disease will. The intensity of natural selection has thus given all animals an extreme drive to reproduce.
Given that humans have successfully outcompeted every other species on the planet, you might think that we’ve mastered this whole reproduction thing. But in fact, human reproduction is inefficient. Extremely inefficient. We are some of the most inefficient reproducers in the animal world because we have errors and flaws throughout almost the entire reproductive process, from the production of sperm and eggs to the survival of our children. I describe
this as inefficient because a breeding pair of two mammals ought to be able to produce way more offspring than humans do, and most other mammals do a better job at it. If you started with two fertile cats, in a year or two, you could have hundreds. If you start with two humans, after a couple of years, you might have one more. Yes, humans take longer to gestate and mature, but that’s not the only limitation in play, as we’ll soon see.
Human reproductive inefficiency is far out of step with the reproductive abilities of other mammals, including our closest relatives. Strangely, there are very few explanations as to why this is. For some aspects of our reproductive difficulties, we understand the cause, but for most, we don’t. Human beings are riddled with fertility problems.
It’s hard to believe that we’re such inefficient baby-makers, given that the world’s human population now exceeds seven billion. But in a way, our shortcomings in this department make our terrific evolutionary success all the more impressive.
Infertile Myrtle
It may be tempting to blame our reproductive inefficiency on one big problem; for instance, on our huge brains, which require huge skulls, which makes childbirth perilous for mother and infant alike. But it is not that simple. The entire reproductive process—from the production of sperm and eggs to the survival of infants—is plagued with problems that highlight a wide range of design defects in the human reproductive system. In practically every part of that system, human beings have more faulty biology than any other mammal we know of. Something is seriously wrong with us in this regard.
You could argue that these inefficiencies are somehow adaptive; perhaps they serve a purpose, such as controlling population growth. Although I will discuss this possibility shortly, it is worth noting now that, if this were indeed the case, it would be a pretty dismal compromise. Other species have achieved the same end through much more elegant means. For instance, helper wolves forgo their own reproduction and instead care for their kin, but there is nothing amiss in their bodies; their reproductive anatomy is just fine. The social structure is such that some wolves choose to remain celibate—a choice that can be reversed if the alpha wolf in their group dies or is vanquished.
Not so with humans. For many individuals, infertility is not a choice, and often it is not reversible without the aid of medical advances, most of them very recent. Besides being biologically incongruent, moreover, any comparison with helper wolves—or worker bees, or drone ants, or other creatures that sacrifice their own reproductive chances for the good of their group—is downright cruel to anyone who has experienced the frustration and pain of infertility. And these people number in the millions. A staggering percentage of humans experience reproductive difficulty, either for extended periods or permanently.
This reality is even more staggering when we consider that infertility can run in families—take a moment to absorb the bitter irony—and that there is usually no outward or inward symptom. Worker bees and helper wolves know their roles and the lack of reproduction that goes with it, and so do their peers. Humans, by contrast, almost never have any idea that they have fertility issues until they try to conceive.
We all know someone who has had trouble reproducing for one reason or another. Estimates vary based on geographic location and precisely how the term infertility is defined, but most studies report that somewhere between 7 to 12 percent of couples trying to conceive have faced persistent difficulties. Fertility problems are equally common in women and men, and in around 25 percent of cases, both partners find that they have reproductive problems.
As many sufferers know, fertility problems have a unique and disproportionate effect on mental health. There are hundreds of diseases and afflictions that are far more physically debilitating but do not cause as much emotional anguish. Even the thought of not being able to have children strikes many people someplace deep in their souls. Most humans are driven to reproduce, and when they fail, it cuts them to the marrow, crushing their spirits and self-confidence—this despite the fact that not even the most heartless among us could possibly assign victims of infertility blame for their condition.
For all the stigma and shame attached to infertility, we’ve all been infertile at one point in our lives. I’m talking, of course, about the infertility that humans experience before they reach sexual maturity—a fallow period that you might not think of as infertility, per se, but that nevertheless has a very similar effect as adult fertility when it comes to the reproduction of the species.
First of all, humans mature rather late compared with most other mammals, even compared with our closest relatives. Humans mature, on average, two to three years later than chimps and four to five years later than bonobos and gorillas. Of course, there are good reasons for this. Given the large size of a human baby’s head, it is important that the pelvis of the female be large enough to accommodate it during birth. If a woman is of small stature, her chance of dying in childbirth is extremely high—even higher than it already is. (More on that later.) This doesn’t explain the lateness of male puberty, which is later even than female puberty, but this is largely without effect in terms of the reproductive capacity of the species. Males and their sperm are never the limiting factor for reproduction of a species even if many or most males happened to be infertile.
The lateness of human female puberty compared with that of other primates results in reduced reproductive efficiency for the species. This is because delayed fertility brings a greater chance that a female will simply not live long enough to reproduce. Remember that as humans were living and dying in the Pleistocene epoch, and even through the Stone Age and the early modern period, life in the wild involved a whole lot of sudden and tragic deaths. This means, in effect, that every year a female was not reproducing increased her chance of dying without leaving any offspring. While this is not such a big deal today, it would have presented a significant challenge to our species for much of its existence. Until the advent of modern medicine, human mortality rates were quite high throughout their lifespans, not just at the far end of it as they are now. For most of our history, many humans died young—and therefore died childless.
The age of sexual maturity is thus the first limiting factor in reproductive capacity, a phenomenon that is true in all species, not just humans. For example, when officials are considering which threatened or endangered species are most in need of regulatory protections, the age of reproductive maturity is a crucial factor. Bluefin tuna, for example, are often cited as an example of a fish species that is in need of protection, not just because of decades of overfishing but because the females do not reach sexual maturity until twenty years of age. This means that a population devastated by overfishing will be very slow to rebound.
But even beyond the extended prereproductive years of the human lifespan, even after humans reach sexual maturity, they often have trouble producing high-quality sperm or eggs, the all-important vehicles of genetic transmission.
Let’s start with men. A 2002 study by the CDC found that around 7.5 percent of men under the age of forty-five had visited fertility doctors. While the majority of those were diagnosed as “normal,” meaning nothing obviously wrong was found, around 20 percent of them had substandard sperm or semen, making reproduction by the old-fashioned route highly improbable or impossible.
Normally, sperm are amazing little swimmers. Although they’re among the tiniest of human cells, they are easily the fastest. After being propelled into a vagina, a sperm must swim around 17.5 centimeters to reach the egg. The sperm cell itself is only around .0055 centimeters (55 micrometers); this is quite a long way, more than three thousand times the length of its body. That’s like a human running over thirty kilometers, almost nineteen miles. Even more impressive, sperm swim at around 1.4 millimeters per second, which would be like a human running twenty-five miles per hour—a speed that would allow an individual to cover those nineteen miles in around forty-five minutes. When you consider that Usain Bolt, the world’s fastest runner, can reach speeds like
this for only a few hundred meters at a time, that feat becomes all the more impressive.
However, men’s sperm take much longer than forty-five minutes to travel the distance from the vagina to the fallopian tube. This is because they waste a lot of time swimming around in random directions.
Human sperm cells, you see, can’t turn left. This is due to the corkscrew nature of their propulsion system; rather than snapping their whiplike tails back and forth and side to side, sperm cells rotate their tails around in a corkscrew motion, like the way you would move your index finger to draw a circle in the air. Because most sperm whip their tails in a right-handed spiral, the rotation pushes them forward and toward the right and they end up swimming in ever-widening circles. This means that it can take as long as three days to reach the egg waiting in the fallopian tube to be fertilized. Very few of the original number get anywhere near their goal. This is one reason why human males produce sperm in such large numbers. You need about two hundred million of them to start with in order to get just one to its destination.
Sperm cells use corkscrew-like locomotion and thus tend to swim in right-hand circles with a random overall trajectory. For this reason, sperm cells actually traverse an extremely long path just to travel the very short distance to the fallopian tubes.
Low sperm count is the most common fertility problem in men. Somewhere between 1 and 2 percent of men suffer from it. These men produce “only” one hundred million (or fewer) sperm per ejaculation. Because the volume of ejaculate varies widely, sperm count is generally measured as sperm per milliliter. While medical professionals do not always agree on what constitutes a healthy sperm count, the average is around twenty-five million per milliliter. Below fifteen million is considered low, and below five million is considered very low; conception by standard means is very unlikely for men who have sperm counts in this range. In some cases, the problem is hormonal or anatomical, and sometimes a combination of medicine and lifestyle or diet changes can restore a healthy sperm count. In the vast majority, however, the best that can be done is a modest increase in sperm count through diet and lifestyle changes.
Human Errors Page 10
