The Violinist's Thumb: And Other Lost Tales of Love, War, and Genius, as Written by Our Genetic Code (25 page)

Ivanov also knew that quite distinct species can reproduce together. He himself had blended antelopes with cows, guinea pigs with rabbits, and zebras with donkeys. Besides amusing the tsar and his minions (very important), this work proved that animals whose lines had diverged even millions of years ago could still have children, and later experiments by other scientists provided further proof. Pretty much any fantasy you’ve got—lions with tigers, sheep with goats, dolphins with killer whales—scientists have fulfilled it somewhere. True, some of these hybrids were and are sterile, genetic dead ends. But only some: biologists find many bizarre couplings in the wild, and of the more than three hundred mammalian species that “outbreed” naturally, fully one-third produce fertile children. Ivanov fervently believed in crossbreeding, and after he sprinkled some good old Marxist materialism into his calculations—which denied human beings anything as gauche as a soul that might not condescend to commingle with chimps—then his humanzee experiments seemed, well, doable.

A modern zonkey—a zebra-donkey mix. Ilya Ivanov created zonkeys (which he called “zeedonks”) and many other genetic hybrids before pursuing humanzees. (Tracy N. Brandon)

Scientists don’t know even today whether humanzees, however icky and unlikely, are at least possible. Human sperm can pierce the outer layer of some primate eggs in the lab, the first step in fertilization, and human and chimpanzee chromosomes look much the same on a macro scale. Heck, human DNA and chimp DNA even enjoy each other’s company. If you prepare a solution with both DNAs and heat it up until the double strands unwind, human DNA has no problem embracing chimp DNA and zipping back up with it when things cool down. They’re that similar.
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What’s more, a few primate geneticists think that our ancestors resorted to breeding with chimps long
after
we’d split away to become a separate species. And according to their controversial but persistent theory, we copulated with chimps far longer
than most of us are comfortable thinking about, for a million years. If true, our eventual divergence from the chimp line was a complicated and messy breakup, but not inevitable. Had things gone another way, our sexual proclivities might well have rubbed the human line right out of existence.

The theory goes like this. Seven million years ago some unknown event (maybe an earthquake opened a rift; maybe half the group got lost looking for food one afternoon; maybe a bitter butter battle broke out) split a small population of primates. And with every generation they remained apart, these two separate groups of chimp-human ancestors would have accumulated mutations that gave them unique characteristics. So far, this is standard biology. More unusually, though, imagine that the two groups reunited some time later. Again, the reason is impossible to guess; maybe an ice age wiped out most of their habitats and squeezed them together into small woodland refugia. Regardless, we don’t need to propose any outlandish, Marquis de Sade motivations for what happened next. If lonely or low in numbers, the protohumans might eagerly—despite having forsworn the comforts of protochimps for a million years—have welcomed them back into their beds (so to speak) when the groups reunited. A million years may seem like forever, but the two protos would have been less distinct genetically than many interbreeding species today. So while this interbreeding might have produced some primate “mules,” it might have produced fertile hybrids as well.

Therein lay the danger for protohumans. Scientists know of at least one case in primate history, with macaques, when two long-separate species began mating again and melded back into one, eliminating any special differences between them. Our interbreeding with chimps was no weekend fling or dalliance; it was long and involved. And if our ancestors had said what the hell and settled down with protochimpanzees permanently, our unique genes could have drowned in the general gene pool in
the same way. Not to sound all eugenicky, but we would have humped ourselves right out of existence.

Of course, this all assumes that chimps and humans did revert to sleeping together after an initial split. So what’s the evidence for this charge? Most of it lies on our (wait for it) sex chromosomes, especially the X. But it’s a subtle case.

When female hybrids have fertility trouble, the flaw usually traces back to their having one X from one species, one X from another. For whatever reason, reproduction just doesn’t go as smoothly with a mismatch. Mismatched sex chromosomes hit males even harder: an X and a Y from different species almost always leave them shooting blanks. But infertility among women is a bigger threat to group survival. A few fertile males can still impregnate loads of females, but no gang of fertile males can make up for low female fecundity, because females can have children only so quickly.

Nature’s solution here is genocide. That is, gene-o-cide: nature will eliminate any potential mismatches among the interbreeders by eradicating the X chromosome of one species. It doesn’t matter which, but one has to go. It’s a war of attrition, really. Depending on the messy details of how many protochimps and protohumans interbred, and then whom exactly the first generation of hybrids reproduced with, and then their differential birthrates and mortality—depending on all that, one species’ X chromosomes probably appeared in higher numbers initially in the gene pool. And in the subsequent generations, the X with the numbers advantage would slowly strangle the other one, because anyone with similar Xs would outbreed the half-breeds.

Notice there’s no comparable pressure to eliminate nonsex chromosomes. Those chromosomes don’t mind being paired with chromosomes from the other species. (Or if they do mind, their quarrel likely won’t interfere with making babies, which is
what counts to DNA.) As a result, the hybrids and their descendants could have been full of mismatched nonsex chromosomes and survived just fine.

Scientists realized in 2006 that this difference between sex and nonsex chromosomes might explain a funny characteristic of human DNA. After the initial split between their lines, protochimps and protohumans should have started down different paths and accumulated different mutations on each chromosome. And they did, mostly. But when scientists look at chimps and humans today, their Xs look more uniform than other chromosomes. The DNA clock on X got reset, it seems; it retained its girlish looks.

We hear the statistic sometimes that we have 99 percent of our DNA coding region in common with chimps, but that’s an average, overall measure. It obscures the fact that human and chimp Xs, a crucial chromosome for Ivanov’s work, look even more identical up and down the line. One parsimonious way to explain this similarity is interbreeding and the war of attrition that would probably have eliminated one type of X. In fact, that’s why scientists developed the theory about protohuman and protochimp mating in the first place. Even they admit it sounds a little batty, but they couldn’t puzzle out another way to explain why human and chimp X chromosomes have less variety than other chromosomes.

Fittingly, however (given the battle of the sexes), research related to Y chromosomes may contradict the X-rated evidence for human-chimp interbreeding. Again, scientists once believed that Y—which has undergone massive shrinkage over the past 300 million years, down to a chromosomal stub today—would one day disappear as it continued to shed genes. It was considered an evolutionary vestige. But in truth Y has evolved rapidly even in the few million years since humans swore off chimpanzees (or vice versa). Y houses the genes to make sperm, and
sperm production is an area of fierce competition in wanton species. Many different protogents would have had sex with each protolady, so one gent’s sperm constantly had to wrestle with another gent’s inside her vagina. (Not appetizing, but true.) One evolutionary strategy to secure an advantage here is to produce loads and loads of sperm each time you ejaculate. Doing so of course requires copying and pasting lots of DNA, because each sperm needs its own genetic payload. And the more copying that takes place, the more mutations that occur. It’s a numbers game.

However, these inevitable copying mistakes plague the X chromosome less than any other chromosome because of our reproductive biology. Just like making sperm, making an egg requires copying and pasting lots of DNA. A female has equal numbers of every chromosome: two chromosome ones, two chromosome twos, and so on, as well as two Xs. So during the production of eggs, each chromosome, including the Xs, gets copied equally often. Males also have two copies of chromosomes one through twenty-two. But instead of two Xs, they have one X and one Y. During the production of sperm, then, the X gets copied less often compared to other chromosomes. And because it gets copied less often, it picks up fewer mutations. That mutation gap between X and other chromosomes widens even more when—because of Y chromosome–fueled sperm competition—males begin churning out loads of sperm. Therefore, some biologists argue, the seeming lack of mutations on X when comparing chimps and humans might not involve an elaborate and illicit sexual history. It might result from our basic biology, since X should always have fewer mutations.
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Regardless of who’s right, work along these lines has undermined the old view of the Y as a misfit of the mammal genome; it’s quite sophisticated in its narrow way. But for humans, it’s hard to say if the revisionary history is for the better. The pressure to develop virile sperm is much higher in chimps than
humans because male chimps have more sex with different partners. In response, evolution has remade the chimp Y thoroughly top to bottom. So thoroughly, in fact, that—contrary to what most men probably want to believe—chimps have pretty much left us guys in the dust evolutionarily. Chimps simply have tougher, smarter swimmers with a better sense of direction, and the human Y looks obsolete in comparison.

But that’s DNA for you—humbling. As one Y-chromosome specialist comments, “When we sequenced the chimp genome, people thought we’d understand why we have language and write poetry. But one of the most dramatic differences turns out to be sperm production.”

Talking about Ivanov’s experiments in terms of DNA is a little anachronistic. But scientists in his day did know that chromosomes shuttled genetic information down through the generations and that chromosomes from mothers and fathers had to be compatible, especially in numbers. And based on the preponderance of evidence, Ivanov decided that chimps and humans had similar enough biologies to push forward.

After securing funds, Ivanov arranged through a colleague in Paris to work at a primate research station in colonial French Guinea (modern Guinea). The conditions at the station were deplorable: the chimps lived in cages exposed to the weather, and half of the seven hundred chimps that local poachers had netted and dragged back had died of disease or neglect. Nevertheless Ivanov enlisted his son (another triple
i:
Ilya Ilich Ivanov) and began chugging thousands of nautical miles back and forth between Russia, Africa, and Paris. The Ivanovs eventually arrived in sultry Guinea, ready to begin experiments, in November 1926.

Because the captives were sexually immature, too young to conceive, Ivanov wasted months furtively checking their pubic
fur each day for menstrual blood. Meanwhile fresh inmates kept piling in, right up until Valentine’s Day 1927. Ivanov had to keep his work secret to avoid angry questions from the Guineans, who had strong taboos against humans mating with primates, based on local myths about hybrid monsters. But finally, on February 28, two chimps named Babette and Syvette had their periods. At eight o’clock the next morning, after visiting an anonymous local donor, Ivanov and son approached their cages armed with a syringe of sperm. They were also armed with two Browning pistols: Ivanov Jr. had been bitten and hospitalized a few days before. The Ivanovs didn’t end up needing the Brownings, but only because they more or less raped the chimps, binding Babette and Syvette in nets. The virgin chimps thrashed about anyway, and Ivanov managed to jam the syringe only into their vaginas, not into their uteruses, the preferred spot for deposits. Not surprisingly, this experiment failed, and Babette and Syvette got their periods again weeks later. Many juvenile chimps died of dysentery in the following months, and Ivanov managed to inseminate just one more female that spring (she was drugged). This attempt also failed, meaning Ivanov had no living humanzees to haul back to the Soviet Union to win more funding.