OF ALL THE HYBRIDS that are theoretically possible, none shocks the mind like a cross between humans and apes, and there has been at least one attempt to create a humanzee. In 1910, Ilya Ivanovich Ivanov, a Russian pioneer in artificial insemination, proposed seeding a female chimpanzee with human sperm. At a zoology conference in Austria, he noted (oh-so quaintly) that this method would avoid the ethical dilemma of forcing the two species to actually have sex. Sixteen years later, with the backing of the Soviet Academy of Sciences, Ivanov traveled to Africa and gave it a try.

The details of Ivanov's experiments only came to light in 2002, when a Rus-sian science historian, Kirill Rossiianov, produced a 39-page paper about the work. The study was published in English in the journal Science in Context, and when I came across it I was astonished. I struck up an e-mail correspondence with Rossiianov, then, late last year, met with him in Moscow. We spoke at a tea shop around the corner from Red Square, the outré topic making me feel like we were Cold War spies trading state secrets.

Over a ten-year period, Rossiianov was able to unearth Ivanov's diaries and lab notes from the Soviet archives. According to Rossiianov, Ivanov and his son, a biochemistry student, set up a lab at the botanical gardens near Conakry, French Guinea. On the morning of February 28, 1927, they wrapped two female chimps in nets and inseminated them with sperm from a local man. On June 25, they inseminated another chimp with human sperm, this time using a special cage and knocking her out with ethyl chloride. None of the three became pregnant.

Rossiianov, a shy man, told me Ivanov's work repulsed him. "What do you think about the ethical dimension of Ivanov's experiments?" he asked. "Because, I dare say, I found them disgustful. Even now I find it terrible difficult to understand."

And Ivanov had plans to take things further. He also asked Soviet authorities for permission to impregnate women in his own country with sperm from an orangutan named Tarzan, who lived at a primate station in the republic of Georgia. He got a green light, and at least one volunteer came forward, but Tarzan died before any tests took place. Ivanov, convicted of counterrevolutionary activities unrelated to these experiments, was sent to the gulag in 1930, ending his career.

So we still don't know whether humans and chimps could successfully hybridize, but it may have happened in the distant past. A paper published in Nature last year offers compelling evidence that our ancestors had prolonged sexual relations with chimpanzees. The study, led by geneticist David Reich, of the Harvard Medical School, compares large stretches of DNA from humans and chimps.

The researchers, who contend that the two species diverged from a common forebear about 5.4 million years ago, found that the chimpanzee and human X chromosomes are more similar to each other than they are to any other chromosomes. The best explanation, they suggest, is that matings between chimps and early humans would have produced fertile female hybrids, who then mated with chimps themselves and had similar-enough X chromosomes to produce fertile male hybrids. They estimate that this went on for 1.2 million years after the initial split between the species. Eventually, only humans mated with the hybrids and the hybrids disappeared, leaving behind nothing but genetic traces in our chromosomes.