The genetic structure of all the great apes is similar. Chromosomes 6, 13, 19, 21, 22, and X are structurally the same in all great apes. 3, 11, 14, 15, 18, and 20 match between gorillas, chimpanzees, and humans. Chimps and humans match on 1, 2p, 2q, 5, 7 - 10, 12, 16, and Y as well. Some older references will include Y as a match between gorillas, chimps, and humans, but chimpanzees (including bonobos) and humans have recently been found to share a large transposition from chromosome 1 to Y that is not found in other apes.
This level of chromosomal similarity is roughly equivalent to that found in equines. Interfertility of horses and donkeys is common, although sterility of the offspring (mules) is nearly universal. Similar complexities and prevalent sterility pertain to horse-zebra hybrids, or zorses, whose chromosomal disparity is very wide, with horses typically having 32 chromosomes and zebras possessing between 44 and 62 depending upon species. In a direct parallel to the chimp-human case, the Przewalski horse (Equus przewalskii) with 33 chromosome pairs, and the domestic horse (E. caballus) with 32 chromosome pairs, have been found to be interfertile, and produce semi-fertile offspring, where male hybrids can breed with female domestic horses.).
In the 1920s the Soviet biologist Ilya Ivanovich Ivanov carried out a series of experiments to create a human/ape hybrid. At first working with human sperm and chimpanzee females, none of his attempts created a pregnancy. In 1929 he organized a set of experiments involving ape sperm and human volunteers, but was delayed by the death of his last orangutan. The next year he fell under political criticism from the Soviet government and was sentenced to exile in the Kazakh SSR during the Great Purge; he died two years later (see below).
As far back as 1977, researcher J. Michael Bedford discovered that human sperm could penetrate the protective outer membranes of a gibbon egg. Among the apes, the gibbon is the farthest from humans. Bedford's paper also stated that human spermatozoa would not even attach to the zona surface of sub-hominoid primate (baboon, rhesus monkey, squirrel monkey), concluding that although the specificity of human spermatozoa is not confined to man alone, it probably is restricted to the Hominoidea.
In 2006, research suggested that after the last common ancestor of humans and chimpanzees diverged into two distinct lineages, inter-lineage sex was still sufficiently common that it produced fertile hybrids for around 1.2 million years after the initial split.
However, despite speculation, no case of a human-chimpanzee cross has ever been confirmed to exist in modern times.
In 1924, while working at the Pasteur Institute in Paris, Ivanov obtained permission from the Institute's directors to use its experimental primate station in Kindia, French Guinea, for such an experiment. Ivanov attempted to gain backing for his project from the Soviet government. He dispatched letters to the People's Commissar on Education and Science Anatoliy Vasilievich Lunacharsky and to other officials. Ivanov's proposal finally sparked the interest of Nikolai Petrovich Gorbunov, the head of the Department of Scientific Institutions. In September 1925 Gorbunov helped allocate US$10,000 to the Academy of Sciences for Ivanov's human-ape hybridization experiments in Africa.
In March 1926 Ivanov arrived at the Kindia facility, but stayed only a month without success. The Kindia site, it turned out, had no sexually mature chimpanzees. He returned to France where he arranged through correspondence with French Guinea's colonial governor to set up experiments at the botanical gardens in Conakry.
Ivanov reached Conakry in November 1926 accompanied by his son, also named Ilya, who would assist him in his experiments. Ivanov supervised the capture of adult chimpanzees in the interior of the colony, which were brought to Conakry and kept in cages in the botanical gardens. On February 28, 1927, Ivanov artificially inseminated two female chimpanzees with human sperm (not his own or his son's). On June 25, he injected a third chimpanzee with human sperm. The Ivanovs left Africa in July with thirteen chimps, including the three used in his experiments. They already knew before leaving that the first two chimpanzees had failed to become pregnant. The third died in France, and was also found not to have been pregnant. The remaining chimps were sent to a new primate station at Sukhumi.
Although Ivanov attempted to organize the insemination of human females with chimpanzee sperm in Guinea, these plans met with resistance from the French colonial government and there is no evidence such an experiment was arranged there.
Upon his return to the Soviet Union in 1927, Ivanov began an effort to organize hybridization experiments at Sukhumi using ape sperm and human females. Eventually in 1929, through the help of Gorbunov, he obtained the support of the Society of Materialist Biologists, a group associated with the Communist Academy. In the spring of 1929 the Society set up a commission to plan Ivanov's experiments at Sukhumi. They decided that at least five volunteer women would be needed for the project. However, in June 1929, before any inseminations had taken place, Ivanov learned that the only postpubescent male ape remaining at Sukhumi (an orangutan) had died. A new set of chimps would not arrive at Sukhumi until the summer of 1930.
During World War II, infamous Nazi doctor Josef Mengele would sometimes torment uncooperative female Jewish prisoners in the Auschwitz concentration camp by showing them photos of chimpanzees and telling them he had impregnated them with the sperm from those chimps. This was only a form of psychological torture and he never actually conducted experiments of the sort, as they served no purpose in his eugenics research.
An episode of Unsolved History broadcast on the Discovery Channel on March 27, 1998, as well as on December 16, 2006, during the program Humanzee, discussed the controversies over Oliver the chimp also detailed some of the rumors and urban legends about "humanzees". One claim was that a common chimpanzee was impregnated by human sperm in a laboratory in China, but died from neglect before giving birth during the Chinese Cultural Revolution in the 1960s. A similar story, reported by University at Albany psychologist Gordon Gallup, alleged that a human-chimp hybrid was successfully engendered and born at the old Yerkes National Primate Research Center in Orange Park, Florida in the 1920s, but was destroyed by the scientists soon after. Gallup claimed he heard the story as a young graduate student, when an elderly academic confided in him that he had been part of the team behind the experiment. Gallup added that he feels the colleague telling him of this genuinely believed the story to be true but that he, Gallup, has never been able to prove it one way or another.