Dr. Eugene McCarthy, a Ph.D in Genetics and an expert in hybridization, speculates in Human origins: Are we hybrids? that human beings are the result of hybridization between chimpanzees and pigs.
A hybrid is produced when two different species mate and produce offspring. Mules are the result of a mating between a male donkey and female horse. Ligers the result of a mating between lions and tigers. Zedonks the result of a mating between zebras and donkeys. Hybrids are not always infertile and fertility can be improved by successive backcrossing with one of the parents. McCarthy speculates that humans may have been the result of the mating of a male boar with a female chimpanzee and that successive matings within a small population gave rise to humans.
McCarthy’s evidence for this is primarily morphological and not genetic. He claims that the multiple generations of backcrossing between pigs and chimpanzees would effectively mask any contribution from pig in nucleotide sequence data. His article lists the similarities between pigs and humans and where humans and pigs differ from nonhuman primates. In his list are included: a layer of subcutaneous fat, thermoregulatory sweating, normal host for the human flea , epidermal lipids contain triglycerides and free fatty acids, lightly pigmented eyes common, eyebrows, heavy eyelashes, earlobes. more lumbar vertebrae, fewer sacral vertebrae , centralized spine, sides of pelvis turn forward, diverticulum at cardiac end of stomach, multipyramidal kidneys, hymen, absence of periodic sexual swellings in female, nipples low on chest, brain lobes (frontal and temporal prominent), primitive premolar, nocturnal activity, extended male copulation time, female orgasm, short menstrual cycle, and tears. Also things common to humans and pigs and rare of absent in nonhuman primates are heart attack, atherosclerosis, and melanoma.
Since McCarthy’s evidence is not genetic, it might simply be that the similarities between humans and pigs are the result of convergent evolution instead of cross-breeding. In other words, the common features are the result of parallel evolutionary processes possibly the result of similar selective pressures. Once humans split from chimpanzees and were forced to adapt to a more terrestrial environment, humans evolved characteristics similar to pigs.