When it comes time to play the old nuclear musical chairs and produce gametes, some types of hybrids do a much better job. Liger females, for example, can produce offspring in backcrosses with both lions and tigers. McCarthy also points out that fertility can be increased through successive backcrossing with one of the parents, a common technique used by breeders. In the case of chimp - pig hybridization, the "direction of the cross" would likely have been a male boar or pig (Sus scrofa) with a female chimp (Pan troglodytes), and the offspring would have been nurtured by a chimp mother among chimpanzees (shades of Tarzan!). The physical evidence for this is convincing, as you can discover for yourself with a trip over to macroevolution.net.

When I asked McCarthy if he could give a date estimate for the hybridization event, he said that there are a couple broad possibilities: (1) It might be that hybridization between pigs and apes produced the earliest hominids millions of years ago and that subsequent mating within this hybrid swarm eventually led to the various hominid types and to modern humans; (2) separate crosses between pigs and apes could have produced separate hominids (and there's even a creepy possibility that hybridization might even still be occurring in regions where Sus and Pan still seem to come into contact, like Southern Sudan).

This latter possibility may not sound so far-fetched after you read the riveting details suggesting that the origin of the gorilla may be best explained by hybridization with the equally massive forest hog. This hog is found within the same habitat as the gorilla, and shares many uncommon physical features and habits. Furthermore, well-known hybridization effects can explain many of the fertility issues and other peculiarities of gorilla physiology.

Many readers are, I'm sure, familiar with the long-standing use of swine in medical research; here is a brief description of the reasons:

Swine have proven to be particularly effective in biomedical research for human disease because of their genetic makeup. Their comparative anatomy and physiology closely resemble that of humans making them excellent matches for research in gene and cell therapy, xenograft and allograft procedures and other types of regenerative medicine.This fact has enabled swine-based research to achieve more relevant and significant findings than research using other animals such as rats or rabbits...

Swine have been used in biomedical research in a variety of areas: cardiovascular, gastrointestinal, metabolic, liver, reproductive and infectious disease. They have also helped researchers learn more about drug addiction and wound healing...

(Via Midwest Research)

SF author Alan E. Nourse, himself a medical doctor, wrote a 1953 story called Family Resemblance predicting this possibility. Unfortunately, I haven't been able to locate a copy of the story to obtain a quote - perhaps a reader can help.