It sounds like a joke… but the punch line might surprise you. According to American geneticist Dr. Eugene McCarthy, the answer is… no offence… you! Well, not just you but all of us – as he claims that: ‘humans evolved after a female chimpanzee mated with a pig’. Unsurprisingly, his hypothesis has not been published in any peer-reviewed journal, but was instead posted on a website that McCarthy himself curates. Nevertheless, these musings have attracted the attention of the press in both the US, and the UK in recent weeks; and have been widely criticized by scientists, creationists and fellow bloggers alike (see links below). As such, I feel that the “monkey BLEEPING* a pig” hypothesis has now been discussed enough – and instead have focused on less controversial examples of hybridisation, which are more common than you might at first think.
A hybrid is any offspring that results from the interbreeding of individuals from two different species, or subspecies. The mule – a cross between a male donkey and a female horse – is probably the best-known animal hybrid; whilst ligers and tigons, are two distinct hybrids of lions and tigers that have been bred in zoos. Further, many plants that end up on our plates originate from past hybridisations. A tangy grapefruit at breakfast would not be possible without an 18th century cross between the Jamaican sweet orange and the pomelo; whereas the lunchtime staple – the sandwich – owes its origins to not one, but two, hybridisation events; as bread wheat is descended from three different wild grass species. Even a soothing cup of peppermint tea is brewed from a naturally occurring hybrid of spearmint and water mint.
Just as you probably share some features with your mum and some with your dad, a hybrid’s traits and characteristics are often a blend of the two parent species. The mule’s long ears resemble those of a donkey, but its height and body shape appear horse-like. However, some of a hybrid’s features are more extreme than those found in either parent. Mules are thought more intelligent than either parent, whilst ligers are the largest of all the living cats.
These more extreme traits, called “transgressive phenotypes”, often result when the mixing of two genomes ‘breaks-up’ groups of genes that would normally have counteracting effects. As such, the hybrid might inherit only the genes with the positive effects (or the negative effects) from both parents. Interestingly, the situation with the liger’s size is a little more complicated, because whilst the liger (male lion x tigress) is bigger than either parent; the tigon (male tiger x lioness) is not – meaning that the effect of a gene from a given species will depend on whether it’s inherited via the male or female parent.
Transgressive phenotypes might benefit hybrids, by allowing them to exploit new niches and colonise new habitats – whilst hybrids can get a further boost in fitness, by masking harmful recessive genes that may have accumulated in either of the parents’ genomes. However, as you probably suspected, hybridisation is not always a boon to evolutionary fitness, and the flipside of this “hybrid vigour” is “hybrid depression”.
Hybrids can be ‘depressed’, or less fit than their parent species, for a number of reasons. Hybridisation is essentially a ‘collision’ between two genomes; and the resulting chaos can break apart groups of genes that must work together to function. Furthermore, hybrids often have reduced fertility compared to their parents. If the two parent species have different numbers of chromosomes – the resultant mismatch in the hybrid makes pairing the chromosomes up and segregating them into egg, sperm or pollen cells troublesome. As such, many hybrids are sterile, including the mule which has 63 chromosomes: halfway between the horse’s 64, and the donkey’s 62.
The reduced fertility or sterility of hybrids is often thought to be a major factor that keeps different species distinct from one another. However, interbreeding between two different species is not always an evolutionary dead-end. Even with reduced fertility, hybrids could interbreed with members of one of the parent species, allowing the flow of new genetic material from one species to the other – and there are even new viable species that have resulted from an hybridization event.
So, whilst a chimp-pig hybrid origin for humans is unpopular and unproven, hybrids represent great natural experiments in evolution, that can teach us a lot about what affects a species’ fitness, and how new species can arise. The widespread nature of hybridization also reminds us that species are not uniquely discrete entities, but instead have changed, and will likely continue to change, over timescales much shorter than we might expect.
* I censored this to avoid causing offence, but if you are anything like me - you were probably more annoyed that a chimp was called a 'monkey'!
Dr. Eugene McCarthy : Human Origins: Are we hybrids? | PZ Myers (scientist and blogger) : The MFAP Hypothesis for the origins of Homo sapiens | Daily Mail : ‘Humans evolved after a female chimpanzee mated with a pig’: Extraordinary claim made by American geneticist | Occam’s Corner at The Guardian : Evolutionary theory that a chimp mated with a pig is pure sausagemeat