It starts off reasonably enough; McCarthy describes the fact that, contrary to our perception of species as being little watertight compartments, hybridization (and thus gene flow between species) is rather common. Not all hybrids are sterile, like the familiar example of the mule; a lot of them are back-fertile to either parental species (an example is the "Brewster's Warbler," which was once thought to be a separate species and is now known to be a hybrid between the Golden-winged and Blue-winged Warblers).
So McCarthy asks an interesting question: are humans a hybrid? The answer, apparently, is yes; recent studies have shown that most human populations show the genetic signature of three ancestral populations -- modern humans, Neanderthals, and Denisovans. (Biologists disagree, however, as to whether these three represent different species -- a distinction that, in reality, probably doesn't mean very much. The concept of species is one of the hardest-to-pin-down terms in all of biological science.)
But, unfortunately, it isn't this intermixing between three proto-hominins that McCarthy is talking about. He thinks we're a much more interesting hybrid than that. He gives his evidence first: humans have low fertility, and males produce a great many abnormal sperm (kind of a surprise given our reproductive success -- you have to wonder, if this is true, how there can be seven billion of us).
What? You want more evidence than that? Sorry, that's it. Guys produce lots of abnormal sperm, and allegedly we have low fertility. So we're hybrids. That's enough, right?
Of course right. So now, if we're hybrids, we have to figure out which two species gave rise to humans. One of them, McCarthy says, was clearly something like a chimp. But he states, in all apparent seriousness, "Many characteristics that clearly distinguish humans from chimps have been noted by various authorities over the years." Can't argue with that. But then he goes right off the edge of the cliff:
One fact, however, suggests the need for an open mind: as it turns out, many features that distinguish humans from chimpanzees also distinguish them from all other primates. Features found in human beings, but not in other primates, cannot be accounted for by hybridization of a primate with some other primate. If hybridization is to explain such features, the cross will have to be between a chimpanzee and a nonprimate — an unusual, distant cross to create an unusual creature.If this sets alarm bells off, good -- because this would require a fertile hybrid being produced from a mating of animals not just from two different genera, or two different families, but two different orders. Entirely possible, McCarthy says, despite the fact that there is not a single example -- not one -- of an interordinal hybrid known from nature. Anywhere. That includes animals, plants, fungi, and so on.
Nevertheless, that doesn't stop McCarthy:
Looking at a subset of the listed traits [unique features of humans are listed in the sidebar on page two of his website; there are too many to list here], however, it's clear that the other parent in this hypothetical cross that produced the first human would be an intelligent animal with a protrusive, cartilaginous nose, a thick layer of subcutaneous fat, short digits, and a naked skin. It would be terrestrial, not arboreal, and adaptable to a wide range of foods and environments.So, let's not dillydally any more; if a chimp is one of our parental species, what's the other?
What is this other animal that has all these traits? The answer is Sus scrofa, the ordinary pig. What are we to think of this fact? If we conclude that pigs did in fact cross with apes to produce the human race, then an avalanche of old ideas must crash to the earth. But, of course, the usual response to any new perspective is "That can't be right, because I don't already believe it." This is the very response that many people had when Darwin first proposed that humans might be descended from apes, an idea that was perceived as ridiculous, or even as subversive and dangerous. And yet, today this exact viewpoint is widely entertained. Its wide acceptance can be attributed primarily to the established fact that humans hold many traits in common with primates. That's what made it convincing... Let us take it as our hypothesis, then, that humans are the product of ancient hybridization between pig and chimpanzee.So, basically, the logic is, "people laughed at Darwin, and he turned out to be right, so if people laugh at me, I must be right?"
But I don't want to be accused of jumping to conclusions ("That can't be right, because I don't already believe it"), so I took what I think is a critical look at the list of allegedly unique features of humans -- ones that, in McCarthy's view, must have come from our other, non-primate parental species. And most of them have to do with quantities and sizes -- "sparse" hair, "large amounts" of elastic fiber in the skin, "richly" vascularized dermis, "narrow" eye opening, "heavy" eyelashes, and so on. Traits involving quantities and sizes are highly responsive to selective pressures, the idea being once you have genes for the production of a feature, it is relatively straightforward to evolve to produce more or less of it.
Of the features he claims are found only in humans and pigs, it appears that in several cases, he is simply wrong. Take multipyramidal kidneys -- he is correct that only humans have this feature amongst primates, but it is hardly unique in the mammalian world. Besides humans and pigs, elephants have multipyramidal kidneys, as do bears, rhinoceroses, bison, and "nearly all marine mammals," according to a paper by M. F. Williams (available here). Williams' contention is that multipyramidal kidneys evolved in animals that lived in coastal or marine environments in order to deal with high levels of salt -- and that each of these lineages evolved it independently, as it represents a unique feature on separate, distantly related branches of the phylogenetic tree (evolutionary biologists call these features "apomorphies").
Then, of course, he has some things on the list of allegedly unique human characteristics that are simply weird. "Particular about place of defecation?" (Has he ever owned a cat?) "Snuggling?" "Extended male copulation time?" "Good swimmer?"
I'm sorry, Dr. McCarthy, but I'm calling bullshit on this.
Now, please understand; it's not like I have any particular problem with our having a checkered ancestry. I'm an evolutionary biologist by training, for cryin' in the sink, I know we're animals. But the idea that Homo sapiens arose when a chimp had sex with a pig... that stretches credulity too far.
Even if you do have a Ph.D.