PZ Myers

The Hybrid Theory

A rebuttal of PZ Myers’ criticisms

EUGENE M. MCCARTHY, PHD GENETICS

The rule is perfect: In all matters of opinion, our adversaries are insane.

Since so much misinformation has been put out about my hybrid theory of human origins, I’ve started writing a series of rebuttals to the various criticisms appearing on the web, lest anyone mistake my silence for an inability to address the objections. The first was in response to a critique by Donald Prothero. The present article is the second in that series.

The critique that I address on this page, entitled The MFAP Hypothesis for the origins of Homo sapiens, is by American biologist Paul Zachary Myers, known on the web as PZ Myers. It was first published on Skepticblog on Jul. 2, 2013. I'm sorry for the long delay in addressing his comments, but I suppose that for a long time, I couldn’t really believe that any intelligent person would take them seriously. However, several people whose opinions I respect have asked me to write this rebuttal. So here it is.

First, please note that PZ Myers is neither trained as a geneticist, nor is he an expert on hybridization. I myself have a masters and Ph.D. in genetics from one of the leading genetics departments in the country (the University of Georgia’s) and I've spent a lifetime investigating hybridization. According to Wikipedia, PZ Myers has a B.S. in biology and a Ph.D. in biology. The same article says that he studies zebra fish in connection with his research in the field of developmental evolutionary biology. I don’t know him, but in reading about him online, I find that he seems to have made his reputation primarily in the evolution-creation debate, in which he is an outspoken proponent of the former. In such debate, he seems to stick to the standard dogmas propounded by the most conservative element within the scientific community and not to take kindly even to fellow scientists who question those dogmas.

What follows is largely a copy-and-paste of PZ Myers’s original article, except in that my responses to Myers’ comments are inserted at the relevant points within his text. I do, however, leave out certain sections that are obscene, ad hominem or merely childish. Some of his comments, I’ve summarized to avoid offending and/or boring the reader. The text of his article begins as follows:

The MFAP Hypothesis for the origins of Homo sapiens

by PZ MYERS, July 2, 2013

I know you’re thinking we’ve had more than enough discussion of one simplistic umbrella hypothesis for the origin of unique human traits — the aquatic ape hypothesis — and it’s cruel of me to introduce another, but who knows, maybe the proponents of each will collide and mutually annihilate each other, and then we’ll all be happy. Besides, this new idea is hilarious. I’m calling it the MFAP hypothesis of human origins, which the original author probably wouldn’t care for (for reasons that will become clear in a moment), but I think it’s very accurate.

My response:

PZ Myers' opening paragraph (quoted above) is, obviously, almost exclusively ad hominem. Although there is an added smattering of speculation about my attitudes and those of Myers' readers, I cannot see that there is anything substantive here to address. I suppose I’d describe it as a bit of demagoguery seemingly intended to titillate the sort of shallow mind that revels in such banter.

Since I try to make this website safe for children, I won’t be quoting Myers' explanation of what the F in MFAP stands for. I'll only say that it represents an old Anglo-Saxon word, which everyone has heard, that means "had sexual relations with." For the rest, I'll be explicit: M stands for "monkey," A for "a," and P for "pig."

One thing I should point out, though, is that his reference to a simplistic umbrella hypothesis seems to imply that he feels there’s something negative about a simple explanation. In fact, however, when rating theories against each other, it’s widely recognized that a simple explanation that explains many different phenomena is much preferable to one that makes up a separate ad hoc explanation for each different phenomenon. So the hybrid theory is to be preferred over theories of human origins based on natural selection, because the former explains all human distinctions by saying one thing (that we are derived from hybridization of the type specified by the theory), whereas the selectionist requires its adherents to make up a separate origin story for each different trait distinguishing us from apes. In other words, under the selectionist view, our naked skin would not be explained by the same factor(s) that explain our multipyramidal kidneys, whereas under the hybridization view both of these traits (and many others) are explained by a single, simple assumption of porcine heritage. So an umbrella theory is good, so long as it turns out to be true. What’s bad is having to make up a new story every time a new trait is discussed.

PZ Myers' comments:

First, the author of this new hypothesis provides a convenient list of all the unique traits that distinguish humans from other primates, listed on the right. It falsely lists a number of traits that are completely non-unique (such as female orgasm and cancer), or are bizarre and irrelevant (“snuggling”, really?). It’s clearly a selective and distorted list made by someone with an agenda, so even though some items on the list are actually unusual traits, the list itself is a very poor bit of data. [Here, Myers appends a list of traits copied and pasted from my website.]

My response:

While I realize that any such list will be affected by the personal biases of the author (in this case, me), I've done my best to avoid such bias by trying (1) to be exhaustive (that is, to include every trait that I find mentioned in the literature) and (2) always to limit the traits listed to those that are recognized not by me, but by experts in the relevant fields (primatologists, comparative anatomists, physical anthropologists, etc.), who list them as human distinctions. In the case of each trait, I provide the citation(s) for the publication where I found the information. While I admit that there may, in fact, be some mistakes in that list, just as there may be mistakes in any human production, the traits that he refers to as false (female orgasm and cancer) are not false.

Take cancer. In fact, I never claimed that cancer does not occur in non-human primates. My only assertion was that the rate of its occurrence is far lower than in humans. And Adolph H Schultz, who I cite, does say that cancers of all types are rare in non-human primates. I suppose PZ Myers, an evolutionary biologist who works with zebrafish, feels himself competent to contradict Schultz, one of the leading physical anthropologists and primatologists of the twentieth century, but I do not. Moreover, when in preparation for this article I rechecked Schultz' claim, I found a survey investigating the incidence of cancer in non-human primates (Puente et al. 2006) that confirms his assessment. In that article, Puente and his colleagues state specifically that:

a number of works have reported that cancer incidence in non-human primates is very low. This fact is especially evident for epithelial neoplasms such as breast, prostate or lung carcinomas, which are responsible for more than 20% of human deaths but whose incidence in great apes is less than 2%.

In short, then, I claimed that cancer is rare in non-human primates and experts in the field do in fact say that cancer is rare in non-human primates. Why, then, does PZ Myers complain? Isn’t it he who’s being false? Indeed, I might point out that while my work is extensively documented (thousands of works cited), his article panning my work contains exactly one (not particularly relevant) citation.

Likewise, in connection with female orgasm, I quote Desmond Morris, a well-known zoologist and ethologist, as follows "female orgasm in our species is unique amongst primates…If there is anything that could be called an orgasm [in nonhuman primates], it is a trivial response when compared with that of the female of our own species." So again, I've attributed the claim to a competent expert. So why does Myers say that I've listed these two traits "falsely"? Indeed, it seems that his own claim about my falseness is false.

And I'm not sure why Myers should consider it "bizarre and irrelevant" that pigs snuggle and chimpanzees do not. To me, it’s just another trait that an expert, in this case Jane Goodall, cites as distinguishing humans from chimpanzees. And it’s also one that links us to pigs.

Next, Myers copies a lengthy excerpt from my website (without my permission, by the way) and pastes it into the text of his article. However, he does not raise any substantive objection to the content of that excerpt. He merely uses it to set up a paragraph of his own where, in an excessively puerile attempt at humor, he defines the acronym MFAP.

Next, he shows a picture of a wild boar and a chimpanzee and says that I claim humans are the product of mating between the two of them, as follows:

PZ Myers' comments:

Let’s be perfectly clear about this. McCarthy’s hypothesis is that once upon a time, these two [i.e., the pictured boar and chimpanzee] met and had sex, And that they then had children that were…us. That’ll learn me. I thought this South Park clip was a joke.

My response:

These callow remarks are merely a simplistic misrepresentation in which he boils down my lengthy discussion of the topic into a couple of sentences. But even as a summary, it misses the mark. The theory I actually propose (a theory, by the way, that accounts for the fact that we share many traits with pigs that we do not share with chimpanzees) is that long ago there was hybridization between a population of pig-like animals and a population of apes (similar to modern chimpanzees and bonobos) and that the resulting hybrid(s) then backcrossed to the ape population, resulting in the production of a mostly apelike population that retained a lot of piglike traits.

PZ Myers' comments:

One thing that struck me in reading McCarthy’s claim is how they are [sic] so similar to the claims of the soggy ape fans [here, Myers refers to proponents of the aquatic ape hypothesis, the theory that humans evolved as apes living in a seaside environment] — they even use the very same physiological and anatomical features to argue for their delusion.

My response:

This, too, is a misrepresentation. While it’s true that quite a few traits that I list are cited also by proponents of the aquatic ape hypothesis (I note in passing that "soggy ape fans" is obviously ad hominem), the list I offer is far longer, that is, their list is only a small subset of mine.

PZ Myers' comments:

For instance, I’ve read aquatic ape proponents’ arguments that the shape of our nose is adaptive for streamlining and for preventing water from flowing into the nostrils while propelling ourselves forward through the water…but compare that to the MFAP. [Here, Myers inserts the following long excerpt from my site without permission.]

Neither is it clear how a protrusive cartilaginous nose might have aided early humans in their “savanna hunter lifestyle.” As Morris remarks, “It is interesting to note that the protuberant, fleshy nose of our species is another unique feature that the anatomists cannot explain.” This feature is neither characteristic of apes, nor even of other catarrhines. Obviously, pigs have a nose even more protuberant than our own. In a pig’s snout, the nasal wings and septum are cartilaginous as ours are. In contrast, a chimpanzee’s nose “is small, flat, and has no lateral cartilages”. A cartilaginous nose is apparently a rare trait in mammals. Primatologist Jeffrey Schwartz goes so far as to say that “it is the enlarged nasal wing cartilage that makes the human nose what it is, and which distinguishes humans from all other animals.” The cartilaginous structure of the pig’s snout is generally considered to be an “adaptation” for digging with the nose (rooting). Rooting is, apparently, a behavior pattern peculiar to pigs. Other animals dig with their feet.

Point, MFAP. Of course, just as I would point out to aquatic ape people, we do have an explanation for the nose: recession of the facial bones associated with reduced dentition, along with retention of the bones associated with the respiratory apparatus. The protuberant nose is simply a ridge made apparent by the receding tide of our chewing apparatus. McCarthy uses evidence as badly as does every wet ape fan.

My response:

First, I would point out that Myers is citing a trait from the subset of traits on my list which are also cited by the aquatic ape hypothesis. So he’s cherry picking (exactly the sort of biased behavior of which he accused me above). His comment does nothing, therefore, to address traits on my list not in that subset, such as the multipyrimidal kidneys shared by humans and pigs. What’s the point of mentioning a single trait?

Second, our "chewing apparatus" is about the same size as that of a bonobo, and bonobos do not have protrusive, rubbery noses like ours. Moreover, apes do not have large nasal bones. In fact, such bones are next to non-existent in apes. But even human microcephalics have the large nasal bones that set humans apart from other primates. As I quote primatologist F. Wood Jones: "In the large size and permanent separation of the nasal bones, man is in marked contrast with all of the anthropoids" The same is true of the rubbery cartilaginous structure of our nose. So Myers' "retreating tide" explanation is not a good one. (The various assertions in this paragraph are documented here.)

In the upcoming paragraphs (quoted immediately below) Myers engages in pure speculation. He doesn’t seem to understand that I offer the theory that we are pig-ape hybrids in order to explain the fact that traits distinguishing us from apes connect us with pigs. His opinions about the likelihood of past events seem largely irrelevant. Moreover, they betray an ignorance of hybridization. When he refers to "behavioral difficulties," he seems to think that different types of animals would never choose to mate, whereas in fact, if we are considering mating alone, it’s well known that even mammals and birds (for example, dogs and geese, or seals and penguins) will copulate. Natural hybridization between distinct types of animals rated as different species is commonplace. And he has no way of knowing, short of some god-like insight, whether a hybridization of this type is "cytogenetically possible." Nor does he have any way of knowing whether chimpanzees would take care of a hybrid infant. The truth of the matter is that we know from direct observation that in many hybrid crosses, the mother does in fact care for her hybrid offspring. Why not chimpanzee mothers, too?

PZ Myers' comments:

Now, why won’t this hybridization claim work? Well, there are the obvious behavioral difficulties, even if it were cytogenetically possible. We’d have to have pigs and chimps having sex and producing fertile offspring, and those human babies (remember, this is a saltational theory, so the progeny would have all the attributes of a third species, ours) would have to be raised by chimps. Or pigs. I don’t think either is a reasonable alternative, and a band of chimps would probably be no more charitable to a helpless fat blob of a baby than Mr Wu’s pigs.

However, no one reasonably expects pigs and chimps to be interfertile. The primate and artiodactyl lineages have diverged for roughly 80 million years [Here, he pictures an evolutionary tree] — just the gradual accumulation of molecular differences in sperm and egg recognition proteins would mean that pig sperm wouldn’t recognize a chimpanzee egg as a reasonable target for fusion. Heck, even two humans will have these sorts of mating incompatibilities. Two species that haven’t had any intermingling populations since the Cretaceous? No way.

My response:

Pure speculation again. I've studied hybridization for thirty years and I am, if anything, less sure about the limits of hybridization than when I began. There’s actually a lot of evidence that distant crosses do sometimes work. And the 80-million-year figure is speculation, too. It depends on the belief that the tree he pictures is accurate. But the accuracy of all such trees depends on the presupposition that hybridization between lineages can be discounted. If it’s rampant, then all such reasoning from trees is gobbledygook. But certainly it can’t be true that different spermatozoa and ova never recognize each other. Otherwise, there would be no such thing as hybridization. But of course, we know there is.

And even if pigs and apes were separated for such a long time as he suggests, how would we know that it was impossible for them to hybridize? What’s magic about 80 million years? We know that crosses between sea turtles (Chelonia x Eretmochelys) that supposedly have been separated for a comparable length of time do produce hybrids. Moreover, such hybrids are at least partially fertile (Karl et al. 1995, p. 265, Lara-Ruiz et al. 2006, p. 265). Karl et al. also found hybrids between another, equally distant pair of turtles (Chelonia x Caretta). So why not pigs and apes, too? What I would say to PZ Myers is: "Stop all the speculating and propounding and explain why the traits that distinguish us from chimpanzees consistently link us with pigs. Offer a different hypothesis accounting for our affinity to pigs. Put up or shut up!"

Myers then goes on for three paragraphs talking about how "chromosomal differences" would prevent such a cross from occurring. But I list various crosses on my website where the parents differ on the chromosomal level as much as do pigs and apes, and those crosses produce hybrids, sometimes fertile ones.

As Annie P. Gray, a leading expert on hybridization in mammals and birds, noted in the preface to her reference work Mammalian Hybrids (1972, p. viii), which compiled information about all known hybrid mammals, “no close correlation was found between the chromosome count or the duration of gestation and the ability of species to hybridize.”

What most people don’t realize, including, it seems, PZ Myers, is that chromosomal incompatibilities are not an absolute block to reproduction. In general, they merely reduce the fertility of hybrids. In other words, even though hybrids affected by such incompatibilities do not produce as many gametes as their pure parents, and even though those gametes are more often deformed and dysfunctional, such hybrids do still occasionally produce offspring. It’s the stereotypic notion of the absolutely sterile hybrid that many people stumble on.

Then he talks about humans and chimpanzees:

PZ Myers' comments:

But we wouldn’t have to even get that far. Human and chimpanzee chromosomes are even more similar to one another, and there are no obvious chromosomal barriers to interfertility between one another. If hybridization in mammals were so easy that a pig and a chimp could do it, human-chimp hybrids ought to be trivial. Despite rumors of some experiments that attempted to test that, though, there have been no human-chimp hybrids observed, and I think they are highly unlikely to be possible. In this case, it’s a developmental problem.

For example, we have bigger brains than chimpanzees do. This is not a change that was effected with a single switch; multiple genes had to co-evolve together, ratcheting up the size in relatively incremental steps. So you could imagine a change that increased mitotic activity in neural precursors that would increase the number of neurons, but then you’d also need changes in how those cells are partitioned into different regions, and changes in the proliferation of cartilage and bone to generate a larger cranium, and greater investment in vascular tissue to provide that brain with an adequate blood supply.

My response:

Why cite rumors? Obviously, there have been few experimental matings between humans and chimps, and those that have been alleged are very poorly documented. For ethical reasons, no scientist conducts such experiments (at least, not publicly). Thus, given that the facts about this cross are largely unknown, why jump to the conclusion that it wouldn’t work? Indeed, even though hybridization of this sort is largely unstudied, there is at least one fairly well documented case of a human-chimpanzee hybrid is on record, though the reported hybrid wasn’t viable. Certain types of crosses produce a high percentage of inviable offspring, but occasionally produce viable offspring as well. But PZ Myers, says "I think they are highly unlikely to be possible." Why? We know that crosses can sometimes work even between forms of life that are rather distantly related. So why not closely related pairs like humans and chimps.

But even if we knew crossing humans and chimps didn’t work, that would tell us nothing about pigs and apes. There are many close crosses that have not been obtained despite assiduous effort on the part of researchers--even Darwin was aware of this--and there are others between types that seem more distant that have been obtained. Granted, greater distance usually means greater difficulties, but it isn’t a hard and fast rule. So how can we be sure a cross between pigs and apes is impossible? Myers supports his claim with nothing more than a metaphor:

PZ Myers' comments:

Development is like a ballet, in which multiple players have to be in the right place and with the right timing for everything to come off smoothly. If someone is out of place by a few feet or premature by a few seconds in a leap, the dancers could probably compensate because there are understood rules for the general interactions…but it would probably come off as rough and poorly executed. A hybrid between two closely related species would be like mixing and matching the dancers from two different troupes to dance similar versions of Swan Lake — everything would be a bit off, but they could probably compensate and muddle through the performance.

Hybridizing a pig and a chimp is like taking half the dancers from a performance of Swan Lake and the other half from a performance of Giselle and throwing them together on stage to assemble something. It’s going to be a catastrophe.

My response:

Personally, I like the idea of a simultaneous production of Swan Lake and Giselle (which, by the way, would be a kind of hybridization!). It seems to promise a new and surreal approach to ballet. So I’d like to see it tried.

But Myers is dancing around the facts. What we know is that in hybrid crosses there are elevated levels of dysfunctionality. More dysfunctional offspring are produced than in ordinary matings. And in distant crosses there are, typically, more produced than in close ones. However, even crosses that produce many dysfunctional individuals may from time to time produce functional ones. What about them? What are their implications? Why couldn’t a rare functional individual pig-ape participate in the foundation of a new population? Even if most individuals from such a cross were non-viable and sterile? In fact, we know that certain crosses produce hybrids that are superior to their parents in certain respects. The best known case, of the many examples of this phenomenon, is the ordinary mule.

PZ Myers' comments:

But here’s the deal: maybe I’m completely wrong. This is an experiment that is easily and relatively cheaply done. Human sperm is easily obtained (McCarthy probably has a plentiful supply in his pants), while artificial insemination of swine is routine. Perhaps McCarthy can report back when he has actually done the work.

My response:

There are several problems here. One is that I don’t think his suggestion about carrying out actual matings is serious. But even if it were, according to what I propose on the website, neither my sperm nor that of any other human being would be necessary. What you’d want would be pig sperm and chimp eggs (and/or pig eggs and chimp sperm). But it wouldn’t be cheap. You’d have to pay for holding facilities, feed, medical care, etc., for the animals, as well as for the animals themselves, caretakers and researchers, not to mention all of the overhead associated with a research facility.

Also, some crosses produce at a rate of less than one hybrid per thousand inseminations. We don’t know what that rate is for pig-chimp hybridization. So even though the production of a single hybrid would, in fact, be very informative, negative results would not be, because you would have to do a huge amount of work (thousands of inseminations) before you could reasonably conclude that such a cross never works.

Also, we only have extant animals to work with. The ancient animals participating in the cross might have been a lot different from a modern chimpanzee and a farmyard hog.

Another issue is that the product of the first cross would be something non-human. Something somewhere between humans and pigs. Intelligent, but not human. Not being Frankenstein, I'm not so sure I'm ready to go there.

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