The MFAP Hypothesis for the origins of Homo sapiens

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.

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.

But set those objections to the list aside for a moment, and let's consider the hypothesis proposed to explain their existence, the MFAP Hypothesis of Eugene McCarthy, geneticist. I will allow him to speak for himself at length; basically, though, he proposes that the way novel traits appear in evolution is by hybridization, by crosses between two different species to produce a third with unique properties.

Many characteristics that clearly distinguish humans from chimps have been noted by various authorities over the years. The task of preliminarily identifying a likely pair of parents, then, is straightforward: Make a list of all such characteristics and then see if it describes a particular animal. 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.

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For the present, I ask the reader to reserve judgment concerning the plausibility of such a cross. I'm an expert on hybrids and I can assure you that our understanding of hybridization at the molecular level is still far too vague to rule out the idea of a chimpanzee crossing with a nonprimate. Anyone who speaks with certainty on this point speaks from prejudice, not knowledge.

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Let's begin, then, by considering the list in the sidebar at right, which is a condensed list of traits distinguishing humans from chimpanzees — and all other nonhuman primates. Take the time to read this list and to consider what creature — of any kind — it might describe. Most of the items listed are of such an obscure nature that the reader might be hard pressed to say what animal might have them (only a specialist would be familiar with many of the terms listed, but all the necessary jargon will be defined and explained). For example, consider multipyramidal kidneys. It's a fact that humans have this trait, and that chimpanzees and other primates do not, but the average person on the street would probably have no idea what animals do have this feature.

Looking at a subset of the listed traits, 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. These traits may bring a particular creature to mind. In fact, a particular nonprimate does have, not only each of the few traits just mentioned, but every one of the many traits listed in th sidebar. Ask yourself: Is it is likely that an animal unrelated to humans would possess so many of the "human" characteristics that distinguish us from primates? That is, could it be a mere coincidence? It's only my opinion, but I don't think so.

Look at the description of the putative non-primate parent in the last paragraph above. What animal are you thinking of? It's probably the same one McCarthy imagined, which is why I've decided that this explanation for human origins must be called the Monkey-Fucked-A-Pig hypothesis, or MFAP for short.

Let's be perfectly clear about this. McCarthy's hypothesis is that once upon a time, these two 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.

One thing that struck me in reading McCarthy's claim is how they are so similar to the claims of the soggy ape fans -- they even use the very same physiological and anatomical features to argue for their delusion. 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.

Neither is it clear how a protrusive cartilaginous nose might have aided early humans in their "savannah 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.

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 — 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.

But further, even if the sperm of one would fuse with the egg of another, there is another looming problem: chromosome incompatibilities. Pigs have 38 chromosomes, chimpanzees have 48. Cells are remarkably good at coping with variations in chromosome number, and even with translocations of regions from one chromosome to another; and further, pigs and people even retain similar genetic arrangements on some of their chromosomes. There are pig chromosomes that have almost the same arrangement of genes as a corresponding human chromosome.

But there are limits to how much variation the cell division machinery can cope with. For instance, with fewer chromosomes than we primates have, that means you need to line up multiple primate chromosomes to match a single pig chromosome (this pairing up is essential for both mitosis and meiosis). Look at pig chromosome 7, for instance: it corresponds to scrambled and reassembled bits of human chromosomes 6, 14, and 15.

Blocks of conserved synteny between pig and human. (a) Pig SSC7 to human chromosomes 6, 14 and 15. (b) HSA13 compared to pig chromosome 11. Block inversions between pig and human are denoted with broken lines. Contig coverage is depicted by bars in the center of SSC7 and HSA13.

Maybe that would work in mitosis within the hybrid progeny — you'd have three chromosomes from the human/chimp parent twisted around one chromosome, but they would be able to pair up, mostly, and then separate to form two daughter cells. But meiosis would be total chaos: any crossing over would lead to deletions and duplications, acentric and dicentric chromosomes, a jumble of broken chromosomes. That would represent sterile progeny and an evolutionary dead end.

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.

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.

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.

Humphray SJ, Scott CE, Clark R, Marron B, Bender C, Camm N, Davis J, Jenks A, Noon A, Patel M, Sehra H, Yang F, Rogatcheva MB, Milan D, Chardon P, Rohrer G, Nonneman D, de Jong P, Meyers SN, Archibald A, Beever JE, Schook LB, Rogers J. (2007) A high utility integrated map of the pig genome. Genome Biol. 8(7):R139.