The Hybrid Hypothesis

8: Closing Thoughts

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Continued from previous section

Now, let’s consider the case of a pig crossing with panins from the standpoint of genetics (a panin is either of the two apes assigned to genus Pan). Some might suppose that in terms of their nucleotide sequences humans are too similar to panins, and too dissimilar to pigs, for them to be pig-panin hybrids. But anyone who does is either unfamiliar with the effects of meiosis at the molecular level, or does not see the ultimate implications of those effects for the genome of a hybrid. The camouflaging effect of gene conversion in backcross hybrids was explained in Section 3 (see the green sidebar there entitled “Genetic Data”) and in the diagram immediately below, (which appeared in Section 3)

Genomic effects of backcrossing:

But an additional consequence of hybrid meiosis, typically, is a jumbling of the genome in later generations. In hybrids, meiosis rearranges chromosomes, and duplicates or deletes them, either in whole or in part. In the present context, then, the expectation that such rearrangements would have occurred makes it more difficult to determine whether genes being compared in pig, human and panin are truly equivalent. To make this clear, it may be worthwhile briefly to describe what happens to chromosomes during meiosis.

In mammals, chromosomes are paired and vary widely in number from one type of organism to another. Each human cell contains 23 pairs — 22 matched pairs (autosomes) and one mismatched pair (the X and Y chromosomes). Pairing is important during meiosis, the process that produces spermatozoa and eggs. Germ cells are produced by cell division. At the beginning of each such division, each chromosome unites with the other member of its pair, a configuration called a tetrad. With chromosomes linked in pairs, the machinery of the dividing cell will distribute one member of each such pair into each of the two “daughter cells” produced by the division. When a tetrad is formed, the two homologous chromosomes composing it actually exchange DNA in a process termed crossing-over. Meiosis is stable under ordinary circumstances when organisms having the same karyotype mate (read some basic information about karyotypes >>). There is little or no tendency for the number of chromosomes to change from one generation to the next. Chromosomes are not subjected to forces that rip them apart, rearrange them, and reassort them. But the picture changes in hybrids, where chromosomes become highly volatile.

When mating occurs between organisms belonging to different chromosets (as is almost invariably the case with organisms treated as separate species), some or all of the chromosomes of the resulting hybrid will be either unpaired or inexactly paired. During meiosis, chromosomes lacking a match do not join to form a tetrad. No cellular mechanism exists to deal with unpaired chromosomes, so they pass into daughter cells at random. In this case, different daughter cells end up with different chromosome complements. Partially matched chromosomes unite to form partial tetrads and exchange lengthy blocks of DNA so that large groups of genes are transposed to new chromosomes. The chromosomes are radically altered both with respect to their genetic content and their overall appearance. (MORE ABOUT CHROMOSOMAL MUTATIONS)

Obviously, the germ cells produced by such mechanisms would vary widely in genetic content. Depending on the cross in question, a larger or a smaller proportion of these gametes, and the later-generation hybrids produced by them, would be inviable. The process of exchange, breakage, repair, loss, and reassortment would continue — in each gamete, in each individual, in every generation, as hybrids mated among themselves or with pure individuals of either of the two parent types — until a generation was reached in which all chromosomes were paired and meiosis became stable once again.

So the karyotype characterizing any such emergent, re-stabilized population would be unlike either of the karyotypes present in the parents that initially crossed to produce it. During the unstable period, chromosomes would have been broken up and reconnected in new configurations as well as reassorted into a new karyotype composed of chromosomes and genes previously present only in separate organisms. If a new type of organism — for example Homo sapiens — emerged from a chromosomal mishmash such as that just described, its chromosomes would differ in structure from those of either of its two parents. Repeated crossing-over, breakage, repair, and reassortment would have created chromosomes in which genes from both parents were mingled. And, as it happens, human chromosomes have in fact been extensively rearranged in comparison with those of panins, a fact that has never been satisfactorily explained in terms of conventional theory.

Backcrossing

Recall that a backcross is a mating of a hybrid with one of the parental types that originally crossed to produce it. If a hybrid backcrosses with an individual of one of those two types, then that parent's DNA will preponderate in the genome of the backcross offspring. If such offspring then backcross again, the progeny will be even more similar genetically to the backcross parent. And an even larger fraction of their DNA will be from the backcross parent. Moreover, as we have seen, even any DNA derived from the other, non-backcross parent becomes more and more like that of the backcross parent with each successive backcross due to the effects of gene conversion (if you need to refresh your memory on this point, please look at the diagram about backcrossing above, showing the genomic effects of backcrossing and read the green sidebar at right above, entitled “Why it may not be easy to evaluate this hypothesis with genetic data”).

In the case of early “humans,” if the initial hybridization were between a boar and a female panin (which, for reasons soon to be stated, seems the likelier possibility), the initial hybrid offspring would be born into a panin troop, which would increase the chances of the hybrid(s) backcrossing with panins. After an initial generation of backcrossing, the probability of additional backcrossing in later generations would be further increased because the backcross hybrids would be more panin-like and would again have grown up among panins. So they would be expected to have an even greater affinity for panins than did hybrids from the initial cross. And, as I have already emphasized, when a hybrid population repeatedly backcrosses to only one of its two parents, the DNA of that parent comes to predominate in the genomes of the hybrids. Long-term backcrossing over many generations can eliminate in the hybrids every genetic trace of the other parent. Of course, when the hybrid becomes genetically (genotypically) identical to one of its parents, it will also be indistinguishable in terms of its physical (phenotypic) traits.

Hybrids can maintain their existence as distinct entities only if at some point they can stop backcrossing and start mating among themselves. To do this, however, they would have to be more fertile than they do in the case of backcrossing. When two individuals of low fertility mate, the chances of producing progeny are much lower than when one such individual mates with a fully fertile parental-type individual. Moreover, in many crosses, hybrids will be of only one sex in the initial generation. In other crosses one sex among the hybrids is sterile. In either of these two cases, backcrossing is an absolute necessity. In some cases, the initial hybrids may be so rare that backcrossing is the only way to find a mate.

At the same time, however, fertility tends to increase in hybrids with each generation of backcrossing. When fertile, or partially fertile, backcross hybrids of both sexes begin to appear, the hybrid population has the option of breaking off and forming a separate (isolated) population. So long as they can produce in every generation a sufficient number of offspring to maintain the population, they can persist even though their fertility may remain poor for many generations thereafter.

In a pig-panin hybrid the chromosomes would be almost entirely unpaired. Meiosis would therefore be severely disrupted. There would be erratic segregation of the chromosomes into gametes, as well as extensive breakage, restructuring, loss, and reassortment of chromosomes — mass mutation. Genetic content would vary widely from one gamete to another. Under such circumstances, many gametes would not even have the genetic wherewithal to reach functional maturity (hybrids, especially ones derived from distant crosses, typically produce far fewer functional gametes than do nonhybrid individuals). Moreover, among those few gametes that did mature, even fewer would develop into mature organisms capable of producing offspring. So even if a rare pig-panin hybrid was actually able to find a mate of its own kind, such a pairing would be highly unlikely to produce viable offspring because, a mating between two individuals of low fertility has a small chance of success. The chances would be greater, if the initial hybrid backcrossed. And this supposition is consistent with observation since in a wide variety of crosses second generation hybrids are produced only in backcrosses, and not by matings among the F₁ hybrids.

The Direction of the Cross

In the case of a pig-panin hybrid backcrossing would most likely have been with panins because the mother in the initial cross would, almost surely have been a panin. There are at least three reasons to reach this conclusion. The first is that a panin penis would probably be incapable of impregnating a sow, but a boar’s penis would be fully capable of carrying out the insemination process with sex roles reversed (Hill 1972, Rodolfo 1934). The second is that a humanlike hybrid would likely require a long period of nurture that a sow would not be able to provide. The third is that during estrus a pink sexual swelling appears on the rump of a female panin. Panin males do not attempt to engage in coitus, even with females of their own kind, unless this swelling is present (Goodall 1983, pp. 190-192; 1986). A boar, on the other hand, will mount any immobile object capable of supporting him, and will voluntarily ejaculate even into an inanimate tubular receptacle if it is of suitable diameter. “It does appear then as if, as far as the boar is concerned, coitus is largely a mechanical process” (Rodolfo 1934, p. 14, see also Sambraus 1990, Abb. 7; Walton 1952, p. 151). When threatened, panin females often attempt to appease the aggressor by crouching down and presenting their genitals.

Thus, if an initial (F₁) hybrid was ever produced from this cross, its mother would almost certainly have been a panin — particularly given that a humanlike hybrid infant would require a mother that could hold it and nurture it during a prolonged period of development. The hybrid would therefore have grown up in a panin troop and, upon reaching sexual maturity, would have been in everyday contact with panins and would have thought of panins as its own kind. Many animals imprint on the animal that raises them and later prefer sex with animals of that type. The same might have been true of proto-humans raised by panins. I say “proto-humans” because any progeny of an initial cross between pigs and panins would have nearly half of their DNA from pigs and would thus be much more similar to pigs than are modern humans (the pig genome is about 10% smaller than that of a panin, so even in the F₁ hybrid, panin DNA would slightly preponderate). Only with successive generations of backcrossing would these piglike traits be reduced, as the backcross hybrids became genetically more similar to panins. An additional reason, then, to suppose that Homo sapiens is the result of backcrossing to panins is the observed preponderance of primate characteristics in humans. And the very fact that such backcrossing seems to have occurred suggests the mother in the initial cross was a panin: When the female in a hybrid cross raises her offspring among her own kind and in the absence of the father, the hybrid usually imprints on, and later seeks to mate with, animals of the same type as its mother.

The series of matings producing humans could perhaps have been completed in only a few generations, but could also have taken many, and have involved a complex mixture of backcrosses and matings of hybrid with hybrid. At this distance in time it’s probably impossible to reconstruct the exact series of events. However, for the reasons just stated, it does seem clear that extensive backcrossing to panins must have occurred. And yet, despite the homogenization of nucleotide sequences resulting from that backcrossing, the hybrids would not become physically indistinguishable from the backcross parent as quickly as their nucleotide sequences did. Under the hybrid hypothesis, this fact that humans have remained physically quite distinct from panins despite their extreme similarity in terms of protein and nucleotide sequences, can be easily explained.

Why are Humans Different from Panins?

Mclean et al. (2011) comment that “the genotypic basis of most human-specific traits remains unknown.” And such is still the case today (2022). Biologists say this because the degree of sequence similarity between humans and panins, which is greater than 98%, doesn’t seem big enough to account for the many obvious physical differences between the two. But under stabilization theory the development of an organism has little to do with point mutations at the nucleotide level. The crucial factor is instead the overall set of genes and regulatory sequences defined by its karyotype. Under that theory, differences between humans and panins would be due primarily to genes being present in different dosages, which would alter the mix of mRNAs and proteins produced during the various stages of development. The human genome, in comparison with the panin genome, would exhibit a shift in dosage toward the dosage seen in pigs, which would cause human morphology and behavior to become less panine and more porcine. Down syndrome is a well-known case in which a dosage difference produces a whole suite of altered morphological traits. No sequence differences exist in such individuals. The altered morphology is due solely to the presence of an extra copy of chromosome 21 (and therefore of an extra dose of the unaltered genes that that chromosome contains).

Nor is there, under that theory, any reason to suppose that the human genome would contain genes sequentially identical to those of pigs. A given type of gene is very rarely present in only a single type of organism. Rather, the typical case for each kind of gene is for a wide variety of organisms to possess slightly altered versions of it. Also, a single type of organism will usually have multiple, differing copies of any given type of gene (these sets of similar but varying genes are called “gene families”). So in the present case, when pig-derived genes underwent crossing-over and gene conversion during hybrid meiosis, they would be expected in most cases to recombine with roughly equivalent panin-derived genes. As a result, successive rounds of backcrossing to panins would cause any such pig-derived genes to become more and more like their panin counterparts.

And yet, all of these converted genes would be expected still to code RNAs and proteins of the same general type that they did originally. The biochemical action of its protein product would not in most cases be greatly altered (for example, the various genes coding for actins, would still code for actins even after conversion). As has already been said, the main factor affecting development of an organism would be the overall set of genes defined by its karyotype, and the associated interactions of genes and regulatory sequences present in different dosages.

A karyotype in a zygote can be thought of as a set of initial conditions. The life cycle, of any stable organism is recursive because at each stage of its life cycle each of the cells in the organism applies the set of rules specified by the karyotype to produce the next stage. Development is the catch-all term for the complex series of events that convert a zygote, during the course of many successive cell divisions, into a mature organism.

Now suppose that a particular zygote contained a panin karyotype, and therefore the particular set of genes contained in such a karyotype, together with the regulatory sequences and all of the other molecules regularly present in that karyotype. Under such circumstances, the zygote begins dividing and its descendant cells go through the series of stages that ultimately produce an adult panin

However, suppose some of these same genes, regulatory sequences, and other molecules were deleted, and that other such genes, sequences and molecules were added, so that a different organism with a different set of gene dosages, to some degree more like the dosages seen in a pig was produced. Then the set of rules governing development would have changed. The interactions of genes and regulatory sequences at each stage of development would differ because those genes and regulatory sequences would be present in different dosages. At any given stage of development some proteins would be produced in greater quantities, others in lesser, so that within the developing organism each cell would respond differently from the way it would respond if it contained a panin karyotype. In this way, the differences at each stage along the path of development would cumulatively result in the production of a human being instead of a panin.

Concluding Remarks

It seems to me that the information thus far presented is consistent with the idea that humans and gorillas originated by hybridization. For humans, the case appears strong, because the hypothesis accounts for such a large number of observations. I consider the gorilla guilty by association, even though far less empirical data is available — both for this animal and, in particular, for one of its two posited parents. I reach this conclusion because 1) the case for human hybridity is persuasive, 2) humans and gorillas both exhibit a pattern of infertility that is otherwise unexplained, and 3) the modicum of genetic and morphological information available for this animal is consistent with the posited hypothesis.

The tentative scenario that I picture is that human beings came into being via hybridization between a pig, whose best modern representative is Sus scrofa, and an ape, best represented today by Pan paniscus. I assume, as a working hypothesis, that before this hybridization event a population of Pan paniscus-like panins was distributed throughout the geographic region now occupied by panins, not just south of the Congo-Zaire-Lualaba river barrier where such animals are found today, but also north, in those areas where only P. troglodytes is now found. Judging from what is known of the African climate in prehistoric times, I think the range of this proto-panin would probably have extended farther north than it does today, particularly in the Nile Valley. It would seem that sometime during the Pliocene, or more probably the Pleistocene, pigs came into contact with panins and hybridization occurred.

Now, it could be that such hybridization occurred only once in very ancient times (perhaps 5,000,000 or more years ago), producing the earliest hominids (australopithecines), and that various early human types hybridized to produce subsequent human types. It may be, too, that various hominid types each arose via a separate cross between a pig and a panin. One possible scenario is that a hybridization event occurred just prior to the time that modern humans first appeared (estimates for this date range from 100,000 to 140,000 years ago), perhaps somewhere in the Nile Valley, followed by an indeterminate number of generations of backcrossing to panins. During this time the hybrids would have improved in fertility, eventually breaking off from the panin population to breed strictly among themselves. During this backcrossing period many pig-derived DNA sequences would become more and more like those of panins. The resulting high level of similarity to panins, together with the fact that primate traits would predominate in hybrids resulting from multiple generations of backcrossing to panins, would explain why humans have invariably been grouped with primates, and would also account for the fact that a connection between pigs and human beings has been overlooked.

It also seems likely that panins crossed with Hylochoerus meinertzhageni, the giant forest hog, to produce the gorilla. This event way well have happened very recently (in fact, it may still be happening); it appears that no fossil remains have been found for the gorilla. The common chimpanzee population (P. troglodytes), also, appears to have been affected by this backcrossing, with some genetic influence carried across from Hylochoerus. This influence seems to be reflected today in such distinctive traits of the common chimpanzee as large body size, heavy jaws and canines, the occasional sagittal crest, and higher levels of genetic and morphological variability (as compared with P. paniscus, which here is considered a genetically isolated residual population). Hybridization with pigs may also have contributed to the increased variability and size of P. troglodytes populations (in comparison with P. paniscus). Thus, as a working hypothesis, P. troglodytes itself can be viewed as a kind of hybrid, but only in the sense that some degree of genetic leakage seems to have seeped through from the human and, especially, the gorilla populations. The human genetic influence seems to be minimal, because backcrossing probably stopped long ago, but in the influence of gorilla heredity on P. troglodytes — since hybridization between gorillas and P. troglodytes appears to continue even today.

I think the question of the gorilla’s hybridity will take longer to resolve than the human case. An immediate, obvious, hindrance is the paucity of information available for the gorilla and, especially, for the forest hog. In addition, several factors seem to indicate that the gorilla may be more highly backbred to panins (particularly to P. troglodytes): 1) Ongoing hybridization seems to be occurring in the case of the gorilla; 2) Gorilla and panin chromosome counts are identical (2n=48), while the human count is lower (2n=46); 3) Fewer morphological differences seem to exist between the gorilla and the two panins than between either of panin and Homo sapiens.

I must admit that I initially felt a certain amount of repugnance at the idea of being a hybrid. The image of a pig mating with an ape is not a pretty one, nor is that of a horde of monstrous half-humans breeding in a hybrid swarm. But the way we came to be is not so important as the fact that we now exist. As every Machiavellian knows, good things can emerge from ugly processes, and I think the human race is a very good thing. Moreover, there is something to be said for the idea of having the pig as a relative. My opinion of this animal has much improved during the course of my research. Where once I thought of filth and greed, I now think of intelligence, affection, loyalty, and adaptability, with an added touch of joyous sensuality — qualities without which humans would not be human.

An anonymous review (from Amazon.com): “Satire, literary scholarship, comedy, investigative reporting, horror — The Department has it all. It’s Confederacy of Dunces + Frankenstein + Moby Dick rolled into one. McCarthy does a great job of populating a university genetics department with clowns, villains, heroes, mad scientists, and monsters while weaving a plot which leads inevitably to the substantial depopulation of that department. He takes a little time getting there, but it’s always a pleasant, informative, and fascinating journey.” View on Amazon.com >>

When it comes to topics like human origins, where the opinions are rigid and the evidence thin, reservation of judgment is best. It is my hope that the arguments presented here will serve as an intellectual springboard allowing the mind to rise above the inflexible creeds of traditional evolutionary thought. Even if the hybrid hypothesis is wrong, any satisfactory theory of human evolution will have to address the facts touched upon in the foregoing discussion. Wrong or right, I believe a final answer is at hand. The obstacles to the acquisition of such knowledge are by no means insurmountable. Scientists around the world are gathering more data every day. If this rising tide of information indicates that the ideas that we have always had about our origins are wrong, we should not hesitate to correct our errors. Time after time, science has dispelled dogma and brought us things that were once beyond imagination. From tiny bacteria to vast galaxies, from telephones to rocket ships, our knowledge has continued to expand. Perhaps we will even at last be able to rend the veil that has so long obscured our own origins. If the hybrid hypothesis is correct, we will be able to find out where we came from. One simple thing is essential to that discovery: In the immortal words of Professor Bernhardt, “It isn’t faith that makes good science, Mr. Klatu. It’s curiosity!”

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