The Hybrid Hypothesis

A new theory of human origins


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Closing Thoughts
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Whatever we see could be other than it is.
Whatever we can describe could be other than it is.
There is no a priori order of things.
—Ludwig Wittgenstein
Tractatus Logico-Philosophicus

BY EUGENE M. MCCARTHY, PHD GENETICS (Continued from previous section) — Now, let's consider the case of a pig crossing with a chimpanzee from the standpoint of genetics. Some might suppose that in terms of their nucleotide sequences humans are too similar to chimpanzees, and too dissimilar from pigs, for them to be pig-ape 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 is explained in the green sidebar at right below (which has also appeared on previous pages). But an additional consequence of hybrid meiosis, typically, is a jumbling 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 chimpanzee 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)

Since some or all of the chromosomes are partially or completely unpaired during this transition process, the mode of inheritance is non-Mendelian. Pairing is an essential presupposition of Mendel's laws.

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 the parent types. This rearrangement and reassortment would continue, generation after generation, until some generation was reached in which all chromosomes were paired and meiosis became stable once again.

It is often asserted that a given type of organism has a particular form and particular habits because it is constrained by the demands of its "habitat". Natural selection is seen as a force that shapes every aspect of an organism to fit its environment. But the stable form created by the chromosomal mechanisms just described would, in large part, only reflect the underlying meiotic (and, thus, reproductive) stability of its karyotype. (this idea is discussed at length elsewhere on this site)

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 chimpanzees, 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 hybrids backcross solely with only one of the two types of parents that originally crossed to produce it, then that parent contributes most of the DNA to the genome of the backcross offspring. If these offspring then backcross again, their progeny will be even more similar genetically to the backcross parent. 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, at right above, showing the genomic effects of backcrossing and read the green sidebar beneath it, 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 chimpanzee (which, for reasons soon to be stated, seems the likelier possibility), the initial hybrid offspring would be born into a chimpanzee troop, which would increase the chances of the hybrid(s) backcrossing with chimpanzees. 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 chimpanzee-like and would again have grown up among chimpanzees. So they would be expected to have an even greater affinity for chimpanzees 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 its own 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-ape 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 of normal fertility). 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-ape 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

Rodolfo (470.15,16) notes "the extraordinary length of the genital tract of the sow and presence of obstructions, such as the sphincter and the rugate thick-walled cervix, call for special adaptations on the part of the boar for the deposition of the semen in a manner such that the spermatozoa may easily be carried into the Fallopian tubes where insemination probably takes place."

In the case of a pig-ape hybrid backcrossing would most likely have been with chimpanzees because the mother in the initial cross would, almost surely have been a chimpanzee. There are at least three reasons to reach this conclusion. The first is that a chimpanzee 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 the female chimpanzee. Chimpanzee 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, chimpanzee 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 chimpanzee — 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 chimpanzee troop and, upon reaching sexual maturity, would have been in everyday contact with chimpanzees and would have thought of chimpanzees 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 chimpanzees. I say "proto-humans" because any progeny of an initial cross between pigs and chimpanzees 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 chimpanzee, so even in the F₁ hybrid chimpanzee 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 chimpanzees. An additional reason, then, to suppose that Homo sapiens is the result of backcrossing to chimpanzees 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 chimpanzee: 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 chimpanzees 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 chimpanzees despite their extreme similarity in terms of protein and nucleotide sequences, can be easily explained.

Why are Humans Different from Chimpanzees?

Mclean et al. (2011) comment that "the genotypic basis of most human-specific traits remains unknown." Biologists say this because the generally accepted figure of 98% sequence similarity between humans and chimpanzees 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 with the overall set of genes and regulatory sequences defined by its karyotype.

S. scrofa H. sapiens P. troglodytes

Estimated number of genes 26,235¹ 37,381² 32,887³

1. www.ncbi.nlm.nih.gov/genome?term=sus%20scrofa, accessed June 17, 2013
2. www.ncbi.nlm.nih.gov/genome?term=homo%20sapiens, accessed June 17, 2013
3. www.ncbi.nlm.nih.gov/genome/?term=pan%20troglodytes, accessed June 17, 2013


An example
Down syndrome is a well-known example of a dosage difference producing a whole a suite of altered morphological traits. No sequence differences exist in such individuals. The altered morphology is due solely to an extra dose of chromosome 21.

In the table above, note that the total number of genes present in the genome differs markedly between pigs, humans, and chimps. Differences in development between these three types of organisms, then, can be attributed to interactions of genes present in different combinations or in different dosages, which would clearly alter the mix of mRNAs and proteins produced. The human set of genes would be derived in part from pigs, others from chimpanzees.

However, there is no reason to suppose that the genes derived from pigs in modern humans would be sequentially similar to those of pigs, because 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 copies of genes within each class of gene (these are called "gene families"). So in the present case, when pig genes underwent crossing-over and gene conversion they would be expected in most cases to find some roughly equivalent gene in the chimpanzee genome. As a result, successive rounds of backcrossing would convert any such pig-derived genes into close variants of their chimpanzee counterparts.

And yet, all of these converted genes would be expected still to code the same types of RNA and protein 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 new combinations or 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. What is called "development" of an organism is merely a specific portion of the overall life cycle of that organism, typically the period during which a zygote develops into a mature organism.

Now suppose that a particular zygote contained a chimpanzee karyotype, and therefore the particular 32,887 genes contained in such a karyotype (this number is the current estimate for the number of genes in chimpanzees — see table above), 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 chimpanzee.

However, suppose some of these same genes, regulatory sequences, and other molecules were deleted, and that other such genes, sequences and molecules were added from pig, so that a different organism with 37,381 genes (i.e., a human, see table above) 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 new combinations and 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 chimpanzee karyotype. The differences at each stage along the path of development would cumulatively result in the production of a human being instead of a chimpanzee.

So, in brief, stabilization theory looks on development as a recursive process because, typically, a zygote with a particular karyotype uses the rules defined by the karyotype to give rise to other cells with a particular karyotype (usually the same one). The daughter cells then follow the rules defined by their karyotype to give rise to other cells with that karyotype, and so forth. The genes and regulatory sequences defined by the karyotype guide the process at every step. Alterations in the karyotype, such as the addition or deletion of genes and regulatory sequences, result in differences at each stage in development that accumulate to alter the ultimate developmental fate of the organism.

A proposed test

One promising option that might well resolve the question of whether humans are pig-ape hybrids would be in silico chromosome painting, a computer-based technique that's powerful yet fairly straightforward. This method visualizes on a computer screen the various chromosomes of a target organism, in this case those of the human genome. To test the question of whether we are hybrids you would take millions of short, randomly chosen nucleotide sequences from pig and chimpanzee and find the best match for each in the human genome. The genomes are now completely sequenced for all three of these organisms. Any regions on human chromosomes showing affinity to pig could be color-coded blue, say, and those similar to chimp could be marked in red. If the genome then showed one or more blue patches, you would have a result that would be inexplicable under conventional theory. Also you would then know where to look for more details confirming a connection to pig. But, again, it would only work if the human genome has not been too thoroughly homogenized toward chimpanzee in terms of its sequences by repeated backcrossing (as described in the green sidebar at right above). I, however, am not as yet set up to do in silico painting. But I am looking for an online site that I could use to carry out such research. If such a site is not available, I will have to write appropriate software myself. Of course, if anyone can provide access to such software and save me that work, I would greatly appreciate it.

Another option — which I myself definitely do not favor — but which several biologists tell me they would like to try, is actual production of a pig-chimpanzee hybrid by artificial insemination. To me, this is a scary, off-limits, even Frankenstein-like option, which, incidentally, I describe at length in my Kindle novel, The Department.

Concluding Remarks

It seems to me that the information thus far presented is consistent with the idea that both humans and the gorilla 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 the pygmy chimpanzee, Pan paniscus. I assume, as a working hypothesis, that before this hybridization event a population of Pan paniscus-like chimpanzees was distributed throughout the range of the chimpanzee, not just south of the Congo-Zaire-Lualaba river barrier where such animals are found today, but also north, in those areas where only the common chimpanzee is now found. Judging from what is known of the African climate in prehistoric times, I think the range of this proto-chimpanzee 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, Sus scrofa, entered the range of the chimpanzee, and at some point hybridization occurred.

Now, it could be that this hybridization occurred only once in very ancient times (perhaps 5,000,000 years ago or more), producing the earliest hominids (australopithecines), and that various early human types hybridized to produce subsequent human types. It may be also that various hominid types each arose via a separate cross between pig and chimpanzee. 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 the chimpanzee. During this time the hybrids would have improved in fertility, eventually breaking off from the chimpanzee population to breed strictly among themselves. During this backcrossing period many pig-derived DNA sequences would become more and more like those of chimpanzees. The resulting high level of similarity to chimpanzees, together with the fact that our primate physical traits predominate, would explain why we have invariably been grouped with primates, and would also account for the fact that a connection between pigs and human beings has always been overlooked.

It also seems likely that the common chimpanzee 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 chimpanzee population (Pan 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 the genetically isolated residual population of pygmy chimpanzees). Hybridization between chimpanzees and Sus scrofa may also have contributed to the increased variability and size of common chimpanzee populations (in comparison with pygmy chimpanzees). Thus, in my working hypothesis, I look on the common chimpanzee, itself, 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 case of the gorilla it may be more substantial — since hybridization between gorillas and chimpanzees 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 the chimpanzee: 1) Ongoing hybridization seems to be occurring in the case of the gorilla; 2) Gorilla and chimpanzee chromosome counts are identical (2n=48), while the human count is lower (2n=46); 3) Fewer morphological differences seem to exist between gorilla and chimpanzee than between chimpanzee 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.

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 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!"


Implications of stabilization theory >>
Works Cited >>
Mammalian hybrids >>
Online Biology Dictionary >>
We shall not cease from exploration
And the end of all our exploring
Will be to arrive where we started
And know the place for the first time.
~T. S. Eliot
Four Quartets, Section V.



The Hybrid Hypothesis - © Macroevolution.net
Genomic effects of backcrossing in a typical hybrid cross:
effects of backcrossing


Why it may not be easy to evaluate this hypothesis with genetic data

In connection with the hypothesis that human origins can be traced to a hybrid cross, it's important to realize that in most mammalian hybrid crosses, the male hybrids are usually more sterile than are the females. This fact means that breeders working with hybrids typically mate fertile females with one of the two parents (that is they "backcross" them). They do not, as a rule, produce new breeds by breeding the first-cross hybrids among themselves.

Often, even after a backcross, the resulting hybrids are still fertile in only one sex. So repeated backcrossing typically occurs. However, after a sufficient number of backcrosses, fertile hybrids of both sexes are often obtained and the new breed can thenceforth be maintained via matings among the hybrids themselves. Repeated backcrossing tends to be more necessary in cases where the parents participating in the original cross are more distantly related and genetically incompatible. So one expects also, in the case of new types of organisms arising via natural hybridization, for backcrossing to be the usual route to fertility and reproductive stability. And the same would hold in the specific case of humans arising via hybridization.

However, consider the effect of such repeated backcrossing on the human genome. The reader may not be familiar with the phenomenon of gene conversion, but its effect on hybrids during backcrossing is to quickly homogenize gene sequences. To understand why this is the case, consider the effects of backcrossing on hybrid DNA.

holliday junction
A Holliday junction. During meiotic recombination, double strands of parental DNA, shown here moving into the junction, separate into two single strands. Each single strand from one parent then joins with a single strand from the other parent. The resulting composite double strands then move out of the junction, undergoing gene conversion in the process.

In the figure above, note that in either of the parental (incoming) double strands each nucleotide in one strand is properly paired with its complementary nucleotide in the other strand, A is always paired with T, and C is always paired with G. So the paired double DNA strand from one parent might look like this:

AGTTCCGACGCG

TCAAGGCTGCGC

while the strand from the other parent might look like this:

AGCTCCGACGCG

TCGAGGCTGCGC

In each of these two double strands all nucleotides are paired with the complementary nucleotide (A always with T, and C always with G). But when one double strand is compared with the other it's clear that they differ at the third nucleotide base position. In the first double strand the nucleotide base pair is T-A, while in the second it is C-G.

Thus, when these strands associate with their new partner strands after passing through the Holliday junction in meiosis, the two resulting double strands will be:

AGTTCCGACGCG

TCGAGGCTGCGC

and

AGCTCCGACGCG

TCAAGGCTGCGC

So T is paired with G in the first outgoing strand and C is paired with A in the second outgoing strand. Gene conversion converts each such mismatched pair into a matched one by replacing one of the two nucleotides with the complement of the other nucleotide (which remains unchanged). Experimental results suggest that the mechanism chooses at random which of the two nucleotides to replace so that the nucleotides derived from either parent survive at equal rates.

So, in any given case of backcrossing, suppose the genomes of the original parents A and B, which produced the first-cross hybrids, differed at one nucleotide position in five (20%). Then the DNA in gametes of the hybrids would differ from A at one position in ten and from B at one position in ten. That is, the first cross hybrid's gametes would be right in the middle between A and B with respect to gene sequence.

However, after one backcross to A, and the resulting gene conversion during meiosis in the backcross hybrids, the gametes produced by backcross individuals would differ from A at only one position in twenty (5%). And gametes produced by second backcross hybrids would differ at only one position in forty (97.5% similarity). It's clear then that it rapidly becomes quite difficult to distinguish, on the basis of nucleotide sequence data, the backcross hybrids from the pure parent A to which backcrossing has occurred. Chimpanzees and humans are about 98% similar in terms of their nucleotide sequences.

The specific genetic underpinnings of the many traits that distinguish humans from chimpanzees are explained, in terms of the present theory, in a separate section entitled Why are Humans Different from Chimpanzees?



Questions or comments about this theory are welcome. Simply send a message to the author through the contact page of this website. He'll be happy to respond!


Evaluating the hypothesis with in vitro fertilization

I think it would be interesting to do in vitro fertilizations in order better to evaluate the chimp-pig hybrid theory of human origins. But I don't know how many fertilizations you would have to do to actually reject the hypothesis, since some hybrid crosses take a lot of inseminations to produce a single hybrid. Consider, for example, the case of domestic hen (Gallus gallus) x capercaillie cock (Tetrao urogallus). The following is the information on that cross, cut and pasted from Handbook of Avian Hybrids of the World, my Oxford University Press book on bird hybrids: "Skjervold and Mjelstad (1992) artificially inseminated 25 hens with semen from 22 capercaillies. Of 1148 incubated eggs, eight embryos developed and three hatched. Surprisingly, seven of the eight showing development, and all of three that hatched and reached maturity, were sired by a single capercaillie. The hybrids had feathered legs down to the feet like capercaillies. When young, they sounded like chickens, when older, more like capercaillies. Sperm survival was drastically reduced for capercaillie semen in hen sperm storage glands (in comparison with that of roosters). Skjervold and Mjelstad say their results strongly indicate a genetic component in sperm survival and that treatment of recipient hens with immunosuppressive drugs would therefore be of interest. In an old report Schröder (1880) says several hybrids were reared. He also alleges that hybrids of both sexes were fertile in backcrosses to domestic fowl, to which, supposedly, the backcross hybrids were very similar. "

There are several factors here that might complicate simple attempts at fertilization as a test of the chimp-pig hypothesis. One is that in some crosses many inseminations are required to produce just a few fertilizations. In the example given, the rate was only eight fertilizations resulting from 1148 inseminations, and, if anything, I would say a chicken and a capercaillie are more similar than a pig and a chimpanzee. So you might think the rate would be lower with a chimp-pig cross, perhaps much lower. There is also apparently some unknown factor that allowed one male among the 22 capercaillie cocks to sire all offspring. So you would have to try lots of different pigs with lots of different chimpanzees before you could start to feel confident in your results. It might be that we are talking about a very, very rare hybrid that is very difficult to produce. And yet, if even one such was ever produced and it backcrossed, that single mating would be enough to get the whole process started. Then again, since no one has ever tried it, this might be a very easy cross to produce. So this method, I would say, would have greater prospects of confirming the hypothesis than of rejecting it. Because you would not be able to reach definitive conclusions by a simple failure to produce the hybrid. I know from the experience of looking at thousands of reports on hybridization that nothing is more common than someone failing to produce a hybrid and claiming it's impossible, only to have someone else later report a success.



Horizontal Transfer - Viral transfer of genes: Not a good explanation

Scientists who think hybridization plays no important role in evolution often speak of "horizontal transfer," which is the transfer, by means of viral infection, of genes from one type of organism to another. Some critics of the theory that humans are derived from hybridization between pigs and chimpanzees have suggested that horizontal transfer of genes from pigs to humans is a more plausible explanation of the many pig-like traits that distinguish us from other primates. The idea is that an initially ape-like animal was modified when infected with viruses of pig origin.

However, in my opinion, there are several problems with this view. One salient difficulty is that close association between humans and pigs is a relatively recent phenomenon, dating back only to the advent of agriculture, a few thousand years ago. But the existence of humans, as documented by paleontology, long predates that time. Of course, it might well have been the case that some animal similar to the domestic pig existed at a much earlier date, but pigs did not at that time live in regular association with humans or, for that matter, with human precursors. So if we somehow absorbed DNA from some other organism via horizontal transfer, why is it that the traits that distinguish us from primates don't connect us instead with dogs? We've been living with dogs far longer than with pigs, and that contact has been more intimate, which would, presumably, make viral infection more likely. The close relationship between Homo sapiens and Canis familiaris dates back to a time when humans were living in caves. But the traits that distinguish us from other primates link us with pigs, not dogs.

In general, retroviruses are proposed as the agents of horizontal transfer, because viruses of this sort actually splice themselves into the DNA of the infected host, and therefore have the potential to be inherited. But for several reasons, retroviruses cannot be used to construct a satisfactory explanation of how humans might have obtained so many porcine traits.

  • The first is that retroviruses carry a very small amount of genetic information. When traveling between cells they must package themselves into a container, called a "capsid." The typical capsid contains only about enough space for, say, 30 kilobases of RNA. This figure varies from one type of retrovirus to another, but by less tnan an order of magnitude. Compare this with the addition of an average human chromosome, which contains on average about 3 billion/46 = 65,000 kilobases of DNA. That is, about two thousand times as much information as the entire genome of a retrovirus. But a single hybridization event typically introduces not one, but many chromosomes into a genome. So all the genetic information required to produce the various pig-like traits of humans could be delivered to a chimpanzee egg by a single pig gamete, whereas, presumably, hundreds of thousands of retroviruses would be required to deliver the same information. Moreover, it is not at all clear why all those retroviruses infecting the precursors of humans would have come only from pigs, and not instead pigs and a variety of other types of animals.
  • Furthermore, much of the 30 kb of RNA that a retrovirus can carry across from one organism to another is it's own genome. Any gene that it picks up from one organism to carry across can be no larger than about half of 30kb, at most. So that means about 15kb. So if you compare that to say, 10 human chromosomes, a typical quantity of DNA in a fairly average hybrid cross, the ratio of information would be (65,000 kb x 10):15kb = 43,333:1. In other words, the amount that can be carried across by a single retrovirus is truly minuscule in comparison with what can be transferred in a single hybrid mating.
  • Moreover, for a retrovirus to become heritable, it must enter the germ line. So it must initially infect a gamete, which is a far rarer event than the mere infection of somatic cells.
  • Further, I find horizontal transfer, as a theory of human origins, unsatisfactory from a philosophical standpoint. In science we are supposed to construct theories, to the greatest extent possible, with known phenomena. It's well known that many new forms of life, treated as species, have been produced by hybridization and the various stabilization processes I describe in the general theory of evolution presented elsewhere on this site. But is there even one known organism treated as a species that has resulted from infection with retroviruses? It seems there is not even a single case. Certainly there are not many. So why frame an explanation of human origins in terms of this apparently unobserved phenomenon?
  • Another objection, of a general nature, is that change through horizontal transfer would take a long time. You don't expect an organism to go out one afternoon and have one of its germ cells infected with 43,333 separate retroviruses. You would expect them to accumulate in the germ line over time, with the passage of many generations. But the fossil record shows that the typical new form of life arises abruptly and remains unchanged up to the time of its extinction. So where does that leave retroviruses, which presumably must be gradually accumulated in vast numbers in the germ line over many generations if they are to have an effect as large as those seen when chromosomes are introduced via sexual hybridization? If you were not trying to account for data of abrupt change followed by stasis, they might be satisfactory. But unless you are going to posit some unknown mechanism that might allow all those virally introduced genes to lie dormant, and then to magically be released to suddenly to have a large effect, I don't see how anyone can expect them to produce the observed phenomenon, that is, the pattern seen in fossil record where new forms typically arise abruptly and then remain unchanged up to the time of their extinctions (i.e., Punctuated Equilibrium). This is not to say that they would have no effect whatsoever. I just can't see why they would be expected to have in any way as large an effect as whole chromosomes introduced by hybridization.
  • One other phenomenon seems inconsistent with the retroviral thesis, and that's the observation that hybrids typically have a reduced fertility in comparison with their parents. When you produce a new organism via hybridization and stabilize a new organism with a new karyotype, the simple fact that it has a new karyotype means that it's offspring in hybrid crosses with preexisting forms will have a reduced fertility. This is because the chromosomes will not be paired in the hybrids. So hybrid sterility is accounted for by stabilization theory. On the other hand, merely sprinkling the genome with retroviruses is not expected to reduce the fertility of the affected organism. Such a hypothesis does not, then, account for the fertility problems actually observed in humans, whereas a presumed hybrid origin does.
The idea of horizontal transfer seems to be favored among people who are uncomfortable talking about sex or with those who suppose that hybrids are somehow always completely sterile, or that they don't occur in a natural setting. Nothing could be further from the truth, though many people imagine otherwise. Anyone who doubts that hybrids occur naturally or that they are often fertile should take a look at my book on bird hybrids (Handbook of Avian Hybrids of the World, Oxford U.P., 2006) or on the mammalian hybrids page of this website, where I'm making public, page-by-page, a lengthy manuscript on hybridization in mammals. I can't imagine that anyone who read the documented information offered there would still think that hybrids are sterile of rare, or that they do not occur in a natural setting. All that is simply an ancient wives' tale that I'm working hard to dispel.



The Department
The Department. Check out the author's kindle novel, The Department, a satire of academic life, which includes an F₁ pig-ape hybrid as one of its major characters.
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What's Big is Pig
— Gene McCarthy


They say we ape the apes alone,
That monkeys' shapes are as our own,
That molecules suggest the same:
They're links to chimps we can't disclaim.

With DNA, wise dons decide:
The gap between is slim, not wide.
They judge what's true. They dole it out.
And what they tout, one dares not flout.

Yes, bigwig prigs will dance their jig.
But, truth to speak: What's big is pig!


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