A reader’s comment: “I was truly enlightened upon reading this. Not only is it so well written that even a layman like me can comprehend the more scientific terminology, but reading it has altered my whole concept of man’s origins.”
Mervyn Sanders, UK
A new theory of human origins
EUGENE M. MCCARTHY, PHD Google+ Profile
8: Closing Thoughts(This is section 8. Go to section 1 >>)
(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 was explained in Section 3 (see the green sidebar there entitled "Why it may not be easy to evaluate this hypothesis with genetic data") and in the diagram immediately below, which also appeared
previously. 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)
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.
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.
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 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 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
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.
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.
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!"
What's Big is Pig
They say we ape the apes alone,
That their form is like our own,
That molecules suggest the same:
They're clear links we can't disclaim.
With DNA wise dons decide:
The gap between is slim, not wide.
They judge what's true and dole it out.
And what they tout, we dare not flout.
Yes, bigwig prigs will dance their jig.
But, truth to speak, what's big is pig.
— Gene McCarthy
A speculative email from a reader:
You might recall the PBS documentary, “Meet the Coywolf.” In that documentary it was suggested that the hybrids have always existed in the territory where the ranges of coyote and wolf overlapped, but the hybrids were never as successful as either parent in that environment. But then the environment changed drastically, with the arrival of humans, such that the hybrids were far better adapted to it than either parent.
I’m thinking that something similar could have happened with chimps, pigs and humans: Humans (or human-like monsters) were always present in the overlapping ranges of chimps and pigs, all through the ages that northern Africa was forested, but they were a marginal population. But then, say five MYA, the forest started to retreat as the climate dried out. Suddenly, in geological terms, those hybrids were at a big advantage compared to either parent, because, unlike chimps, they could forage out on the savanna due to their upgraded cooling systems, and they were smarter than pigs and furthermore they could use found objects as rudimentary weapons. So the chimps disappeared from nearly all of their former range while humans were left behind by the retreating tide, so to speak, still living in the same territory as pigs (as shown by the Laetoli footprints), hunting them more and more efficiently and perhaps still occasionally mating with them.
That speculative scenario suggests to me that “humans” are still being produced in the overlapping ranges of chimps and pigs; they just haven’t been noticed because no one has been looking for them. Primatologists tend to study small groups intensively rather than trying to suck up all the data about thousands of animals.