“This has to be a joke,” I thought to myself, as I read the Phys.org article on Dr. Eugene McCarthy, the geneticist who believes that human beings originated as a result of hybridization between a pig and an ape. But it wasn’t. The guy is serious, and he is a bona fide geneticist who specializes in the study of hybridization and who has taught at the University of Georgia. In 2006, his Handbook of Avian Hybrids of the World (Oxford University Press, 2006), was published and favorably reviewed. The book provides information on nearly 4,000 distinct types of hybrid crosses among birds and cites more than 5,000 publications. During his years of work as a geneticist, however, Dr. McCarthy had become increasingly dissatisfied with Darwin’s theory of evolution. In 2007, he attempted to publish another book with Oxford University Press, titled, On the Origins of New Life Forms. The book was made a very favorable impression on one anonymous reviewer (see here for that review), but other reviewers vehemently objected to McCarthy’s bold dissent from Darwinism, so the editor eventually decided to terminate his contract with the book’s author. Following this editorial rejection, McCarthy made the decision to post his entire book online, under the slightly longer title, On the Origins of New Forms of Life: A New Theory.
More recently, Dr. McCarthy has published another online book, The Hybrid Hypothesis: A new theory of human origins, in which he defends the view that human beings trace their origin back to an event several million years ago, probably in North Africa, when a wild boar such as the one pictured above (image courtesy of Richard Bartz and Wikipedia) – an animal that will mate with just about anything, including a tube – copulated with a female ape that looked much like today’s pygmy chimpanzee (or bonobo). McCarthy hypothesizes that pygmy chimps were distributed much further north a few million years ago than they are now, and he points out that female bonobos often “present” their genitals to males as a way of defusing aggression. The offspring was then raised among chimps, and inter-bred with them. The genes of the hybrid offspring would have been hideously jumbled as a result of a cross between a 38-chromosomed pig and a 48-chromosomed pygmy chimp, and over the next few generations, it and its descendants continued to inter-breed with pygmy chimpanzees, losing most (but not all) of their pig DNA in the process, and making them look more and more like chimpanzees. At some point, however, the hybrid population’s genes became stable enough to allow them to interbreed with one another instead of having to interbreed with chimpanzees: they were finally able to produce viable, fertile offspring of their own, in sufficient numbers. The hybrids (who were by then about “98% chimp” in their DNA) then broke away from the chimpanzee population in which they had been raised, and formed their own breeding population. Because they’d inherited an excellent internal cooling system from their pig ancestors that prevented their heads from over-heating, they were quickly able to develop into large-brained creatures: us. McCarthy speculates that each of the different species of hominids in the fossil records may have originated from separate hybridization events between pigs and apes. You can read his closing thoughts about his hypothesis here.
Well, that’s Dr. McCarthy’s theory of human origins in a nutshell. I’ve tried to summarize it as best as I can, in a single paragraph. Unfortunately, Dr. McCarthy doesn’t specify in detail how we got from the 450-cc brain of Australopithecus africanus to the 1250-cc brain of Homo sapiens (successive matings by boars with ever-more advanced hominids?) – which is a real problem for his theory, since he insists (as we’ll see below) that species (or life-forms, as he prefers to call them), once generated, remain more or less static over time, until they die out.
Dr. McCarthy’s theory of human origins: why it won’t work
One thing I’d like to say right up front is that McCarthy’s hypothesis that a boar may have mated with a bonobo in the distant past, simply won’t work, for purely mechanical reasons. Here’s why (warning: the subject matter is highly distasteful and definitely not recommended for children or minors.) (H/t: Darren Naish and David Marjanovic.)
The key points of Dr. McCarthy’s theory
What I’d like to discuss in this post, however, is his theory of how species originate. Here’s a non-technical summary of the bare bones of his theory, which he refers to as stabilization theory. The theory has five main ingredients:
(a) Anti-essentialism: According to Dr. McCarthy, the term “species” doesn’t denote anything objective, and the higher taxonomic categories (e.g. genus, family, order, class, phylum, kingdom) are not natural categories but mere terms of convenience.In this context, McCarthy approvingly quotes Charles Darwin’s letter to Joseph Hooker dated December 24, 1856:
I have just been comparing the definitions of species … It is really laughable to see what different ideas are prominent in various naturalists’ minds, when they speak of ‘species’; in some, resemblance is everything and descent of little weight — in some, resemblance seems to go for nothing, and Creation [is] the reigning idea — in some, descent is the key — in some, sterility an unfailing test, with others it is not worth a farthing. It all comes, I believe, from trying to define the undefinable.
(Darwin, F. ed. 1887. The life and letters of Charles Darwin. London: John Murray, vol. I, p. 88.)
Dr. McCarthy doesn’t like to even use the term “species”; he prefers the term “life-form.”
(b) Stasis: Life-forms are stable over time, from the time when they originate until the time when they eventually die out. Once established, species don’t undergo evolutionary change at all;
(c) Life-forms control their environment, rather than being controlled by it:
“Under stabilization theory, the living organism is the initiative force and creative power — a positive agent, selecting and reconstructing its environment. It is not mere wax upon which an indifferent environment writes at will.”
(d) stabilization theory: certain genetic processes which are known to disrupt the normal reproductive cycle are the typical source of new types of organisms. Each new life form originates suddenly when its recombinant karyotype becomes genetically stabilized. (A karyotype can be defined as “the set of chromosomes present in a particular type of organism, viewed in terms of their characteristic size and structure.”)
Usually (but not always) new life-forms originate as a result of following a hybridization event between two distinct ancestors. Different species hybridize to spawn novel, stable forms nearly instantaneously, which will then persist unchanged for long periods of time until there is another hybridization event.
A wide variety of organisms produce hybrids in a natural setting. Such hybrids occur in large numbers, and they often are not sterile. The ability of hybrids to produce viable offspring varies from cross to cross, and from individual to individual. Within the context of stabilization theory, this fact is important since it opens up the possibility of natural selection among differing hybrids. Therefore, this phenomenon will be considered here at some length. Variation of this sort has long been recognized. After noting that fertility is usually lower in hybrids than in their parents, Stebbins (1969: 33) pointed out that:
the majority of interspecific hybrids, however, are not completely sterile, but are able to produce at least a small percentage of viable gametes, at least of one sex. In the F2 or back cross progeny of such hybrids a great range of fertility exists, and in nearly all such progenies there exist some individuals which are more fertile than was the F1. This fact was already demonstrated many years ago by Müntzing (1930) in his research on Galeopsis tetrahit × bifida, and has been found in the progeny of many other F1 interspecific hybrids (Stebbins, 1950; Grant, 1963).
(e) There is no tree of life, but rather a tangled web. Any two distinct life-forms will be found to share a multitude of different ancestors, rather than springing from a common stock.
Dr. McCarthy has a special Web page on mammalian hybrids, to bolster his claim that different species can and do inter-breed, eventually producing fertile offspring. However, his best examples are from animals that diverged very recently. His documented cases are as follows: Phalangerid hybrids (Family Phalangeridae), Kangaroo hybrids (Family Macropodidae), Pygmy possum hybrids (Family Burramyidae), American opossum hybrids (Family Didelphidae), Armadillo hybrids (Family Dasypodidae), Anteater hybrids (Family Myrmecophagidae), Sloth hybrids (Family Bradypodidae), Elephant-shrew hybrids (Family Macroscelidae), Rabbit hybrids/Hare hybrids (Family Leporidae), Squirrel hybrids (Family Sciuridae), Beaver hybrids (Family Castoridae), Old World Monkey hybrids (Family Cercopithecidae), Gibbon hybrids (Family Hylobatidae) and Pig hybrids (Family Suidae).
All very impressive at first glance, but if you take the animals that have been observed to hydridize and then plug their names into Timetree.org to find out how diverse they are and when they last shared a common ancestor, you typically come up with answers like: four million years ago. They’re not that far apart. However, a pig and a chimp are supposed to have diverged 80 million years ago. (It doesn’t matter for the moment if you believe Timetree.org’s figures as Gospel; what matters here is that the numbers roughly show the degree of genetic diversity between the two species.) Professor P. Z. Myers has some telling comments to make here in a critical review of Dr. McCarthy’s theory (2 July 2013):
However, no one reasonably expects pigs and chimps to be interfertile. The primate and artiodactyl lineages have diverged for roughly 80 million years — just the gradual accumulation of molecular differences in sperm and egg recognition proteins would mean that pig sperm wouldn’t recognize a chimpanzee egg as a reasonable target for fusion. Heck, even two humans will have these sorts of mating incompatibilities. Two species that haven’t had any intermingling populations since the Cretaceous? No way.
To make matters worse, Dr. McCarthy shows himself to be unduly credulous. On the very same page where he lists accounts documented hybrids, he also lists articles about other cases, including “Domestic Dog × Primate (Canis familiaris × Primates sp.)” Hmmm. Dog and primates? When you click on the link, it turns out that it describes cases alleged from the eighteenth century (and given some credence by naturalists at the time) of monkeys inter-breeding with dogs, and then goes on to list cases documented from the fifteenth century (way back before Vesalius) of human beings inter-breeding with dogs, including the following account (several decades after the alleged event) from the surgeon Ambroise Pare: “In the year 1493 a child was conceived and engendered of a woman and of a dog, having, from the navel up, upper parts similar in form and shape to the mother, and it was very complete, without Nature’s having omitted anything; and, from the navel down, all its lower parts were also similar in form and shape to the animal, that was the father: which (just as Volatranus writes) was sent to the pope who reigned at that time [Alexander VI].” I’m sorry, but I do think that this is one example of where the adage, “Extraordinary claims require extraordinary proofs” applies. One would certainly want more documentation than that, to establish such a claim. Dr. McCarthy does himself no favors by publishing such medical rubbish on his Web page.
Hybridization: a destructive rather than a creative process
Professor P. Z. Myers, in his trenchant review of Dr. McCarthy’s theory (see here for an earlier post of his on other problems with the theory), actually made (unintentionally, I’m sure) a lot of ID-friendly points when discussing why hydridization between radically different species simply won’t work as a mechanism for generating evolutionary novelty:
…[T]here are limits to how much variation the cell division machinery can cope with. For instance, with fewer chromosomes than we primates have, that means you need to line up multiple primate chromosomes to match a single pig chromosome (this pairing up is essential for both mitosis and meiosis). Look at pig chromosome 7, for instance: it corresponds to scrambled and reassembled bits of human chromosomes 6, 14, and 15.
Maybe that would work in mitosis within the hybrid progeny — you’d have three chromosomes from the human/chimp parent twisted around one chromosome, but they would be able to pair up, mostly, and then separate to form two daughter cells. But meiosis would be total chaos: any crossing over would lead to deletions and duplications, acentric and dicentric chromosomes, a jumble of broken chromosomes. That would represent sterile progeny and an evolutionary dead end.
But we wouldn’t have to even get that far. Human and chimpanzee chromosomes are even more similar to one another, and there are no obvious chromosomal barriers to interfertility between one another. If hybridization in mammals were so easy that a pig and a chimp could do it, human-chimp hybrids ought to be trivial. Despite rumors of some experiments that attempted to test that, though, there have been no human-chimp hybrids observed, and I think they are highly unlikely to be possible. In this case, it’s a developmental problem.
For example, we have bigger brains than chimpanzees do. This is not a change that was effected with a single switch; multiple genes had to co-evolve together, ratcheting up the size in relatively incremental steps. So you could imagine a change that increased mitotic activity in neural precursors that would increase the number of neurons, but then you’d also need changes in how those cells are partitioned into different regions, and changes in the proliferation of cartilage and bone to generate a larger cranium, and greater investment in vascular tissue to provide that brain with an adequate blood supply.
Development is like a ballet, in which multiple players have to be in the right place and with the right timing for everything to come off smoothly. If someone is out of place by a few feet or premature by a few seconds in a leap, the dancers could probably compensate because there are understood rules for the general interactions…but it would probably come off as rough and poorly executed. A hybrid between two closely related species would be like mixing and matching the dancers from two different troupes to dance similar versions of Swan Lake — everything would be a bit off, but they could probably compensate and muddle through the performance.
Hybridizing a pig and a chimp is like taking half the dancers from a performance of Swan Lake and the other half from a performance of Giselle and throwing them together on stage to assemble something. It’s going to be a catastrophe.
Now the interesting thing is that Dr. McCarthy substantially concedes the above points. Hybridization between two dissimilar species is a terribly chaotic process that disrupts genes and causes widespread mortality in offspring. What he proposes, however, is that given the vast number of matings that occur over time, and the vast number of combinatorial possibilities, it is not surprising that a very tiny number survive, and that of these, a tinier number manage to stabilize themselves genetically, by back-crossing with one of the parent populations over the next few generations, before a population of inter-fertile hybrids then emerges and breaks away from the parent group. But he fails to supply detailed calculations to support his contention, and more tellingly, he fails to document even a single case of such an event happening. A good scientific hypothesis needs better evidence than that.
Dr. McCarthy’s anti-essentialism is old-fashioned nonsense
Dr. McCarthy, in his book on macroevolution, maintains that the concept of a species is no more real than the outdated scientific concept of the ether. I do wish he had taken the time and trouble to familiarize himself Dr. Branko Kozulic’s 2011 paper, Proteins and Genes, Singletons and Species. Dr. Kozulic adduces a wealth of evidence showing that each and every species is distinguished from other species by not one but hundreds of singleton proteins and genes, which are chemically unrelated to other proteins and genes. Dr. Kozulic proposes that these unique genes and proteins, singletons, define the very character of each and every species. If he’s right, then it seems to me that essentialism is alive and well, after all: species are real. (The real question we need to address, in my opinion, is how we should define, in a scientifically rigorous fashion, the higher taxa, such as genus, family, order, class, phylum and kingdom – but that’s a topic for another post.) Here are some relevant excerpts from Dr. Kozulic’s paper:
A recent study, based on 573 sequenced bacterial genomes, has concluded that the entire pool of bacterial genes – the bacterial pan-genome – looks as though of infinite size, because every additional bacterial genome sequenced has added over 200 new singletons . In agreement with this conclusion are the results of the Global Ocean Sampling project reported by Yooseph et al., who found a linear increase in the number of singletons with the number of new protein sequences, even when the number of the new sequences ran into millions . The trend towards higher numbers of singletons per genome seems to coincide with a higher proportion of the eukaryotic genomes sequenced. In other words, eukaryotes generally contain a larger number of singletons than eubacteria and archaea. [Eukaryotes are organisms whose cells have a nucleus, unlike bacteria – VJT.] (p. 16)
When a relative to a singleton is found, together the two proteins create a family. In the absence of biochemical data, nothing can be said about biological function of that protein family as long as no established domain or structural motif is discernible from the amino acid sequences. Such proteins of obscure function, or POFs, make about 25% of the proteins found in each genome [113, 114]. POFs tend to be shorter than the proteins of defined function . (p. 16)
Today, almost ten years since the announcement of the first draft of the human genome sequence, no structural assignment is available for about 38% of human proteins : at present we thus lack basic information about a large fraction of the proteins of human proteome . (p. 16)
Based on the data from 120 sequenced genomes, in 2004 Grant et al. reported on the presence of 112,000 singletons within 600,000 sequences . This corresponds to 933 singletons per genome. In 2005, Orengo and Thornton reported on the presence of about 150,000 singletons in 150 sequenced genomes . In 2006, within 203 sequenced genomes and 633,546 nonidentical sequences Marsden et al. identified 158,798 singletons ; thus the singletons made 24% of all sequences and there were on average 782 singletons in each genome. In 2008, Yeats et al.  found around 600,000 singletons in 527 species – 50 eukaryotes, 437 eubacteria and 39 archaea – corresponding to 1,139 singletons per species. No information about the number of singletons is available in the most recent summary of the data from over 1100 sequenced genomes encompassing nearly 10 million sequences . In spite of the missing recent data on singletons, the results of the above calculations are sufficient for an unambiguous conclusion: each species possesses hundreds, or even thousands, of unique genes – the genes that are not shared with any other species. This conclusion is in full agreement with the power-law distribution of protein families discussed above. (p. 17)
Figure 3 shows how the number of unique genes (singletons), expressed as an average per each sequenced genome, was changing with the total number of the genomes sequenced. Evidently, the number of singletons tends to increase, from several hundreds to more than one thousand. The presence of a large number of unique genes in each species represents a new biological reality. Moreover, the singletons as a group appear to be the most distinctive constituent of all individuals of one species, because that group of singletons is lacking in all individuals of all other species. The conclusion that the singletons are the determinants of biological phenomenon of species then follows logically. In [his] System of Logic, John Stuart Mill outlined his Second Canon or Method of Difference : “If an instance in which the phenomenon under investigation occurs, and an instance in which it does not occur, have every circumstance in common save one, that one occurring only in the former; the circumstance in which alone the two instances differ, is the effect, or the cause, or an indispensable part of the cause, of the phenomenon.” (p. 18)
In short: the evidence for the objective reality of species is massive, and it is scientifically quantifiable. Dr. McCarthy needs to address this evidence.
Dr. McCarthy’s theory fails to account for the origin of new functional information at the level of proteins
Even if we were to allow (very generously, for the sake of argument) that organs with new functions might be generated once in a blue moon, as a result of the genetic swapping and shuffling that occurs during hydridization, Dr. McCarthy’s theory fails to account for functionality at the bottom level in living things. I’m referring to the functionality of proteins themselves, of course. To quote Dr. Kozulic again:
The experimental data of Keefe and Szostak indicate – if one disregards the above described reservations – that one from a set of 10^11 randomly assembled polypeptides can be functional in vitro, whereas the data of Silverman et al.  show that of the 10^10 in vitro functional proteins just one may function properly in vivo. The combination of these two figures then defines a “macromolecular miracle” as a probability of one against 10^21. For simplicity, let us round this figure to one against 10^20. (p. 8)
It is important to recognize that the one in 10^20 represents the upper limit, and as such this figure is in agreement with all previous lower probability estimates. Moreover, there are two components that contribute to this figure: first, there is a component related to the particular activity of a protein – for example enzymatic activity that can be assayed in vitro or in vivo – and second, there is a component related to proper functioning of that protein in the cellular context: in a biochemical pathway, cycle or complex. (p. 8)
To put the 10^20 figure in the context of observable objects, about 10^20 squares each measuring 1 mm^2 would cover the whole surface of planet Earth (5.1 x 10^14 m^2). Searching through such squares to find a single one with the correct number, at a rate of 1000 per second, would take 10^17 seconds, or 3.2 billion years. Yet, based on the above discussed experimental data, one in 10^20 is the highest probability that a blind search has for finding among random sequences an in vivo functional protein. (p. 9)
Each unique gene, and accordingly each novel functional protein encoded by that gene, however, represents a major problem for evolutionary theory because unique proteins are as unrelated as the proteins of random sequences – and among random sequences functional proteins are exceedingly rare. Experimental data reviewed here suggest that at most one functional protein can be found among 10^20 proteins of random sequences. Hence every discovery of a novel functional protein (singleton) represents a testimony for successful overcoming of the probability barrier of one against at least 10^20, the probability defined here as a “macromolecular miracle”. More than one million of such “macromolecular miracles” are present in the genomes of about two thousand species sequenced thus far. Assuming that this correlation will hold with the rest of about 10 million different species that live on Earth , the total number of “macromolecular miracles” in all genomes could reach 10 billion. These 10^10 unique proteins would still represent a tiny fraction of the 10^470 possible proteins of the median eukaryotic size. (p. 21)
If just 200 unique proteins are present in each species, the probability of their simultaneous appearance is one against at least 10^4,000. [The] Probabilistic resources of our universe are much, much smaller; they allow for a maximum of 10^149 events  and thus could account for a one-time simultaneous appearance of at most 7 unique proteins. The alternative, a sequential appearance of singletons, would require that the descendants of one family live through hundreds of “macromolecular miracles” to become a new species – again a scenario of exceedingly low probability. Therefore, now one can say that each species is a result of a Biological Big Bang; to reserve that term just for the first living organism  is not justified anymore. (p. 21)
In short: Dr. McCarthy’s hypothesis is an interesting one, and I agree that it should at least have been published. But it is, like neo-Darwinism, a failure. It fails to address the origin of functional information at the biochemical level, from the bottom up (i.e. proteins). Until it can do that, it does not merit the title of a “theory.” Neither does neo-Darwinism.