BOOKS—Human Origins: Are we hybrids?
From ANIMAL PEOPLE, September 2013: (Actually published on October 8, 2013)
Human Origins: Are we hybrids? by Eugene McCarthy Free download from: http://www.macroevolution.net/human-origins.html#.Ud5d8GSgn6k
Recalling my April 2012 review of evolutionary geneticist Eugene McCarthy’s provocative opus On the Origins of New Forms of Life: A New Theory, British ANIMAL PEOPLE online reader Mervyn Sanders wrote recently to mention that “Dr. McCarthy is now claiming that humans are a hybrid of chimp and wait for it, pig. Yet he appears to be able to back his claims up with well researched evidence.”
As with On the Origins of New Forms of Life, there are many ways to read McCarthy: as earnest crackpot, as subtle scientific satirist, as author of mind-experiments that many find disturbing on multiple levels, as an imaginative re-assembler of data that he believes has been misconfigured by conventional wisdom, and––as I see him––as a theorist who may be ahead of his time, but is rarely taken seriously because he insists on literal interpretations of insights that may be closer to metaphors.
McCarthy, a former faculty member at the University of Georgia in Athens, established his expertise about hybridization as author of the Oxford University Press Handbook of Avian Hybrids of the World, published in 2006.
Central to both On the Origins of New Forms of Life and McCarthy’s new near-book-length essay Human Origins: Are we hybrids? is McCarthy’s contention that hybridization is not nearly as rare as is conventionally believed, and though seldom recognized, is the major engine of species evolution. Gradual adaptation leading to natural selection for useful traits, as postulated by Charles Darwin, may also have a role. But since it is hard to see how evolution might have favored long sequences of initially not very useful small changes in body structure, McCarthy believes hybridization provides a better explanation of how mutations emerge and convey a survival advantage to the species who have them.
McCarthy in On the Origins of New Forms of Life built on the “punctuated equilibrium” theory presented by Niles Eldridge and the late Stephen Jay Gould in 1972. The gist of “punctuated equilibrium” is recognizing that while evolution usually proceeds at a glacial pace, it may accelerate abruptly in response to catastrophic events such as a comet hitting the Earth. McCarthy adds to the “punctuated equilibrium” hypothesis the idea that catastrophes may at once stimulate cross-species mating, from loss of access to more suitable mates, and increase the likelihood that cross-species liaisons might produce viable offspring, better suited to the changed conditions than their parents.
Among the mechanisms which might enhance the success of hybridization during a “punctuated evolution” episode are increased exposure to radiation; abrupt changes in diet, with effects on the immune systems of species; and the effects of disease.
I pointed out in reviewing On the Origins of New Forms of Life that McCarthy dismisses most of the possibilities of genetic mixing through the actions of pathogens. This is also a weakness in Human Origins. There are in fact retroviruses, capable of infecting both pigs and people, which through repeated exposure might accomplish the effects of hybridization without need for any contact closer than a sneeze.
A peer’s review
Fellow geneticists who take McCarthy seriously, outrageous as some of his suggestions seem to be, include John Hewitt, who directs the Institute for Behavioral Genetics at the University of Colorado and is executive editor of the journal Behavior Genetics.
Assessed Hewitt in a recent essay for the online science journal Phys.Org, McCarthy “has amassed an impressive body of evidence suggesting that human origins can be best explained by hybridization between pigs and chimpanzees. Extraordinary theories require extraordinary evidence and McCarthy does not disappoint. Rather than relying on genetic sequence comparisons, he instead offers extensive anatomical comparisons, each of which may be individually assailable, but are startling when taken together.” McCarthy “argues that humans are probably the result of multiple generations of backcrossing to chimpanzees,” Hewitt continues, “which in nucleotide sequence data comparisons would effectively mask any contribution from pig.
“It is not yet clear if or when genetic data might support, or refute, our hybrid origins,” Hewitt added. “The list of anatomical specializations we may have gained from porcine philandering is too long to detail here. Suffice it to say, similarities in the face, skin, and organ microstructure alone are hard to explain away. A short list of differential features, for example, would include multipyramidal kidney structure, presence of dermal melanocytes, melanoma, absence of a primate baculum (penis bone), surface lipid and carbohydrate composition of cell membranes, vocal cord structure, laryngeal sacs, diverticuli of the fetal stomach, intestinal ‘valves of Kerkring,’ heart chamber symmetry, skin and cranial vasculature and method of cooling, and tooth structure.”
McCarthy’s own words
Writes McCarthy, “The chimpanzee is plausible in the role of one of parents that crossed to produce the human race because they are generally recognized as being closest to humans in terms of their genetics (I use the term ‘chimpanzee’ loosely to refer to either the common chimpanzee or to the bonobo.) But then the question arises: If an ancient cross between the chimpanzee and some parental form ‘X’ produced the first humans, then what was that parent? Does it still exist? What was it like?
“Many characteristics that clearly distinguish humans from chimps have been noted by various authorities over the years. The task of preliminarily identifying a likely pair of parents, then, is straightforward: make a list of all such characteristics and then see if it describes a particular animal. One fact, however, suggests the need for an open mind: as it turns out, many features that distinguish humans from chimpanzees also distinguish them from all other primates. Features found in human beings, but not in other primates, cannot be accounted for by hybridization of a primate with some other primate. If hybridization is to explain such features, the cross will have to be between a chimpanzee and a nonprimate––an unusual, distant cross to create an unusual creature.
“The other parent in this hypothetical cross that produced the first human would be an intelligent animal with a protrusive, cartilaginous nose, a thick layer of subcutaneous fat, short digits, and a naked skin. It would be terrestrial, not arboreal, and adaptable to a wide range of foods and environments. These traits may bring a particular creature to mind. In fact, a particular nonprimate does have, not only each of the few traits just mentioned, but all of the simple, non-synergistic traits distinguishing humans from their primate kin.
“Any attempt to account for these details in terms of natural selection seems inadequate,” McCarthy says. “It is difficult to see what selective pressures could have caused human beings and pigs to converge in so many different respects. Perhaps it is all just a coincidence, but after a certain point coincidence begins to assume the color of relationship.
“No claim whatever is made that it is actually a fact that humans somehow arose through hybridization of pigs with chimpanzees,” McCarthy cautions. “I merely propose an evaluation of two distinct hypotheses by the usual scientific criterion: the hypothesis less consistent with available data should be rejected.”
McCarthy notes “the frequent use of pigs in the surgical treatment of human beings. Pig heart valves are used to replace those of human coronary patients. Pig skin is used in the treatment of human burn victims. Serious efforts are now underway to transplant kidneys and other organs from pigs into human beings. Why are pigs suited for such purposes? Why not goats, dogs, or bears––animals who in terms of taxonomic classification are no more distantly related to human beings than pigs?”
Relevant to McCarthy’s Human Origins hypothesis is that in August 2000 the Roslin Institute of Scotland and Geron Bio-Med of California, two long-time leaders in genetic research, suspended efforts to produce transplantable organs for humans in pigs to prevent the risk of accidentally transmitting pig endogenous retroviruses into humans. Called PERVs for short, pig endogenous retroviruses do not harm pigs, and may not harm people, but British virologist Robin A. Weiss demonstrated in 1997 that cross-species infection can occur.
Because PERV invades cells much as does HIV, integrating itself into the genetic program of the host, the Roslin Institute and Geron Bio-Med sought to avoid the potential liability if a PERV strain ever attacks humans.
PERV is no longer seen as quite the threat to human health that it appeared to be then, but the discovery of PERV hinted that there may exist a diverse array of retroviruses capable of mixing and mingling genetic material among species, with unpredictable consequences. Like ordinary swine flu, such retroviruses might lie seemingly dormant for decades, centuries, even millennia, before unique combinations of circumstance permit them to emerge and begin shuffling their host species’ DNA.
Should genetic research clearly establish that pigs made a substantial contribution to the specific traits that make us human, whether through direct hybridization among far distant ancestors or with disease as intermediary, the implications of the relationship will be immense.
Even without the ideas McCarthy outlines in Human Origins, pigs––and many bird species, including chickens––would appear to have the intelligence to be accorded whatever enhanced appreciation humans extend to nonhuman primates, dogs, whales, dolphins, etc. Among those people who argue for ethical consideration of other species based on perceived likeness to humans, discovering a direct kinship to pigs might either push pork permanently off the menu, or resurrect rationales for cannibalism.
Meanwhile there is the question of how seriously to take McCarthy, whose ideas about human evolution have already been disparagingly compared by some reviewers to the “aquatic ape” hypothesis advanced by marine biologist Alistair Hardy (1896-1985) and by science writer Elaine Morgan (1920-2013). This is probably the best known previous theory that suggests the major anatomic features McCarthy notes evolved together for a common purpose.
Hardy formulated his “aquatic ape” ideas in 1930, but did not publish them until 1960 in anticipation of the attacks that followed. Morgan published her version of the “aquatic ape” hypothesis in 1972, then turned to literary writing while enduring decades of ridicule. But it is to be appreciated that Hardy presented his ideas, near the end of his professional career, almost 10 years before the discovery of Lucy (Australopithicus). Morgan wrote The Aquatic Ape only two years afterward. Neither Hardy nor Morgan had access until much later in life to a fraction of the fossil evidence we now have about human evolution.
It is now widely believed that some early humans, including Australopithicus, lived mainly along shorelines and fed heavily on shellfish. Hardy and Morgan appear to have been right about that, and perhaps about some relevant adaptations, such as bipedal stance. Many of the adaptations that McCarthy believes came from pigs, such as the subcutaneous fat layer and sparse body hair, might have been helpful to creatures who spent their lives foraging along seashores. Whether these traits were acquired by literal hybridization or via retroviruses or by some other means, they could have conveyed an immediate survival advantage.
Like Hardy and Morgan, McCarthy may be far off in some of what he says, but to dwell excessively on the possibly erroneous details is to miss the greater point. Consider Albrecht Durer’s armor-clad rhinoceros, drawn from second-hand descriptions in 1515. Every detail is wrong; but if all you knew about a rhino was that drawing, and you met one, you would nonetheless know immediately what you had encountered, because the overview––the general features and proportional anatomy––were generally correct.