BY EUGENE M. MCCARTHY, PHD GENETICS
This article is a little different from others on this site, because it’s about the findings of my own research. I’m a geneticist whose work focuses on hybrids and, particularly, the role of hybridization in the evolutionary process. Here, I report certain facts, which seem to indicate that human origins can be traced to hybridization, specifically to hybridization involving the chimpanzee (but not the kind of hybridization you might suppose!). You can access detailed and documented discussions supporting this claim from links on this page. But I’ll summarize the basic reasoning here, without a lot of citations and footnotes. (If you would like to read an even briefer summary, click here; read about some objections to the theory here; also, a recent news story).
So why do I think humans are hybrids? Well, first of all, I’ve had a different experience from most people. I’ve spent most of my life (the last thirty years) studying hybrids, particularly avian and mammalian hybrids. I’ve read thousands, really tens of thousands, of reports describing them. And this experience has dispelled some mistaken ideas I once had about hybrids, notions that I think many other people continue to take for granted.
For example, one widespread, but erroneous, belief is that all hybrids are sterile. This idea keeps a lot of people from even considering the possibility that humans might be of hybrid origin. This assertion is absolutely false — though I have in fact heard lots of people make it. For instance, in reviewing the reports I collected for my book on hybridization in birds (Handbook of Avian Hybrids of the World, Oxford University Press, 2006), which documents some 4,000 different kinds of hybrid crosses among birds, I found that those crosses producing partially fertile hybrids are about eight times as common as crosses known to produce sterile ones. The usual result is a reduction in fertility, not absolute sterility. My current work documenting hybridization among mammals shows that partially fertile natural hybrids are common, too, in Class Mammalia. And yet, it seems most people base their ideas of hybrids on the common mule (horse x ass), which is an exceptionally sterile hybrid, and not at all representative of hybrids as a whole.
I should, perhaps, also mention that differences in parental chromosome counts, even rather large ones, do not preclude the production of fertile hybrids. While differences of this sort do bode ill for the fertility of the resulting progeny, it is only a rule of thumb. For example, female geeps, the products of hybridization between sheep (2n=54) and goats (2n=60), can produce offspring in backcrosses. Likewise, female zeedonks (Burchell’s Zebra, 2n=44 x Ass, 2n=62) have also been fertile in backcrosses. There are many other examples of this sort among mammalian hybrids. Therefore, such differences between the parents in a cross do not in any way guarantee an absolute sterility in the hybrid offspring. (For those readers who do not know, backcross hybrids are produced when hybrids from a first cross mate with either of the two types of parents that produced them. When the resulting progeny mate again with the same parental type, the result is the second backcross generation, and so forth.)
A second so-called fact, which might make it seem impossible for humans to have had a hybrid origin, is the equally erroneous notion that hybrids, especially successful hybrids, do not occur in a state of nature. A third is the mistaken idea that only plants hybridize, and never animals. In fact, however, natural, viable, fertile animal hybrids are abundant. A wide variety of such hybrids occur on an ongoing basis (read a detailed discussion documenting these facts). For example, of the 5,000 different types of hybrid crosses listed in my book on hybridization in birds, approximately half are known to occur in a natural setting (download a PowerPoint presentation summarizing data on hybridization in birds). My current research indicates a comparable rate for mammals.
Sequence data. And I must now emphasize a fact that I, as a geneticist, find somewhat disappointing: With nucleotide sequence data, it can be very difficult to identify later-generation backcross hybrids derived from several repeated generations of backcrossing (to understand the basic problem, see diagram at right). Instead, as is the case with other later-generation backcross hybrids, the most revealing data is of an anatomical and/or physiological nature. And this is exactly the kind of hybrid that it looks like we are — that is, it appears that humans are the result of multiple generations of backcrossing to the chimpanzee.
The thing that makes backcross hybrids hard to analyze using genetic techniques is that, in terms of nucleotide sequences, they can differ very little from the parent to which backcrossing occurs. It’s important to realize, however, that a lack of such differences does not prevent them from differing anatomically. Sequence differences are not necessary for anatomical differences to be present. An obvious example of this phenomenon is Down’s syndrome. Individuals affected by Down’s regularly exhibit certain distinctive anatomical features, and yet in terms of their nucleotide sequences they do not differ in any way from other humans. To detect someone with Down’s syndrome, sequence data is completely useless. But with anatomical data, detecting affected individuals is easy. This issue is discussed in more detail in a subsequent section. But for the present, take a careful look at the diagram explaining the genomic effects of backcrossing (at right above).
Human infertility. Another observation that appears significant in connection with the hypothesis under consideration is that it has been well known for decades that human sperm is abnormal in comparison with that of the typical mammal. Human spermatozoa are not of one uniform type as in the vast majority of all other types of animals. Moreover, human sperm is not merely abnormal in appearance — a high percentage of human spermatozoa are actually dysfunctional. These and other facts demonstrate that human fertility is low in comparison with that of other mammals (for detailed documentation of this fact see the article Evidence of Human Infertility). Infertility and sperm abnormalities are characteristic of hybrids. So this finding suggests that it’s reasonable to suppose, at least for the sake of argument, that humans might be of hybrid origin. It is also consistent with the idea that the hybridization in question was between two rather distinct and genetically incompatible types of animals, that is, it was a distant cross.
Methodology. The chimpanzee is plausible in the role of one of the parents that crossed to produce the human race because they are generally recognized as being closest to humans in terms of their genetics (here, I use the term chimpanzee loosely to refer to either the common chimpanzee or to the bonobo, also known as the pygmy chimpanzee; the specific roles of these two rather similar apes within the context of the present hypothesis will be explained in a subsequent section). 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?
As the reader might imagine, if the assumption is correct that one of our parents is the chimpanzee, then it should be possible actually to identify the other parent as well. A hybrid combines traits otherwise seen only separately in the two parental forms from which it is derived, and it is typically intermediate to those parents with respect to a wide range of characters. Naturalists routinely use these facts to identify the parents of hybrids of unknown origin, even backcross hybrids.
First they posit a particular type of organism as similar to the putative hybrid (in the present case, this organism is the chimpanzee). They then list traits distinguishing the hybrid from the hypothesized parent, and this list of distinguishing traits will describe the second parent. A detailed analysis of such a triad will often establish the parentage of the hybrid. The traits in question in such studies are generally anatomical, not genetic. DNA evidence is used in only a very small percentage of such identifications (and even then, rarely in efforts to identify backcross hybrids), and yet firm conclusions can generally be reached.
So in the specific case of humans, if the two assumptions made thus far are correct (i.e., (1) that humans actually are hybrids, and (2) that the chimpanzee actually is one of our two parents), then a list of traits distinguishing human beings from chimpanzees should describe the other parent involved in the cross. And by applying this sort of methodology, I have in fact succeeded in narrowing things down to a particular candidate. That is, I looked up every human distinction that I could find and, so long as it was cited by an expert (physical anthropologist, anatomist, etc), I put it on a list. And that list, which includes many, many traits (see the lengthy table on the right-hand side of the next page), consistently describes a particular animal.