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Human origins: Are we hybrids?

Eugene M. McCarthy, Ph.D. Genetics

Published: July 6, 2013

This section is a little different from others on this site, because it's about the findings of my own research. I am a University of Georgia trained geneticist (M.S., Ph.D.) who worked in various genetics laboratories at the University of Georgia and conducted research there from 1989 to 2007. During those years I also taught biology and genetics at UGA.

My 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 the table of contents. But I'll summarize the basic reasoning here, without a lot of citations and footnotes (see below).

My theory of human origins: Rationale

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 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. The reality, however, is something quite different. 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 more than 4,000 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.

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