There may need to be a significant revision in the recent description of one of humanity’s oldest ancestors. Ardipithecus ramidus (or “Ardi” for short), the 4.4 million year old hominid fossil discovery, has been a godsend to paleoanthropologists (pun intended). But one of the key researchers has made what could be a serious error in his interpretation.
Christopher Ryan, who writes for Psychology Today at his blog Sex at Dawn (also the title of his forthcoming book) has discovered evidence that could undermine Owen Lovejoy’s argument about human sexual evolution ever since Ardi:
In a nutshell, Lovejoy argues that the evidence he and his colleagues presented indicates an absence of sperm competition in the human line and shows male provisioning of females that eventually led to the modern nuclear family. Simple enough. But to make this all-too-familiar argument, Lovejoy misleads, misunderstands, and mis-states his own findings to the point where, if this were a graduate-school paper, his professor would demand a re-write.
Ryan demonstrates multiple factual errors in Lovejoy’s paper “Reexamining Human Origins in Light of Ardipithecus ramidus” (the 11th and last in the special series, free access here) and has submitted a letter (along with Todd Shackelford) to the editors of Science.
Two of the most damning points have to do with evidence pertaining to sperm competition. Lovejoy states that the ratio of human testes size to body size is similar to that of gibbons, a monogamous “lesser” ape. However, Ryan points out that the actual figures show something completely different. Citing the authoritative textbook Primate Sexuality by Alan Dixon (which I recently used as a teaching assistant for a class of the same name at Duke) he points out that gibbons have a testes to body ratio of 1 to 1000. Humans come in at 1 to 160. Dixon also shows that chimpanzees and bonobos lie at a ratio of 1 to 36, significantly closer to humans than gibbons are. [UPDATE: A reader has pointed out that these values are in error. See comments below.] This is a glaring oversight on the part of both a distinguished scholar as well as the journal itself.
Another very troubling error is in the case of human sperm production. Lovejoy states that human spermatogenesis efficiency (the daily sperm production per gram of testes) is only a measly 0.06 x 10^6 and he cites Peirce and Breed in their 2001 paper in the journal Reproduction as his source. Using this figure Lovejoy then concludes:
The estimated corresponding value in chimpanzees is greater than that of humans by two orders of magnitude.
That would be a significantly large difference, except that it appears Lovejoy made a mistake because the actual figure from that paper is 6 x 10^6, not 0.06 x 10^6. This, interestingly enough, is two orders of magnitude greater than the figure he cites, making chimpanzees and humans identical. This completely undermines his argument that humans have low sperm production.
Here’s a screen capture of Peirce and Breed’s paper for those who can’t access it past the pay wall:
And here’s the direct quote from Lovejoy’s paper:
Another measure, spermatogenesis efficiency (daily sperm production per gram of testes), “varies from about 2.65 x 10^7 in rabbits to < 0.06 x 10^6 in humans" (46).
Why does this matter? Testes size and sperm production are both key predictors of primate mating strategies. Chimpanzees and bonobos, for example, have a highly promiscuous society where females mate with multiple males during their period of oestrus. In the arms race that developed to outcompete other males, chimpanzees and bonobos have evolved extremely large testicles with correspondingly high sperm volumes to “wash out” an earlier male’s contribution. In contrast, gibbons are socially monogamous (though not strictly sexually monogamous, both partners are known to have “extra-pair copulations” with neighbors) and have correspondingly small testes and reduced sperm production as a result.
By mischaracterizing these values Lovejoy has given a false impression about the evolution of human mating systems. We are much, much closer to chimpanzees and bonobos than we are to gibbons. It’s actually extremely shocking that Lovejoy would have made such a major mistake on what is common knowledge among evolutionary anthropologists.
Ryan suggests the following possibility for why these blatant mistakes weren’t identified in the rigorous peer review process:
Perhaps editors, fact-checkers, and general readers are over-eager to accept even the weakest arguments, as long as these arguments support the notion that sexual monogamy is characteristic of our species’ evolutionary past. This is what they expect and hope to be told.
In Pablo Neruda’s poem “The Enigma” the brilliant Chilean writer questions the ability of the human species (a handful of genes short of being a chimpanzee) to obtain unbiased information about themselves from the natural world.
I walked around as you do, investigating
the endless star,
and in my net, during the night, I woke up naked,
the only thing caught, a fish trapped inside the wind.
Neruda suggests that all of us, poets and paleontologists alike, cast our net out into the world to collect and assemble various facts, but all we end up finding is our own ideas reinforced. While I personally think the scientific method is the antidote to Neruda’s enigma, his point is well taken that our own personal biases can frequently shape the way we interpret (or, rather, misinterpret) information to fit our own views. The value of science as a cultural tool is to root out these errors and to mercilessly critique one another’s results to expose these personal biases. I, for one, will be eagerly awaiting the response from Lovejoy and the editors at Science about these mistakes in such a major research paper and look forward to untangling the enigma of Ardipithecus.
For more on this see my post: Reexamining Ardipithecus ramidus in Light of Human Origins.
Lovejoy, C. (2009). Reexamining Human Origins in Light of Ardipithecus ramidus Science, 326 (5949), 74-74 DOI: 10.1126/science.1175834
Peirce, EJ and Breed, WG. (2001). A comparative study of sperm production in two species of Australian arid zone rodents (Pseudomys australis, Notomys alexis) with marked differences in testis size Reproduction, 121, 239-247. DOI: 10.1530/rep.0.1210239