What’s that, you say? Baculum is the technical term for the penis bone. Many mammals have one – presumably to aid in sexual intercourse. For mammals that mate infrequently, prolonged intercourse ups the chances that a particular male sires some babies. For mammals that must mate quickly, the baculum provides immediate rigidity. And for all mammals, keeping the urethra straight while copulating is imperative, so maybe it’s there to prevent a kink in the works, so to speak. The truth is, there are a lot of hypotheses about what bacula do but – as you might imagine – they’re kind of difficult to test. Regardless, our nearest evolutionary neighbors, the great apes, all have bacula, as do most other primates. Gilbert and Zevit cite this– the fact that our baculum is missing – as evidence for their argument. Which goes like this:
- A rib seems like an unlikely origin for Eve because male and female humans have the same number of ribs.
- Ribs also lack “intrinsic generative capacity”, which penises have “in practice, in mythology, and in the popular imagination”.
- Most mammals – and especially primates – have bacula, humans do not.
- It is therefore “probable” that Adam’s baculum was removed to make Eve, and not a rib.
The authors then continue to support their argument with alternate translations of the Hebrew word for “rib” (which they say could mean “support beam”) and claim the raphe of the human male scrotum is what Genesis 2:21 is referring to when it says “The Lord God closed up the flesh.”** I’m almost convinced!
Almost. Lots of evolutionary innovation occurs through gaining functions, but losing functions (or appendages) also happens. Humans are different from the other great apes in a lot of ways – did you know we’re the only ones with chins? Just because we’re related but lack an otherwise common trait doesn’t mean God took it from us. It’s also interesting to note that some species – like the walrus – have gigantic bacula (like 22 inches gigantic and the largest fossilized baculum from an extinct walrus species comes in at 4 feet). Great apes have much, much smaller bacula – and the closer they are to us, the smaller it is (Figure 1).
Why do humans lack a baculum? Well, there are several theories. Richard Dawkins has hypothesized that sexual selection is responsible, as erectile function may be an honest signal of a potential mate’s health***. Perhaps our mating system – which allows for more and shorter copulations instead of infrequent and longer copulations – made them costly and useless enough to be selected against. Or maybe the bacula serve no purpose – they’re vestigial in great apes. There is a lot of speculation about the “missing” human baculum on the internet and scientific literature – I’m almost embarrassed to be adding to the load – but the point here is that this argument is an odd mix of science and creationism and the end result is a story that makes less sense than if the authors had stuck to one or the other. They invoke phylogenetic concepts to justify their religious opinion – basically, they’re saying “Our nearest evolutionary relatives have bacula (as do most members of our clade Mammalia), so if we don’t have one, God must have taken it – to produce female human beings.”
That last clause there – the part where it creates the second gender, is the part I get least when I consider the distribution of bacula across the animal kingdom. I know humans are special but still – why do some animals have bacula at all? I’m trying to not be disrespectful and snarky – but seriously, this argument is inconsistent with the natural world. Bacula ossify by a different mechanism than, say, your femur – it’s not part of the main skeletal system. This may allow it to be more easily lost and/or gained through time and could help explain why we (and spider monkeys and whales and hyenas and ungulates) aren’t really “missing” it, we just lost it.