Saturday, February 4, 2012

So I spend a lot of time thinking about human evolutionary origins, or at least in the context of the morphological novelties we possess that set us apart from other species. Fitting, as this is essentially what my research focuses on, albeit in beetles.

One thing that just baffles me is the patchy distribution of hair on our bodies. There are many theories that try to explain this phenomena, but I think most of them are largely incomplete. P.E. Wheeler puts the onus on thermal regulation coupled with bipedality, Markus Rantala claims it had to do with a fitness advantage against ectoparasites, and Elaine Morgan even claims it was an adaptation to aquatic environments! Mine may seem like a fatuous criticism, and that is a valid complaint, but my criticism really hinges on what I consider an outstanding problem in evolutionary biology.

Far too often, evolutionary biology is focused on an ultimate explanation or description of a phenomenon, and generally doesn't possess the proximal explanation. For those unfamiliar with this distinction, let me explain. An ultimate phenomenon is essentially the "why" explanation. An example close at hand is the ultimate explanation for the evolution of beetle horns. As horns are used as weapons in battles for reproductive access to females in all species studied, this is put forward as the explanation for their origin.

You see this sort of thinking in a few of the common (and uncommon) theories regarding the evolutionary loss of hair in the human lineage. Wheeler proposes that hair loss provided a fitness advantage in the arena of body temperature homeostasis as it allowed us to cool through sweating, something other primates don't do. That is, loss of hair allowed us to keep cool more easily. There is some decent data to support this, but I feel I am echoing Gould a bit when I caution away from the easy appeal to fitness to explain every evolutionary novelty.

Unfortunately this explanation is a bit lacking in the mechanics of how this could have happened- the proximal explanation. If we accept that a loss of hair led to a competitive fitness advantage, is it even possible for us to lose hair in the exact pattern we did? I.E. a great reduction in body hair, while retaining thicker pubic, armpit, leg, head, and facial hair? If losing hair led to a fitness advantage, why are we not more completely glabrous? If it functioned in heat dissipation, why did our legs retain denser hair relative to our torso? To ape Dawkins, I am reminded of this quote of his:
"The power of a scientific theory may be measured as a ratio: the number of facts that it explains divided by the number of assumptions it needs to postulate in order to do the explaining."
Replace "theory" with "hypothesis", and I think this quote can be used to argue my point, instead of hammer Rick Perry about creationism. To put it another way, I think the heat dissipation hypothesis just raises more questions than it explains.

Similar problems arise when one investigates other hypotheses of human hair loss, and that is before we even get to the ridiculous "aquatic ape hypothesis".

To return to my beetle example and describe what a proximal explanation might look like, we can turn to some preliminary work by my competitor's lab. They are investigating a few hypotheses that postulate that horns arose out of a co-option of the existing genetic architecture for limb-patterning. To contrast this with the ultimate explanation, this explanation provides a mechanism for how the horns arise that doesn't need to make the initial jump to fitness benefits, and it doesn't need to assume that the horns have always been used in sexual contests (keep Dawkin's quote in mind).

Now, a true evolutionary theory for novelty has both a proximal and an ultimate explanation. In my case, I am searching for other proximal mechanisms to explain the ultimate phenomena of beetle horn morphology and diversity. The question is essentially "what genetic change (proximal) led to the emergence of horns, which were then shaped by sexual conflict (ultimate)?" While most researchers investigating the problem of human hair pattern evolution can honestly claim this is their end goal, my main point of contention lies in the fact that I believe the proximal explanation should be attempted before making assumptions to explain the ultimate.

Now, in strict defiance of all I just said, I am going to propose a new quasi-ultimate explanation for the patterning in human hair evolution, but I think mine is a potentially better avenue for uncovering proximal mechanisms.

We have strong evidence that the human population underwent quite a few bottleneck events in our past, and anyone with even a small background in population genetics can tell you what kind of effects that can have on genetics. I postulate that our ancestors went through one such bottleneck effect, and the resulting buildup of mutations due to the low population size led to the emergence and sweep to fixation of the hairless phenotype. Once established in our population, the various preserved hair patterns were then further shaped by sexual (and potentially natural) selectional forces to encourage facial hair patterns in men (and I could handwave about cultural forces shaping the evolution of this phenotype by citing a few religious texts that discourage the trimming of beards).

Now, there are obvious holes and problems with my hypothesis, but my point is that even if I was to test this, and found a shift towards down regulation of human hair genes around the time of one of our bottleneck events, that would still not be enough of a proximal explanation for this, so maybe I just will never be satisfied.

DISCLAIMER OF BIAS: Everything in this post is "armchair", and I hesitate to call it that. I have done very little formal research on this topic, and would welcome anyone who could provide me some sources to peruse. A further admission of bias is that this entire post is intended as a framework to argue the superiority of initially basing evolutionary origins on expansions of neutral drift theory as a proximal explanation and adding in directional selective forces shaping phenotype as a more ultimate mechanism. Perhaps more critically, it is a plea for researchers to start with the assumption of neutrality per the Dawkins quote above, and only move onto selective pressure when neutrality has already been established for the emergence of traits, or ruled out entirely.

Before I leave this post, I must mention I found a recent review by Wheeler on this subject that attempts to explain this phenomenon by incorporating more genetic analyses, but unfortunately I can't quite get to it. If anyone has a copy, I would appreciate it!

1 comment:

  1. I agree that people are far too prone to go for the ultimate explanations. Of course, a very important reason they do is that it's more sensationalist and gets them more publicity, in both popular and scientific media. At least this has been the case historically, maybe in your field the scientific community is becoming more critical from lessons learned. I know that in my field (psychology) this happens all. the. time. This is largely due to hacks who like making money on it and also the psychological science world having loads of scientists who aren't very scientific, sadly.

    I'd like to add that I also think a big issue is that oftentimes researchers don't only want to find plausible ultimate explanations, they want to find THE ultimate explanation. There must have been a bunch of advantages/disadvantages of less body hair and hey, maybe there wasn't just this one advantage that was critical in the development. Pursuing parsimony is generally good, but not to the point where you're oversimplifying things so much that your hypothesis/theory becomes misleading. I'm sure everyone would agree on this if they explicitly discussed it, and maybe it's something deeply involved biological researchers always keep in the back of their heads, but from what I've read and heard this tendency seems to be a real problem. Especially at for example high schools, though I understand teachers might avoid discussing this so as to not make students go "what so in the end we don't know anything anyway? what's the point, laaaaame let's go shoot ourselves up with heroin instead lol". Have you reflected much on this?