The non-biodegradability of plastic and the evolution of the human hand

Yesterday, on New Year's eve, we reprised a post we had done a couple of years ago.  That was itself a follow-up to our previous day's post on the nature of evolutionary speculation and reconstruction.  Today, on the actual holiday, we follow up on yesterday's repeated installment.  Yes, you might conclude that we're getting desperate to fill our pages over the holidays!

Well that's not really so, because our two previous posts were intended to be humorous, but also to make a point that is completely serious: our biological nature is often attributed to forces of natural selection in arguments that, to put it bluntly, do not always reflect the tightest of scientific reasoning--indeed, 'explanations' that sometimes are just fitted to a strongly held prior belief, an assumption that is accepted but not tested.  We think that is often not good science, or even not science at all.

Because the points are so highly technical and relate to the key subject of understanding human evolution and our behavior in particular, we'll take them in order:

Plastic is not biodegradable
Plastics are polymers made from natural carbon compounds such as we (today) take from fossil fuels.  They are resilient molecules with strength and durability, which is why we use so much of them.  In particular, however, they cause an environmental problem:  they are not biodegradable.  Discarded plastic litters our garbage dumps, but also our beaches, roadsides, picnic sites, etc.  Perhaps only chewing gum lasts longer (at least, under desks and on movie theater floors).

Everlasting evidence of almost anything we use! http://www.debgoesgreen.com/?p=1349

Unfortunately, because plastics hardly degrade naturally, they last for thousands of years, and with current rates of use that means a huge litter problem, danger to wildlife, and unsightly vistas.  But there can be a beneficial side-effect of the non-biodegradability of plastics:  as has widely been pointed out, our plastic remains will tell future archeologists much about how we lived today!  By the same token, we can turn that to our advantage today, by seeing what plastics tell us about our own past--in particular, our evolutionary past, and in ways you probably hardly realized!

The human hand evolved 'for' something
A characteristic of human beings that has attracted a lot of attention from paleoanthropologists is our hand.  Our dextrous hand enables us to do many things, such as type, pick our noses, and use tools of various sorts.  It is so very handy, so to speak, that it simply must have had some origin due to natural selection.  It couldn't have evolved just by chance.

Yesterday, in a reprise of a post from 2011, we examined the clearly dextrous nature of the fossil hand-bones of the species named Australopithecus sediba.  We argued that the species was wrongly named, in articles that hyped its importance in some ways on the thinnest kind of evidence.  In particular, the long-fingered hand was assumed to be related to early evidence of tool use, despite the minor irritating detail that no tools were found at the site.

The ritual explanation for the human hand's evolution, as an adaptation for tool use, that is offered by anthropologists shows aspects of our culture more than it does of our understanding of evolution.  Natural selection works only if a trait's presence leads to greater reproductive success than its absence.  That means that, whatever the reason, the hand must have evolved (if selection was involved) for some use that increased reproductive success.  Aesthetics alone would not be enough!

The usual handy explanation. Source  

Instead of tool use, we suggested that it was equally, if not more plausible that the hand evolved for its usefulness in masturbation.  Self-pleasuring is a way of generating sexual excitement and readiness, we noted, and would be thus very closely connected to reproductive fitness.  It's a much more direct and hence plausible explanation for the origin of the dextrous hand than is the ability to hurl rocks at fleeing wildebeests or to bring down a bunch of berries otherwise too high to reach by mouth.  So we suggested that the species would be better named Australopithecus erotimanis, to recognize the fundamental role that self-attention played in the evolution of our hands (that, we quickly acknowledge, most clearly were used for tool-making at some later time in our evolutionary history).

Now, one might argue that to suggest that our hand evolved for self-enjoyment was demeaning or silly, trivializing the gravity of the need for true evolutionary explanations.  But this is not because the argument was weak--after all, sexual arousal is even more closely tied to reproductive success than hunting and gathering by stoned ancestors, which might just have evoked laughter in their intended prey.  But topics, or even words, like 'masturbation' are awkward in our society and not likely to be taken seriously in real science....or are they?

To make our point, back to plastics!  We wanted to stiffen our discussion of the use of the hand, based on the actual evidence of the A. sediba finds.  So what could plastics possibly have to do with it?

The plastic tool--that wasn't!
We launch our argument by noting that the original sediba investigators essentially inferred tool use among the creatures they found, despite the absence of tools at their fossil site.  But even the tool-use assumption has been made impotent, one may say, by the restricted definitions that are applied, by which 'tool' refers to hunting, gathering, or warfare.  It may seem natural, but as default arguments so often do, that has become such a reflex explanation that it led investigators to fail to recognize the vital importance of what else wasn't found at the site!

We ourselves fell into the trap.  We failed to mention, in our original post on these finds, that not a single dildo or vibrator was reported among the artifacts at the erotimani site.  Yet, as we noted above, plastics are not biodegradable.  If our ancestors had been using their hands to wield those tools for self-stimulation, surely we would have found them!  But neither their casing nor any of their metal electronic parts (the ones that vibrate) were found.  Not a trace!  Not even a travel case!

Now this is as hard a piece of evidence as one could want for our hypothesis.  Unlike the absence of stone tools and tool-use, the absence of vibrators shows that they must have been using their hands--not tools.  Supporting this fact is that not only were there no stone axes, but there were no stone dildos, either, which of course would have survived to be found today.   You might say that, as practiced tool-users, the erotimani would perhaps have fashioned wooden ones instead of stone, and wood would have decayed and not be found today.  But that argument doesn't hold (so to speak), because wooden ones would have led to very painful splinters which certainly would not have been good for reproductive behavior.  Ouch!

The best explanation must be true....mustn't it?
Now there is so much of a tendency to offer, and uncritically to accept, just-so adaptation stories, that surely we must agree that one of them must be the true one.  But how do you decide which is best?  And why would our effort at explanation be dismissed out of hand, so to speak?  How can one assert that we have not fingered the truth?  Perhaps the reason is not scientific at all, but instead is cultural--reflecting both our need for simple explanations and our particular sensitivities.  This possibility is easy to see.

If someone were to find a structure in beetles, or even oysters, that the organisms routinely used to stimulate their genital organs, and this were related to reproductive behavior, the evolutionary argument would be totally compelling and would be front-page material in the Times and Nature or Science.  Do you doubt that?  So why not in humans--unless this is about our cultural squeamishness rather than science!  Hunting tools are respectable in mixed company in our society, but humping tools aren't.

Upon close inspection, and seriously, our explanation is in every way as good as the usual ones.  We didn't write it just to wet--or rather, whet--your appetite with our suggestive, er, post (sorry! Many words on this topic have potential double entendres, and I'm finding it very hard to work on my post, which drains the pleasure out of it).  Think carefully about how present-day behavior is typically assumed to be the past's selective reason, about the often near total lack of actual evidence for invoking a specific adaptive explanation, especially in regard to vague things such as behavior, but often about structure as well.

So, we challenge anyone to seriously say that our explanation is not at least as good, and at least as closely tied to evolutionary fitness--that is, reproductive success--as other explanations.  On what grounds?

Life is complicated by the fact that most structures have more than one function today, and not all of them need have evolved at the same time, though the tendency is to pick one of them and insist it was in the long, distant past the reason for the structure's existence today.  Fragmentary or statistically weak evidence, today or from the past, is too often accepted if a nice story can be told by it.  We think this isn't the best that science should offer and that, except perhaps in sexual affairs, restraint would be a better policy.

Even indirect evidence, such as the non-biodegradability of plastic, may tell the tale.  You never know.