The evolution of peculiar human traits. Sigh

In one short window of Twitter feed on a Sunday morning, people are tweeting and retweeting links to a  a story in the Guardian called "Why do humans cry?", a blog post called "Why do I sigh?".  And, on our ride back to Pennsylvania from Tennessee after the physical anthropology meetings on Saturday, the BBC played a piece on why humans kiss.  The question of why we do X usually has two meanings -- what biological function does X serve, and why did it evolve, though, surprisingly, only the first question is addressed re. the sigh (but we can fix that).  And 'why' always means the natural selection explanation, as is routinely assumed to be necessary and appropriate.

Why do humans cry?  Darwin couldn't think of a biological purpose, but scientists today can, writes Mark Honigsbaum in the Guardian.

In recent decades, scientists have offered several accounts of how the capacity for tears may have given early hominids an adaptive advantage. These range from the aquatic ape theory, according to which tears were an adaptation to saltwater living, to the notion that by blurring our vision tears may serve as a "white flag" to potential aggressors – a signal that the crier is incapable of harm. Then there are the straightforward biological theories, such as the claim that tears evolved to keep the eye moist and free of harmful bacteria.

But perhaps the theory enjoying the widest currency at the moment is the notion that tears are a form of social signalling that evolved from mammalian distress calls – a clear visual signal in other words that someone is in pain or danger and needs help.

Now a new book, Why Only Humans Weep, by Ad Vingerhoets, explains that adult humans are the only mature mammals who continue to signal distress, by crying, even if it might also signal their presence to a predator.  He further suggests that the human's relatively large visual cortex evolved to perceive the nuances of emotion we show on our faces.  "In addition, crying is an emotional expression that signals appeasement and supplication in adults – something that he argues would have been advantageous in early human communities as a means of promoting greater mutual trust and social connectedness."  We also cry tears of happiness and joy, and empathy.

Gustav Klimt; The Kiss (Wikimedia Commons)

Why do we kiss?  This is answered by the BBC's "The Why Factor".  The biological reason is that it's pleasurable, and appeals to all five senses.  And it evolved because our primate relatives signal sexual receptivity with bright red, swollen genitalia, but when our ancestors began to walk upright, their sexual receptivity signal switched   to their full, pink lips, which have a 'come hither quality about them'.  This is called "the genital echo" (the reddened genital signal moved upwards, and this also explains why human breasts evolved to be larger than the mammary glands of our non-human primate relatives -- they look like sexually appealing buttocks).  This could be when lipstick evolved as well, but it's hard to say since it doesn't preserve well in the fossil record.



Why do we sigh?  One researcher who addressed this question received the IgNobel Prize for his efforts.  KH Teigen in 2011, though, in fairness (and sounding a touch defensive) he told one blogger he was doing the work to make the point to his students that there were subjects that haven't yet been studied by psychologists.

The answer is that sighing helps your breathing when you're stressed, is an expression of annoyance, and it's a "mental reset button".  How do we know?  Teigen and colleagues at the University of Oslo used a questionnaire to elicit the feelings people associate with sighing, feelings about others sighing vs themselves sighing (we do it when we're frustrated but others when they are sad).  And they gave participants two puzzles, one easy and the other one impossible, and asked people to try to solve them, but to quit whenever they wanted to.  They counted sighs as people worked on the puzzles.  Seventy seven percent of people sighed, most 4 times.

Researchers at the University of Leuven (Vlemincx et al.) also studied sighing, and have just published a new review on the subject.

The causes and consequences of sighing are reviewed and integrated in the proposed model in which sighing is hypothesized to function as a resetter in the regulation of both breathing and emotions, because it restores a balance in respiratory variability fractions and causes relief.

So, researchers have answered the question of what biological function sighing might serve, but we're astonished that they haven't addressed the evolutionary question which would be, essentially, what is it about sighing that caused our early ancestors who sighed best to have more children?  Since no one else seems to have answered this, we may as well.

If it's true that when we see someone sigh, we think that person is sad, then clearly it evolved for much the same reason crying evolved.  (Except that it involves the auditory part of our brain, not the visual, which is a bit of a problem.  Why isn't our auditory cortex more highly evolved?)  So, as we evolved living in small bands, we were empathetic and when someone close to us was sad, we would have wanted to help.  And, well, it's a short path from helping someone feel happier to mating with them -- sometimes it's one and the same act -- so this may well explain the origins of sighing.

Oh, except that cats sigh, dogs sigh, hell, maybe even lizards sigh.  So, we need an explanation that predates our earliest hominin ancestors.  Well, dogs are empathetic, so maybe our explanation still holds water.

Prairie dogs kissing (Wikimedia)

And as for kissing, although it has a magical, neurological, 5-sense etc. evolutionary origin, as explained by the interviewees on The Why Factor, and is therefore also part of our unique Darwinian heritage, well, yes, it's true that it's not universal and has only spread worldwide in more recent times.

Nothing like water tight explanations of everything in Nature!  It seems anybody, journalist or professor (anthropologist or not) is qualified to write books or be interviewed internationally, with essentially nobody questioning either the explanations offered, or the offerer's credentials, or whether anybody has such credentials when it comes to some of these evolutionary 'explanations'.

Sure, it's possible that crying, sighing and bussing were naturally selected.  That is, that those who cried, sighed and kissed best had the most children, and therefore that the traits became fixed in the human population.  But this is something that can't be tested, nor can the reason these traits evolved be ascertained.  But that doesn't stop people from speculating.

And it's possible that these traits have a significant biological function.  But these can't be ascertained with certainty, either.  And, they have to be shown to serve the same purpose cross-culturally, which Darwin certainly tried to do with his speculations about emotions, but which is very difficult.

Speculation is fun, not new, and it pays the bills, so why bother to complain about it, except for the myth that science is about knowledge, and science programs are about education. Supposedly.