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What makes us human?  @psychohistorian.org

What makes us human?

posted: 4785 days ago, on Sunday, 2007 Jan 14 at 06:07
tags: psychology, books.

The evolutionary history of our species has given us an astounding repertoire of behavioural oddities, which seem to be unique to us and also remarkably divergent from other species. They appear to bear witness to our singular position in Nature. Yet how this evolutionary feat was achieved is not at all obvious. In fact, faced by an equally astounding lack of discernibly unique biological features, such as how cognitive process transform experience into meaning, what makes us human is still a wide-open question.

There is a great deal of neurological evidence that certain abilities or qualities of the mind reside specifically in certain physical areas of the brain. The neocortex – the convoluted mass that looks like old chewing-gum – for example, is clearly the centre of higher-level thinking. However, current attempts to identify the neurobiological correlates of consciousness are still speculative. There are several models, but no consensus – beyond the fact that they all agree that the centre of consciousness is not in the brainstem or cerebellum.

Leaving neurology aside, what are the unique, defining traits that make us human? Science journalist Chip Walter points out that zebra have stripes, hammer-head sharks have weird heads, and elephants have trunk, and suggests that such specific traits may say a lot about how such creatures came to be the way they are.

In a fascinating new book, Thumbs, toes and tears, and other traits that make us human, he suggests six defining physical-behavioural traits that seem to be entirely unique to us folk: the big toe, opposable thumbs, voices, kissing, laughing and crying.

The big toe is usually not given much credit, and is a sorely neglected digit. Thumbs-up for launching us on an evolutionary winning streak is usually reserved for our opposable thumbs. However, without the big toe, we would probably never have developed thumbs in the first place.

The big toe, or hallux, makes it possible to stand upright. When you take a step, 40% of your weight is supported by your number-one digit. Walking would be quite difficult, if not impossible, without it, and certainly running, jumping and hop-scotch would be out of the question.

Once we had a toe enabling us to stand upright, all sorts of things were set in motion, including freeing our hands, which made the development of opposable thumbs possible. An upright posture also changed our sexuality, because we were now front-facing, and not on all fours (vide habits of your Canis lupus familiaris), which lead to all sorts of new and stimulating possibilities.

Being upright, our throats elongated, and our voice boxes dropped down, leading to the development of the pharynx with its unique muscles that can make intricate noises.

Incidentally, this also made it possible for us to choke, because our nose and mouth now meet in one place, requiring the invention of the epiglottis. Other primates wisely declined this whole business, leaving us as the only primate with this ability.

Crying, too, is another unique human feature. When we're highly emotional – be it elation or despair – we leak from our eyes. While other animals have similar structures that generate tears, we're the only creatures that cry when we're feeling intense emotions.

Not only are our emotional tears unique to us, but the chemical make-up of emotional tears differs from that of the kind that lubricates or clears debris out of the eye.

Some researchers suggest that crying removes the hormones and chemicals that are associated with the emotion, in an attempt to restore balance, but even a strong emotional response only delivers about a teaspoon-full of tears.

Another suggestion is that crying is the autonomic nervous system's way of getting us back to "normal". Crying may be a wake-up call, alerting us that we're too absorbed in our current emotional state and need to stop being upset and start focussing again on the surroundings – which is a good thing if you want to survive (it's a jungle out there, you know).

After a traumatic experience, a good cry leaves us with a sense of relief and we experience a cathartic moment. But crying also gets us a sympathetic shoulder – it is a powerful form of non-verbal communication that lets others know something is wrong and that we're in trouble.

Tears seem to play an important clue to others about our mental state, as investigation by Randy Cornelius (Vassar College, USA) suggest. His sneaky experiment involved showing test subjects photographs of people crying, with tears visibly rolling down their cheeks. Respondents were able to correctly identify the displayed emotion. Then, he photoshopped out the tears, and asked test subjects to identify the "doctored" images. Responses ranged from boredom, amazement, discomfort, to laughter – way off the mark. This suggests that tears help observers correctly identify our inner turmoil, being literally a cry for help.


  1. Butler, A.N., Manger, P.R., Lindahl, B.I. & Arhem, P. (2005) Evolution of the neural basis of consciousness: a bird-mammal comparison. Bioessays, 27(9), 923-36.
  2. Pribram, K.H. (2006) What makes humanity humane. J. Biomedical Discovery and Collaboration, 1:14.
  3. Tattersall, I. (2006) Karl Pribram, the James Arthur Lectures, and What Makes Us Human. J. Biomedical Discovery and Collaboration, 1:15.


  1. Book review of Thumbs, toes and tears (Science a GoGo)
  2. Chip Walter's Weblog

nothing more to see. please move along.

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