Paragon of Animals?

by Deb on March 19, 2012

Human Anatomy

We fascinate ourselves, we want to know all about us. But we have a blind spot, an assumption that we must be the best. There’s something to back this up – after all, we dominate an entire planet and have reached beyond. Surely this means we are the pinnacle of creation? The topmost twig of the evolutionary tree?

Maybe not.

There are several things that make us unique – our brains, the way we walk and our life cycle. But unique does not mean the best or even a particularly good design.

Brain

The human brain is the world’s most amazing and advanced computer. It contains billions of cells that are interconnected in complex networks, each cell affecting dozens more. With input from sensory nerves all over the body it allows humans to build up a good, if not always accurate, view of the world around them. We have become the dominant life form, with profound effects on the chemistry and biology of the planet.

Unfortunately we are programmed with many quirks. Humans are generally bad at dealing with risk – sometimes we are very risk averse and other times we gamble [1]. Unfortunately ‘stay away from anything that might be a lion’ is a lot tougher these days than when our brains were evolving; we get the lions and the gazelles mixed up when they don’t come with manes or horns.

Likewise, when we developed we only had to cope with a small tribe rather than enormous numbers, so we put far more weight on things that happened to people we know, or feel like we know, personally [2]. The cult of the celebrity might be harmless if it’s choosing a skirt or trousers, but not good when we’re choosing the country’s leaders or deciding what to do about climate change.

These are only a couple of our glitches, there is an entire field, psychology, demonstrating that we don’t think the way we think we do.

Locomotion

Humans started as a quadruped that stood upright, this is still visible in many aspects of our anatomy and not just locomotion. For example, our internal organs are supported by hanging from the back wall of the abdominal cavity. This makes perfect sense if the spine is horizontal and the organs are hanging down, but when the spine is vertical – not so much.

To accommodate this 90 degree change in attitude, humans developed a unique method of locomotion that appears incredibly hazardous to an outside observer but somehow works. Basically we repeatedly fall forwards but catch ourselves at the last minute.

Because we only use two legs we have to spend part of each step balancing on one, unfixed pole. This is difficult and we will inevitably fall, so we control the direction of fall by deliberately pushing ourselves over then swinging our other leg forward to catch ourselves. This method of walking developed long before our extremely over-sized brains, however,  the earliest walkers do have larger brains than their closest relatives of the same size. It’s possible that this early increase was part of the re-wiring needed to stop us from falling over [3]. (Nina Jablonski was my lecturer at UWA while writing this paper. It’s strange reading it and hearing her lecture series in my head, I’ve remembered this for 20 years. Yes it was a particular area I was interested in and later studied, but she was also an awesome lecturer.)

It takes human babies a long time to develop the strength and dexterity needed to move in this peculiar way, as well as completing the anatomical changes needed. Babies are born with disproportionately short, weak legs and spines that curve like a C. Keeping this would be disastrous for upright walking because our centre of gravity wouldn’t be over our legs and we would fall over whenever we tried to stand. The short legs are a bit of a blessing – when babies predictably fail at the ‘catching themselves’ part of the step they don’t hit the ground as hard.

First babies develop the strength to hold up their very heavy heads and begin the cervical curve in their neck. Later when they crawl or creep quadrupedally and then stand, they develop the lumbar curve lower down. When they are completed, the spinal curves allow us to stand and balance our centre of gravity over our legs. It isn’t anywhere near as stable as three or four legs would be, but it is possible. However, it means our spine and legs need to constantly adjust to allow us to balance when standing still, and on top of all this one leg has to be able to support our entire weight when we are moving.

Our spine is being pulled forward and down by our organs, is curved to stop us falling over,  is constantly adjusting to balance all the weight on top of it and shifting from side to side as we change legs. No wonder humans are prone to back problems.

Reproduction

Human reproduction is yet another system that shows signs of jury-rigging and appears to work in spite of, rather than because of, its design.

Men use the same organ for some waste removal and sperm delivery, while the female reproductive and excretory openings are next to each other. This unfortunate arrangement leads to a high risk of infections in women’s reproductive and urinary systems that can lead to infertility and even be life threatening.

Once pregnancy is acheived, problems arise from spinning the body 90 degrees to upright.  There is nothing to help support the uterus except a thin sheet of muscles underneath it – a sheet the baby pushes through to be born, weakening it for later pregnancies. The weight of the baby in front pushes out the woman’s centre of gravity, which led to further changes in women’s spines so they can stand up during pregnancy as well. [4]

During birth the consequences of upright posture are brutally obvious. In order to attach the muscles that support the upright spine and mechanically difficult gait, the pelvis has been drastically remodelled from its quadrupedal origins and squashed from front to back. Combined with the enlarged brain, human babies have an extremely difficult and dangerous birth. In quadrupeds, even apes, their heads fit easily through the pelvis with room to spare. Human babies cannot fit through a woman’s pelvis without deformation and twisting during birth, and some babies do not fit at all. Even with the most modern technology women and babies still die in childbirth. In low income countries pregnancy and birth are in the top ten killers of females [5].

Growth and Development

It is clear that human pregnancy and birth are difficult and dangerous, and human babies are born as soon as practical to minimise the risks and make it possible at all [6]. Early birth means that for several months babies should be considered ‘external foetuses’ and are completely helpless – they cannot even hold onto their mothers or be left in a nest. Immature babies are very difficult and tiring for mothers to care for and this has almost certainly driven all sorts of social changes to help support them. Humans are social animals, the reason for all the different organisations we’ve developed is to allow our babies to grow up.

The change to an early birth does have a positive side. Because babies are born early, their brains are still developing as quickly as if they were still inside for several months. They are getting sensory feedback and stimulation while their brain is still forming, which encourages neural connections and complexity [7].

As I mentioned above, our original increase in brain size is possibly linked to the complexity of standing and walking upright, which led to earlier births while brains were smaller so babies could fit through the modified pelvis. But this gave babies stimulation while their brains were still growing quickly, which encourages brain development.

Now we’re into the realms of speculation, but could this be linked to our later massive brain increases? Our upright posture primed our mental development – are our brains both the driver and the consequence of our early birth?

Conclusion

Human brains are advanced but still make serious errors, a legacy of living in and defending small groups. Our bodies show all the hallmarks of jury-rigged anatomy – rather than being designed as an independent animal, they are a clumsy and dangerous modification of the elegant and safe quadruped model. If this were a purposeful design, the engineer would be fired.

But somehow, serendipitously, it works.

 

[1] Nobel Prize-Winning Research on Risky Decision Making http://www.spring.org.uk/2007/03/nobel-prize-winning-research-on-risky.php

[2] Availability Heuristic http://en.wikipedia.org/wiki/Availability_heuristic

[3] Origin of habitual terrestrial bipedalism in the ancestor of the Hominidae http://www.anthro.psu.edu/projects_labs/JablonskiLab/printable_files/biped.pdf

[4] Female lower back has evolved to accommodate strain of pregnancy http://news.harvard.edu/gazette/story/2007/12/female-lower-back-has-evolved-to-accommodate-strain-of-pregnancy/

[5] WHO: Women’s Health http://www.who.int/mediacentre/factsheets/fs334/en/index.html

[6] The Human Baby as an External Foetus http://science-at-home.org/the-human-baby-as-an-external-foetus/

[7] Children and Brain Development: What we know about how children learn http://umaine.edu/publications/4356e/

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