A Little (More) Evolutionary Neuroanatomy, Part 2

TL;DR: Today we’re going to finish up our very brief look at evolution. We took this little detour so that we could eventually talk about the neuropsychology of interaction with the common understanding that the way humans experience the world is ridiculously complicated. Most of us  think that basic human senses like vision or hearing or taste or touch must be pretty straightforward, but none of them are. 

Last time

When we left off last time we were discussing Mr. Darwin’s concern that the complex structure and functions of the human eye might disprove his theory of how evolution happens. The problem was that he couldn’t imagine how anything as complex as an eye would evolve, because that would require an eye to appear in stages. In other words, a partial eye would have to prove more useful than not having one at all, so that it could spread through the generations. Then that partial eye could stay in play long enough to eventually evolve into an increasingly useful eye, until we reach an eye that is fully functional.

If you’ve had similar concerns about evolution, please allow me to help ease your worries. The eye did evolve slowly, and a lot of evidence exists for intervening forms that appeared along the way. In fact, these days, those who study the eye use the ridiculous complexity and overlapping functions as a strong argument in favor of incremental evolution.

The evolution of the eye

The first life forms on earth appeared about 3.7 billion years ago. These were cyanobacteria. They exist now as fossils called stromatolites, but some of their direct descendants still live on, and we can study them in order to gain insights into their progenitors.

Some cyanobacteria developed the ability to absorb different frequencies of solar radiation at different rates. To use an overly simple analogy, it was beneficial to be able to be warmed by the infrared end of the sun’s spectrum of light without getting sunburned by the ultraviolet end.

Spots with greater sensitivity to light started to have a long-term effect on how life could express itself on earth long before the separation between plants and animals. In fact, it is now understood that the eyes of all modern creatures can be traced back to the small photosensitive spots that allowed some cyanobacteria to “lean towards the light” like sunflowers and sunbathers do today. For a detailed look at this (sorry), you could check out Dr. Russel D. Fernald’s very informative and nicely-illustrated article in the October 2006 issue of Science: “Casting a Genetic Light on the Evolution of Eyes.”

Some of you won’t take the time to read that, so let me sum it up here. The basic chemistry required for vision seems to have evolved several times. The basic structure of eyes that we usually think of – the kind you see in the mirror, if you are able to do so – evolved at least twice. The eye used by an octopus looks similar to yours or mine, but ours grew outwards from the nervous system, and theirs grew inwards from the skin. Cool, eh? Now don’t you want to read that paper? How do we get from patches of light-sensitive cells to complex modern eyes? We get there through a combination of small errors in gene replication and long periods of time.

Tornado in a junkyard

Now, some folks have said that thinking that a fully complete human could evolve by accident would be like believing that a tornado could pass through a junkyard and accidentally shape the scraps into a showroom-quality 1965 Jaguar XJ6. The worst flaw in that argument, ignoring many, many others, is that you’re working backwards from a perfect, high-performance, finished product. Humans are neither perfect, high-performance, nor finished. Our bodies are full of mistakes. We grow too many teeth for the size of our jaws. Babies are born too big to pass through the birth canal unless the two bones that make up the mother’s pubic symphysis dislocate. Feet, knees, hips, lower backs, and necks all degrade with normal everyday use because we are using them in a posture unlike the postures in which they evolved. Other animals can regrow heart cells, and many of them can regrow severed nerves and missing limbs. We can regrow a little bit of missing bone or skin and most of our liver, but not much else.

Some of the subsystems within our bodies are better than anything we can currently build, but they still have ridiculous flaws. The cartilage in our joints has incredibly low friction, but once torn, it heals poorly and slowly, if at all. Our brains are fantastic processing systems, but because they sit at the whip-end of our spines, many injuries to them come from crashing around inside the skulls that should be protecting them. 

So, evolution has made many great components, but please don’t think that we are driving around in a high-performance machine.

Errors and time

We have about 37.2 trillion cells in our bodies, and each one grew from the division of other cells; that is, from the splitting and recombining of the DNA in other cells. Each cell has so much information in it that, even if the replication only makes one error in every billion steps of recombination, that would still result in about one error every time a cell splits. Most of these errors get lost in the crowd, but sometimes they’re significant. Sometimes they express themselves in a way that will make it easier or harder for us to survive and pass them on to the next generation. Right now, the average reader of this blog is walking around with about 100 personal mutations. 

So our eyes aren’t the “organ of extreme perfection” that Mr. Darwin thought they were. They are the latest step in one path of trial by error that took about 4×105 generations to branch and evolve from something like the eye pit in an earthworm to something like the eyes that are reading these words. And it’s not just the eyes. By the same token, I hope you can infer that our ears aren’t perfect microphones. They are a weird mix of overlapping accidents that eventually combined to allow you to hear in a really foolish way. They are another complex means of translating information from the outside world around us into the inner world of our thoughts and feelings. The same is true of all of our other senses; smell, taste, balance, time, hunger, dehydration, acceleration, deceleration, and our many, many senses of touch.

What’s that? You’ve heard we only have 5 senses? Well, we’ll have to deal with that oversimplification another time.

What does this have to do with interaction?

What this means is that the reality we are all living in is actually virtual reality. When you smell a warm chocolate brownie in the oven, you are not smelling warmth or even chocolate. Your senses are giving you a wide variety of chemical signals, and some unconscious parts of your brain are working together to fit that chemical data into a story that the slower parts of your brain can understand. 

Next time we’ll talk about how the whole body is an interface, and look at how these kinds of translations are fundamental for almost every single thing a human does.

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