The human brain (and, I believe, the brains of all mammals, and for all I know all vertebrates) is divided into two hemispheres, joined where they meet the spinal cord, and otherwise unconnected except by a small band of nerve tissue called the corpus callosum (which can be cut without impairing brain function, and sometimes is cut surgically to treat epilepsy).
Why is that? What is the evolutionary advantage to dividing the brain into two parts? I can’t think of any reason why that should make it work better.
As far as I know there’s never been a real answer to this question other than to say it is survival benefit due to evolution. The human body can survive often when one side of the brain is damaged in many cases. Stroke victims for example may have limited use of one side of their body. Yet are quite capable of surviving although they may suffer physical disabilities. The brain is also quite amazing in that often when parts of it are damaged it will sometimes find alternate pathways and “rewire” itself through undamaged neurons.
Since we are…damn I can’t recall the term for being bi-lateralized physically. Meaning both right and left sides are identical. :smack: damn I hate it when this happens.
Anyway our bodies are “split” from top to bottom with exact opposites and the brain simply developed the same way because it is centered and as it grows (as we grow) it develops from the center outwards.
With the exception of a few internal organs everything about out physique is bilateral. Feel the ridge in the top of your mouth, your sternum and backbone, your face, etc…
I can’t cite my advanced biological psych text with a link but that’s pretty much what it has to say on the matter.
I could give you several examples where people have had severe injuries to the brain and yet they survived but I assume you already have access to this info.
I still don’t see the point in the separation of hemispheres – if the brain is adaptable enough that one part can take over the function of another part that’s damaged, so there are two mirror-reflection halves, I get that – but why do they have to be separate? Wouldn’t it work more efficiently as a single unit, and if any part gets damaged the neighboring part takes over? We got any neurologists here?
Evolution doesn’t work that way. There very may well be “better” or “more efficient” ways for our brains to have evolved, but the fact is, they didn’t. We have what we have not necessarily because there’s a reason for it, but because there’s no reason for it not to be this way. Take something simpler, like our hands. Why five fingers? If five is good, then oughtn’t six to be better? There’s no particular reason we have five fingers other than the fact that our evolutionary ancestors had five digits. Just the same way, there’s no logic behind the separation of our brain hemispheres, because there’s no logic in evolution. Sometimes it’s just a case of “We do it this way because we’ve always done it this way”.
All living things on Earth are survivors. We are the last in a long line of ancestors. Many of those ancestors had offspring that DIDN’T survive. But, we’re the end result of all those offspring of all those generations that DID survive. What allowed us to survive? Hard to say. We’re pretty convinced that our ability to think and use language has served us well, so that when we couldn’t CATCH a rabbit, we could outsmart it. Beyond that, it’s a bit harder to figure out. A lot of who we are may just be random stuff left over that never hurt us or weeded us out. (Current thinking about DNA suggests that it’s mostly extraneous information). So, I think what you’re looking for is specualtion on how bilaterally differentiated cerebral hemispheres may have aided in our adapting to changing times. I’d like to know what Stephen Pinker would say about that. But, of course, it’s all just speculation, isn’t it? xo C.
Bilateral symmetry seems to be fairly common in humans (and many other species).
You have 2 arms, 2 legs, 2 eyes, 2 ears, 2 nostrils, 2 breasts, 2 ovaries or testicles, etc.
And the same internally: you have 2 lungs, 2 kidneys, 2 sides to your heart, etc.
It’s been argued that this is just evolutionaly ‘byproducts’, due to the way cells divide, and so forth.
It’s also been argued that it is valuable, as ‘spares’ for critical parts, and so would provide some evolutionary benefit, and would thus have been ‘selected for’.
Geneticists argu back and forth on this; as far as I know, there is no ‘generally accepted consensus’ on it.
Not quite true. The individual hemispheres will continue to function, but a variety of odd disabilities occur because they can no longer communicate with each other. I’m having trouble finding a good article online. But, the subject is a staple of psychology textbooks. Try searching on callosal syndrome.
I would like to add to teela’s post. In 1995, I severely bruised my right-frontal lobe in an automobile accident. After my brain stopped swelling and it looked like I would survive they began rehabilitation. They put me on the floor with stroke survivors, the end result of a TBI is very similar. I had little use of the left side of my body for about 18 months, but since I could use half of my body, I could assist in my upkeep rather than just lay there and my wife was able to bring me home rather than put me in a nursing home. It was kind of like Hell for a few years but things have worked out. I, for one, am grateful for a bimodal brain. I woud have died by the side of the road in June1995 without one. Now, I do my part to make the world a better place.
They did some pretty neat tricks with a picture of half a babyface (on the left) and half an old man face (on the right), but stuck together to look like one head. They would line the picture up with the field of vision so one eye would only see one “halfface” . The right eye would pick up the old man, and send it to the left side of the brain, the left eye picks up the baby sends it to the right brain. I’ve already forgotten a bit but here’s the gist. Since the hemispheres can’t communicate, the way we can express what we saw is pretty odd. You basically can write the you saw the old man, but not say it, and you can say you saw the baby but not write it. I don’t think it was writing, because IIRC that uses both sides, but that was basically it.
