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Where did our natural defences go?


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#21 Raina

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Posted 28 June 2011 - 10:42 PM

View PostRaymond, on 28 June 2011 - 09:22 PM, said:

It's not all good design - running a toxic waste channel right through a pleasure centre wouldn't get past the planing stage today.

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#22 Rhea

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Posted 29 June 2011 - 09:58 PM

Heh. I just realized that I'd discussed the topic of vaccination up-thread in a different context. Sorry, folks - senior moment. :blush:

Actually, that might be an argument that our big brains don't do us much good when they die cell by cell as we get older. :p

Edited by Rhea, 29 June 2011 - 10:00 PM.

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#23 Delvo

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Posted 26 August 2011 - 01:53 PM

View PostJudasRimmer, on 15 May 2011 - 07:25 PM, said:

We have no fangs or claws
Stone tools, including not just knives but also axes (both shaftless and shafted) and spears, have been around since before this species was. Thus, no human population anywhere as ever lacked them. That makes them a part of our basic nature; the natural state of humans is to have them. They're our claws.

View PostJudasRimmer, on 15 May 2011 - 07:25 PM, said:

relatively poor physical strength even compared to our nearest ape* relatives
In a test of grabbing and pulling, yes. That approximates how they move around in the trees. We have no use for that ability. Also, although you could back up to pointing out that they have a higher muscle mass as a percentage of body mass in overall, that greater muscle mass comes with an increase in food requirements, so having smaller muscles contributes to our ability to survive on less food.

View PostJudasRimmer, on 15 May 2011 - 07:25 PM, said:

average eyesight/hearing
Hearing, maybe, but much better vision than usual. (The usual counterexample is raptors, for the amount of detail they can make out at a distance, but we're better at focusing up close and seeing in dim light.)

View PostJudasRimmer, on 15 May 2011 - 07:25 PM, said:

We can't even run very quickly,and not that far either.
At a sprint we're bad, but for distance, the only other type of non-flying animal that might beat us or even comes particularly close is the canines, which tend to use that ability to keep chasing prey they didn't catch at first until the prey is too exhausted to continue. Generally, faster sprinters than us get tired doing it and need more recovery time, and are at more risk of things like hyperventilation, hyperthermia, and heat stroke. (I once disputed an article that was posted somewhere around here that said we're designed for "running", but have since then come to the conclusion that that was just a bad word choice and the author must have meant what is usually called "jogging".) Also, while we're on locomotion, I have to add what I believe is the origin from which our ability to jog longer distances than most other animals was derived: we're primarily outstanding as walkers. It's not a fast pace and others might even walk faster, but we can keep doing it all day after they would have quit. That gives us a wide range to collect food from, and increases our capacity to seasonally migrate and/or find what we need when times are tough (like additional water sources during a drought).

View PostJudasRimmer, on 15 May 2011 - 07:25 PM, said:

unprotected stomachs [no ribcage] and throats
Mobility! (And expansion ability, not just for pregnancy but also for fat storage, which contributes to our ability to survive through fluctuations in the available food supply)

View PostJudasRimmer, on 15 May 2011 - 07:25 PM, said:

no separation of breathing/eating passages
That was not as much of a problem before we stood up and rotated our heads down and started speaking, as has already been said here, but it's still pretty odd even in other vertebrates, just not as much. But there's a reason why all vertebrates are stuck with it: when our ancestors were fish who could benefit from an additional source of oxygen along with water over their gills, some surface which could be exposed to the air was needed for gas exchange to work through, and an internal surface was better than an external one, and the inside walls of the digestive tract were the only surface fitting both of those requirements. So they started absorbing oxygen from swallowed air in their digestive systems (as some fish still do today), then developed an area of the digestive tract where the wall expanded out and formed a separate pouch just for gas exchange, then kept modifying that until it became what we would now recognize as lungs. So the connection is necessary because when the precursors to lungs got started, there was nowhere else to put them and no other opening but the mouth for them to use, and they are essentially a part of the digestive system.

View PostJudasRimmer, on 15 May 2011 - 07:25 PM, said:

why did we lose such useful features
In some cases, like the digestive and respiratory tubes joining and the lack of abdominal ribs, what you're lamenting our lack of is things we didn't lose; we never had. (And other vertebrates generally don't either.)

View PostJudasRimmer, on 24 June 2011 - 07:01 PM, said:

evolution has tended to give us duplicates of most of our important organs,such as eyes,ears,lungs,kidneys etc for redundancy.
This is the part that made me want to answer the thread. Having backups is not the reason why those things are paired.

Some paired structures, particularly eyes and ears and limbs, gain abilities that way, that a single one wouldn't have. In some other cases, like ribs, part of their function just requires being on both sides.

But more fundamentally, the reason for pairing is that a vertebrate's body doesn't know any other way to grow and develop. In a bilaterally symmetrical organism, there's a central plane through the body defined by a couple of linear front-to-back features positioned above & below each other: the digestive tract and central nervous system. Anything else that isn't in that plane and derived from those defining central features has to be in pairs because those are the only ways for anything in a bilaterally symmetrical body to grow, the only places from which any body part can originate. So the central plane is the only part of the body where there can be just one of anything. To expect the bilaterally symmetrical body to produce something from any other source or in any other way is like expecting a centipede not to be made out of repeating segments from front to back or expecting a plant's stem not to be pretty much the same all the way around in a circle.

So strong is the ontogenic impulse to duplicate on the right and left sides, that even some things that start out as part of the singular central plane still split anyway; you have two lungs but they come from the single central digestive tract, and two eyes and a partially split brain which come from a single front end of the central nervous system. Things we have only one of that's on one side or the other instead of in the middle, like the liver, gall bladder, pancreas, and spleen, are central-plane structures that just got pushed around out of place (mostly as a result of the intestines getting longer than the body so they had to get all squiggly in order to fit). In some species, when it's turned out to be useful to reduce to one of something that was paired, like snakes' lungs, the left-right splitting tendency was so irrevocably built in that the only way they could get it down to one was to let them separate as usual early in fetal development and then suppress growth for one of them; just not starting with the left & right pair in the first place was not a possibility.

And even some centrally-located structures that we have only one of actually arise from fusion of pairs anyway; for example, you have one frontal bone, but before you were born, there was a time when you had right and left frontal bones, and that silly shape between your nose and upper lip is a result of the halves of your face fusing. Another way to see how strong is our built-in tendency for splitting ourselves as if with a mirror: look at similar fusions that result even though they aren't supposed to, when the two halves of a pair come too close together during ontogeny. You get things like mermaid syndrome, cyclopia (don't look for pictures of that one if you're easily grossed out or disturbed), and horseshoe kidney. These same structures wouldn't fuse due to contact with other distinct body parts, but they do fuse with their own reflections, which shows that the body is in some way treating paired parts as really the same thing in two places.

So it's not that we have two of a lot of things because those individuals with a backup could still carry on after something happened to one. What's actually going on is that individuals with just one of most body parts would never have existed at all because the most of the body just doesn't have any way to grow just one of anything.

View PostJudasRimmer, on 24 June 2011 - 07:01 PM, said:

However we only have one heart-the most important organ to keep us alive. Why do we not have two hearts as well?
Development of the heart starts with a pair of blood vessels which fuse at the body's mid-plane. Some twisting and subdividing happen after that, but fundamentally, what it is is a right-&-left pair of structures already.

Edited by Delvo, 26 August 2011 - 02:05 PM.


#24 Mark

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Posted 26 August 2011 - 06:37 PM

View PostRaymond, on 28 June 2011 - 09:22 PM, said:

It's not all good design - running a toxic waste channel right through a pleasure centre wouldn't get past the planing stage today.

Mark: Oh, I don't know about that...we don't seem to mind polluting our rivers and lakes that we bath and use for water sports. Mankind has an ongoing history of bad planning, and not learning from history.
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#25 Mark

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Posted 26 August 2011 - 06:41 PM

Delvo:.

Quote

.. but we're better at focusing up close  

Mark: Excellent post Delvo! ...with just that one exception I listed above. You'll have to speak for yourself when it comes to focusing up close. :lol: My new glasses surely haven't helped my situation. I guess it's time to bi-focal it.
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