evolution: hard to fathom instances

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BlackSails
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Re: evolution: hard to fathom instances

Postby BlackSails » Tue Jul 20, 2010 2:11 pm UTC

Lutetium wrote:- Ever seen how easy a water birth can be? There's reduced pain from the mother and fewer bacterial infections of the child as they don't start trying to breath til the get out of the water


This is entirely false. Water births are dangerous and dirty and associated with a much higher risk to the newborn.

Id like to know how DNA coiling evolved. I mean, without the ability to uncoil it, coiling is pretty much a death sentence.

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Re: evolution: hard to fathom instances

Postby gmalivuk » Tue Jul 20, 2010 2:17 pm UTC

Is it really necessary to revive a two-years-dead discussion about the aquatic ape quackery?

In any case, do you have a citation for a difference in risks associated with water births?
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Re: evolution: hard to fathom instances

Postby BlackSails » Tue Jul 20, 2010 2:21 pm UTC

gmalivuk wrote:Is it really necessary to revive a two-years-dead discussion about the aquatic ape quackery?

In any case, do you have a citation for a difference in risks associated with water births?


I had no idea this was a necroed thread - it was on the front page, and the date (except the year) was reasonably close.

And yes, I have a citation. http://www.bmj.com/cgi/content/full/319/7208/483

http://www.sciencebasedmedicine.org/?p=2713

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Re: evolution: hard to fathom instances

Postby qetzal » Tue Jul 20, 2010 4:10 pm UTC

BlackSails wrote:Id like to know how DNA coiling evolved. I mean, without the ability to uncoil it, coiling is pretty much a death sentence.


An inability to coil (condense) DNA would also be a death sentence. If an organism couldn't condense its DNA somehow (e.g. histones in eukaryotes, histone-like proteins in bacteria), I doubt a single cell could contain anywhere near enough DNA for a workable genome. So however coiling/uncoiling evolved, I imagine it must have been a stepwise process, and it must have evolved well before anything like existing cells.

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Re: evolution: hard to fathom instances

Postby BlackSails » Tue Jul 20, 2010 7:22 pm UTC

qetzal wrote:
BlackSails wrote:Id like to know how DNA coiling evolved. I mean, without the ability to uncoil it, coiling is pretty much a death sentence.


An inability to coil (condense) DNA would also be a death sentence. If an organism couldn't condense its DNA somehow (e.g. histones in eukaryotes, histone-like proteins in bacteria), I doubt a single cell could contain anywhere near enough DNA for a workable genome. So however coiling/uncoiling evolved, I imagine it must have been a stepwise process, and it must have evolved well before anything like existing cells.


Well, current theory has the first cells being RNA, and afaik, RNA does not have coiling or supercoiling.

I would guess that it went RNA->DNA->Coiled DNA, but I cant see any sort of useful intermediate between coiled and uncoiled.

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Re: evolution: hard to fathom instances

Postby qetzal » Tue Jul 20, 2010 10:13 pm UTC

Maybe I'm misunderstanding what you mean by coiling. I thought you were talking about DNA condensation into chromatin, but maybe you're just referring to the 'standard' double-stranded DNA helix? If so, that's not unique to DNA; it's a property of stranded-ness. DNA and RNA both form helical coils whenever they're double stranded. It's just that RNA isn't ordinarily fully double-stranded in a cell, while DNA usually is.

RNA can be "condensed" as well. E.g. ribosomal RNAs get "condensed" into compact structures (ribosomes) through the action of the ribosomal proteins. True, it's not exactly like DNA being condensed into chromatin, but the principle is similar. Multiple proteins bind to long floppy nucleic acids to form much more compact structures.

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Re: evolution: hard to fathom instances

Postby Sizik » Tue Jul 20, 2010 11:07 pm UTC

gmalivuk wrote:Is it really necessary to revive a two-years-dead discussion about the aquatic ape quackery?


cspirou wrote:Hi everyone.

I was thinking about something and I thought it would be better to resurrect this thread instead of starting a new one.

What is the evolutionary pressure that led to the left side of the brain controlling the right side of the body and vice versa?
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Re: evolution: hard to fathom instances

Postby gmalivuk » Tue Jul 20, 2010 11:28 pm UTC

I don't mean reviving the thread, I mean reviving a particular discussion therein that came and went 2 years ago.
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Re: evolution: hard to fathom instances

Postby Interactive Civilian » Wed Jul 21, 2010 12:45 am UTC

BlackSails wrote:I would guess that it went RNA->DNA->Coiled DNA, but I cant see any sort of useful intermediate between coiled and uncoiled.

Well, the transition from RNA to DNA most likely overlapped with the decrease in RNA acting as ribozymes and increase in them acting as code templates (or genes or cistrons, if you prefer) for proteins and enzymes which helped maintain their living(nearly living?) systems. Since DNA is more stable than RNA, it made a better information storage molecule. The replicators could stop doing their own work and just hold the code for things to do their work for them.

While making this transition, it's not hard to imagine selection favoring more compact, stable structures for the DNA. Aside from holding the genetic code, DNA doesn't have much of a function (though selfish gene theory and natural selection dictates that any DNA, even those bits without apparent function, which can manage to get replicated into following generations will grow and propagate in a population). It just sits there holding the instructions for everything else. A tightly bundled structure is smaller, easier to store, and easier to protect from would-be harmful chemical attackers. An uncoiled plain bit of "naked" DNA runs the risk of being broken down, deformed, co-opted, and so on. Of course, you have to pay for all of this with the cost of working with genes stored this way, but apparently the benefit outweighed the cost.

Stepwise, even a small amount of coiling around even an inefficient histone precursor would provide some advantage, simply by giving the DNA that much more protection. Little by little over the generations, the mechanisms and pieces for coiling and uncoiling grow more efficient by natural selection.

This is one possibility (and to me, the simplest).
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Re: evolution: hard to fathom instances

Postby crayfish » Wed Jul 21, 2010 6:58 pm UTC

I think a good concept to consider in terms of the odds of evolution is convergent evolution. For example the flying squirrel evolved completely separately of the sugar glider, and though they are far from each-other in the evolutionary tree they are remarkably similar. Consider the odds of one animal evolving to be a small vertebrate quadruped and then another animal evolving to be the same thing, but without the two being even close to related (one is a mammal, the other a marsupial). It seems improbably that the two would evolve to be so similar, the fact that they reside on earth at the same time seems incomprehensible.

You see other examples of convergent evolution everywhere, like bats and birds both having wings, or porcupines and hedgehogs having their backs lined with hard quills (though porcupines have the advantage of releasing them). One could argue that such specific traits evolved because certain niches which appeared in different locations across the world allowed such a specialization to survive. However, when you consider the frequency of such occurrences it seems as though there are other forces at work.

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Re: evolution: hard to fathom instances

Postby crowey » Wed Jul 21, 2010 7:16 pm UTC

minor pedantry: marsupials are mammals. there are three subclasses of mammal, eutheria: placental, methatheria: marsupial and prototheria: monotremes.

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Re: evolution: hard to fathom instances

Postby qetzal » Wed Jul 21, 2010 8:46 pm UTC

crayfish wrote:One could argue that such specific traits evolved because certain niches which appeared in different locations across the world allowed such a specialization to survive. However, when you consider the frequency of such occurrences it seems as though there are other forces at work.


It may also have to do with what sorts of adaptations are possible or reasonable. In many cases, there may only be certain ways to achieve a given phenotype, and some may be easier to reach evolutionarily than others.

If you're already a small, tree climbing mammal, and you frequently need to escape aerial or arboreal predators like raptors or snakes, the ability to jump to a far away branch or tree is an obvious response. Evolving flaps of loose skin between fore and hind legs is a pretty simply adaptation that could easily enhance such an escape response. The skin's already there, it's just a matter of it being a bit looser. That strikes me as probably a relatively easy adaptation that different mammals could achieve via any of a wide range of different genetic changes that nevertheless result in a very similar phenotype.

In contrast, I'd expect evolution of something like bat wings might be a much more restricted pathway.

All speculation on my part though.

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Re: evolution: hard to fathom instances

Postby Omegaton » Wed Jul 21, 2010 9:52 pm UTC

qetzal wrote:
crayfish wrote:One could argue that such specific traits evolved because certain niches which appeared in different locations across the world allowed such a specialization to survive. However, when you consider the frequency of such occurrences it seems as though there are other forces at work.


It may also have to do with what sorts of adaptations are possible or reasonable. In many cases, there may only be certain ways to achieve a given phenotype, and some may be easier to reach evolutionarily than others.

If you're already a small, tree climbing mammal, and you frequently need to escape aerial or arboreal predators like raptors or snakes, the ability to jump to a far away branch or tree is an obvious response. Evolving flaps of loose skin between fore and hind legs is a pretty simply adaptation that could easily enhance such an escape response. The skin's already there, it's just a matter of it being a bit looser. That strikes me as probably a relatively easy adaptation that different mammals could achieve via any of a wide range of different genetic changes that nevertheless result in a very similar phenotype.

In contrast, I'd expect evolution of something like bat wings might be a much more restricted pathway.

All speculation on my part though.

I think qetzal is going in the right direction. To top it off, you've also got gliding frogs, gliding ants (although I don't know if this actually involves any morphological specialization), and even gliding snakes.

Convergent and parallel evolution happen a lot, yes, but I can't imagine other forces being at work.

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Re: evolution: hard to fathom instances

Postby qetzal » Wed Jul 21, 2010 10:20 pm UTC

Omegaton wrote:Convergent and parallel evolution happen a lot, yes, but I can't imagine other forces being at work.


I think crayfish was wondering if the frequency of convergent evolution is based on more than just the frequency of similar animals with the possibility to adapt into similar niches. I think he's right, in a sense. It's also based on the fact that some evolutionary paths are easier to follow than others. E.g., flying squirrels and sugar gliders both evolved gliding skin flaps, not just because of the survival advantage of gliding, but also because evolution of gliding skin flaps is relatively 'easy' in some genetic sense. Easier than evolving true wings, for example.

Your examples of gliding frogs & snakes fit well with this, since they're also examples of gliding that evolved through the use of flattened body surfaces.

I don't care for the "other forces at work" phrasing, though. Sounds too reminiscent of intelligent design, though perhaps that's not what crayfish intended.

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Re: evolution: hard to fathom instances

Postby Charlie! » Thu Jul 22, 2010 6:57 am UTC

qetzal wrote:If you're already a small, tree climbing mammal, and you frequently need to escape aerial or arboreal predators like raptors

First thought: Image

Second thought: Image
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Re: evolution: hard to fathom instances

Postby qetzal » Thu Jul 22, 2010 2:32 pm UTC

Ha! I expected someone would take it that way, given Randall's well-known obsession with raptors. :D

Arboreal velociraptors that could drop onto your head from above would be both awesome and freakishly terrifying. Plus an excellent SyFy Original Movie!

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Re: evolution: hard to fathom instances

Postby Telchar » Fri Jul 23, 2010 4:49 pm UTC

Interactive Civilian wrote:That doesn't make sense to me. Even if the head rotates why would that affect things in the head, like the "wiring" of the eyes?

That said, I have no idea what evolutionary pressure or adaptations would have caused the cross wiring. How is it in other verterbrates? Do fish, for example, have the same cross wiring? If so, then what about invertebrates like arthropods and molluscs?

It's definitely an interesting question, though. Is it actually an adaptation? Or is it more of a fixed accident from the early evolution of more centralized nervous systems?


Lateralization is seen in all vertebrates (The sources I can find say all, but I don't know how extensive the studies are) and is generally thought to have evolved to cut down on redundancy in structures. Why have both halves deal with speech vocalization when it could be just one area? One study I found (here) seems to think it evolved due to a kind of "social" pressure due to asymetrical individuals in populations interacting.

As for the cross-lateralization, I'm pretty sure I remember there being a reason, but I'll have to find my biopsych lecture notes to find it. The internet is not giving me anything useful.
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Re: evolution: hard to fathom instances

Postby radams » Fri Jul 23, 2010 9:54 pm UTC

Not all the optic nerves cross over. The nerves attached to the left half of both retinas go to the left brain. The nerves attached to the right half of both retinas go to the right brain.

This means the left brain receives the image from the right-hand side of both eyes' fields of vision, and the right brain receives the left-hand side of both eyes' fields of vision. (Remember: the image on the retina is inverted.)

You want both images going to the same place in the brain so that the processing for depth perception and all that 3D gubbins can be done. Given that, the way it evolved is the way that involves the least crossing over.

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Re: evolution: hard to fathom instances

Postby gmalivuk » Fri Jul 23, 2010 10:35 pm UTC

No, because nerves from every other part of the body cross over more in the current system than they would if the right brain controlled the right body and the left brain controlled the left body. The eyes could just as easily have crossed the other way, with the right visual field (from both eyes) going to the right brain instead of the left.
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Re: evolution: hard to fathom instances

Postby Telchar » Sat Jul 24, 2010 12:01 am UTC

Interesting information from an article on the trigeminal nerve (5th Cranial never for you A&P suckers)

Wiki wrote:It is noteworthy that the secondary neurons in each pathway decussate (cross to the other side of the spinal cord or brainstem). The reason for this is because initially Spinal Cord forms segmentally. Later on, decussated fibres reach and connect these segments with the Higher Centres. The main reason for Decussation is that optic chiasma occurs(Nasal fibres of the Optic Nerve cross so each cerebral hemisphere receives the contralateral vision) and to keep interneuronal connections short(responsible for processing of information) all sensory and motor pathways converge and diverge respectively to the contralateral hemisphere.(Courtesy H. Balram Krishna, excerpt from Cunningham's Manual of Practical Anatomy)



I still haven't been able to find out the advantage of the decussation in vertebrates. Most of the research has to do with motor nerves from the pyramidal and extra pyramidal tracts, so I imagine it's a motor advantage, but I can't find anything conclusive.

Edit: Found this, but you can't view it all unless you pay.

Abstract] wrote:Contralateral central nervous control may be an evolutionary consequence of dependence on the image-forming eye, especially in large organisms. As a result of the topological transformation of the visual stimulus in the pupillary eye, the external environmental hemispace impinges directly upon the contralateral internal organismal hemispace. Selective pressure leads to the development of central connections capable of the most rapid and precise functional association of the internal milieu with the organism's environment. The consequence is contralateral central sensorimotor control. Previous hypotheses are discussed, including those based on bilaterality, binocularity, the optic chiasm and avoidance behaviors.


Essentially, it's so that each hemisphere of the brain has a complete picture of the visual field. Crossing over allows both the left and right proccesing centers (lateral geniculare nuclei) to receive the entire picture which allows for the infill of the blind spot due to binocular vision.

Why motor function flips I still haven't figured out.
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