Quote:
Originally Posted by Michaelangelica
Very interesting conversation Pyrotex and biochemist. Could you break it down a bit for "bears of little brain"...
|
[This post has been edited to correct a stupid mistake]
Mangel,
I would never in 984,625 years ever think of you as a bear of little brain. Poo!
But, assuming that there might be a few lurking around, I will give it my best shot.
Evolution: Body Plans and DNA
DNA does not code for body parts. There are no genes for a trunk. You cannot, even in principal, splice some genes from an elephant into a giraffe, and get a giraffe with a trunk. What DNA does is code for chemistry -- and 99% of that is code for producing proteins. Proteins have no relation to body parts or body plans. Proteins encode for something else that we might call "Effect" -- this includes the shape of the protein, its ability (if any) to fold or shift into an alternate shape, the charge-density map of its shape, its sensitivity to external electric fields, and its electro-mechanical behavior in the presence of certain "trigger" molecules or levels of acidity (pH).
Proteins (by and large) are the Universe's smallest
machines.
[hypothetical scenario] Along comes
an amino acid a nucleotide and a specific protein will respond to its (and only its) electric field and will change shape and "grab" the
amino acid nucleotide. This alters the protein's charge-density, which is detected by another protein which attractes protein #1 over to a nest of tightly coordinated proteins that are building a segment of RNA. Protein #1 is leveraged into position by electric fields. A free electron is pumped into Protein #1 and it releases its
amino acid nucleotide and reverts to its default shape. Protein #1 drifts away to start the process over again, and the
amino acid nucleotide is strung onto the end of the RNA strand.
So, at a higher level of abstraction, we can say that proteins have functionality. They "do" things; they make things happen.
Now look at a primitive strand of DNA in an organism from the Cambrian Era. The DNA creates proteins in a certain sequence. The proteins take on certain shapes and do certain functions. One patch of DNA generates the shape of a body segment. Another patch of DNA generate a string of proteins that triggers the first patch at a certain "time" in the sequence. Patch #2 typically generates a string of 4 trigger proteins; they trigger the creation of 4 body segments: head, thorax1, thorax2, tail.
Thorax1 and thorax2 are nearly identical, containing buds for a pair of legs.
Head is quite similar; it also contains buds, but they will develop into antenna/eye complexes. Tail also contains buds, but they develop into orifices.
You have the standard 4-leg body plan: lizards and mammals.
A mutation occurs, and patch #2 now generates a string of 5 trigger proteins. Bingo, you now have the 6-leg body plan, containing thorax1, thorax2 and thorax3. Thorax2 is the "new" segment, and is identical to thorax 1.
Later on, other mutations occur in DNA patch #2, yielding the 8-leg, 10-leg, 24-leg, and 96-leg body plans: spiders, squids, worms, centipedes.
Now slip ahead a few tens of Millions of years. The DNA of each body plan has evolved tremendously. The individual "body segments" have to some extent merged, and to some extent, each gone their own way, developing their own refined attributes. Thorax1 and thorax2, among the lizards, have specialized, with thorax1 focused on heart and lung development. Thorax2 focused on digestion and reproduction. The division between these two segments has blurred and disappeared, unless you notice the diaphragm inside.
NOW--consider what would happen if we had another mutation in DNA patch #2 at THIS point of evolution. When it happened in the Cambrian Explosion, it resulted in primitive critters with 6, 8, 10, or more legs. Which survived, and each evolved in their own independent direction.
NOW--it produces a lizard with six legs, two hearts, two pair of lungs, two diaphragms, a really screwed up spine and nervous system and the agility of a mushroom.
It's dead, Jim!
Why? Several reasons. The lizard is now 1,000 times larger than its Cambrian ancestor. Mechanical repetition of body segments doesn't scale up. Each segment has evolved its own functions which cannot be improved with mere organ duplication. Each segment has evolved its own internal mechanical, nervous and fluidic structures which can only be botched if you throw in extra components.
And so, whereas the proto-critter back in the Cambrian could alter its basic body plan with a single mutation, its descendents tens of MYrs later CANNOT execute the same kind of change and survive. Too much DNA superstructure and organ evolution, and specialization has taken place, and you collapse that "house of genetic cards" if you start slipping in a new pair of arms at the "bottom of the house".
A body plan can therefore be "frozen in" by all the changes that take place afterwards. At one time, our arms could have had as many joints as you can imagine. Look at the joints in antennae which are modified limbs. But once 3 joints (shoulder, elbow and wrist) had been around for a while, evolution of muscles, nerves, tendons, veins took place on top of that, to take whatever mechanical and fluidic advantages there were in each arm segment. Each joint evolved to take advantage of its special place and function in the arm. So now, 3 joints is also "frozen in".
Given that there are people with 6 fingers and cats with 7 toes, I conclude that the "body plan" of the hand (or paw) is not yet "frozen in". But if specialization and functional extention of the the fingers/toes takes place over a long enough time, then that too will become "frozen in" and a mutant with 6 fingers may not even be able to survive!
That, in a nutshell, is how and why body plans can only be created at very early stages in the evolution of any creature. At a stage when no superstructure has been built upon, and is tightly dependent upon, a certain number of: legs, segments, vertabrae, eyes, whatever. But after that superstructure has evolved, the body plan underneath is "frozen in" and cannot change.
Metaphor: think of an evolving organism as a personal computer. When it was simple (back in the 80's) you could slap in a new, faster CPU and take off. By the late 90's, peripheral extensions, such as video boards, sound boards, array processors, external drives, etc., had progressed so far that making a new, faster CPU chip that was universally backward compatible was nearly impossible.
Thanks for the question!!!
