Transitional life forms are the ones that 'evolution' says should be there
And they are, insofar as our definition runs.
, as it is trying to get all the parts an animal needs in in order to survive
.
Yes and no. Some mutations are irrelevant, some permit an organism merely to compete more effectively with its conspecifics, some allow an organism to exploit a new resource or environment, and some affect species survival over the long term.
So at some point legs had to come about with all the systems to make that leg work.
Well, what kind of leg? A flatworm's marginal edge is still a leg, by the definition that it permits movement. Given the immense amount of time we're talking about and the simplicity of early locomotive instruments, why should we be surprised that
Muscle is no good with out the bones to attach to or blood or veins or brain to control them. These all would have to be done in a trial and error way. Because 'evolution' doesn't know it making anything. Millions of these in all shapes and configurations , should be in the fossil record. But they are not there.
Ah - but they are.
The definition you're using - legs - is a difficult one, because it's not well understood sometimes. Look around you: at your cat, your hamster, a bird flying by. These are all variations on a simple tetrapod body plan: four limbs, two in front, two in back. It appears that this plan arose as the result of several macro-evolutions (well, deletions, I think anyway) in a number of regulatory body development genes, turning large numbers of radial fin elements into a few, strong bones. Clearly, not all mutations did this, and thus they were not suited for the continued expansion of a species onto land -
but, those that did
were suited for this expansion. Consider the advantages to amphibian fitness: a vast, unexploited land ripe for colonization, with no predators. And these mutations allowed amphibs to become
truly amphibious, dragging their corpulent selves onto land. Sounds almost too good to be true, doesn't it? Well, it was: their predators eventually caught the same mutations (or else evolved from them) and they got fast enough to start catching them and so on and so on. Predator-prey relationships "caught up" to the newly-exploited niche.
Now, amphibs came from fish, and fish didn't really have "limbs" until the lobe-fins came around, our beloved ancestors. Prior to this, they had fins. The problem is that it's harder to pin down the development of fins because the fossil record is harder to work with the further back you go. But the evolution of fins themselves is not really such a complex process either.
http://www.sciencedaily.com/releases/2009/03/090323212021.htm
http://ngm.nationalgeographic.com/2006/11/evolution/zimmer-text
As you can see, many of the genetic elements involved with fin production simply ended up being involved with limb production. What you have to remember is that much of biology is labile. If you're a lobe-finned fish and your strange mutant gene starts off building you finger-like things instead of radial arches (fins), then your blood vessels are simply going to try to expand out along what one might call their developmental "best guess". ("Neotenic best guess"?) So they'll go here and there and to be honest the whole thing will actually probably work moderately well. (Your descendants with new mutations allowing better interplay between blood vessels and bones will probably be a little more fit than you, but that's all to the good.) Same with nerves, really.
This wiki link is a good one for observing the intermediary stages of limb/tetrapod development. It's only a starter, so I recommend checking out the links too.
http://en.wikipedia.org/wiki/Tetrapod
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