Suppose man is evolved from ealier species as has been put forth by science...
OK. Let's suppose this is so. There's a bunch of misunderstandings and assumptions in the message. It's not your fault. Science is badly taught for the most part.
The first thing to keep in mind is that there's no Grand Destiny that says species "give way" to "higher" or "more evolved" species. Evolution just means change. Some populations of a species may change over time until they are so different from other related species that we say "Let's call them something else". Other populations might resemble there ancestors more closely. It says nothing about who is more "advanced" or anything like that.
The descendants of some fish colonized dry land. It didn't mean that fish populations still in the water suddenly got their pink slips.
Why our current form? While we are adaptable and have a great survival instinct, could we not have been "better".
The second thing to realize is that evolution doesn't design. It doesn't optimize. Things that successfully produce offspring successfully produce offspring. Things which don't don't and are removed from the board.
Evolution throws up some wonderful features. It also throws up a bunch of horrible kludges that just happen to give their possessors enough of a leg up to do slightly better than whatever else is out there. Many times it's a slight change in the rate at which something develops. A tiny alteration in the development of the jaw and teeth in lizards produces bird beaks. Teeny genetic shifts in the rates at which cells are laid down produce a huge variety of beaks.
Stuff also gets reused. Lots of enzymes result from slight genetic variations on existing ones. They have make reactions in cells happen at different rates which can have dramatic effects on overall biochemistry. It probably wouldn't be the way you'd design it if you had a purpose in mind. But what happened worked. It allowed the individuals who had the trait to be slightly more successful at passing on their DNA. So it stuck until things changed that made the individuals with the trait a tiny bit less successful at the Great Game.
Two legs doesnt seem like much of a balance thing... why not at least a tail to help even things out?
In our the ancestors of todays Great Apes lost their tails. I don't know the history of how it happened. There is probably someone who could give you an good biochemical and anatomical rundown on how it happened. In any case, by the time our ancestors started spending more time on the ground they didn't have tails anymore. No population of our ancestors a) re-evolved tails and b) did better enough because of it that they had more descendants than other proto-humans who didn't have tails.
Note, I'm not looking for a "Science vs Creationism" debate... but rather veiws on why we would have evolved into a complex yet fairly imperfect form we are now, and what "improvements" we could have developed had to make us "better"... camouflage, venom, claws, ability to regrow a bitten off limb, etc...
and why?
The simple answer is that we did well enough with what we ended up with.
Complexity is a red herring. It only gets brought up because the Creationists like to blather on about how impossible it is to evolve complex structures. In fact, what we often see is a reduction in the number and functions of earlier less specialized structures. Insect antennae, legs and mouth parts are all essentially the same structure with slight developmental differences that make them more suited to specialized tasks. Silverfish are closer to the ancestral stock. Their legs are more numerous and less specialized. Many early insects had extra winglets. Earlier ancestors had more body segments.
Another part of the answer is that there is a biological cost to any new structure. If the cost reduces reproductive success enough or pure dumb luck wipes out those who have the genes for the trait it doesn't get conserved. Evolution doesn't have any way of knowing what you're great to the fifth grandchildren will be doing. It only tells you whether you've managed to have children.
Consider the eye. Creationists ask "What good is half an eye?" The answer is, quite a lot. Very primitive creatures have light-sensing structures. If there's light, being able to detect it is useful. In some otherwise very similar species there are slight biochemical differences which cause the light-sensitive spots to be formed close together. The increased discrimmination has advantages. The development of a clear cuticle over the eyespot results from very small chemical and developmental changes. And in fact there are many lineages which have the first adaptation and closely related ones which have the second. And so on. Stephen Gould did a masterful telling of that story in one of his essay collections.
It doesn't mean that there are no other structures which could function as eyes. Trilobites ended up with eyes made of calcite crystals. It doesn't mean there couldn't be better eyes. In fact, given the hit-or-miss nature of evolution that's almost certain. It means that our ancestors developed structures which functioned as eyes well enough to get by.
Or you can
look at the wing. In vertebrates the wing developed at least three different times. With bats it came from longer fingers with big webs between them. Pterosaurs got a long fourth finger. Birds developed the most specialized structures so far with reduction in the number of digits, reduction in the bones in the remaining ones, fusion of the wrist and other modifications. Three different wings which all work well enough but which are all clearly modifications of structures in flightless animals.
