"I go to church, but I hate how schools aren't proving what they are teaching." Why does it work for one but not for the other?
Evolution was already accepted by the Church well before Darwin's theory. What Darwin did is explain how evolution worked. There were three problems (as far as the Church was concerned) with his theory:
1. It showed that evolution was passive. In the sense that the properties that a race would end up having resulted from the weaker specimens being culled by the environment before having offsprings, rather than by the individuals themselves evolving with the environment and THEN passing on the favorable property to their offspring.
For example: if the climate gets colder, specimen that are more adapted to the cold will survive better. They will have a higher probability of having offsprings who will carry whatever gene made their parents better adapted to the cold. Under the old understanding, it would be possible to endure oneself to the cold, then pass on this endurance to one's descendants.
(The Church would have preferred an active evolution, and one that could be more directly governed by God)
2. The theory showed that it was possible for humans to have evolved from some ancestor who was not quite human. This ancestor could have a common ancestor with other primates.
There were at least five species of hominids that descended from this common ancestor, of which two (at least) were "intelligent" (H. Sapiens Sapiens -- that's us -- and H. Sapiens Idaltu) It is unclear if H. Neanderthalensis was a member of the H. sapiens subspecie.
(The Church would have preferred that humans be directly created by God, instead of being part of the evolution process)
3. The evolution, following this theory, takes time. For example, we took 200,000 years to evolve from our hominid ancestor.
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Of course, the biggest problem comes with number 2, which means that we were not created directly by God and, therefore, it may become far more difficult to determine who (or what) has a soul and who does not.
For example, when Christian explorers arrived in North America, they deemed that the inhabitants did not have souls. It was therefore OK to kill them (it was not a sin).
Back to the theory.
Any attack on points 1 and 2 have failed on scientific grounds. All scientists agree that the DNA evidence does show that we share far more in common with some animals (because we share common ancestors) than with others. Our body also shows vestigial organs that are useless to us in our present environment, but would have been useful for an ancestor in a different environment. These ancestors were known to be quite remote from our present form.
This leaves point 3. If we could just show that the time is not available, then the theory would be rendered useless (it needs all three points -- and some others which I have not added here).
The major thrust was to resurrect the idea that the world is young. When the idea of a world that was created in 4004 BC was issued by Ussher, it was already dismissed by the Christian churches everywhere. No one knew the age of the world, but everyone already knew that it was much older than that.
In order to dismiss the idea that the world is very old (old enough for evolution to have had time to work the way it could have), even the Big Bang is being "tossed aside".
When it was first formulated, the Big Bang theory was ridiculed by scientists. Another theory was presented (to explain expansion, which could not be denied) called the Steady State theory. Its greatest promoter was Fred Hoyle, probably the greatest astrophysicist at the time.
Fred was an atheist and proud to be. His main attack of the Big Bang theory (he's the one who gave it this awful name) is that it allowed that the universe could have been created. The great advantage of the other theory, he said, is that an eternal universe does not need to be created, therefore it does not need a Creator.
The Big Bang theory, at first, was defended by a priest (Lemaitre) and a few of his friends. It was often touted as a weak attempt to bring God back into the picture. The only reason the theory survived is that it is the one that best explains what we see.
For example, at the beginning, the universe was homogeneous and isotropic (this is a requirement -- it means that the universe was everywhere the same, and it looked the same in any direction from any position).
Therefore, once matter was created and (much later) allowed to assemble into galaxies, the spin axes of galaxies had to be randomly distributed.
If all the galaxies were spinning in the same direction, then that would show that the Big Bang theory ain't working.
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Evolution takes time and is the result of pressures from the environment. Various animals have evolved from common ancestors. For example, lions, tigers and domestic cats have evolved from a common feline ancestor. As Earth is getting more and more covered with humans and their cities, the domestic cat is doing a lot better. Is the domestic cat better than the lion or tiger? No. Just better adapted, and a little bit intelligent.
However, the specie that evolved into hominids is no longer present. In order to have animals evolve into humans, they would first have to evolve into this common ancestor (for a long time, it was called "the missing link"). This ancestor, or at least a link in the missing chain, is Australopithecus afarensis.
Unless some other animals first evolve into A. afarensis once again, then we can't have new sub-species of H. sapiens.