Evolutionists, elaborate upon human evolution

Ancestors of humans occupied a much more similar niche as humans. Competition isn't going to occur if two species stay away from one another. For one, it probably isn't a coincidence that our closest extant (alive) relatives (chimps and gorillas) don't inhabit tropical grasslands, at least not the type we are thought to have come from.

Evolution is a process of a lot of things, namely natural selection, gene flow, genetic drift and mutation.

However, natural selection is the one that is relevant here. The neat thing (IMO) is that all it really requires are three things, and once it gets going, it has to keep going (it is spontaneous):
1. A heritable material. This is DNA.
2. Variation in the environment and genetically, and a correlation between those two. For example, height is highly heritable, and you know this because tall people tend to have tall kids.
3. Differential selection for that variation. If for whatever reason, short people are more fit - survive better, or have more kids, you would expect that population to get shorter, generation after generation. Note that this wouldn't happen without 1 and 2. That's selection.

Yes, Lamarck's theory was thrown out. However, some "Lamarckian" evolution does occur in "cultural" evolution and "epigenetics." But these are quite different from what Lamarck envisioned.

If you are really interested in learning about it, you should read "The Ancestor's Tale" by Richard Dawkins. This traces the ancestry of humans backwards from modern humans to single celled creatures and is a fascinating journey. It is probably the best book he has written and he's written some very good ones.

Instead of typing it all out, i'll refer you to the wiki page that I've helped to maintain:
http://en.wikipedia.org/wiki/Human_evolu…