I see.
So what you are saying is a species 'evolves' if it's environment changes, in order for it to survive.
Ok. I can see that.
But it still doesn't explain why neanderthal and cromagnon man no longer exist.
Was it environment that caused their extinction, because of their stubborness to adapt to that environment, or where they just too stupid to move?
Maybe a catyclismic change that was unavoidable?
BTW. Thanks for explaining these things to me in a way that even a dumb 'layman' like myself could understand.
Dimensio, you got suckered.
The exact answer to this question may never be known, and even if we knew it for some of our hominid cousin species we still wouldn't know the answer for all of the untold billions of species that have gone extinct over geological time. Some things we can only speculate about, but that doesn't make the evidence for evolution less convincing.
Species go extinct because insufficient members survive to child-rearing age. Cataclysmic change can make this happen very rapidly to a large number of species; witness the current effects of habitat destruction by man. Species that cannot adapt fast enough to the changed circumstances will die out. Or even in unchanged environmental circumstances a new similar competitor can lead to rapid extinction. A few decades ago a single pregnant grey squirrel must have arrived in the UK somehow, which only had red squirrels (in their millions). Now red squirrels are all but extinct, surviving only on islands that the greys haven't reached, and in Scotland where the greys advantages are not so crushing.
Let me amplify on this a little. If you are truly interested, this example may help.
Modern humans left Africa some 60,000+ years ago. Much of the subsequent expansion of these initial groups actually came quite a bit later. All human races (outside of Africa) developed in that time span due to adaptation to local environmental conditions through the mechanism of natural selection.
I did a major post on this yesterday on another thread, and don't want to go through the labor again. Short version:
Skin color is an adaptation to reduce ultraviolet radiation (dark) or augment vitamin D production (light). Tanning is a method to split the difference.
Nose form (or more precisely the nasal chambers of the skull) is related to increasing the temperature and humidity of air reaching the lungs, and the variation worldwide corresponds well to local climate. However, this changes slowly, so some migrations, such as North American Indians, mostly retain the cold-weather adapted form.
Body shape also corresponds to temperature, with large or rounded bodies better suited to cold climates and smaller or linear bodies to hot climates.
In all of these traits there are clines, or gradual changes from one environment to another, allowing us to examine the intermediate forms. For example, skin becomes progressively lighter from north Africa to the sub-arctic (but the Eskimos have do not have the extremely light skin of northern Europe because there is no way to get enough vitamin D with the low light levels, nor could they expose enough skin to the sunlight anyway; they rely on fish oils as supplements).
Now these examples, and many others just within our species, show adaptation to local environments through the mechanism of natural selection. Given enough time, this can result in speciation.
Hope this helps.
Neanderthal probably didn't survive the last great ice age even though there's an unproven theory that they were assimilated and even contributed the red hair gene to our current population. Cromagnon was an early Euroasian who's descendants currently populate the earth.
http://www.freerepublic.com/focus/f-news/1505777/posts?page=56#56