Because that’s an inevitable consequence of natural selection. Perhaps my “Brown Bear example” (originally posted on a music forum - don’t ask) will be of some assistance in understanding how natural selction can bring about an entirely new species:
[i]The primary reason why life on Earth is so successful is its ability to adapt to a wide variety of environments. A species that is “adept” is more likely to survive than one that isn’t. This is known as natural selection: the individuals best suited to the environment in which they find themselves will theoretically survive and pass on their genes to their offspring, the weaker individuals will perish.
For instance, assume that there is a large population of brown bears living in the United States. All of a sudden drought strikes and some of the bears are forced to move up north to frosty Canada in order to find food and water. Once they find themselves in this colder environment, natural selection kicks in. Those with the thicker coats will be better able to keep warm, those with the intelligence to crack open the tops of frozen ponds to find fish will be better able to feed themselves. Those who have neither a thick cover of fur or a great abundance of intelligence will simultaneously freeze and starve.
The bears who have these qualities will be able to pass their unique genetic make up onto their offspring and, over the period of tens of thousands of years, the brown bears who moved up to Canada slowly change in appearence and demeanor. They will have a much thicker coat of fur than they did before, and they will also be much more intelligent. They gradually become a distinct species of bear.
Now remember that some of the brown bears decided not to move up to Canada, but to remain in America and stick out the drought. They have been isolated from the Canadian bears for tens of thousands of years, with no breeding between the two groups. While the Canadian bears have developed a thicker coat and a greater intelligence, the bears who remained in America have stayed virtually the same because they haven’t had to evolve (and this is another basic rule of thumb with evolution: that animals generally don’t evolve unless they have to. For instance, sharks are so perfectly adapted to their environment that they are virtually the same animal now as they were 350 million years ago). So, using this example, we can see that even though a species evolves, it doesn’t mean that the species it evolved from has to die out. Just because the American brown bear evolves into the Canadian bear when a small group of them venture up north, it doesn’t mean that the brown bears who remain in America die out.
This is the same with human evolution. Africa used to be a heavily forested continent, with few of the savanas that exist today. Then the world went through a climatic change and the forests began to dry up, leaving large areas of open grassland.
This was a problem for our primate ancestors. Where we before had eked out a happy existence swinging around in the trees, all of a sudden we were left exposed. There was a lot of competition for food in Africa at that time, and we were well down on the food chain. We weren’t particularly big creatures, nor did we have anything about us physically (like big claws or poisonous fangs) that could allow us to compete with the bigger animals (like the lions and so forth).
All we had when the forests dried up and we were left exposed was the fact that we were relatively intelligent and the fact that we were pack hunters. This allowed us to create communication between individuals so as to co-ordinate attacks on antelope or what have you. The taller individuals would be more likely to see over the tops of the tall grass to see oncoming predators while the smaller individuals would fall victim to these same animals. This was why we began to assume a more upright stance.
As a result of necessity, we became to become taller and more intelligent than our other primate cousins, a direct result of being forced out into the African grasslands.
These qualities also gave us the ability to roam, and Neanderthal man started to spread into Europe and Asia probably some 200,000 years ago and lived out happy lives for many a millenia.
But this is something that a lot of people fail to grasp. When neanderthals expanded into Asia and Europe, there were none of the dangers in these places that there were in the African plains. They didn’t need to evolve any more because they were basically on top of the food chain. There were no real predators more capable than the neanderthals in Europe or Asia, so we were free to kill and eat whatever we wanted. We didn’t need to evolve because, effectively, we were already too well suited to our environment.
However, the Neanderthals who remained in Africa during this time had no such luxury. They were still competing fiercely for scarce food supplies. They had to keep on evolving to stay alive. After 140,000 odd years of evolution, the Neanderthals in Africa had evolved into Homo Erectus…almost indistinct from modern man (homo sapien).
Homo erectus eventually ventured from Africa into Europe and Asia some 60,000 years ago. Of course, the Neanderthals, who hadn’t had to evolve for the past 140,000 years, were still there. There were two distinct species of human being living side by side in Europe and Asia.
Being such highly advanced predators, there wasn’t room for them both. Homo erectus, after having gone through 140,000 years of evolution in Africa, was far more intellegent, strong, mobile and adaptable than the more primitive Neanderthals. Homo erectus drove the Neanderthals to extinction about 40,000 years ago and essentially took over Europe and Asia as the main predator, above all other life on the food chain. But it should be remembered that for at least 20,000 years, there were two different species of human living side by side, which is almost unintelligable today.
Anyway, after all that, my point is the same as the one that Ruby_Slippers said before. We didn’t actually evolve from apes. Humans and all other primates evolved from the same common ancestor, but in reality we are seperated from all other primates by 2 million years of evolution. And as I explained before, just because one species evolves from another, it doesn’t mean that the original species has to die out.
And we’re still evolving even now. In the past 200 years, we’re an average of 5 inches taller than the humans of 1800. Like Carl Sagan said, man is a transitional animal. It would be arrogant to think of ourselves as the pinicle of creation or evolution, we’re still evolving and will continue to do so until we become extinct. [/i]
The last part of that little essay is irrelevant, but I’m sure you can see how the first part is pertinent to our discussion. Specialisation occurs, because if animals find themselves in an environment that differs from the one that they were originally based (either through migration, or more commonly some rapid climate change) then they either have to acquire the attributes necessary to exist in this environment (which is done slowly over several generations) or else they perish. Specialization serves as evidence of the evolutionary position, not as a hinderence to it.
The qualms you have about the evolution of the eye and the acquisition of “useless” traits are based on similar principles, but seeing as I have an exam tomorrow morning, I’ll have to leave them until another time.
