Darwin liven in a world where it was forbidden to doubt, and the accepted view at the time was that we lived in a world created directly by God, just a few thousand years ago, and that everything was created, just the way it was, so that man could exist and be tested in his faith and obedience to the church.
The social structures of the time were more or less stable, but they were based on this world view. Darwin and many others were questioning that, and evidence brought by journeys around the world made more obvious that the world was there for much, much, much longer. In fact, unimaginably longer. And that, for all we could see, it was not created to harbor humans anymore than to harbor tortoises or any other actual or possible living thing. We were not designed from scratch, we simply happened, just like a baby appears, among countless possibilities, there it is, this one, specifically. So man, had things turned out just a little bit different millions of years ago, would probably not exist, something would have existed, how close to our present outlook is entirely unknown.
Now this new perspective of things brings a new question: if we were not designed, then, by which processes are species selected, developed, maintained. How can we explain that things are the way they are.
Now, my friend, it should be plain and simple to see that the answer to that would be to describe in detail everything that has ever happened on this planet. Then we would know exactly what processes brought us here. We could categorize them in all sorts of scales, etc. It would be a huge task, obviously impossible to our limited minds, even if we had complete access to the data. That this is indeed so difficult can be easily see when we try to reconstruct the reason why something fairly simple happened. For instance an airoplane motor blows up. Now you recover the pieces, we have the black box of the plain, but to know exactly what happened - was something that entered the engine and made it blow, was it a design failure, a terrorist action, a maintenance problem? Or think of a disease - what caused it, how does it work?
Everything in the natural world is so complex that it seems utter impossible to say why that has happened instead of some other thing, and much less could we dream of finding a complete explanation of the whole history of all the species on animals and plants on the entire planet over thousands of millions of years.
And yet, that was what Darwin proposed, an explanation of the kind of thing that happens. Of course it is not a detailed explanation of the events, but it is supposed to be a detailed and complete explanation of the kinds of events that intervene to shape the process of evolution over time (the process that brings about the complex species was we know them).
Now the process that Darwin proposed is very simple and has only two steps:
1. Diversification of the genetic code (either by mutations or sexual combination of genes in the offspring)
2. Selection - the less adapted reproduce less and so the more adapted will have more offspring.
It is a very simple process, obviously real, with effects easy to understand, but it has two important frailties. First of all, when we look around us, it does not seem that the most important factor in evolution today, now, is this kind of stuff. We would say things like "decisions", "values", things like "courage", "fear", "prejudice", "willingness to learn", or simply "random events", are much more important in dictating history in the short term, than any of this. Of course, if history in the short term is so affected by all these things, then in the long run it cannot be impervious to them.
The second thing is that we cannot actually look to the past and see if that was the way it happened. Of course, both diversification and selection still occur today and we have no reason to suppose that they would not have happened in the distant past, since the conditions, for genetic mutation for instance, were there from the start. But today we see ourselves as having a role in shaping our future. We think we are in some measure responsible for what will happen next. Each of us has a role to play, each of us can make a small contribution, either for the best or for the worst.
For instance, regarding offspring, a man may be very much more endowed in many respects than another man. But the willingness to have offspring, and many other factors that do not have to do with genetics but with choices and will, may influence the quantity of children that each man has.
Now, how much of this has been happening in the past? Does the history of mammals allow for things like will, courage, persistence, etc, to have a role, even a big role, in the history of evolution?
The question may seem a bit ridiculous, because, from a third person perspective, the scientific perspective, we can't even know that our fellow human beings have freedom in the relevant sense. It is obvious that we do see ourselves as having freedom, but this may be just a way to describe our behavior, and certainly does not imply that we can in fact get loose of the genetic and cultural constraints that give us our human shape.
In any case it seems both absurd to say that we do not have freedom in this real, incompatible with determinism, sense, for our life would seem to be meaningless and we would be nothing but robots. Where would the judicial system, for instance, get its foundation? But it also seems absurd to attribute to people the ability to change the laws of nature, for it seems that macroscopic stuff like cells, including brain cells, and tissue and muscles, etc, work in a way that can be predicted. That is deterministic. So, from a scientific perspective, we are just very, very, very complicated robots. Much more complex than the ones we are able to build with current technology but physical stuff nevertheless, predictable and, fundamentally, controllable and predictable.
If we are in such a predicament concerning human nature, the same does not apply to the rest of our brother animals. Be it chimps, dolphins, horses, dogs, etc, the scientific view is the same: they are mere machines, working out a program, that may be influenced by the environment but they are, nevertheless, machines. This perspective has been quite practical, for we can therefore distinguish them from us in a clear cut sense: animals can be killed, used in experiments in whatever way necessary, a species may be decimated, used for any kind of purpose, altered in any way, etc. Humans can't. And we, humans, feel quite safe with that distinction.
But the distinction between "us" and the rest of nature has two problems: first it is completely unwarranted. As philosopher Peter Singer notes, a human baby has much less awareness of himself and the world around him than a typical orangutan. So it seems this podium in which we place ourselves is warranted only by our own needs.
The second problem is that this distinction makes the world quite irrational: up until man arrives the whole world worked mechanically, with nothing interesting going on. Then man arriaves and, alas, we have the moral conflicts, the moral fight of good and evil, and everything that makes life worthwhile. Now, it would seem that the universe took a very long time until something interesting like us finally came up!
Although this view that mankind has created puts mankind in a glorious place, it seems far from true. It is easy to spot the same kind of choices and moral dilemmas in animals we know well, like pets. The only difference is that «they don't talk», which implies many other differences, like «they cannot inherit their ancestors experiences and insights», they must learn anything anew. If we think it that way, it becomes easy to see that they are not much less intelligent, a child with no ability to talk, to absorb her large cultural experience accumulated by generations and generations would not be much smarter than most mammals! This is offensive to most people, because we see nature as something ugly, less valuable, that can be used and abused according to our needs. So to say that I am not much different from any other mammal is akin to say that I am not much different from a madman. In both cases I'm comparing myself to someone who is outside of the circle of those that have the luxury of having social rights. That can be dangerous, in the very least it sounds dangerous. And being angry with such a comparison is a good way to place ourselves back in the restricted circle: «not me, I'm not an animal, no sir!»
Now, if we are really searching for the truth, we cannot really say that we are different. Our only difference is in terms of language, and that has made us into artificial entities. People are artifacts made over countless generations of accumulated experiences and insights. Our intelligence was manufactured and carefully preserved and transfered into each new child. After the program is in place we can say: "I am such and such, want such and such and will do such and such to have it. This is the world I live in, and this is how it works, and these are the things that are valuable, and those are the things that are forbidden." In a very literal sense we are artificial intelligences running on biological hardware. However, choices, deisres, courage, weakness, persistence, altruism, love, and other qualities of this kind, are not taught, they are there from the start, in the brain, and we have no reason to think that the large majority of animals have them too.
So, this leads us with a fundamental unanswered question: «do we have free will?» (in the incompatibilist sense) If we do have free will then it seems to play a significant role to shape events. It would be a huge force in shaping evolution, specially over millions of years. If we do not have free will, that Darwin's account must apply also to current events, and we must say that the ultimate reason that explains World War II is diversification and selection.
What we cannot do is to divide the world between the human and the pre-human era, as if they work in completely different sets of rules.