During the reading "Origin of species", there are used so many examples. I have to say, that in every example there is a situation that is about killing one specie in order to survive. For example, he talked about the animals of the same species. He says that if one is not eating the other they both will die. So, the one of the two animals has to eat the other in order to stay alive. And as we know, the main point of pessimism is death. All examples and whole book is about killing each other. That is why I think that Darwin's Origin is pessimistic book.
We, as modern humans are, in relative, spoiled into thinking that every single life is going to be lived to the fullest, and end naturally.
It is true, that if I was in a developing country, practically hunting and gathering to survive, I might disagree with my earlier statement, but through four billion years of evolution, complex multicellular life developed, giving life more subjective purpose than before, due to conscious sentient beings appearing, such as we Homo Sapiens Sapiens. It is not particularly optimistic, but definitely not pessimistic.