Why is there death and cancer?

In human history have been the natural aging has been the presence of death, then we may assume that the natural aging death will be a very well-deserved things. But not like this, the current study did find some organisms (lower organisms) have the ability to eternal life. So, the question is, eternal life is the ability to evolve the ability death or senescence evolved come from?

From an evolutionary point of view we have to try to interpret what Darwin this issue. If eternal ability evolved, imagine what the meaning of existence of this ability, through what kind of selection pressure to make some creature has the ability to eternal life? I think for a long time, did not expect what selection pressure allows a species to adapt to the environment and the screening of individuals of the species have eternal life and the ability to reproduce it. So I made another hypothesis, not an innate ability of biological aging death, it may be the ability of an organism evolved under natural selection pressures gradually acquired are filtered out! The following is an open brain hole time, we have to analyze at roughly what kind of natural selection pressure screening out aging death this phenotype!

Darwin's theory of evolution is generally summarized as follows: overgrowth, survival competition (between the two species and within species), natural selection, survival of the fittest.