From one point of view, evolution is an extraordinarily cruel process. It needs thousands of deaths until a species develops one very important feature.
Darwin called one of the principles involved "Survival of the fittest".
As a matter of fact this is only part true:
Every time an individual procreates, it passes on it's (faulty) genes to the next generation (along with some random mutations).
If the next individual develops a mutation that prevent's it from procreating, this individual dies and the faulty gene causing it is not spread. It does not matter, weather the mutation is harmful (like developing a disease) or actually superior to the ancestoral gene code (like developing a smarter brain but with the problem of worse chances to procreate).
This is called sexual evolution:
The more one mutation promotes spreading of the gene before the end of life, the greater the preassure of selection for this very gene.
This fault in the process of evolution actually describes why we have not evolved to live much longer. (I'll come to that later.)
Here is a practical example:
Two children are born, each with a different mutation. The first one (A) is "programmed" to be a physically perfect individual with common intelligence.
The second one (B) is not physically perfect and therefore does not stand out of society, but developed a greater intelligence which is crutial for the survival of this species in their modern world.
As the species evolved, every member tried to select the physically best counterpart for procreating.
This leaves B with very few mating partners and therefore fewer chance of spreading the "intelligence gene".
(You could also substitute the "intelligence gene" by some "longevity gene", etc.)
This theory has long been discussed and mathematically, but not biologically proven. (The math behind the theory is actually quite simple and can be derived from the example above.)
However, for the first time, biologists have proven the validity of this principle and extended the lifespan of C. elegans by 50% just by switchig off one of those falty genes A spread.
Go on an read this fascinating story...