Sex and death are intimately related.
Before sex, life was immortal, or sort of. Before nature thought of gene swapping as a way to expedite new attributes by way of DNA mixing, life was only single cells that reproduced by dividing. One cell simply split in two taking half the genetic material with it and growing the other half after division to become two perfect replicas. One cell became two, two became four etc. and no one had to die. Unfortunately we humans can't do the divide thing and I'm not sure I would like to have myself as a roommate anyway. Yes, twins, triplets etc. happen, but still at the single cell level and still require two to happen.
The disadvantage of immortal cell division is slow adaptability to environmental changes as cells wait on random faulty genes to produce, by chance, an attribute that will help them adjust to their new environment. Fortunately single cells are good at playing the numbers game and produce billions of cells very quickly by doubling. If two cells become four in one minuet it doesn't take long before ten thousand cells become twenty thousand cells in a minuet etc.
Gene swapping probably started when single cells with one attribute found it advantages to hang around with another group of cells with another attribute and began to swap DNA as a way to make the union permanent, a kind of primitive group sex. These cell parties soon led to the first multi cellular life forms and DNA transfer methods were perfected as multi cellular life became more complex.
Even plants have sex, of a sort, and plants like animals all die after, hopefully, reproducing. Immortality is reserved for the single cellular, sequential replacement for the multi cellular, and with it sex and death. At the single cell level one becomes two. At the multi cellular level two becomes one. It makes one wonder if sex is worth dying for.
