As it is the case for non-coding regions, the coding regions of organisms can be expanded or shrunk during evolutionary processes. However, the dynamics of coding regions are expected to be more correlated with functional complexity and diversity than are the dynamics of non-coding regions. Hence, it is interesting to investigate the increase of diversity in coding regions--the origin and evolution of new genes - because this provides a new component to the genetic variation underlying the diversity of living organisms. Here, we examine what is known about the mechanisms responsible for the increase in gene number. Every mechanism affects genomes in a distinct way and to a different extent and it appears that certain organisms favor particular mechanisms. The detail of some interesting gene acquisitions reveals the extreme dynamism of genomes. Finally, we discuss what is known about the fate of new genes and conclude that many of the acquisitions are likely to have been driven by natural selection; they increase functional complexity, diversity, and/or adaptation of species. Despite this, the correlation between complexity of life and gene number is low and closely related species (with very similar life histories) can have very different number of genes. We call this phenomenon the G-value paradox.