The human male is characterized by extremely poor semen quality as reflected in the number, morphology and motility of the spermatozoa and a high incidence of nuclear and mitochondrial DNA damage. As a consequence of these factors, defective sperm function is thought to be a major contributor to the aetiology of human infertility, as well as childhood diseases including dominant genetic mutations such as achondroplasia and cancer. Factors associated with the origin of poor semen quality include: (i) a lack of selection pressure for high fecundity genes in developed countries, (ii) an evolutionary lineage associated with the deterioration of several male fertility genes in humans and their close ancestors, (iii) genetic factors including, but not limited to, Y-chromosome deletions (iv) paternal age and (v) environmental factors. A model is proposed whereby factors such as ageing or environmental toxicants initiate DNA strand breakage in the spermatozoa of affected males, eventually leading to a mutation in the embryo. This hypothesis stresses the importance of discovering the identity of those environmental factors that are capable of damaging DNA integrity in the male germ line. Such information could make an important contribution to understanding of the origins of both male infertility and a variety of pathological conditions that affect humans, including cancer and dominant genetic disease.