We report the first detailed and systematic study in a mammalian system to unravel the mystery of the beginnings of life. The fertilizing ability of damaged spermatozoa at various levels of disintegration (cellular and molecular) has been investigated in homologous (mouse) and heterologous (human spermatozoon, hamster oocyte) models. Live pups were produced after destruction of spermatozoa at various cellular and molecular levels followed by injection into oocytes. We demonstrate that with damaged spermatozoa, the key point in the fertilization process is the activation of the oocyte by injection of cytosolic sperm factor. A similar fertilization rate as that using live intact spermatozoa can be achieved following activation. However, the integrity of the genetic material influenced in-vitro development of the embryos and live fetuses. This study contributes to a better understanding of the fertilizing ability of damaged spermatozoa. These findings can be applied clinically to patients with necrozoospermia or very severe oligozoospermia and in wildlife research where damaged spermatozoa from rare species can be used to regenerate young, and hence propagate the species. Also implied is the possible contribution of sperm DNA strand breakage to early pregnancy loss.