Neurogenesis, or the birth of new neuronal cells, was thought to occur only in developing organisms. However, recent research has demonstrated that neurogenesis does indeed continue into and throughout adult life. On going neurogenesis is thought to be an important mechanism underlying neuronal plasticity, enabling organisms to adapt to environmental changes and influencing learning and memory throughout life. A number of different factors that regulate neurogenesis have been identified. Physicial activity and environmental conditions have been known to affect proliferation and survival of neurons. Hormones have also been found to influence the rate of neurogenesis in vertebrates (e.g. testosterone) and invertebrates (eg: ecdysone). Serotonin is believed to play a key role in neurogenesis.