At the moment of conception it is past kamma that conditions the initial consciousness that vitalizes the fetus. It is this invisible kammic energy, generated from the past birth that produces mental phenomena and the phenomenon of life in an already extant physical phenomenon, to complete the trio that constitutes man. For a being to be born here a being must die somewhere. This enigmatic statement may be better understood by imagining life as a wave and not as a straight line. Birth and death are only two phases of the same process.
Birth precedes death, and death, on the other hand, precedes birth.
A first beginning of beings, who, obstructed by ignorance and fettered by craving, wander and fare on, is not to be perceived. This life-stream flows ad infinitum, as long as it is fed by the muddy waters of ignorance and craving. When these two are completely cut off, then only, if one so wishes, does the stream cease to flow, rebirth ends as in the case of the Buddhas and arahats. The Buddha has here referred merely to the beginning of the life-stream of living beings. It is left to scientists to speculate on the origin and the evolution of the universe.