Suzuki, in Zen Mind, Beginner's Mind, gives a wonderful metaphor for life, birth, and death – a waterfall. If you think of a river flowing downstream, it is all one river. When the river reaches the edge of a waterfall, it breaks up into billions of drops of water. The individual life is like one of those drops of water. Our friends and family are those drops of water around us, that fall with us. We see ourselves as separate, individual. Then, at the bottom of the waterfall, we become the river again.
Life happens between the top and the bottom of the waterfall. Birth is that moment we become separate, the span of a life is the time we fall as individual drops of water, death is the moment we merge back together. Unity before birth, separation during life, unity again at death.