No one really knows for sure when life in general begain.
If this question is in regards to procreation, and the development of an egg to a baby, I would argue that life begin at conception: From a biological prospective, a sperm and an egg individually have 50% of the required DNA to develop into a fully functional human being; Separately, they can never do that, for they each will stand alone with their 50% of DNA; however, at conception, the individual 50% combine to make the 100% DNA needed for propagation, cell proliferation, etc.
This zygote is its own biochemical system, with its own chemical reactions taking place based on the DNA that it inherited. Granted, it rely on its mother's nutrients for food, but even after birth, a child still rely on its mother's natural nutrients (breast milk), or their mother to provide some alternative food source. -There are times when the mother's body may naturally abort the zygote which may happen for a number of reasons, but more often than none, the development is not going well.
This is not an opinion, but it is a Biological fact! Life begins at conception!