On the Question "When Does Human Life Begin?"

When does an oak tree's life begin? I am not asking this frivolously.

I would say: An oak tree begins life as an acorn.

But an acorn is not the same thing as an oak tree. Then when does an oak tree become an oak tree? That's a much harder question.

The question is not, When does human life begin? That's easy: at conception. The question is, When does a human become a human? That's a much harder question.