That's not a very good definition of life or when life begins. In biology we think of life as a system of complex symmetry, where biochemical processes regulate the very processes that define life. Those biochemical processes begin when a sperm and egg fuse. We even use a name for when the individual animal begins it's life long process. It's called a zygote. A zygote certainly exists. If an unborn human doesn't exist, then there can't be an abortion, or a miscarriage, or a birth. Science moved past spontaneous generation/abiogenesis long ago, when experimenters such as Pasteur (given most of the credit) and earlier
Francesco Redi provided evidence that life doesn't simply spring up from nothing.
There are many species that cannot live on their own at some phase of their development. That doesn't disqualify them from being a living member of their species.