This is a good question and there's lots of possible answers. 1. Multiple forms of life did arise at about the same time, but in the end there was one. 2. Mul

This is a good question and there's lots of possible answers. 1. Multiple forms of life did arise at about the same time, but in the end there was one. 2. Multiple forms of life did arise and ended up cross breeding or doing lateral gene transfers or any number novel things. We are the sum result of the earliest forms of life, but it's not like we have everything mapped out there. 3. The potential for new life is all around us, but can't compete with existing life that has millions of years head start. Regardless of where you pick a point after life is already established, a million years later, when the dinosaurs went extinct, 1901, the problem is the same. Something else is already there has been evolving for ages. 4. New life does periodically arise but it's a drop in the ocean and finding that particular drop isn't easy. Assuming it even survives at all for any length of time. 5. We've already seen independent life, but just haven't recognized it as such.