Question

When did primitive life begin on earth?

Answer 1

At least `3.8` billion years ago. Possibly more, but it's hard to tell.

Explanation:

We have seen evidence of life as early as `3.8` billion years ago, approximately `700` million years after the Earth formed.

Finding earlier evidence is tricky...

The oldest rocks we have are about `4` billion years old, but some contain crystals of zircons as old as `4.4` billion years. We can measure certain things in these zircon crystals, such as the ratio of isotopes of some elements. The problem seems to be that these only offer somewhat circumstantial evidence of the existence of life on Earth at that time.