According to the fossil record how many years ago did life first seem to appear on Earth?

The Cambrian Explosion, a period between about 540 and 530 million years ago when many major animal groups appeared in the fossil record. This is when animals first developed hard shells and skeletons, so they became much more easily fossilised.

Nobody is quite sure what caused the Cambrian Explosion, but increased oxygen levels in the air after the melting of a huge ice age around 600 million years ago may have led to an increase in the size of animals in the oceans; in turn they developed skeletons and shells to support their larger bodies. The evolution of predators may have inspired prey animals to develop shells, as well.

We do have fossils before the Cambrian Explosion, but these are simple, soft-bodied, small and rare, so the origins of animals are still shrouded in mystery.