The earliest known fossil footprints on land actually predate the earliest definitively land-based animals fossils by a considerable margin: 170 million years. It appears that our early ancestors may have explored the land before they moved there permanently. In fact, fossil records suggest that this exploration began before there were even terrestrial (as opposed to aquatic) plants – which may account for this peculiarity: no plants would have meant no food.
The first known animals to leave the ocean for good were members of the superclass Tetrapoda, a large group that includes all amphibians, reptiles, birds, and mammals, living and extinct. Including us.