circa 530,000,000 BCE — The first animals leave the ocean

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.

circa 4,200,000,000 BCE — The Earth’s oceans form

The majestic oceans of planet Earth were formed neither quickly nor simply. It took literally millions of years between the first surface water’s appearance and the creation of the primordial sea.

Several factors contributed to this: the gradual cooling of the Earth was the first and most important, but also important was the slow release of water from existing minerals, the condensation of steam, and even the addition of water in the form of ice from occasional cometary collisions with the planet.

The first waters soon became the habitat of early prokaryotes – whose biochemical processes led to the formation of still more water. Indeed, it is possible that the majority of water on the planet today exists as a result of these organisms.