The Earth is really old. Really, really, really old. We’ve already seen that scientists estimate it’s about 4.6 billion years old. That’s much older than you can think back. But what’s even more interesting is the fact that it was so hot that there wasn’t even any solid rock for about 500 million years. This means that Earth was a ball of boiling hot liquid with no land to speak of. After a few million years, the outer layer of the Earth cooled down enough to be solid (like the crust of a pizza) but the inner part of the Earth was still hot liquid magma. Of course, that magma wanted to get out of the middle of the Earth. So, the magma bubbled up through the crust (these are volcanoes), and so gases and water vapor ended up coming out of the hot Earth and landing in the atmosphere. That water vapor cooled down enough to become the ocean, which is where it’s believed life began.
So there were these tiny, microscopic living things in the ocean. Amazingly, some of them actually turned into fossils so that scientists know that at least 3.5 billion years ago, these small organisms existed! Furthermore, scientists find these organisms inside of certain types of rocks that only existed during a certain time period. By doing all sorts of chemical testing, scientists can estimate how old a particular type of rock is, and once they know that, they know how old the fossils are inside of that rock. The guides which help scientists all over the world figure out how old rocks are is called a rock sequence.
One of the tests that scientists can do to figure out the age of rock is called radiometric dating. Radiometric dating uses processes that happen naturally to be able to tell how old certain things are. For example, you have a squirrel and an apple tree. Well, this squirrel happens to like eating apples, but will only eat one per day and leaves behind the core of the apple. One day, you happen to go past your apple tree and notice that there are 90 apples still on the tree and 10 apple cores on the ground. Assuming that nothing else happened with the apples or the cores, you know that the squirrel has been eating apples for 10 days. If you went by the tree seven days later, you would expect to see 17 apple cores instead of 10.
Radiometric dating is very much like this. There are certain chemicals, like uranium, which are radioactive and break down over time. They actually form other chemical elements, as uranium will form lead, just like the squirrel in our example turned the apple into a core. Scientists know how long it takes for uranium to break down into lead, so if they find some uranium and lead together, they can figure out how long that uranium has been around, and therefore how old the rock is. For example, if 90% of the rock is uranium, then it’s older than rock that’s 95% uranium.