Think about the last time you ran really fast or watched someone run all-out.  Lots of body parts were working hard.  You could see legs moving fast and arms swinging.  What was happening inside that you could not see?  At the end of the race, the runner was breathing hard, very hard.  Why?

Look at me! I'm running so fast I hope I don't run out of gas.

The runner's body needed more air, so it was pulling as much as it could into the lungs.  The air that goes into our lungs has many different gases in it, but our bodies do not need all of them.  The part of air our body really needs is a gas called oxygen.  This gas makes up about one-fifth of the air we breathe.  All the cells in your body need it, and your muscles run out of it fast when you work them hard.  After you pull it into your lungs, it gets carried all over your body by the blood pumped by your heart.

In fact, most of the living things that you can see on Earth need oxygen to do their everyday jobs.  People, animals, plants, and even some tiny bacteria use it.  Most life forms need it to live, and they get it from the air around them, but it was not always like this.  Long ago, there was not much of that gas around Earth, but there were still living things.

The Earth formed more than four and a half billion years ago.  Then it took about another billion years before any kind of living thing showed up.  The air was much different than it is now.  There was very little oxygen, so we think that the first living things did not use it.  You may not know it, but inside your body you have tiny organisms like this who help you break down your food.  These organisms are called anaerobic, which means they do not need oxygen to live and grow.  In fact, too much of it can even kill some of them.

We think the first living things were very simple and got the energy they needed from what was around them.  Then some began to change, and some of them started to make their own food.  They took the chemicals and gas around them and started using them for energy.  We think that some of the first life found what it needed to live in the ocean.  When what they used for energy started to get used up in the ocean, some life forms were able to take light and make food they could use for energy.

Simple creatures like this amoeba proteus aren't very pretty but you'll need a microscope to see them.

Making food started happening more than a billion years after the first life showed up on Earth.  After all that time, living things were not as simple as they once were.  They could do more.  Some could use photosynthesis, which is when a living thing takes the light from the sun and turns it into energy it can use.  When a living thing does this, oxygen is given off as waste.  As this happened more and more, the air slowly changed into what it is like today.

Not only are strawberries delicious, the plant produces oxygen too.

More oxygen made it possible for new life forms to develop that could use this gas.  Living things that need oxygen are called aerobic.  Over time, they grew and changed.  They were far different from the simple living things that first appeared on Earth.  This change in the air helped make it possible to have all the different kinds of life on Earth today.

That fast runner we talked about at the beginning of this story is happy to have that air to breathe.  You do not have to be speedy to be thankful there is enough oxygen in the air.  Think about what life was like when Earth first formed.  Can you see why it took so long for more complex living things to show up?  We have come a long, long, long way from those first tiny life forms.