What's your favorite animal?  Are you a dog or cat person?  Do you love snakes or butterflies?  Do you have a poster of a horse or wolf?  No matter which organism is your favorite, the question is the same.  Where did it come from?

Earth was formed more than four and one-half billion years ago.  There was no life for another billion years.  The first life was very simple.  Slowly, it changed and developed.  How did it start in the first place?  Scientists are not sure, and they do not all agree, but they have some ideas.

One thing scientists do agree on is what was needed for life to begin: water, energy, and certain building blocks.  An early idea was that life began in what we call the primordial soup.  Primordial soup is what scientists call the air and oceans of early Earth.  You can think of this as just like a soup you eat.  Soup has lots of different things in it: vegetables, meat, and broth.  The "soup" that some life might have started in had a mixture of different gases and liquids.  But how did life pop up in that "soup"?  Again, no one is sure.

Who doesn't love soup?

Many people have tried to figure this out,  Years ago, some scientists tried making life in their own "soup" using electricity.  They set it up so that the electricity would go through the gases that they thought were in the air and ocean of the early Earth.  After a week, there was some soupy stuff that had the building blocks of life in it, but now, many people think the test may have used the wrong gases.

Other scientists think that the energy added to the "soup" may have come from under the ocean.  Deep in the ocean, there are holes in the floor that let out heat and gases.  Next to these holes we have seen living things that are not found anywhere else on Earth.  Different kinds of plants and animals live near these holes.  Maybe billions of years ago those gases used heat energy to mix with stuff in the ocean and form the building blocks that started life.

The building blocks found in the ocean may have been helped along by something that, at first, seems impossible.  Long, long ago the sun was not as bright as it is now, so the oceans may have been frozen over.  The water under the ice cover would have been cold, and some scientists think that the cold could have helped the first organisms live longer than if it had been very hot.  Also, the ice cover could have protected living things from the harmful rays of the sun as well as from large rocks falling from space and crashing into Earth.

On the other hand, there are people who think that some of the building blocks for cells came from space.  They might have been brought on comets, which are balls of frozen gas, rock, and dust that go around the sun.  Sometimes they are called "dirty snowballs," though they are much bigger.  They are likely made of gas, dust, ice and rock left over when our sun and its planets were formed.  Some people think this may have happened because they have found bits of the building blocks of life in the dust of comets.

Can comets wag their tails? I don't think so.

ESO, CC BY 4.0 , via Wikimedia Commons

Have you ever made a snowball?  You pack the ice and snow with your hands, but other things can become part of it by accident, such as grass and dirt.  When snowballs hit something, they often fall apart, and the stuff spreads out.  Something like this may have happened on our planet.  During the first billion years after Earth was formed, it was hit again and again by these icy rocks that broke up when they landed.  It was a huge snowball fight!  The building blocks and water (in the form of ice) they brought may have started life on the planet.

Dirty snowballs were not the only things hitting Earth long ago.  Large and small lumps of rock called meteors rained down on the planet for thousands of years.  Some were so large they would have left big dents in the earth.  But since the earth always moves and changes, a bit at a time, none of these holes are still around.  However, pieces of these meteors have been found deep under the ground.

These rock showers could have set the stage needed for life to start.  It's called a meteor impact when one of these space rocks hits Earth at a very high speed.  This would give off lots of heat energy that could have put together the building blocks of life.  The air in rooms for people is often between 20 and 25 degrees C (68 and 77 degrees F).  Some animals, like certain penguins, can live where it is always very cold.  However, the first life forms may have needed it to be about three times as hot as your classroom.  The crashing space rocks brought that heat.

You won't need your umbrella in a meteor shower.

When you look at the world full of living things such as kittens, owls, honey bees, trees, and turtles, it's hard to picture a time when there was no life on planet Earth.  It's everywhere you look.  Some life is so small you cannot see it with your eyes, but it's there just the same.  Most life on Earth is not that simple, but it all had to start somehow.  Scientists are still searching for answers.