Dear Madeleine Isaacs
You might like to have a look at the letter I wrote to Sue Lee Hooker (if she doesn't mind sharing). But I'll answer your question as best as I can and tell you what most scientists think at the moment, more or less.
When the earth was young it was hotter than it is now--on the surface at least. The atmosphere was full of chemicals like ammonia, hydrogen, water vapour and methane.
Eventually the surface of the earth cooled enough for water to be in liquid form (not steam). It gathered in ponds and oceans. This hot water dissolved lots of chemicals from the air and from the rocks and from clay--and maybe even chemicals from comets that crashed into the young earth. Lightning flashed.
It was like the ultimate laboratory for the mad professor!
What scientists don't agree on yet is what happened next. But although no one knows for sure how life started on Earth, we're getting closer to the answer all the time...and coming up with new ideas, too.
What we do know is that eventually at least one tiny cell was formed in the hot chemical soup of the early earth. And this tiny cell could reproduce itself. And each of those cells could reproduce themselves...and so on. Some simple maths can show that soon there would be zillions of them. Although not all would be perfect copies.
Over millions and billions of years different tiny creatures evolved to take advantage of the changes that were happening on the earth. Some of these creatures excreted oxygen--eventually enough to make up more than a quarter of the entire atmosphere of the earth. Having oxygen around allowed plants and animals to develop. Animals could breathe it. The oxygen in the air was poisonous, of course--because oxygen reacts with most things. But animals and plants soon worked out ways to protect themselves from the oxygen.
This process has continued for an unbelievably long time. Some organisms have been so successful they have continued the same for millions or even billions of years. And some animals and plants have evolved very recently. Like us, for instance.
Well, Madeleine, another one of my long answers. I hope it was helpful.