Chloe Olliver, Wendelien Bakker and Kylie Welsh, of Diamond Harbour School, asks :-
Oxygen is essential for life. Where did the first oxygen come from?
Andy Pratt, a chemist at the University of Canterbury, responded.
Hydrogen is the simplest atom of all. There are many more hydrogen atoms in the universe than atoms of any other element. In stars, like our sun, huge amounts of hydrogen atoms are compressed together. They fuse together to make a second element, helium. There is lots of helium in the sun. This process of making new elements produces a lot of energy. This makes the sun very hot and it is how we get sunlight. In big stars the helium fuses together to make more elements. When three heliums fuse they make the sixth element, carbon, which is essential for all living creatures. The carbon which is produced can fuse with another helium to make the eighth element which is oxygen.
The oxygen on earth was originally made in stars that eventually exploded. When such stars explode the oxygen atoms, along with lots more stuff, are spread far and wide. The solar system was made when a dust cloud of interstellar debris, including oxygen atoms, collapsed together to make the sun and the planets.
When the earth was formed some of the oxygen was already present as water. This water was gradually squeezed to the surface of the earth and became the oceans. Once the oceans had formed it was possible for life to develop, and all life depends on water. You are right that the oxygen in the atmosphere has been produced by living creatures (plants and algae). The oxygen in the air is produced from water as well as from carbon dioxide.
There probably wasn't any free oxygen about when the Earth was first formed as it is very reactive. Any oxygen which was around reacted with other things to make rocks. This is where lots of the oxygen on earth is found, even today. For example, sand and clay are mainly made from oxygen that has reacted with silicon (an element similar to carbon).