The total mass of microorganisms on the Earth exceeds that of all other life.
More than one kilogram of a human's mass is made up of microorganisms.
Microorganisms have been found living at depths of over a kilometre below the bottom of the ocean floor.
http://www.newscienti...-of-earths-crust.html
When they were first observed, nobody at the Royal Society of London (the leading scientific establish of the day) believed they existed.
http://en.wikipedia.o...tonie_van_Leeuwenhoek
A bit high brow for a nine year old but could be good if reworked.
All multicellular life is composed of cells that are the result of an endosymbiotic relationship between two ancient microorganisms. Without the mitochondria living inside eukaryotic cells, large lifeforms would be impossible.
See the Origin heading:
http://en.wikipedia.org/wiki/Mitochondrion