The question is a little unclear but the oldest form on history as in life is dug out of Ice in antarctica. They keep finding more and more really old bacteria over there.
How about the basic study of basic bacteria, get a house hold germ
(they're the ones with the apron on and a duster, aparently there is a difference between bacteria and house hold bacteria who knew??)
Show them the growth of the bacteria, get 4 or 5 dishes and implant certain conditions...