These organisms were among the first to appear on Earth. The oldest known fossils are those of bacteria-like organisms. Bacteria were unknown to people until the 1600s. Later Antony Van Leeuwenhoek observed them in his newly-made microscope. The cells of bacteria are different from plants and animals in different ways. The bacteria lack a nucleus and other membrane-organelles. Bacteria are found almost everywhere on Earth, including in the seas and lakes on all continents.