Cells!

Cells are the basic units of all living things. The first cell appeared on Earth about 3.5 billion years ago. Today, there are many different kinds of cells. The differences in the cells of organisms are sometimes used to classify them into groups. Although cells may vary in their size, shape, contents and organisation, they all perform functions that are involved in keeping the organism to which they belong alive.A whole new world was discovered just over 400 years ago when an English inventor and scientist used magnifying lenses to observe the basic units of which all living things are made. This led to a new way of thinking about living things that required a new scientific language, new classifications and new inventions to find out more about this new world.In the seventeenth century, Robert Hooke looked at thin slices of cork under a microscope (= very small + view) that he had made himself from lenses. He observed small box-like shapes inside the cork. He called the little boxes that he saw cells. Microscopes opened up a whole new world that had never been seen before.Using microscopes to carefully observe different living things showed that they also were made of these tiny basic units. As the magnification provided by microscopes increased, it could be seen that, although the basic structure of cells was similar, there were quite a few differences. Different groups of organisms often contained different types of cells. It was also discovered that different types of cells could be found within an individual organism.

-From Science Quest 8 Australian Edition(3.1 Cells)