It is possible to understand life as the existence of a living being, and as the co-existence of all the living beings that obey the specific laws of nature. It is difficult to grasp how all the living beings can be different and simultaneously have something essential in common. History provides us with a good example of one way to explain this phenomena: how Saint Patrick explained God's unity and trinity by using the shamrock as a symbol. One excellent symbol to use when explaining the unity and diversity of life is a rainbow — each color of a rainbow can exist separately, but in the color spectrum, all the colors are organized in a special order and create a unity.
Explain to your audience that atoms, molecules at the chemical and biochemical levels, and cells at the biological level, are basic elements of all the living beings. The idea that all the Universe consists of small non-divided units like atoms was widespread among the thinkers of Antiquity. But the perspectives of the philosophers on the basic element differed. For example, Heraclitus supposed that the main element creating the Universe was fire, while Anaximander thought that it was apeiron. Titus Lucretius Carus composed a treatise called "On the Nature of Things" where he discussed the basic elements of the universe comprehensively.
Stress that all living beings are systems. This is the main principle of the unity of life. A system has a unity that is not equal to the sum of its parts. A striking example that can be used to explain the value of a system or wholeness, is the famous Indian "Gatha of Theri Subha" from "Tipitaka." In the story, a young libertine has fallen in love with a beautiful righteous woman and tries to seduce her by telling her that her beautiful eyes drive him mad. She plucks out her eye, demonstrating that outside the whole, a piece has no real value. All the biological organisms function like systems. Some of the elements in the systems are essential, others are valuable, but none of them functions outside the system.
Emphasize that the diversity of life has its roots in different conditions under which different biological organisms evolve and exist. The appearance of separated twins may be a testimonial supporting this argument. People who are very close and similar according to their origin and nature grow up into different individuals if separated. Furthermore, embryos of multiple biological species are very similar, but the adult organisms differ, because diverse environments in which they live, modify them. Hence, diverse external conditions determine the diversity of life. Charles Darwin advocated this statement through all his scientific career. He provided his earliest sketch on life evolution and diversity in his "The Voyage of the Beagle" and the full version of his theory he presented in "The Origin of Species."