As well as thinking about daily tasks like cooking and eating, humans have always thought about the meaning of existence. There are many answers to the question 'What is the meaning of life?'.
For example, most people would agree that both animals and plants are living things. Animals and plants reproduce either asexually or sexually. But these are not the only methods of reproduction - viruses reproduce by 'injecting' their genes into another cell and hijacking the host so that it makes more viruses.
As viruses can't reproduce independently, can they be said to be alive? The reproduction theory might imply that viruses are only alive part of the time - when infecting a cell.
And should the presence of DNA be the only criterion for being alive? If so, perhaps humans are no more 'alive' than a cold virus, an anthrax bacterium or a worm. Are humans just receptacles for DNA - biological machines blindly perpetuating our genes into the future? The controversy has raged many years, and predates the discovery of the structure of DNA in 1953 by Francis Crick and James Dewey Watson. Explore these arguments further in the following DNA scene.
SCENE: Positions on life