From a biological standpoint, life has to have a few important characteristics. They must respond to outside stimuli, they must be able to regulate, they have to grow, to reproduce, use energy, evolve, and it must be made of cells. These are the basic characteristics for all forms of life on this planet. In order to be considered “life” a species of organisms must be able to do all of these things. Though there may be some exceptions, such as infertile organisms, all forms of life display all of these characteristics. If a group of particles cannot perform all of these they are not considered to be life. Viruses, for example display several of these key concepts, but as a group they do not display all of them, so they are not considered to be alive.
The smallest unit of life on earth is the cell. It is the smallest and most basic organism that fulfills all seven characteristics. It is also the oldest form of life on the planet. Cells can function independently as their own organism, or combine to create multicellular organisms. Every larger organism is made out of cells. Multicellular organism are made up of many specialized cells, these cells coordinate with each other to performs complicated tasks that one cell could not do.
What I find most fascinating about life, other than its existence, is the amount of diversity among it. The ability of life to change from one small bacteria to billions of unique organisms is simply incredible. Even among the same species the amount of diversity is staggering. I think that diversity is one of the most important things about life. Without that diversity, life probably wouldn’t have survived very long at all. All it would take is one major disaster and all of life would have died out. The diversity ensures that organisms are capable of adapting to different situations, ensuring the survival of life if anything disastrous did happen.