Life can only be felt but is difficult to define. Everyone is very easy to say whether something is alive or dead. When one looks at a tree that is day to day high, leafy green, then branched, it is easy to know that the tree is alive. when the man has seen the dry tree, its leaves fall, does not grow big watered or fertilized, it is easy to know that the tree is dead, no longer alive. Humans and animals.
When humans and animals appear to grow, all members of the body work, and can multiply.
Everyone easily concludes people and animals who then live rather than die. This is the meaning of life biologically. In the sense of life like that, humans and animals. Eat together, drink, move, breed, love children, and see others.