Life

What defines a life?

• the existence of an individual human being or animal. (google)

But that’s the definition for things that are in front of us, what about those who are dead, how do we know if they lived or not?
Are we really here just to exist, eat, sleep and die? 

What if a person is born in an stranded island and spend his whole life there wandering on the island and eventually leaves the world. How do I know if he lived there, why should I know. Why do I need to know who is living next to my house? But there are people who I know lived a life and alot of people talk about them.