I think you’re talking about what is a human life. Not what is the biological definition of life. That’s pretty simple—just a series of a certain kind of chemical reaction. The kind we learn about in organic chemistry.
More specifically, I think you are actually asking what the meaning of a human life is, not how we might describe a human life.
My answer, as always, is that meaning-making is something that human beings do. In fact, it is the thing we do that it seems like no other form of life does. We ponder and try to find some satisfying reason for our awareness of our own condition. We need a purpose because that helps us decide what to do, and it makes our lives feel more satisfying. It’s probably built into us by evolution.
How do we make meaning? Actually, it’s pretty arbitrary. Mostly we do it in a social context. So we look around for a strong social group and we affiliate ourselves and take on its purpose. For many, a religiously organized group fills this need.
Others of us want to make our own meaning. We make our own choices. I think that’s where the greatest angst arises, because without reference to a social group, we have to be pretty damn sure of ourselves to make a choice about meaning purely based on our own preferences.
However, even without reference to any specific religion, I find my own choices are pretty similar to those from religions. I want to help people. I want to be useful. I want to make other people’s lives better. I derive a lot of meaning and satisfaction out of this.
I don’t need people’s thanks, but thanks always reinforce my desire to help and also helps me think I am actually helping (as opposed to fantasizing that what I am doing is helpful).
I also get meaning by feeling connected to others, especially on a kind of non-cognitive level. Music and dance help me achieve these connections, but again, I see that many religious traditions have “spiritual technologies” that enable their adherents to achieve similar states of bliss. Bliss is good. Bliss makes me feel like I’m doing the right thing. It makes me feel connected—not just to other people, but to the entire universe.
Then there’s evolution, itself. It is very meaningful to me to have children and a family. This is related to connection and to work (doing good). Children are an evolutionary legacy that will validate, I hope, the choices I have made in my life. My children are their own people, but I hope I have been a positive influence on them. If they do not validate me, I will be disappointed, but not angry. It is their choice. I am just trying to do the best I can, but it is possible I am on the wrong track. I trust evolution to be self-correcting over the long run, even if I turn out to be a dead-end, evolutionarily.
It is trying that provides meaning to me.
What gives meaning to me is only that which gives meaning to me. Other people will be different, and every person’s choices are, to my mind, equally valid. I don’t have any way of saying this is good or that is bad, except in reference to myself. I have no objective way to determining this. If someone chooses to be a murderer, I cannot say that is bad, in general. I can say it is bad in specific—it hurts me or people I love. But from a universal perspective, I cannot know. Perhaps we need all people to die for some reason I don’t understand.
I don’t think that is likely, though. I think what is likely is that there is no universal imperative. All we have is our own preferences, and we use those to choose what is meaningful to us and to decide how to live, and from all our actions, the future will arise.