The animals that live their lives, they hunt, they mate, reproduce, fight for their territory. What would make their life meaningful? Of course there is the fact that they do not comprehend philosophy and would not think as much as we humans do. But truth of the matter is that they also have lives and when talking about the meaning of life, then we should also be considering the meaning of life as a whole; not just meaning of life pertaining to human beings. They live because they are alive, not for any other reason. Only humans try to lead a life that means anything or try to find meaning in living. In many ways that becomes our hubris.
If you have read some of my previous posts, there is one named "Happiness". The purpose of every man in life is to attain a state of happiness, there could be different ways of obtaining it but the crux of all that we do boils down to happiness. We help others because that makes us happy, we buy an item of luxury that makes us happy and so on. So if the purpose of life for all of us is to be happy; then can that mean that the value of somebody's life be measured in the amount of happiness that they possess. But that again raises some problems does it not? Firstly, there is no measurement of happiness as such. We can't really gauge how happy a person is. Secondly, there is the problem of when exactly are we measuring the happiness of an individual. There are so many stages in life and if the only thing that is common to us in life is being born and dying, then that means that the only time that we could ever measure the meaningfulness of life of an individual is when he is dead because obviously a new born does not yet understand life or for that matter anything that is happening around him.
That which unites us all becomes the factor that determines the meaning of life. When we are all staring at the face of death, we are united and we look back to think whether we have fulfilled the purpose of life; whether we are truly happy. And when in my mind, if I think I am happy and have fulfilled the purpose of life; then I alone for that moment am the man with the most meaningful life in the world and would gladly accept death.