Human life only matters through our influences in the environment around us. We influence life on Earth with our contributions to climate change, whether this is through lack of regulation or the fact we are dealing with overpopulation (etc.) [Climate change is not our only influence in the environment though. Fighting diseases, efforts to keep species from going extinct, etc] In a sense, we only matter to other life, without other life, we would not matter at all, but without other life, we would not exist. We mattered to the West African black rhinoceros, as we affected its very existence to the point of extinction.
The purpose of life is to reproduce and die, according to my environmental science teacher some years ago. On a more philosophical level, we have to create our own purpose in life. An example would be if someone desired their purpose in life to be helping others, so they make it their purpose through efforts to help others.