The importance of human life is determined by individual perception, and is usually subject to that individual's values, beliefs and culture. Over the centuries, many people have written and spoken about this subject, but there is no single universal answer.
Existential questions about why humans exist, what their purpose is and how they should function within the world continue to be sources of intrigue for the species. How people attempt to answer these questions often depends on what religions and philosophies they follow.
Many modern religions believe each human life's importance is based on that person's service to a higher power. Some existentialist philosophers, such as Friedrich Nietzsche, concluded that life has no significance beyond what one could accomplish within one's lifetime. Biologists and environmental scientists judge all life on how effectively it exists within its ecosystem; some judge human lives by their carbon footprints. Fiction works also attempt to explore these topics, as in the novel "Cloud Atlas" and movie by the same name, in which it is implied a human life is as significant as the impacts that person has on others, even if those impacts do not manifest until decades after their death.
Historically, humans have not been able to agree on how to rank lives according to significance. Some determine importance based on social status, wealth or race, while others use age or gender. In debates about the issue of abortion, participants often disagree on how to define the importance and life of the unborn child. Once again, differences of opinion on this issue are usually based on core values.