As living organisms ourselves, we humans are fascinated by life and all its processes. Life is not a simple or clear-cut phenomenon; it is difficult to define precisely and essentially impossible to know for sure how it arose on our planet. We can describe the attributes of living organisms and reach an understanding of life, but there will still be gray areas (such as viruses) and even legal debates (such as when is a person no longer alive). We can also explore many theories on the origin of life on earth, but biology is a science and in our study of biology we are concerned with scientific explanations based on the best scientific data that are available. Life arose slowly as inanimate chemicals underwent reactions and gradually became more complex and accumulated in the oceans. Fossil and isotopic evidence indicates that true, cellular organisms existed on earth by about 3.5 billion years ago, when the planet was already about 1 billion years old. The original transition from nonliving, complex molecular aggregates to living prokaryotic cells was a gradual one, and the continued evolution and diversification of life into all the forms that exist on earth today have also been gradual. Conditions that led to the initial origin of life on earth no longer exist here; today, all life on earth comes from already existing life, not from inanimate chemicals.