Life refers to all physical entities that have complex internal structures that allow them to interact with their environment, intake and utilize nutrients, grow, reproduce, and evolve. This includes all animals, plants, fungi, protists, archaea and bacteria. Some definitions also include non-cellular life forms such as viruses.
It is currently thought that the age of the earth is about 4.54 billion years. The oldest physical traces of life that have been found date back about 3.7 billion years, although some scientists hypothesize that it may have started even earlier. While various theories have been developed, it is still not known how life initially developed, including whether it originated just once, with all species evolving from that one-time event, or whether it originated multiple times. It is also not known whether life is unique to Earth, or also exists elsewhere.
Although the number of catalogued currently extant species of life is between 1.2 million and 2 million, the total number is uncertain, and may be vastly greater. Moreover, largely as a result of several mass extinctions, this current number is only a tiny fraction of the total number of species that have ever existed, perhaps even much less than one percent.