Living things have the following six characteristics that define life: they are made of cells, they use energy, they grow, they reproduce, they respond to their environment and they self-regulate. A seventh characteristic is often added to the list, which is that living things have biological organization.
Single-celled organisms such as bacteria are considered living things since they possess all of the characteristics. Viruses, which are infectious agents, usually aren't considered because they don't use energy, don't grow, can't reproduce independently and are unresponsive to the environment. Multicellular beings, such as humans, often have other organisms living within their bodies.
Organisms are classified based on their shared characteristics and common descent. These classifications are divided into categories which are further divided by their subcategories. These divisions include domain, kingdom, phylum, class, order, family, genus and species.
Life on Earth is believed to have started at least 4.25 billion years ago, around 290 million years after the Earth's formation. The earliest evidence humans have found for a life form is a 3.7 billion year old biogenic graphite. While Earth is the only known planet to contain life, other life forms are hypothesized to be present within the solar system and beyond.
