How do we know that something is alive? The answer seems obvious and intuitive, but when you ask that question in the context of searching for life on other planets, it becomes more difficult to pin down. In the unfamiliar environments of other planets, we need to find a way to distinguish between a phenomenon that isn’t alive as compared to one that is. One way to address this is by considering the characteristics of living things, which reveals how much all living things have in common. All life on Earth adheres to a common biochemistry. In all living things, cells are the main unit of organization, cellular membranes are made up of molecules called phospholipids, genetic information is made up of molecules called nucleic acids, and functions within and between cells are mostly carried out by molecules called proteins. This means that a fly has the same basic biochemistry as an elephant!

Beyond this shared biochemistry, all life has certain general traits in common, too. Here are some of the key traits of life as we know it:
1) All life is highly ordered and structured. Not only do all living things that we know of have cells and cellular structures, but many living things also have larger-scale structure such as bilateral symmetry (in humans) or radial symmetry (in starfish).
2) All life reproduces itself, either sexually (as animals do) or asexually (such as budding in yeast or one cell splitting into two identical daughter cells via binary fission as bacteria do).
3) All life grows and develops to reach maturity, such as from a caterpillar to a butterfly.
4) All life takes in and utilizes energy to carry out the functions of its cells, which results in growth and development. Mechanisms for energy intake are vastly different across all species, and can range from eating food like humans do, to converting sunlight into sugars like plants do, to the harnessing of the energy produced when rocks radioactively decay like some bacteria do.
5) All living things exhibit homeostasis, which is the ability to maintain a steady internal environment regardless of their external environment. For example, most humans maintain a body temperature of 98.6 degrees Fahrenheit regardless of whether they are out playing in the snow or hiking in the hot desert. Homeostasis is achieved because of strict biochemical regulations in cells and organs.
6) All living things respond to their environment by sensing external stimuli and changing their biochemistry and/or behavior. For example, when cuttlefish sense danger, they can instantaneously change their colors to match whatever background they are against to avoid being seen by a predator.
7) Finally, all living things adapt to external pressures, and evolve because of them. Adapting is much like responding to a stimulus in the environment, but takes it to the next level. In evolutionary adaptation, one cuttlefish will have the ability to change colors more quickly and effectively than another (because of its genetic makeup), and it will inherently be more likely to survive than another one that doesn’t do it as well or as quickly. The first one is more likely to pass on its genes to its offspring, and that offspring will pass it on to their offspring, and so on. Over time, the population of cuttlefish descended from that one who changed colors more quickly and effectively is more highly adapted to its environment. They have undergone the process of natural selection and are more likely to survive. Their genes were “selected for” by the external pressures of the environment.

Something that is alive will exhibit all of these traits, while phenomena that we do not consider to be alive can exhibit some, but not all of them. For example, a fire exhibits some of these traits – it consumes energy (wood and oxygen) and gives off by-products such as CO~2~ and heat, it grows in size as it consumes more and more fuel, and it may appear to reproduce as it spreads. But because it doesn’t exhibit all of these traits, we don’t consider fire to be alive. Defining life from the viewpoint of examining its characteristics reveals how much life on Earth has in common, and helps distinguish between living and non-living things. If there is other life out there in the cosmos and it’s like the life that we know, then we would expect it to also show these traits of living things.