Life

What exactly is life? This question is surprisingly difficult to answer. Scientists and philosophers have struggled with it for centuries. In fact, at least 123 different definitions of life have been compiled. This difficulty exists because life is a process rather than a simple substance. Defining it is also hard because we do not know what life might look like on other planets.

Most modern biologists use descriptive definitions to identify life. They look for specific traits that help an organism survive in its environment. One key trait is homeostasis, which is the regulation of an internal environment to maintain a constant state. An example is sweating to lower body temperature. Another trait is metabolism, the process of transforming energy. This involves anabolism, which converts chemicals into cellular components, and catabolism, which decomposes organic matter. Living things also show growth by increasing in size and structure. They use adaptation to become better suited to their habitats through evolution. Finally, they respond to stimuli, such as a plant's leaves turning toward the sun through phototropism.

Life is organized in a complex, hierarchical way. It spans from tiny molecular machines to individual cells and organs. These parts work together to form organisms, populations, and entire ecosystems. All these living parts together form the biosphere. Within a specific place and time, the collection of living things is called a biota.

Human understanding of life has changed many times throughout history. In ancient Greece, Empedocles proposed materialism. He believed life was a mixture of four eternal elements: earth, water, air, and fire. Aristotle offered a different view called hylomorphism. He argued that living things consist of both matter and form. To Aristotle, the form of a living thing was its soul. He categorized souls into three types: vegetative for plants, animal for moving creatures, and rational for humans.

Later, in the 1740s, Carl Linnaeus developed binomial nomenclature. This was a formal system used to name and classify species.

All life on Earth shares a common chemical foundation. Living things are composed of biochemical molecules made from core chemical elements. Every organism contains two essential types of macromolecules: proteins and nucleic acids. Nucleic acids, such as DNA and RNA, carry the genetic information for a species. This information includes the instructions for making proteins. Proteins then act as the molecular machinery that carries out chemical processes. The cell serves as the basic structural and functional unit of life.

Organisms are categorized based on their cellular structure. Prokaryotes, such as bacteria and archaea, are small organisms made of single cells. Eukaryotes are larger and can be single-celled or multicellular with complex structures.

Some life thrives in extremely harsh environments. These specialized organisms are known as extremophiles.

While we only know of life on Earth, scientists believe extraterrestrial life is probable. We also study artificial life, which is simulated by engineers and scientists.

Viruses remain a controversial subject in the study of life. Many scientists view them as gene-coding replicators rather than true living organisms. They are often called organisms at the edge of life. This is because they possess genes and evolve through natural selection. However, viruses cannot perform metabolism on their own. They must use a host cell to replicate through a process called self-assembly.

This unique way of functioning helps scientists study how life might have originally started.