Life

Life is a special kind of matter that can grow and change. To be alive, something usually needs to eat, breathe, and react to the world around it.

All living things are made of tiny parts called cells. Some life is so small you cannot see it without a microscope, like bacteria.

An amazing "wow" fact is that life has been on Earth for over 3.5 billion years! Scientists use a system created by Carl Linnaeus to give every living thing a special name. Life can even be found in very hot or cold places, like deep under the ocean.

Life is defined by several important biological processes. These include homeostasis (keeping a steady internal state, like sweating to stay cool), metabolism (turning food into energy), and reproduction.

All living things are made of cells, which are the basic building blocks of life. Inside these cells is DNA, a molecule that holds the instructions for how an organism grows and functions.

Life on Earth started at least 3.5 billion years ago. Over time, species changed through a process called evolution. This created a "tree of life," showing how all creatures are related to a common ancestor.

While most life needs oxygen and water, some "extremophiles" live in harsh places like deep oceans or inside rocks.

Scientists even wonder if life exists on other planets! Today, we also study "artificial life," which uses computers and robots to copy how living things work.

Life is matter that carries out biological processes such as signaling and self-sustenance. Scientists describe life using several key traits: homeostasis, organization, metabolism, growth, adaptation, response to stimuli, and reproduction. Homeostasis is the ability to maintain a constant internal state, such as a human body regulating its temperature. Metabolism involves transforming energy to build cellular components (anabolism) or break down organic matter (catabolism).

The study of life has changed over thousands of years. Ancient philosophers like Aristotle believed living things had "souls," while modern science focuses on biochemistry. In the 1740s, Carl Linnaeus created the system of binomial nomenclature we use to name species today.

All living things are made of cells, which contain macromolecules like proteins and nucleic acids (DNA and RNA). DNA carries the genetic information needed for a species to survive and reproduce. There are two main types of cells: prokaryotes (like bacteria) which are simple and small, and eukaryotes (like plants and animals) which have a nucleus and complex parts.

Life on Earth is incredibly old, beginning at least 3.5 billion years ago. Scientists believe all living things share a universal common ancestor. Over time, evolution has led to the millions of species we see today, though more than 99% of all species that ever lived are now extinct.

Life is found almost everywhere on our planet, from 19 kilometers underground to 64 kilometers high in the atmosphere. Some organisms, called extremophiles, can survive in the vacuum of space or in boiling hot water.

Defining life remains a challenge. For example, viruses have genes and evolve, but they cannot reproduce without a host cell and do not have their own metabolism. Because of this, they are often called "organisms at the edge of life." Scientists are also exploring artificial life through computer simulations and synthetic biology to create new biological functions that do not exist in nature.

Life is a complex process rather than a single substance, making it notoriously difficult to define. Biologists generally use a descriptive definition involving seven traits: homeostasis, organization, metabolism, growth, adaptation, response to stimuli, and reproduction. However, at least 123 different definitions of life have been compiled. From a physics perspective, life is an open thermodynamic system that uses gradients in its surroundings to create imperfect copies of itself. NASA often uses a simplified definition: "a self-sustained chemical system capable of undergoing Darwinian evolution."

The history of biological thought has evolved from ancient materialism and hylomorphism to modern molecular biology. Empedocles argued in 430 BC that life was a mixture of four elements: earth, water, air, and fire. Aristotle later proposed hylomorphism, suggesting that living things possess souls—vegetative souls for plants, animal souls for motion and sensation, and rational souls for humans.

For centuries, many believed in "spontaneous generation," the idea that life could arise from mud or garbage, until Louis Pasteur disproved it in 1859. Vitalism, the belief in a non-material life force, was also popular until 1828, when Friedrich Wöhler synthesized urea from inorganic materials, proving that the chemistry of life follows the same laws as the rest of the universe.

At the molecular level, all known life is composed of biochemical molecules formed from core elements: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur (CHNOPS). The primary macromolecules are proteins and nucleic acids (DNA and RNA). DNA consists of two polynucleotide strands forming a double helix, using four nucleobases: cytosine, guanine, adenine, and thymine.

The cell is the fundamental unit of life. Prokaryotes, such as bacteria and archaea, are single-celled and lack a nucleus. Eukaryotes, which include all multicellular organisms, possess a nucleus and membrane-bound organelles like mitochondria. It is believed that eukaryotes evolved through endosymbiosis, where one prokaryote began living inside another.

Life on Earth originated between 3.5 and 4.28 billion years ago, shortly after the planet's formation 4.54 billion years ago. This led to a universal common ancestor (LUCA), from which all modern biodiversity evolved. In 2016, scientists identified 355 genes likely belonging to LUCA. Evolution, driven by natural selection and genetic drift, has produced a vast array of species, though the fossil record shows that over 99% of all species are now extinct.

The biosphere encompasses all ecosystems on Earth, showing life's incredible range of tolerance. Microbes have been found 1.2 kilometers below the seafloor in 120 °C sediments and as high as 77 kilometers in the mesosphere. Extremophiles can survive radiation, freezing, and desiccation.

Viruses remain a point of controversy; they possess genes and evolve but lack metabolism and must hijack host cells to replicate, placing them at the "edge of life."

Looking beyond Earth, the search for extraterrestrial life focuses on "habitable zones" around stars where liquid water might exist. Scientists use the Drake equation to estimate the probability of alien civilizations and have proposed the Confidence of Life Detection (CoLD) scale for evaluating evidence of life on other worlds. Simultaneously, the field of synthetic biology is working to design and build entirely new biological systems, expanding our understanding of what life can be.