Life

Life is matter that has biological processes, such as signaling and the ability to sustain itself. It is defined by the capacity for homeostasis, organisation, metabolism, growth, adaptation, response to stimuli, and reproduction. All life eventually reaches a state of death, and none is immortal.

Life has been studied since ancient times. Theories such as Empedocles's materialism and Aristotle's hylomorphism tried to explain what life means. Life originated at least 3.5 billion years ago, evolving into all the species that exist today.

Living things are made of biochemical molecules, mainly from a few core chemical elements. All living things contain proteins and nucleic acids, including DNA and RNA, which carry important information. The cell is the basic unit of life, with smaller organisms like bacteria consisting of single cells, and larger ones possibly made of many cells.

Definitions

The definition of life has been difficult for scientists and philosophers to pin down. This is because life is a process, not a single substance, and we don't fully know what kinds of living things might exist beyond Earth.

In biology, life is often described by certain traits. Living things usually maintain a stable internal environment (like sweating to cool down), are made of cells, use energy, grow, adapt to their surroundings, respond to stimuli, and can reproduce. From a physics standpoint, life can be seen as a system that uses energy to create copies of itself and evolve over time. Death marks the end of these life processes, though deciding exactly when it happens can be tricky. There is also debate about whether viruses are considered alive, as they can replicate and evolve but need host cells to do so.

History of study

Main article: Materialism

Some of the earliest ideas about life were materialist, meaning they said that everything, including life, is made of matter. Ancient thinkers like Empedocles thought the world was made of four elements: earth, water, air, and fire, and that different mixes of these elements created different kinds of life. Others, like Democritus, thought that tiny particles called atoms made up everything, including the soul that makes something alive.

Main article: Hylomorphism

Another idea came from Aristotle, who believed that everything has two parts: matter and form. For living things, the form is like a soul that gives them their special qualities. Plants have a simple soul that helps them grow, animals have a soul that lets them move and feel, and humans have a soul that lets them think and reason.

Main article: Spontaneous generation

For a long time, people believed that living things could just appear from non-living matter, like mice from garbage. This idea was finally proven wrong by scientists like Louis Pasteur, who showed that life comes only from other life.

Main article: Vitalism

Some thinkers thought there was a special, invisible force that made living things different from non-living things. This idea was proven wrong when scientists showed that life could be explained by normal chemistry and physics, without any mysterious forces.

Development

Main article: Abiogenesis

Main article: Evolution

Main article: Fossils

Main article: Extinction

Life on Earth began at least 3.5 billion years ago. Scientists study how simple molecules could have formed the first living cells through a process called abiogenesis. Over time, life has changed and evolved through a process called evolution. This evolution happens when traits that help organisms survive are passed on to new generations.

Fossils are the remains or traces of ancient organisms preserved in rock layers. They help us learn about life from millions of years ago. Many species have gone extinct, meaning they no longer exist. In fact, over 99% of all species that ever lived are now extinct. Mass extinctions sometimes cleared the way for new types of organisms to thrive.

Environmental conditions

The variety of life on Earth comes from how living things interact with their world and with each other. For much of Earth's history, tiny microorganisms have been the most common form of life. As these tiny organisms grew and changed, they changed the air and water around them over millions of years. For instance, some early cyanobacteria made oxygen, which changed the whole planet and helped new kinds of animals and plants develop.

The biosphere includes all the places where life exists — from deep in the soil to high in the sky, from inside rocks far underground to the deepest parts of the ocean. Life can survive in very tough places, like very hot or very cold spots, showing how adaptable living things can be. Some tiny organisms, called extremophiles, can even live through extreme conditions like freezing temperatures or very dry environments. These amazing survivors help us understand how life can exist in many different places on Earth.

Classification

Main article: Biological classification

Main article: Aristotle's biology

Long ago, a Greek thinker named Aristotle grouped living things into plants and animals. He separated animals into those with blood and without blood, similar to how we now think of animals with backbones and without backbones.

Later, a scientist named Carl Linnaeus created a way to name plants and animals using two words, making it easier to identify them. Over time, scientists discovered many new kinds of tiny living things and created new ways to sort them, leading to our modern understanding of how all living things are related.

Composition

Main article: DNA

All living things need certain basic chemical elements, such as carbon, oxygen, and phosphorus, to work properly. These elements help build important molecules like nucleic acids and lipids, which are key parts of life. Carbon is especially important because it can form many different stable connections, allowing for the wide variety of molecules found in living organisms.

DNA, or deoxyribonucleic acid, is a special molecule that carries the genetic instructions needed for living things to grow, develop, and reproduce. Most DNA is made of two long strands twisted together in a shape called a double helix. These strands contain the information needed to make new copies of themselves, which is important when cells divide and make new cells. Inside cells, DNA is organized into structures called chromosomes. Every living thing is made up of tiny units called cells, which come from other cells through division. There are two main types of cells: prokaryote cells, which do not have a nucleus, and eukaryote cells, which do have a nucleus and other special parts called organelles. Most plants, animals, and fungi are made of eukaryote cells.

In the universe

Main articles: Extraterrestrial life, Astrobiology, and Astroecology

While we only know for sure that life exists on Earth, many scientists believe that life might also be found elsewhere in space. They study other planets and moons in our Solar System and beyond, looking for signs that simple life might have existed or could exist. Projects like SETI work to detect signals from possible alien civilizations.

Scientists also study how life on Earth can survive in extreme conditions, which helps them understand where else life might be found. For example, some organisms can survive in conditions similar to those on Mars. The area around a star where Earth-like planets could support life is called the habitable zone, and its size depends on the type of star. The chances of life developing may also depend on where a star is located within a galaxy.

Artificial

Main articles: Artificial life and Synthetic biology

Artificial life is the simulation of life processes using computers, robotics, or biochemistry. Synthetic biology is a field of biotechnology that mixes science with biological engineering. Its aim is to create new biological functions and systems that do not exist in nature. This includes designing biological systems that can process information, manipulate chemicals, create materials, produce energy, provide food, and improve human health and the environment.