Life

Life is matter that can carry out biological processes, such as signaling and keeping itself going. Life can keep itself balanced, be organized, use energy, grow, change over time, respond to things around it, and make more of itself. All life ends in death, and nothing lives forever.

People have wondered about life for a very long time. Ideas from thinkers like Empedocles and Aristotle tried to explain what life is. Life began at least 3.5 billion years ago and changed over time into all the species we see today.

Living things are made of special biochemical molecules from a few main chemical elements. All living things have proteins and nucleic acids, such as DNA and RNA, which hold important information. The cell is the smallest part of life. Small organisms like bacteria have just one cell, while larger ones can have many cells.

Definitions

The definition of life is hard to explain. This is because life is a process, not one single thing, and we do not know all the kinds of living things that might exist beyond Earth.

In biology, life is often described by some special traits. Living things usually keep a balanced inside (like sweating to cool down), are made of cells, use energy, grow, change to fit where they are, respond to things around them, and can make more of their own kind. From a physics point of view, life can be seen as a system that uses energy to make copies of itself and change over time. Death ends these life processes, though deciding exactly when it happens can be hard. There is also debate about whether viruses are considered alive, as they can copy themselves and change but need host cells to do so.

History of study

Main article: Materialism

Some of the earliest ideas about life 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. They believed 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. He 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. 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 proven wrong by scientists like Louis Pasteur. They 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 more than 3.5 billion years ago. Scientists study how simple molecules formed the first living cells, a process called abiogenesis. Over time, life has changed and evolved. This happens when traits that help organisms survive are passed to new generations, a process called evolution.

Fossils are the remains or traces of ancient organisms preserved in rock layers. They help us learn about life from long ago. Many species have gone extinct, meaning they no longer exist. Most species that ever lived are now gone. Sometimes big events let new types of organisms 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 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, showing how adaptable living things can be. Some tiny organisms, called extremophiles, can 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 thinker named Aristotle grouped living things into plants and animals.

Later, a scientist named Carl Linnaeus created a way to name plants and animals using two words. This made it easier to identify them. Scientists later found many new kinds of tiny living things and created new ways to sort them. This led to our modern understanding of how all living things are related.

Composition

Main article: DNA

All living things need basic chemical elements, such as carbon, oxygen, and phosphorus, to work. These elements help build important molecules like nucleic acids and lipids. Carbon is very important because it can form many different connections.

DNA, or deoxyribonucleic acid, is a special molecule that carries the genetic instructions for living things to grow and develop. Most DNA looks like two long strands twisted together in a shape called a double helix. Inside cells, DNA is organized into structures called chromosomes.

Every living thing is made of tiny units called cells. Cells come from other cells. 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. Most plants, animals, and fungi are made of eukaryote cells.

In the universe

Main articles: Extraterrestrial life, Astrobiology, and Astroecology

We know that life exists on Earth, but scientists think it might also be found elsewhere in space. They study other planets and moons in our Solar System and beyond, searching for signs that simple life might have existed or could exist. Projects like SETI try to find signals from possible alien civilizations.

Scientists also study how life on Earth can survive in very harsh conditions. This helps them understand where else life might be found. For example, some tiny creatures can live in conditions like 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 using computers, robotics, or biochemistry. Synthetic biology is a part of biotechnology that mixes science with biological engineering. It tries to make new biological systems that do not exist in nature. These systems can help process information, make chemicals, create materials, produce energy, provide food, and improve health and the environment.