History of life

The history of life on Earth is a wonderful story about how living things have changed over billions of years. Earth formed about 4.54 billion years ago, and life probably began soon after. The oldest signs of life are tiny fossils found in rocks from Greenland and Western Australia. These fossils show that very small organisms were living more than 3 billion years ago.

In the beginning, most life was made of tiny microbes, like bacteria and archaea, living in the oceans. A big change happened when some bacteria learned to make food from sunlight through photosynthesis. This process released oxygen into the water and air, changing the planet and helping more complex life to develop.

Later, cells with nuclei called eukaryotes appeared. Some of these cells began living together, leading to new types of cells that give us energy today. Over time, these cells joined together to form multicellular organisms with different kinds of cells doing different jobs. Plants started to move onto land, and animals with bodies that look the same on both sides, called Bilateria, also appeared.

Big events, such as mass extinctions, sometimes cleared the way for new groups of animals to thrive. For example, after the dinosaurs disappeared, mammals became the main land animals. Today, scientists think there could be many different kinds of tiny organisms on Earth, though we have only named and described a small part of all the species that have ever lived.

Earliest history of Earth

Main article: History of Earth

Earth formed about 4.54 billion years ago. Scientists studied rocks and ancient lead to learn this age. At this time, Earth crashed into a big object called Theia. This may have helped make the Moon.

Early Earth went through a very hot time called the Hadean. But scientists found special stones called zircons. These show Earth’s surface cooled and got its first oceans and atmosphere just 100 million years later. This early world might have been ready for life.

Earliest evidence for life on Earth

Main article: Earliest known life forms

Scientists have found very old rocks that might show the earliest signs of life on Earth. The oldest clear evidence of life is from about 3 billion years ago and looks like tiny fossils of bacteria. Some even older rocks, about 3.5 billion years old, have been studied, but it’s hard to tell if what scientists see in them was truly life or just natural processes. Researchers keep looking for more clues to learn when life first began.

Origins of life on Earth

Further information: Evidence of common descent, Common descent, and Homology (biology)

Most biologists think that all living things on Earth came from one last universal ancestor. This is because it would be very hard for more than one group of life to develop all the same complex parts.

Life on Earth uses carbon and water. Carbon is good because it can make stable structures for complex chemicals and is easy to find, like as carbon dioxide. Water is also important because it can dissolve many things and stays liquid in cold and warm temperatures. Scientists are still learning how life might have started from non-living materials.

Independent emergence on Earth

Main article: Abiogenesis

One idea is that life began with simple molecules that could copy themselves, like RNA. These molecules might have been the first steps before cells formed. Another idea is that tiny bubbles made of fats came first and helped other molecules to form inside them.

Replication first: RNA world

Main articles: Last universal common ancestor and RNA world

Some scientists think that RNA molecules might have been the first life because they can copy themselves and help build other molecules. Later, DNA took over because it is more stable.

Membranes first: Lipid world

Experiments show that fat-like molecules can form bubbles that can copy themselves. These bubbles might have helped RNA form inside them.

The clay hypothesis

Main articles: Graham Cairns-Smith § Clay hypothesis, and RNA world

Some types of clay can help RNA form. These clays can copy their own patterns and might have helped the first molecules to come together.

Metabolism first: Iron–sulfur world

Main article: Iron–sulfur world hypothesis

Experiments show that simple proteins might have formed near hot underwater vents using minerals like iron sulfide as helpers.

Metabolism first: Pre–cells (successive cellularization)

In this idea, early life was made of many different types of simple cells. Over time, these cells changed and became the three main groups of life we know today.

Prebiotic environments

Geothermal springs

Wet and dry cycles in hot springs help simple molecules to form and stay together. These places have the right mix of minerals and temperatures for life’s building blocks.

Deep sea hydrothermal vents

Minerals around deep-sea vents can help form organic molecules. Experiments show that RNA and fat-like molecules can form in these conditions.

Carbonate-rich lakes

Some scientists think that life might have started in lakes rich in carbonate rocks. These lakes could have kept important chemicals like phosphate available for life to use.

Life "seeded" from elsewhere

Main article: Panspermia

Some people think life on Earth might have come from somewhere else in space. This idea goes back to ancient times and has been suggested by many scientists. Experiments show that some tiny living things can survive in space, and some rocks from Mars show signs that might have been made by life.

Environmental and evolutionary impact of microbial mats

Main articles: Microbial mat and Great Oxidation Event

Microbial mats are thin layers made of tiny life forms, mostly bacteria. They create their own small worlds with different chemicals. These layers work together like a food chain, where one group’s waste becomes food for another.

One special type of microbial structure is called a stromatolite. These build up over time as tiny organisms move upward to stay on top of sand and mud. Stromatolites in Australia are very old, dating back over 3.5 billion years.

In modern underwater mats, the top layer often has cyanobacteria. These use sunlight to make food and produce oxygen. This oxygen helps other organisms live. The bottom layer has no oxygen and may contain gases like hydrogen sulfide. The rise of oxygen in Earth’s air about 2.4 billion years ago was very important for the development of more complex life.

Diversification of eukaryotes

Main article: Eukaryote

Eukaryotes are complex cells that have a nucleus and other special parts called organelles. Scientists think they started when smaller cells lived inside bigger ones and helped them survive. Over time, these smaller cells became important parts of the bigger cells, like the mitochondria that give cells energy.

One big change happened when some eukaryotes began to make their own food using sunlight. This was thanks to a special group of cells called plastids. This change helped create many different kinds of plants and algae that later filled the oceans and lands.

Sexual reproduction and multicellular organisms

Evolution of sexual reproduction

Main article: Evolution of sexual reproduction

Sexual reproduction in living things involves meiosis and fertilization. These processes mix genes from both parents, creating new traits in their young. In contrast, asexual reproduction does not mix genes, although genes can sometimes move between types of bacteria through horizontal gene transfer. Even though sexual reproduction needs two parents, most animals, plants, fungi, and protists use this method. Scientists think sexual reproduction began very early and has stayed important.

Multicellularity

Main article: Multicellular organism

Multicellularity means an organism is made of many cells. This evolved in groups like sponges, fungi, plants, and brown algae. Having many cells helps organisms share nutrients, protect themselves, and reach for sunlight or food. It also lets cells do different jobs, making organisms more complex. Sexual reproduction helps keep these organisms healthy.

Fossil evidence

Fossils from long ago show some of the earliest multicellular life. For example, the Francevillian biota, dated to about 2.1 billion years ago, might be some of the first clearly multicellular organisms. Another important fossil is Bangiomorpha, dated to 1.2 billion years ago. It is the oldest known organism with specialized cells and the oldest known to reproduce sexually. These fossils help us learn how life on Earth became more complex.

Emergence of animals

Further information: Animal, Ediacaran biota, Cambrian explosion, Burgess Shale-type fauna, and Crown group § Stem groups

Animals are living things made of many cells that can move. They have special parts to help them work. They are different from plants because their cells do not have walls. In 2019, scientists found a tiny creature named Caveasphaera in very old rocks. This suggests animals may have started to change and evolve much earlier than we thought.

The first clear fossils of animals look like today’s jellyfish and sea anemones. Before a big change called the Cambrian explosion, there were strange, flat animals called the Ediacara biota. These were very different from animals today, and scientists are still studying them. During the Cambrian period, many new animal groups appeared quickly, including some with hard shells to protect themselves. This time was very important for many animal groups we see today.

Deuterostomes and the first vertebrates

Main articles: Chordate and Evolution of fish

See also: Chordate genomics

Most animals from the Cambrian explosion were a group called protostomes. Another group, called deuterostomes, includes animals like starfish and sea urchins, and animals with backbones called chordates. Some of the earliest chordates, such as Haikouichthys and Myllokunmingia, lived in China and had early backbones. The first animals with jaws appeared later, in the Late Ordovician.

Colonization of land

Life on land was very different from life in water. Early animals and plants had to find new ways to survive. They needed to stay moist, support themselves, and breathe differently. They also had to change how they reproduced, since water was no longer there to help carry eggs and sperm.

The first plants and animals on land appeared during the Ordovician period. But the land ecosystems we know today began in the Late Devonian period. Some signs suggest life might have first appeared on land as early as 3.48 billion years ago. There is evidence of bacteria on land from 3.22 billion years ago. A fossilized fungus found in the Canadian Arctic may have lived on land a billion years ago.

Life on land needed big changes. Plants grew more complex with systems for photosynthesis, absorbing nutrients through roots, and staying upright. Animals developed new ways to find food and excretory systems. Most animals also evolved internal fertilization. Senses like vision, movement, and hearing also changed to work better on land. These changes helped life flourish in many new places, leading to the rich ecosystems we see today.

Mass extinctions

Main article: Extinction event

Life on Earth has had big events called mass extinctions. During these times, many species died out suddenly. Even though these events were sad, they sometimes helped new types of life to grow. When one group of animals becomes more common, it’s often because an extinction event removed the old group, giving the new group a chance to thrive.

We know about these events from fossils. The time between big extinctions has been getting longer, and fewer species are dying out now. This might be because the oceans became better homes for life, with more oxygen and richer ecosystems. Also, because very old fossils are often incomplete, scientists sometimes thought they found new species when they were really just pieces of the same old species.