History of life

The history of life on Earth tells the amazing story of how living things have changed and evolved over billions of years, from the very first simple cells to the huge variety of plants and animals we see today. Earth formed about 4.54 billion years ago, and scientists believe that life began not long after that. The earliest signs of life are tiny fossils found in rocks from Greenland and Western Australia, showing that very small organisms were living over 3 billion years ago.

In the early days, the most common life forms were tiny microbes, like bacteria and archaea, living in the oceans. One big change happened when some bacteria learned to make food from sunlight through photosynthesis, releasing oxygen into the water and air. This oxygen changed the whole planet and helped create conditions for more complex life to develop.

Over time, cells with nuclei called eukaryotes appeared, and some of them began living together in ways that led to new types of cells, like those that give us energy today. Eventually, these cells joined together to form multicellular organisms with different kinds of cells doing different jobs. Plants began to move onto land, and animals with bodies that look the same on both sides, called Bilateria, appeared too.

Huge events, like massive die-offs called mass extinctions, cleared the way for new groups of animals to thrive. For example, after the dinosaurs disappeared, mammals grew to become the dominant land animals. Today, scientists estimate there could be as many as one trillion different kinds of tiny organisms on Earth, though we have only named and described a tiny fraction 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 have studied pieces of rock and ancient lead deposits to figure out this age. Around this time, Earth collided with a large object called Theia, which may have helped form the Moon.

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

Earliest evidence for life on Earth

Main article: Earliest known life forms

Scientists have found very old rocks that might contain 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 closely, but it’s hard to tell if what scientists see in them was truly life or just natural processes. Researchers continue to look for more clues to understand when life first began.

Origins of life on Earth

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

Most biologists believe that all living things on Earth share a single last universal ancestor. This idea makes sense because it would be very hard for two or more separate groups of life to develop all the complex parts that every living thing needs.

Life on Earth uses carbon and water. Carbon is great because it can form stable structures for complex chemicals and is easy to find, especially as carbon dioxide. Water is also important because it can dissolve many things and stays liquid over a wide range of temperatures. Scientists are still studying 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 both 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 eventually formed 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, layered communities of tiny life forms, mainly bacteria, that create their own tiny worlds with different chemicals. These layers work together like a food chain, where one group’s waste becomes food for the next.

One special type of microbial structure is called a stromatolite. These are built up over time as tiny organisms move upward to avoid being covered by sand and mud. Stromatolites found in Australia are very old, dating back over 3.5 billion years.

In modern mats underwater, the top layer often contains cyanobacteria, which use sunlight to make food and produce oxygen. This oxygen helps other organisms live better. 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 forms.

Diversification of eukaryotes

Main article: Eukaryote

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

One important change happened when some eukaryotes began to carry out photosynthesis, thanks to a special group of cells called plastids. This allowed them to make their own food using sunlight. This development helped create many different kinds of plants and algae that would later fill the oceans and lands.

Sexual reproduction and multicellular organisms

Evolution of sexual reproduction

Main article: Evolution of sexual reproduction

Sexual reproduction in eukaryotes involves meiosis and fertilization, which mix genes from both parents. This mixing creates new combinations of traits in offspring. In contrast, asexual reproduction does not mix genes, though sometimes genes can move between different types of bacteria through horizontal gene transfer. Even though sexual reproduction has some downsides, like needing two parents to make offspring, most animals, plants, fungi, and protists use this method. Scientists think sexual reproduction began very early in the history of complex cells and has stayed important.

Multicellularity

Main article: Multicellular organism

Multicellularity, where organisms are made of many cells, evolved in many different groups, including sponges, fungi, plants, and brown algae. Being made of many cells helped organisms share nutrients better, protect themselves from being eaten, and reach for sunlight or food more effectively. Multicellularity also allowed cells to specialize in different tasks, which helped these organisms become more complex. Sexual reproduction seems to be important for multicellular organisms because it helps prevent rogue cells from taking over.

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, which is the oldest known organism with specialized cells and is also the oldest known to reproduce sexually. These fossils give us clues about how life on Earth became more complex over time.

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 and have special body parts to help them function. They are different from plants because they do not have walls around their cells. One important discovery in 2019 was a tiny, many-celled creature named Caveasphaera found in very old rocks. This finding suggests that animals might have started to evolve much earlier than scientists once thought.

The first clear animal fossils are from creatures similar to today's jellyfish and sea anemones. Before a big change in life called the Cambrian explosion, there were strange, flat animals known as the Ediacara biota. These were unlike any animals alive today, and scientists are still learning about them. During the Cambrian period, many new animal groups appeared quickly, including some with hard shells to protect themselves from being eaten. This time was very important for the development of 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 belonged to a group called protostomes. Another group, called deuterostomes, includes animals like starfish and sea urchins, as well as animals with backbones known as chordates. Some of the earliest chordates, such as Haikouichthys and Myllokunmingia, lived in China and had early forms of backbones. The first animals with jaws appeared later, in a time called the Late Ordovician.

Colonization of land

Adaptation to life on land presented major challenges for early organisms. Land animals needed ways to avoid drying out and develop structures to support themselves against gravity. Systems for respiration and gas exchange had to change, and reproductive methods could no longer rely on water to carry eggs and sperm.

The earliest evidence of land plants and animals dates back to the Ordovician period, but modern land ecosystems appeared later in the Late Devonian period. Recent discoveries suggest life may have first appeared on land as early as 3.48 billion years ago, with possible evidence of bacteria living on land 3.22 billion years ago and a fossilized fungus found in the Canadian Arctic that may have grown on land a billion years ago.

Life on land required significant changes. Plants needed to become more complex with systems for photosynthesis, root absorption, and support. Animals developed new feeding and excretory systems, and most evolved internal fertilization. Vision, movement, and hearing also adapted to life away from water. These changes allowed life to thrive in new environments and led to the diverse ecosystems we see today.

Mass extinctions

Main article: Extinction event

Life on Earth has experienced big events called mass extinctions, where many species died out suddenly. Even though these events were terrible at the time, they sometimes helped new types of life to develop. When one group of animals becomes more common than another, it’s usually because an extinction event removed the old group, giving the new group a chance to grow.

We can see from fossils that the time between these big extinctions has been getting longer, and that fewer species are dying out overall. This might be because the oceans became better places for life to survive, with more oxygen and richer ecosystems. Also, because fossils from very old times are often incomplete, scientists sometimes thought they found new species when they were really just pieces of the same old species.