The history of life on Earth shows how living and extinct organisms changed over time, from the first life forms to today. Earth formed about 4.54 billion years ago, and evidence suggests life began before 3.7 billion years ago. All living species today share similarities, which means they evolved from a common ancestor through the process of evolution.

The earliest clear signs of life come from carbon traces and stromatolite fossils found in 3.7-billion-year-old rocks in Greenland. In 2015, possible signs of life were discovered in 4.1-billion-year-old rocks in Western Australia. Older evidence includes fossilized microbes in rocks from the Nuvvuagittuq Belt, which may be as old as 4.28 billion years. These early fossils might have formed from non-living processes.

During the early Archean eon, microbial mats made of bacteria and archaea were the main life forms. Many key evolutionary steps happened in this environment. Cyanobacteria developed photosynthesis around 3.5 billion years ago, producing oxygen as a waste product. Oxygen built up in the oceans first, then in the atmosphere, leading to the Great Oxygenation Event about 2.4 billion years ago. The earliest eukaryotes (cells with organelles) appeared around 1.85 billion years ago, likely from the merging of ancient microbes. Eukaryotes diversified more when mitochondria (energy-producing cell parts) allowed more energy production. Some eukaryotes gained photosynthesis through symbiosis with cyanobacteria, becoming algae that later replaced cyanobacteria as the main producers.

Around 1.7 billion years ago, multicellular organisms began to form, with specialized cells working together. Most large organisms, including plants and animals, reproduce sexually by combining male and female cells to create offspring. Scientists are still studying how sexual reproduction evolved.

Microorganisms created the first land ecosystems at least 2.7 billion years ago. Plants evolved from green algae about 1 billion years ago. Microbes helped plants move to land during the Ordovician period. Land plants became so successful that they may have caused the Late Devonian extinction by reducing carbon dioxide levels and changing ocean conditions.

Animals with left and right sides (bilateria) appeared by 555 million years ago. Ediacara biota lived during the Ediacaran period, while most modern animal groups, including vertebrates, appeared 525 million years ago during the Cambrian explosion. During the Permian period, synapsids (early mammal ancestors) were the dominant land animals.

The Permian–Triassic extinction event 252 million years ago killed most complex life. After this, archosaurs became the most common land animals, and dinosaurs ruled during the Jurassic and Cretaceous periods. After the Cretaceous–Paleogene extinction 66 million years ago, mammals grew larger and more diverse. These extinctions may have helped new species evolve by creating opportunities for survival.

Only a small number of species have been identified. Scientists estimate Earth may have 1 trillion species, but identifying all microbes is very difficult. About 1.8 million species have been named, and only a tiny fraction of all species that ever lived are still alive today.

Earliest history of Earth

The oldest meteorite pieces found on Earth are about 4.54 billion years old. This information, along with dating ancient lead deposits, suggests Earth is about the same age. The Moon has the same composition as Earth's outer layer but lacks an iron-rich core like Earth's. Many scientists believe that about 60–110 million years after the Solar System formed, the early Earth collided with Theia, a Mars-sized planet that shared Earth's orbit. This impact sent large amounts of Earth's outer layer into space, where it gathered to form the Moon. Another idea is that Earth and the Moon formed at the same time, but Earth's stronger gravity pulled most of the iron particles in the area toward itself.

Before 2001, the oldest rocks found on Earth were about 3.8 billion years old. This led scientists to think Earth's surface was molten until that time. They named this early period in Earth's history the Hadean. However, analysis of zircons formed 4.4 billion years ago shows Earth's crust solidified about 100 million years after Earth formed. This suggests Earth quickly developed oceans and an atmosphere, which might have supported life.

Evidence from the Moon shows it was hit by leftover debris from the Solar System's formation between 4 and 3.8 billion years ago. Earth likely experienced an even stronger bombardment because of its stronger gravity. While there is no direct evidence of Earth's conditions during this time, it is likely Earth was also affected by this heavy bombardment. This event may have removed Earth's early atmosphere and oceans. However, gases and water from comet impacts, along with volcanic activity, may have helped replace them. If life had already developed underground by this time, it might have survived the bombardment.

Earliest evidence for life on Earth

The earliest known living things were very small and simple in structure. Their fossils looked like tiny rods that are hard to distinguish from shapes formed by non-living natural processes. The oldest confirmed signs of life on Earth, believed to be fossilized bacteria, are about 3 billion years old. Other findings in rocks dated to around 3.5 billion years ago were also thought to be bacteria, and chemical evidence from rocks about 3.8 billion years old suggested life might have existed earlier. However, these findings were carefully examined, and scientists found that non-living processes could create similar signs of life. While this does not prove the structures were non-living, they cannot be considered clear proof of life. Chemical evidence from rocks formed 3.4 billion years ago has been interpreted as possible signs of life.

Fossils of tiny microorganisms, possibly 3.77 to 4.28 billion years old, were found in the Nuvvuagittuq Greenstone Belt in Quebec, Canada. However, this evidence is debated and considered uncertain.

Origins of life on Earth

Most scientists believe that all living things on Earth share a single last universal ancestor. This is because it would be extremely difficult for two or more separate groups to develop the same complex biochemical processes independently.

Another idea suggests that a single first cell or early cell-like structure never existed. Instead, early life may have started with a group of simple, non-cellular structures called pre-cells. These pre-cells gradually evolved into the three major groups of life (domains) through a process of diversification. This means the formation of cells happened in stages over time.

Life on Earth depends on carbon and water. Carbon is useful because it can form stable, complex molecules and is easy to find in the environment, such as in carbon dioxide. No other element is as effective as carbon for building life. Silicon, which is below carbon on the periodic table, does not form as many stable molecules. Its compounds are often not water-soluble, and it forms hard solids like silicon dioxide, making it harder for life to use. Other elements like boron and phosphorus have more complex chemistries but face other challenges. Water is a good solvent and has special properties: ice floats, helping aquatic life survive in cold water, and its molecules have positive and negative ends, allowing them to form many different compounds. Other solvents, like ammonia, are only liquid at very low temperatures, where chemical reactions may be too slow for life to develop. Life based on different chemistry might exist on other planets.

Scientists study how life might have formed from non-living chemicals by focusing on three possibilities: self-replication (the ability to make copies of itself), metabolism (the ability to take in energy and repair itself), and external cell membranes (which control what enters and exits a cell). Research on this topic, called abiogenesis, is still in early stages, as scientists are just beginning to connect theories with experiments.

Even the simplest living organisms today use DNA to store genetic instructions and rely on RNA and proteins to carry out these instructions for growth and reproduction. The discovery that some RNA molecules can help copy themselves and build proteins led to the idea that early life might have been based entirely on RNA. These RNA-based life forms could have existed in an "RNA world" before DNA took over. RNA was later replaced by DNA because DNA can store longer, more stable genetic information, improving inheritance and increasing the abilities of living things. Today, RNA remains a key part of ribosomes, which help build proteins in cells. Evidence suggests the first RNA molecules formed on Earth more than 4.17 billion years ago.

Although scientists have created short, self-replicating RNA molecules in labs, there are questions about whether RNA could form naturally without life. Some early RNA molecules may have been made from simpler molecules like PNA, TNA, or GNA before being replaced by RNA.

In 2003, researchers proposed that certain metal sulfide materials could help form RNA at high temperatures near hydrothermal vents. Under this idea, cell membranes would appear later, and early cell-like structures would remain in the spaces between these materials.

Some scientists think that double-walled lipid "bubbles," similar to cell membranes, might have formed first. Experiments show that lipids can form these bubbles and even copy themselves. Though lipids do not carry genetic information like RNA, they could have been shaped by natural selection to last longer and reproduce. RNA might have formed more easily inside these bubbles than outside.

RNA is complex, and scientists are unsure if it could form naturally in the environment. Some clays, like montmorillonite, may have helped speed up RNA formation. These clays can grow by copying their structure, change based on their environment, and help form RNA. Though this idea is not widely accepted, some scientists support it.

In 2003, experiments showed that montmorillonite could also help fatty acids form bubbles that could trap RNA. These bubbles could grow and divide, possibly helping early cells form. A similar idea suggests that iron-rich clays might have helped create the building blocks of life, like nucleotides, lipids, and amino acids.

Experiments starting in 1997 showed that iron sulfide and nickel sulfide could help form proteins from simple inorganic materials like carbon monoxide and hydrogen sulfide. Most steps required temperatures around 100°C and moderate pressure, though one step needed much higher temperatures and pressure. This suggests that protein formation might have occurred near hydrothermal vents.

In this scenario, early life evolved from a group of pre-cells, which were simple, evolving structures with different traits and shared genetic material through horizontal gene transfer. From this group, the ancestors of the three domains of life (Bacteria, Archaea, and Eucarya) developed one after another.

For cells to form, pre-cells needed protective barriers like cell membranes or walls. For example, bacteria developed rigid cell walls made of peptidoglycan, which helped them survive and spread to many environments.

This idea explains why similar traits appear randomly across the three domains of life and why basic features like the genetic code are shared by all. It also explains the close relationship between Archaea and Eucarya. A diagram of this pre-cell scenario shows key evolutionary steps.

Wet-dry cycles in certain environments may have played a role in these processes.

Environmental and evolutionary impact of microbial mats

Microbial mats are layered groups of bacteria and other small organisms that are usually only a few millimeters thick. Despite their small size, these mats have many different chemical conditions, each of which supports different types of microorganisms. In some ways, each mat forms its own food chain because the waste products of one group of microorganisms often act as food for another nearby group.

Stromatolites are short, pillar-like structures formed when microorganisms in mats slowly move upward to avoid being covered by sediment carried by water. Scientists have debated whether some stromatolite fossils found in rocks older than 3 billion years are real or could have formed through non-living processes. In 2006, new stromatolite fossils were discovered in Australia, found in rocks dated to 3.5 billion years ago.

In modern underwater mats, the top layer is often made of cyanobacteria that use sunlight to produce oxygen, creating an environment rich in oxygen. The bottom layer lacks oxygen and is often filled with hydrogen sulfide, a gas produced by the organisms living there. Oxygen is harmful to organisms that cannot use it, but it greatly improves the energy production of those that can. Oxygen-based photosynthesis by bacteria in mats increased the amount of life on Earth by up to 1,000 times. The oxygen used in this process comes from water, which is more available than the substances needed for earlier, non-oxygen-based photosynthesis. From this time onward, life itself created most of the resources it needed, rather than relying on geological processes.

Oxygen became a major part of Earth’s atmosphere around 2.4 billion years ago. Although eukaryotic cells may have existed earlier, the rise of oxygen in the atmosphere was necessary for the development of more complex eukaryotic cells, which are the basis of all multicellular life. In microbial mats, the boundary between oxygen-rich and oxygen-free layers would shift upward when photosynthesis stopped at night and downward when it started again in the morning. This change likely encouraged organisms in the middle layer to adapt to oxygen, possibly through endosymbiosis, where one organism lives inside another, and both benefit.

Cyanobacteria have the most complete sets of biochemical tools among mat-forming organisms. This makes them highly self-reliant and well-suited to live independently as floating mats or as the first phytoplankton, forming the foundation of most marine food chains.

Diversification of eukaryotes

Eukaryotes may have existed before the atmosphere became rich in oxygen, but most eukaryotes today need oxygen. Oxygen is used by mitochondria, the cell parts that produce ATP, the energy source for all known cells. In the 1970s, an intense discussion concluded that eukaryotes developed through a series of endosymbiosis events between prokaryotes. For example, a predator microorganism may have invaded a large prokaryote, likely an archaeon, but instead of destroying it, the invader lived inside and eventually became mitochondria. Later, one of these combined organisms may have swallowed a cyanobacterium, a photosynthesizing prokaryote, but the cyanobacterium survived inside and became the ancestor of plants. Similar events occurred repeatedly. After each endosymbiosis, the organisms involved reduced unnecessary genetic duplication by rearranging their DNA, sometimes sharing genes between them. Another idea suggests that mitochondria originally used sulfur or hydrogen for energy and later adapted to use oxygen. Alternatively, mitochondria may have been part of eukaryotes from the beginning.

Scientists debate when eukaryotes first appeared. The presence of steranes in Australian rocks suggests eukaryotes existed 2.7 billion years ago. However, a 2008 study found these chemicals may have entered the rocks less than 2.2 billion years ago, providing no clear evidence about eukaryote origins. Fossils of algae called Grypania, found in 1.85 billion-year-old rocks (originally dated to 2.1 billion years ago but later revised), show eukaryotes with organelles had evolved. A variety of fossil algae were discovered in rocks dated between 1.5 and 1.4 billion years ago. The oldest known fungal fossils are about 1.43 billion years old.

Plastids, a group of organelles that includes chloroplasts, are believed to have originated from endosymbiotic cyanobacteria. This process occurred around 1.5 billion years ago and allowed eukaryotes to perform oxygenic photosynthesis. Three types of photosynthetic plastids evolved: chloroplasts in green algae and plants, rhodoplasts in red algae, and cyanelles in glaucophytes. Soon after the first plastid endosymbiosis, rhodoplasts and chloroplasts spread to other bikonts, forming a group of phytoplankton by the end of the Neoproterozoic Eon.

Sexual reproduction and multicellular organisms

Sexual reproduction in eukaryotes involves two key processes: meiosis and fertilization. These processes lead to genetic recombination, which means offspring receive 50% of their genes from each parent. In contrast, asexual reproduction does not involve recombination, but some organisms, like bacteria, can transfer genes horizontally. Bacteria also exchange DNA through a process called bacterial conjugation. This allows them to spread resistance to antibiotics and other toxins, and to use new nutrients. However, conjugation is not a form of reproduction and can occur between different species, including bacteria and plants or animals.

Bacterial transformation is a process that allows bacteria of the same species to transfer DNA. This process involves many bacterial genes and is sometimes called a bacterial form of sex. It occurs naturally in at least 67 prokaryotic species across seven different groups. Scientists believe that sexual reproduction in eukaryotes may have evolved from bacterial transformation.

Sexual reproduction has disadvantages. Genetic recombination can break up helpful gene combinations. Also, because males do not directly increase the number of offspring, asexual populations can outcompete sexual populations in as few as 50 generations. Despite this, most animals, plants, fungi, and protists reproduce sexually. Evidence suggests that sexual reproduction appeared early in eukaryote history and has changed little since then. How sexual reproduction evolved and survived remains a mystery.

The Red Queen hypothesis suggests that sexual reproduction helps organisms resist parasites. Parasites find it easier to overcome genetically identical clones than sexual species, which have varied genetic traits. Some experiments support this idea, but questions remain. For example, studies found that sexual geckos had more mites than asexual geckos in the same habitat. Also, research on plant disease resistance did not find clear evidence that parasites drive sexual reproduction.

Alexey Kondrashov’s deterministic mutation hypothesis (DMH) suggests that harmful mutations in organisms can be reduced through sexual recombination. This process removes harmful mutations from the gene pool by isolating them in individuals that do not survive. However, evidence shows that many species have fewer than one harmful mutation per individual, and no species shows strong synergy between harmful mutations.

The random nature of recombination causes genetic traits to change between generations. This genetic drift alone is not enough to make sexual reproduction advantageous, but when combined with natural selection, it may help. Natural selection favors lineages with beneficial traits that become genetically linked. However, if good traits appear with bad traits, their benefits are reduced. Sexual recombination allows good traits to link with other good traits, which may offset the disadvantages of sexual reproduction. Scientists continue to study how different theories explain the benefits of sexual reproduction.

The purpose of sexual reproduction remains a major unsolved question in biology. John A. Birdsell and Christopher Wills reviewed competing models. All these models rely on the benefits of genetic variation from recombination. Another idea is that sex evolved as a way to repair DNA damage, with genetic variation being a byproduct.

Multicellular organisms are defined as having multiple cells. Some examples include colonial cyanobacteria like Nostoc and green algae like Volvox, which have specialized cells for reproduction. Multicellularity evolved independently in many groups, such as sponges, fungi, plants, and algae. This article focuses on organisms with highly specialized cells, even though this approach may seem human-centered.

Early advantages of multicellularity included better nutrient sharing, protection from predators, and the ability to grow in new environments. These traits also helped other organisms diversify by creating varied habitats. However, multicellularity disadvantages individual cells, as most lose the chance to reproduce. In asexual multicellular organisms, rogue cells that can reproduce may take over. Sexual reproduction prevents this by removing rogue cells from the next generation, making it essential for complex multicellularity.

Eukaryotes evolved earlier than bacteria and archaea but remained rare until a rapid diversification around 1 billion years ago. Eukaryotes have greater variety of forms than bacteria, and sexual reproduction helped them exploit this by creating multicellular organisms with specialized functions.

Scientists compared gene regulation in unicellular and multicellular organisms. They found that multicellular organisms have new types of gene families and regulatory networks essential for development. These findings suggest a possible mechanism for how multicellular life evolved at the genetic level.

Fungi-like fossils were found in vesicular structures.

Emergence of animals

Animals are living things made up of many cells, and they are different from plants, algae, and fungi because they do not have cell walls. All animals can move, even if only during certain stages of their lives. Except for sponges, all animals have bodies made of different types of tissues, such as muscle tissue, which helps move parts of the body, and nerve tissue, which sends and processes signals. In November 2019, scientists found a multicellular organism called Caveasphaera in rocks that are 609 million years old. This organism is not clearly an animal or a non-animal and may be linked to one of the earliest examples of animal evolution. Studies of Caveasphaera fossils suggest that animal-like development began much earlier than the oldest clearly identified animal fossils. This could mean that animal evolution started about 750 million years ago.

The earliest widely accepted animal fossils are cnidarians, such as jellyfish, sea anemones, and Hydra, which may be about 580 million years old. However, fossils from the Doushantuo Formation are only roughly dated. These fossils show that the cnidarian and bilaterian groups had already split into separate lines.

The Ediacara biota, which lived about 40 million years before the Cambrian period, were the first animals larger than a few centimeters. Many of these animals were flat and had a "quilted" look, which made scientists consider placing them in a separate group called Vendobionta. Others, like Kimberella, Arkarua, Spriggina, and Parvancorina, were thought to be early mollusks, echinoderms, or arthropods. Scientists still debate how to classify these fossils because they lack clear features found in modern animals. However, Kimberella is likely a triploblastic bilaterian, meaning it was more complex than cnidarians.

The small shelly fauna are a mix of fossils found between the Late Ediacaran and Middle Cambrian periods. The earliest, Cloudina, shows signs of defense against predators, suggesting the start of an evolutionary arms race. Some Early Cambrian shells belonged to mollusks, and other fossils, like Halkieria and Microdictyon, were later identified as belonging to animals with armor plates when more complete remains were found in Cambrian lagerstätten that preserved soft-bodied animals.

In the 1970s, scientists debated whether the modern animal groups appeared suddenly or gradually, but this was hard to determine because of a lack of Precambrian fossils. A re-examination of fossils from the Burgess Shale lagerstätte showed animals like Opabinia, which did not fit into any known group. At first, these fossils were seen as evidence that modern animal groups evolved quickly during the Cambrian explosion. Later discoveries and new theories showed that some of these unusual animals were related to modern groups, such as Opabinia being a lobopod, a group linked to arthropods and possibly related to modern tardigrades. Scientists still debate whether the Cambrian explosion was truly sudden and why it happened.

Most animals involved in the Cambrian explosion debate were protostomes, one of the two major groups of complex animals. The other group, deuterostomes, includes echinoderms like starfish and sea urchins, as well as chordates. Many echinoderms have hard calcite shells, which appear in the Early Cambrian small shelly fauna. Other deuterostomes are soft-bodied, and important Cambrian deuterostome fossils come from the Chengjiang fauna in China. Chordates, another deuterostome group, have a distinct dorsal nerve cord. They include soft-bodied invertebrates like tunicates and vertebrates, which have backbones. While tunicate fossils are older than the Cambrian period, fossils like Haikouichthys and Myllokunmingia from the Chengjiang fauna appear to be true vertebrates. Haikouichthys had distinct vertebrae, which may have been slightly hardened. Jawed vertebrates, such as acanthodians, first appeared in the Late Ordovician period.

Colonization of land

Adaptation to life on land is a major challenge. All land organisms must avoid drying out. Larger organisms need special structures to support their weight. Systems for breathing and exchanging gases must change. Reproduction cannot rely on water to move eggs and sperm. The earliest clear evidence of land plants and animals dates back to the Ordovician period (488 to 444 million years ago). Some microorganisms reached land much earlier. Modern land ecosystems appeared in the Late Devonian period, about 385 to 359 million years ago. In May 2017, scientists found evidence of life on land in 3.48-billion-year-old geyserite and related minerals in Western Australia. In July 2018, scientists reported that bacteria may have lived on land 3.22 billion years ago. In May 2019, scientists discovered a fossilized fungus, Ourasphaira giraldae, in the Canadian Arctic, which may have grown on land a billion years ago, long before plants lived there.

Oxygen began to build up in Earth's atmosphere over 3 billion years ago, as a result of photosynthesis in cyanobacteria (blue-green algae). Oxygen can cause harmful chemical reactions that were toxic to many earlier organisms. Natural antioxidants, both inside and from food, helped protect against this damage. For example, brown algae store large amounts of minerals like rubidium, vanadium, zinc, iron, copper, molybdenum, selenium, and iodine, which act as antioxidants. These minerals function as essential elements in marine life, helping cells manage chemical reactions.

When plants and animals moved to land and rivers about 500 million years ago, the lack of these marine antioxidants posed a challenge for life on land. Terrestrial plants gradually developed new antioxidants, such as ascorbic acid, polyphenols, flavonoids, and tocopherols. Some of these, like those in fruits and flowers, appeared more recently, in the last 200 to 50 million years. Angiosperms, the most common plants today, and their antioxidant pigments evolved during the Late Jurassic period. Plants use antioxidants to protect themselves from harmful chemicals produced during photosynthesis. Animals also face these same chemicals and have developed their own antioxidant systems. Iodine, in the form of iodide, is an important antioxidant in the diets of both marine and land organisms. It helps protect cells by acting as an electron donor.

Before life moved to land, there was no soil. Land surfaces were either bare rock or shifting sand. Water and nutrients would drain quickly. In the Sub-Cambrian peneplain in Sweden, weathering from the Neoproterozoic era created kaolin deposits up to 5 meters deep, while later deposits from the Mesozoic era were much thicker. It is believed that without plants, erosion by sheet wash was a major process during the late Neoproterozoic era.

Films of cyanobacteria, which are not plants but use photosynthesis, have been found in modern deserts where vascular plants cannot grow. This suggests that microbial mats may have been the first organisms to colonize dry land, possibly in the Precambrian era. Mat-forming cyanobacteria may have developed resistance to drying out as they spread from the seas to intertidal zones and then to land. Lichens, which are partnerships between fungi and algae or cyanobacteria, are also important colonizers of lifeless environments. They help break down rocks and form soil where plants cannot survive. The earliest known ascomycete fossils date back to the Silurian period (423 to 419 million years ago).

Soil formation was very slow until burrowing animals appeared. These animals mix soil's mineral and organic parts, and their waste provides organic material. Burrows have been found in Ordovician sediments and are linked to annelids (worms) or arthropods.

In aquatic algae, most cells can perform photosynthesis and are nearly independent. Life on land requires plants to become more complex. Photosynthesis is most efficient at the top, roots extract water and nutrients from the ground, and middle parts support and transport materials.

Spores of land plants similar to liverworts have been found in Middle Ordovician rocks (about 476 million years ago). Middle Silurian rocks (about 430 million years ago) contain fossils of true plants, such as clubmosses like Baragwanathia. Most were under 10 centimeters tall, and some were closely related to vascular plants, the group that includes trees.

By the Late Devonian period (about 370 million years ago), trees like Archaeopteris grew so densely that they changed river systems from braided to meandering. This caused the "Late Devonian wood crisis" because:

Animals had to change their feeding and excretory systems, and most land animals developed internal fertilization of eggs. The difference in how light bends between water and air required changes in their eyes. However, movement and breathing became easier in some ways, and better sound transmission in air encouraged the development of hearing.

The oldest animal with clear evidence of air-breathing is Pneumodesmus, a millipede from the Early Devonian (about 414 million years ago). Its air-breathing ability is shown by spiracles, openings to tracheal systems. Earlier trace fossils from the Cambrian-Ordovician boundary (about 490 million years ago) may show tracks of large amphibious arthropods on coastal sand dunes, possibly made by euthycarcinoids, which are related to myriapods. Trace fossils from the Late Ordovician (about 445 million years ago) suggest land invertebrates existed. There is clear evidence of many arthropods on coasts and alluvial plains near the Silurian-Devonian boundary (about 415 million years ago), including signs that some arthropods ate plants. Arthropods were well-suited for land because their jointed exoskeletons protected against drying out, provided support, and allowed movement without water.

The fossil record of other major invertebrates on land is limited. No fossils of non-parasitic flatworms, nematodes, or nemerteans have been found. Some parasitic nematodes are preserved in amber. Annelid worm fossils are known from the Carboniferous period, but they may still have been aquatic. The earliest land gastropod fossils date to the Late Carboniferous, and this group may have waited until leaf litter was abundant enough to provide the moist conditions they needed.

The earliest confirmed fossils of flying insects date to the Late Carboniferous, but insects may have developed flight earlier, in the Early Carboniferous or Late Devonian. This ability allowed them to access more ecological niches, escape predators, and adapt to environmental

Mass extinctions

Life on Earth has experienced major extinction events at least since 542 million years ago. While these events were harmful at the time, they sometimes helped speed up the development of life. When one group of organisms stops being the main group in an ecosystem and another takes over, it is usually not because the new group is "better" than the old one. Instead, it often happens because a mass extinction event removes the old dominant group, allowing the new group to become dominant.

The fossil record suggests that the time between major extinction events is increasing, and the usual rate of species dying off is getting lower. These patterns could be explained in several ways:

The number of different types of species (called "genera") that existed at any time, based on the fossil record, shows a different pattern. It increased quickly from 542 to 400 million years ago. Then, it slightly decreased from 400 to 200 million years ago, with the Permian–Triassic extinction event playing a major role in this decline. After that, the number of species increased quickly again from 200 million years ago until now.