History of Life on Earth

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History of Life on Earth

Life on Earth has a history stretching back billions of years, from its beginnings in simple chemical systems to the extraordinary biological diversity of the modern world. Fossils, ancient rocks, molecular evidence and geological dating allow scientists to reconstruct major stages in this history, including the emergence of cells, oxygen-producing photosynthesis, complex multicellular organisms, animals, plants, vertebrates and eventually humans.

The history of life has not been a steady progression. Evolution has repeatedly been shaped by environmental change, continental movement, climatic shifts, biological innovations and catastrophic mass extinctions. Entire groups of organisms have disappeared while surviving lineages diversified into newly available ecological roles. The resulting fossil record provides a chronological archive of more than three billion years of evolutionary change.

Origins of Life and the Earliest Organisms

Life appears to have emerged relatively early in Earth's history. Scientists continue to investigate how nonliving chemistry produced self-replicating systems and eventually the first cells. Water, carbon, nitrogen, phosphorus and other chemical building blocks necessary for life were available on the young Earth.

Possible environments for life's origin include oceans, hydrothermal systems, volcanic regions, mineral surfaces and other chemically active settings capable of providing energy and concentrating organic molecules. Membrane-forming compounds may eventually have enclosed self-replicating chemical systems, creating primitive cells in which biological evolution could operate more effectively.

Natural selection may have begun even before modern cells existed as different self-replicating chemical systems competed for resources. Once cellular organisms became established, evolutionary processes produced increasingly diverse forms of microbial life.

Microbial Earth and the Rise of Oxygen

For much of Earth's history, life was overwhelmingly microbial. These organisms were small, but their collective biological activity eventually transformed the planet itself.

One of the most consequential developments was oxygen-producing photosynthesis. Early photosynthetic microorganisms released oxygen that gradually accumulated in oceans and the atmosphere. Geological evidence indicates that localized oxygenation occurred before the major atmospheric transition known as the Great Oxidation Event roughly 2.4 billion years ago.

Rising oxygen concentrations created both an environmental crisis and an evolutionary opportunity. Organisms adapted to oxygen-free conditions faced a potentially toxic substance, while other organisms evolved ways to tolerate and eventually exploit oxygen for highly efficient metabolism. Increasing oxygen availability ultimately helped create conditions capable of supporting larger and more energetically demanding organisms.

The Rise of Complex Cells and Multicellular Life

Another fundamental transition was the emergence and diversification of complex eukaryotic cells. Chemical evidence preserved in ancient rocks suggests eukaryotic organisms may have become ecologically important long before large complex organisms became conspicuous in the fossil record.

Multicellularity subsequently opened additional evolutionary possibilities. By the Ediacaran Period, large multicellular organisms inhabited marine environments. Exceptionally preserved fossils reveal organisms capable of movement, reproduction and increasingly complex ecological interactions.

Ediacaran discoveries have also blurred the traditional boundary between this period and the Cambrian explosion. Evidence increasingly indicates that important evolutionary innovations associated with later animal groups had already begun developing before the Cambrian Period.

The Cambrian Explosion

Beginning more than 500 million years ago, the Cambrian Period witnessed one of the most significant episodes of biological diversification in Earth's history. Many major animal groups became prominent in the fossil record, while ecosystems developed increasingly sophisticated predator-prey relationships, sensory systems and modes of movement.

Exceptional fossil deposits such as the Burgess Shale, Qingjiang assemblage and other Cambrian sites preserve soft-bodied organisms that normally would have disappeared without leaving recognizable fossils. These deposits reveal remarkable experimentation in animal body forms and provide evidence about the origins and relationships of major evolutionary lineages.

Research also suggests that the Cambrian explosion was neither instantaneous nor completely isolated from earlier evolution. Important developments began during the preceding Ediacaran Period, while diversification and extinction continued throughout the Cambrian.

Colonization of Land and Vertebrate Evolution

Life eventually expanded from predominantly aquatic environments onto land. Plants fundamentally transformed terrestrial environments, while arthropods and later vertebrates developed adaptations allowing them to survive away from permanent bodies of water.

Roots, vascular tissues and other innovations helped plants colonize terrestrial habitats. Among vertebrates, evolutionary changes involving limbs, lungs and eventually the amniotic egg reduced dependence on aquatic environments for movement and reproduction.

The Paleozoic Era consequently witnessed dramatic diversification in both marine and terrestrial ecosystems. Fishes expanded into numerous ecological roles, plants spread across continents, and increasingly diverse animals established themselves on land.

Dinosaurs and the Mesozoic Era

Dinosaurs became one of the dominant vertebrate groups of the Mesozoic Era and underwent approximately 150 million years of evolutionary diversification. Fossils reveal enormous variation in dinosaur size, anatomy, feeding strategies, defensive structures and geographic distribution.

New discoveries continue to reshape dinosaur evolutionary trees. Fossils from Africa, Asia, Europe, Antarctica and the Americas demonstrate that dinosaur communities were diverse and geographically widespread.

Birds emerged from within the dinosaur evolutionary tree, meaning dinosaurs did not disappear entirely at the end of the Cretaceous. Modern birds represent the surviving descendants of one branch of dinosaur evolution.

Other reptiles also produced remarkable evolutionary experiments. Pterosaurs independently evolved powered flight and developed neurological adaptations comparable in function, though not origin, to those found in birds.

Mass Extinctions and Evolutionary Change

Mass extinctions repeatedly transformed the history of life. Scientists traditionally recognize five especially severe extinction events during the Phanerozoic, although researchers continue debating whether other biological crises should also qualify as major mass extinctions.

The Permian-Triassic extinction, often called the Great Dying, was the most severe known biological crisis in Earth's history. Enormous proportions of marine and terrestrial biodiversity disappeared.

The end-Cretaceous extinction approximately 66 million years ago eliminated non-avian dinosaurs along with many other organisms. Mammals also experienced substantial losses, but surviving mammalian lineages subsequently underwent major diversification.

Extinction therefore has a paradoxical evolutionary role. Biological crises destroy ecosystems and evolutionary lineages, yet surviving organisms may subsequently expand into ecological opportunities left vacant by extinct groups.

Fossils and Reconstructing the History of Life

The fossil record is one of the principal sources of evidence for understanding life's history. Fossilized organisms, tracks, nests, eggs and other traces reveal which organisms lived in the past and provide evidence concerning their anatomy, behavior and environments.

Successive rock layers preserve fossils in recognizable chronological patterns. The discovery that particular fossil assemblages occur in predictable sequences helped establish the principle of faunal succession and made fossils important tools for correlating and dating sedimentary rocks.

Radiometric dating complements these relative dating methods by assigning numerical ages to rocks. Together, fossil succession and radioactive dating allow scientists to place evolutionary events within an increasingly precise geological chronology.

Evolution provides the framework connecting these observations. Similarities and differences among organisms, their chronological appearances and their geographical distributions allow paleontologists to reconstruct evolutionary relationships extending across deep time.

Continents, Climate and the Evolution of Life

Biological evolution has always occurred within a changing planet. Continents have moved, oceans have opened and closed, mountains have risen, and Earth's climate has shifted between dramatically different states.

The distribution of related fossils across continents now separated by oceans helped provide important evidence for continental drift and eventually plate tectonics. Continental movement repeatedly divided and reunited populations, creating opportunities for isolation, migration, extinction and diversification.

Climate change has likewise influenced biological history. Geological evidence documents periods much warmer and colder than today, while abrupt environmental disruptions have sometimes coincided with major extinction episodes.

Ancient Lineages and Evolutionary Persistence

Some modern organisms belong to evolutionary lineages extending hundreds of millions of years into the past. Sharks and their relatives, for example, possess a history reaching back roughly 450 million years and have survived multiple mass extinctions.

Species sometimes described as "living fossils" retain anatomical features resembling those of distant ancestors. The phrase can nevertheless be misleading. These organisms have continued evolving genetically and biologically even when their overall body forms have remained comparatively conservative.

Their survival demonstrates that evolution does not necessarily require constant dramatic changes in external appearance. Successful biological structures can persist while populations continue adapting to changing environments.

Human Evolution

Human evolution represents one recent branch within the much larger history of life. Fossils, archaeological evidence, genetics, behavior and environmental records document a complex evolutionary history extending millions of years into the past.

Rather than a straight progression toward modern humans, hominin evolution involved numerous species and branching evolutionary lineages. The earliest stages documented by fossils occurred in Africa.

Thousands of hominin fossils reveal changes in anatomy, geographic distribution and evolutionary relationships. Fossil discoveries indicate that Homo sapiens had emerged in Africa by approximately 300,000 years ago.

Human history therefore occupies only a very small portion of geological time, following billions of years during which microorganisms, complex cells, multicellular organisms, animals and vertebrates had already transformed the planet.

Reading Deep Time

Understanding life's history requires thinking across timescales vastly exceeding recorded human history. The geological time scale divides Earth's past into eons, eras, periods and smaller intervals, many of whose boundaries correspond to major biological and geological transitions.

The Phanerozoic Eon encompasses the interval in which abundant complex organisms are especially well represented in fossils. It is divided into the Paleozoic, Mesozoic and Cenozoic eras.

This chronology demonstrates the immense separation between familiar prehistoric organisms and humans. Non-avian dinosaurs, for example, disappeared tens of millions of years before humans evolved.

Recent Discoveries and a Changing History of Life

The scientific history of life is continually revised as new fossils are discovered and existing specimens are reexamined. Recent discoveries have provided new evidence about Ediacaran organisms, Cambrian ecosystems, dinosaur diversity, reptile evolution and ancient terrestrial predators.

New analytical techniques can also change interpretations of fossils discovered decades earlier. Anatomical imaging, molecular comparisons, chemical fossils and improved geological dating allow researchers to test evolutionary relationships that previously depended primarily on visible skeletal anatomy.

Consequently, the evolutionary tree of life should not be understood as a completed historical diagram. It is an evolving scientific reconstruction that becomes more detailed—and sometimes substantially rearranged—as new evidence emerges.

Evolution, Extinction and the Modern World

The fossil record is relevant not only to understanding the past but also to interpreting biological changes occurring today. Paleobiologists use hundreds of millions of years of fossil evidence to study natural extinction rates, ecological disruption and patterns of recovery following environmental crises.

Ancient extinction events demonstrate both the resilience and vulnerability of life. Biodiversity has repeatedly recovered after catastrophic losses, but recovery generally occurs over geological rather than human timescales and often produces ecosystems fundamentally different from those that existed beforehand.

Understanding the history of life therefore provides a long-term perspective on modern environmental change. It reveals how profoundly climate, geology, biological innovation and extinction can reshape the biosphere.

Conclusion

The history of life on Earth is a story spanning billions of years of biological and planetary change. Life began with comparatively simple organisms, remained predominantly microbial for immense stretches of time, transformed Earth's atmosphere through photosynthesis and eventually produced complex cells, multicellular organisms and increasingly elaborate ecosystems.

The Cambrian diversification, colonization of land, rise of vertebrates, evolution of dinosaurs and mammals, repeated mass extinctions and eventual emergence of humans represent only some of the major transitions preserved in fossils and geological evidence.

Life's history also demonstrates that evolution is neither a straight line nor an inevitable march toward greater complexity. It is a branching process shaped by natural selection, environmental change, contingency, extinction and survival. Every living species represents a surviving branch of this immense evolutionary history, connecting the biological world of the present with organisms that lived billions of years ago.

Origins of Life and the Earliest Organisms

| NASA | NASA Science | May 13, 2026

Life may have appeared on Earth nearly four billion years ago, surprisingly soon after the planet itself formed. This overview examines what scientists know about when life began and the evidence preserved from the earliest chapters of biological history.

| NASA | NASA Science | May 13, 2026

The chemical ingredients necessary for life were present very early in Earth's history. The article examines water, carbon, nitrogen, phosphorus and other elements that supplied the raw materials from which the earliest organisms could develop.

| NASA | NASA Science | May 13, 2026

Scientists continue to investigate where life first emerged, considering environments including oceans, volcanic regions, rock surfaces and other energy-rich settings on the young Earth.

| NASA | NASA Science | May 13, 2026

The emergence of primitive cells was a critical transition between chemistry and biology. Membrane-forming molecules may have enclosed self-replicating chemical systems and created protected environments in which early cellular life developed.

| NASA | NASA Science | May 13, 2026

Evolution may have begun with competing self-replicating chemical systems even before fully developed cells existed. Natural selection eventually became the mechanism responsible for much of life's enormous diversification.

| Natural History Museum | Natural History Museum | n.d.

A broad history of life from its chemical beginnings to microorganisms, multicellular organisms, animals, plants, dinosaurs and humans. The article provides a useful chronological framework for understanding more than three billion years of biological evolution.

| University of California Museum of Paleontology | UC Berkeley | n.d.

This extensive History of Life Through Time exhibit surveys the development and diversification of major biological lineages over roughly 3.7 billion years and connects fossils with the evolutionary history of modern organisms.

| University of California Museum of Paleontology | UC Berkeley | n.d.

Fossils demonstrate that organisms have changed through geological time and provide crucial evidence for reconstructing evolutionary relationships. The resource introduces the fossil record as a historical archive of life on Earth.

Microbial Earth and the Rise of Oxygen

| NASA | NASA Science | July 18, 2024

Microorganisms dominated the earliest history of life and profoundly altered Earth's environment. Scientists use chemical and geological evidence to reconstruct microbial evolution because microscopic organisms generally leave few conventional fossils.

| NASA Astrobiology | NASA | April 20, 2023

Earth's atmosphere once contained very little free oxygen. The Great Oxidation Event transformed atmospheric chemistry and ultimately helped make the evolution of large, energy-demanding organisms possible.

| NASA Astrobiology | NASA | October 6, 2021

Evidence suggests small amounts of atmospheric oxygen existed before the dramatic rise associated with the Great Oxidation Event approximately 2.4 billion years ago, showing that planetary oxygenation was a complex process.

| NASA Astrobiology | NASA | September 5, 2019

Researchers can use modern DNA and reconstructed ancient proteins as molecular fossils to investigate organisms that lived billions of years ago, including cyanobacteria associated with the oxygenation of Earth's atmosphere.

| NASA Astrobiology | NASA | April 11, 2019

The development of oxygen-producing photosynthesis was one of the most important biological innovations in Earth's history. Research into manganese chemistry helps scientists investigate how organisms adapted to the increasingly oxygen-rich planet.

| NASA Astrobiology Institute | NASA | March 5, 2019

Ancient rocks provide clues that portions of Earth's oceans became oxygenated even before the major atmospheric oxygen increase known as the Great Oxidation Event.

| NASA Astrobiology | NASA | October 23, 2018

Researchers have investigated whether early photosynthetic organisms may have relied on pigments different from modern chlorophyll, illustrating the diversity of possible evolutionary pathways during Earth's microbial past.

| NASA Astrobiology | NASA | January 19, 2016

The rise of oxygen created an evolutionary crisis for organisms adapted to an oxygen-free world. Some microbes evolved mechanisms for surviving and eventually exploiting oxygen as an efficient source of metabolic energy.

Multicellular Life and the Ediacaran World

| American Museum of Natural History | AMNH | May 20, 2026

Exceptionally preserved Ediacaran fossils provide evidence about some of Earth's earliest complex organisms and illuminate the development of movement, reproduction and ecological interactions before the Cambrian Period.

| Natural History Museum | Natural History Museum | April 2, 2026

Fossils from China suggest important groups of complex organisms had already evolved before the traditional beginning of the Cambrian explosion, adding evidence for a more gradual transition toward modern animal ecosystems.

| Natural History Museum | Natural History Museum | July 9, 2025

Fossil evidence demonstrates that sophisticated animals existed during the Ediacaran Period before the famous Cambrian diversification, although their soft bodies often made fossil preservation unlikely.

| Natural History Museum | Natural History Museum | May 17, 2024

Ediacaran ecosystems represent a crucial stage in life's history when large multicellular organisms expanded and animals increasingly interacted with both their environment and one another.

| Jason Daley | Smithsonian Magazine | September 6, 2019

A roughly 550-million-year-old fossil accompanied by a preserved trackway offers evidence for mobile, bilaterally organized animals before the Cambrian explosion.

| Smithsonian Magazine Staff | Smithsonian Magazine | August 13, 2018

Unusual Ediacaran fossils demonstrate how difficult it can be to determine where Earth's earliest complex organisms belong on the evolutionary tree and illustrate the strange ecosystems preceding the Cambrian Period.

The Cambrian Explosion

| Natural History Museum | Natural History Museum | June 1, 2026

Research suggests increasingly sophisticated sensory abilities developed around the beginning of the Cambrian radiation, potentially transforming how animals searched for food, avoided threats and interacted with their environments.

| Smithsonian Magazine Staff | Smithsonian Magazine | April 7, 2026

Newly described fossils suggest that parts of the evolutionary diversification traditionally associated with the Cambrian explosion had begun during the preceding Ediacaran Period.

| Smithsonian Magazine Staff | Smithsonian Magazine | February 2, 2026

A fossil deposit in China provides evidence concerning an early Cambrian extinction episode and helps researchers reconstruct ecological changes occurring during one of the most important evolutionary radiations in Earth's history.

| Smithsonian Magazine Staff | Smithsonian Magazine | July 26, 2024

Fossils of the unusual Cambrian arthropod Odaraia help reveal the extraordinary experimentation in body forms that accompanied the rapid diversification of animal life more than half a billion years ago.

| Smithsonian Magazine Staff | Smithsonian Magazine | May 3, 2023

Exceptionally preserved fossils from Wales show how Cambrian organisms and ancestors of modern animal groups coexisted during a major period of biological diversification.

| Smithsonian Magazine | Smithsonian Magazine | March 21, 2019

The Qingjiang fossil assemblage preserves soft-bodied Cambrian organisms in remarkable detail and offers new evidence concerning the pace and character of the Cambrian evolutionary radiation.

| Josh Davis | Natural History Museum | February 19, 2019

Research on trilobite evolution suggests the especially rapid phase of the Cambrian explosion may have lasted roughly twenty million years, followed by a comparatively steadier pace of diversification.

| Smithsonian Magazine | Smithsonian Magazine | n.d.

The Burgess Shale's extraordinary preservation of soft-bodied organisms revolutionized scientific understanding of Cambrian ecosystems and revealed a striking diversity of extinct body plans.

| Hayley Dunning and Emily Osterloff | Natural History Museum | n.d.

An accessible overview of the Cambrian Period and the rapid evolutionary diversification during which many of the major groups of animals represented in modern ecosystems first became conspicuous in the fossil record.

Colonization of Land and Vertebrate Evolution

| University of California Museum of Paleontology | UC Berkeley | n.d.

Fossils appear in a recognizable chronological sequence through rock layers, allowing scientists to reconstruct the order in which major groups of organisms appeared during Earth's history.

Mass Extinctions

| NASA | NASA Science | January 28, 2002

The Permian-Triassic extinction was the most severe known biological crisis in Earth's history. Enormous numbers of marine and terrestrial species disappeared near the end of the Paleozoic Era.

| American Museum of Natural History | AMNH | n.d.

Earth's fossil record preserves evidence for several catastrophic mass extinctions. These episodes repeatedly eliminated large portions of biodiversity while creating ecological opportunities for surviving groups to diversify.

| University of California Museum of Paleontology | UC Berkeley | n.d.

The disappearance of non-avian dinosaurs is placed within the broader history of mass extinction, including the still larger Permian-Triassic extinction that devastated life long before dinosaurs disappeared.

Dinosaurs and the Mesozoic Era

| American Museum of Natural History | AMNH | July 2012

Dinosaur research contributes far beyond knowledge of prehistoric animals, helping scientists investigate evolution, biogeography, geological history and major changes in Earth's environments.

| American Museum of Natural History | AMNH | n.d.

Dinosaur fossils document roughly 150 million years of evolutionary history. The visualization shows where different dinosaur groups lived, when major lineages appeared and how birds emerged from within the dinosaur evolutionary tree.

Fossils and Reconstructing the History of Life

| NASA Earth Observatory | NASA | May 8, 2008

William Smith's discovery that particular fossil assemblages occur in predictable sequences helped establish the principle of faunal succession, one of the foundations for dating sedimentary rocks and reconstructing Earth's biological history.

| American Museum of Natural History | AMNH | n.d.

Paleontology uses fossilized plants, animals and microorganisms to reconstruct organisms and ecosystems that existed millions or billions of years before recorded human history.

| American Museum of Natural History | AMNH | n.d.

Fossils preserved in successive rock layers allow scientists to determine when organisms appeared and disappeared, turning Earth's strata into a chronological record of biological history.

| University of California Museum of Paleontology | UC Berkeley | n.d.

Robert Hooke recognized centuries ago that fossils were remains of once-living organisms and that the fossil record demonstrated both the appearance and extinction of species through Earth's history.

| University of California Museum of Paleontology | UC Berkeley | n.d.

Evolution provides the fundamental framework through which paleontologists organize fossil evidence and reconstruct the interconnected history of organisms through geological time.

Human Evolution

| Smithsonian Human Origins Program | Smithsonian Institution | June 25, 2026

The Smithsonian's Human Origins Program brings together fossils, archaeological remains, behavioral evidence and environmental records to reconstruct the final several million years of the history of life leading to modern humans.

| Smithsonian Human Origins Program | Smithsonian Institution | July 9, 2024

Human evolution represents a branching history involving numerous hominin species rather than a simple progression toward modern humans. The earliest known portions of that history are documented entirely in Africa.

| Smithsonian Human Origins Program | Smithsonian Institution | January 3, 2024

Thousands of fossil individuals provide evidence concerning human evolutionary history, including changing anatomy, geographic distribution and relationships among extinct hominin species.

| American Museum of Natural History | AMNH | November 2018

Fossils spanning roughly seven million years document the evolutionary history of hominins, including extinct relatives and ancestors that help scientists understand the eventual emergence of Homo sapiens.

| Smithsonian Human Origins Program | Smithsonian Institution | June 8, 2017

Fossils from Africa demonstrate that Homo sapiens had evolved by approximately 300,000 years ago, pushing the documented origin of our species much farther into the past than earlier evidence had suggested.

| Smithsonian Human Origins Program | Smithsonian Institution | n.d.

This interactive timeline places major hominin species and evolutionary developments into chronological context alongside environmental changes during the last several million years.

Recent Discoveries in the History of Life

| Natural History Museum | Natural History Museum | June 29, 2026

Fossil evidence from Antarctica adds to evidence that the continent once supported rich forests and diverse dinosaur communities, illustrating how dramatically Earth's geography and climate have changed through time.

| Natural History Museum | Natural History Museum | June 4, 2026

Reanalysis of an ancient reptile once regarded as an early turtle relative challenges earlier interpretations of turtle evolution and demonstrates how new fossil evidence can reshape evolutionary family trees.

| Natural History Museum | Natural History Museum | June 3, 2026

Fossils of Praearcturus gigas reveal a giant scorpion-like predator more than 400 million years old, offering insight into the early diversification of large animals on land.

| Josh Davis | Natural History Museum | March 20, 2026

A juvenile dinosaur fossil from South Korea represents a newly named species and expands knowledge of dinosaur diversity in Cretaceous Asia.

| American Museum of Natural History | AMNH | March 9, 2026

Research into the inner ears of living and extinct reptiles shows how anatomical structures preserved in fossils can help scientists reconstruct evolutionary relationships.

| Natural History Museum | Natural History Museum | March 4, 2026

The newly described Tanyka amnicola represents an unusual surviving branch from an earlier period of evolutionary experimentation, highlighting the persistence of ancient biological lineages.

| James Ashworth | Natural History Museum | February 19, 2026

Fossils from Niger reveal a newly described Spinosaurus species with an elaborate cranial crest, adding to evidence for the remarkable diversity of large predatory dinosaurs in Cretaceous Africa.

| Smithsonian Magazine | Smithsonian Magazine | February 6, 2026

Research suggests ecosystems began recovering rapidly after the asteroid impact that ended the Cretaceous Period, offering insight into how surviving organisms rebuilt biological communities after mass extinction.

Evolution of Dinosaurs and Other Reptiles

| American Museum of Natural History | AMNH | December 3, 2025

Fossil skulls reveal that pterosaurs and birds independently evolved brains capable of supporting powered flight, providing an example of convergent evolution in two very different vertebrate lineages.

| James Ashworth | Natural History Museum | August 27, 2025

Newly discovered fossils of Spicomellus afer reveal extraordinary armor and meter-long spikes, showing that armored dinosaurs experimented with elaborate defensive structures early in their evolutionary history.

| Natural History Museum | Natural History Museum | August 22, 2025

A newly described iguanodontian from Britain's Isle of Wight adds evidence that Early Cretaceous dinosaur communities were highly diverse and included species with unusual body structures.

| James Ashworth | Natural History Museum | June 25, 2025

Enigmacursor mollyborthwickae helps resolve a long-standing taxonomic problem and provides new information about small herbivorous dinosaurs living in Cretaceous North America.

| American Museum of Natural History | AMNH | n.d.

Fossilized bones, eggs, nests and trackways allow paleontologists to reconstruct dinosaur anatomy, growth and behavior and place dinosaurs within the broader evolutionary history of vertebrates.

| Natural History Museum | Natural History Museum | n.d.

The Museum's dinosaur resources explain how new fossil discoveries continually alter scientific understanding of dinosaur appearance, behavior, diversity and evolutionary relationships.

Life Through Mass Extinctions

| Riley Black | Smithsonian Magazine | July 12, 2024

Scientists traditionally recognize five major mass extinctions during the last half-billion years, but researchers continue debating whether additional biological crises deserve recognition as major extinction events.

| Smithsonian National Museum of Natural History | Smithsonian Magazine | April 22, 2019

Fossil exhibits trace the long history of life through environmental transformations and mass extinctions, including the end-Permian and end-Cretaceous crises.

| Smithsonian Magazine | Smithsonian Magazine | August 4, 2016

Research into mass extinction examines how volcanism, climate disruption and other planetary changes repeatedly reshaped ecosystems and redirected evolutionary history.

| Maris Fessenden | Smithsonian Magazine | June 22, 2016

Studies suggest that mammals also suffered enormous losses during the end-Cretaceous extinction, meaning the subsequent rise of mammals followed a severe evolutionary bottleneck rather than an easy transition.

| American Museum of Natural History | AMNH | n.d.

More than 22,000 trilobite species are known from fossils. Their long evolutionary history and eventual extinction make them important organisms for studying diversification and biological crises during the Paleozoic Era.

| American Museum of Natural History | AMNH | n.d.

The end-Cretaceous asteroid impact dramatically transformed terrestrial and marine ecosystems and eliminated non-avian dinosaurs along with many other groups, opening ecological opportunities for surviving organisms.

The Paleozoic Era

| U.S. Geological Survey | USGS | n.d.

The Paleozoic Era, from roughly 541 to 252 million years ago, witnessed the diversification of marine animals, the rise of fishes and the first major colonization of land by plants and vertebrates.

| U.S. Geological Survey | USGS | n.d.

The Phanerozoic Eon encompasses the portion of Earth's history in which abundant complex life is especially well represented in fossils and is divided into the Paleozoic, Mesozoic and Cenozoic eras.

| Natural History Museum | Natural History Museum | n.d.

Sharks and their relatives have an evolutionary history stretching back roughly 450 million years, surviving repeated mass extinctions while diversifying into a wide variety of marine forms.

From Water to Land

| David Quammen | National Geographic | 2018

For most of Earth's history life consisted primarily of microscopic organisms. Fossils document the later rise of large multicellular forms and the extraordinary diversification of animal body plans beginning near the end of the Precambrian.

| Ed Yong | National Geographic | December 24, 2008

The maximum size of organisms increased enormously during life's history, particularly following the emergence of complex cells and later the development of multicellular organisms.

The Rise of Complex Cells

| National Geographic | National Geographic | June 15, 2023

Chemical fossils preserved in ancient rocks suggest eukaryotic organisms may have been ecologically important hundreds of millions of years before their familiar body fossils became common.

| Chloe Berge | National Geographic | April 27, 2023

Australia's Ediacaran fossil beds preserve communities from roughly 550 million years ago and provide rare evidence about the transition from predominantly microscopic life to complex animal ecosystems.

| Michael Greshko | National Geographic | March 21, 2019

The Qingjiang fossil deposit in China preserves more than 500-million-year-old organisms in extraordinary detail and reveals previously unknown diversity during the Cambrian explosion.

Origins of Life

| National Geographic | National Geographic | March 8, 2024

Scientists investigate several possible environments for life's origin, including hydrothermal vents and chemically active surface environments, while fossil evidence indicates microbial life existed billions of years ago.

| National Geographic | National Geographic | June 2, 2013

Every living organism shares an evolutionary history reaching billions of years into Earth's past, ultimately linking modern biodiversity through common ancestry.

Fossils as a Record of Life

| U.S. Geological Survey | USGS | June 13, 2001

Fossils occurring in predictable sequences allow geologists to correlate rock layers and reconstruct the relative chronology of biological evolution across different regions.

| Lucy E. Edwards and John Pojeta Jr. | U.S. Geological Survey | 1993

Fossils provide evidence for both the organisms that lived in Earth's past and the geological sequence in which biological and environmental changes occurred.

| U.S. Geological Survey | USGS | n.d.

Radiometric dating allows scientists to assign numerical ages to rocks and thereby place fossils and major evolutionary events into an increasingly precise geological chronology.

Continents, Climate and Evolution

| U.S. Geological Survey | USGS | July 11, 2025

The distribution of identical and related fossils on widely separated continents helped scientists recognize continental drift and eventually develop the theory of plate tectonics.

| Michon Scott | NOAA | n.d.

Geological and fossil evidence demonstrates that Earth has experienced climates dramatically warmer than today and that abrupt climate changes have sometimes coincided with major extinction events.

Ancient Lineages Surviving Today

| National Geographic | National Geographic | March 13, 2024

Organisms commonly called “living fossils,” including horseshoe crabs and other ancient lineages, retain anatomical similarities to distant ancestors while continuing to undergo genetic evolution.

| National Geographic | National Geographic | August 22, 2012

The concept of a “living fossil” can be misleading because apparently ancient-looking species have continued evolving even when their overall body forms remain relatively conservative.

Reading Deep Time

| William L. Newman | U.S. Geological Survey | 1977

The geological time scale divides Earth's enormous history into intervals that partly reflect major changes in the kinds of organisms represented in successive fossil assemblages.

| U.S. Geological Survey | USGS | n.d.

Geological chronology demonstrates the enormous separation between non-avian dinosaurs and humans, illustrating why understanding deep time is essential for interpreting the history of life.

Evolution, Extinction and the Modern World

| U.S. Geological Survey and Smithsonian Institution | USGS | December 25, 2025

Smithsonian natural-history collections document Earth's transformation from its early geological history through billions of years of biological evolution and changing ecosystems.

| Seth Finnegan et al. | NOAA Repository | 2024

The marine fossil record provides hundreds of millions of years of evidence about ecological change, extinction and recovery and can help researchers understand the vulnerability of modern marine organisms.

| Smithsonian National Museum of Natural History | Smithsonian Magazine | October 22, 2020

Paleobiologists use patterns preserved in the fossil record to establish natural extinction baselines and investigate how modern biodiversity losses compare with biological crises of the geological past.