History of Life

Time span of life on Earth

It is more than 3.5 billion years.

Age of Earth

It is approximately 4.54 billion years.

Geological Time Scale

It is the system used by scientists to organize Earth's approximately 4.54-billion-year history into progressively smaller chronological units based on rock and fossil records.

Hierarchical units of the Geological Time Scale

They are the following: 1. Eon 2. Era 3. Period 4. Epoch 5. Age.

Division boundaries of the Geological Time Scale

They correspond to major changes recorded in rocks and fossils, including the appearance or disappearance of important groups of organisms, major climatic changes, and mass extinction events.

Precambrian Time

It represents nearly 90% of Earth's history, is commonly known as the Age of Bacteria, and includes the Hadean, Archean, and Proterozoic Eons.

Eons of Precambrian Time

They are the following: 1. Hadean Eon (~4.54–4.0 Ga) 2. Archean Eon (4.0–2.5 Ga) 3. Proterozoic Eon (2.5 Ga–538.8 Ma).

Hadean Eon

It is the earliest eon of Earth's history, spanning from approximately 4.54 to 4.0 billion years ago (Ga).

Notable events of the Hadean Eon

They are the following: 1. Formation of Earth from material in the early Solar System 2. Formation of the Moon, likely following a giant impact 3. Formation of an extremely hot surface undergoing intense geological activity 4. Cooling of Earth leading to crust formation and stable bodies of liquid water 5. Absence of confirmed fossil evidence of life.

Evolutionary significance of the Hadean Eon

It established the physical and chemical conditions from which life would eventually emerge.

Archean Eon

It is the eon spanning from 4.0 to 2.5 billion years ago (Ga) during which microbial prokaryotic life first established and diversified.

Notable events of the Archean Eon

They are the following: 1. Stabilization of Earth's crust 2. Establishment of oceans 3. Atmosphere containing very little free oxygen 4. Emergence of the earliest widely accepted fossil evidence of microbial life 5. Evolution of early cyanobacteria 6. Common occurrence of stromatolites.

Stromatolites

They are layered microbial structures, primarily formed by early cyanobacteria, serving as common fossil evidence of ancient microbial communities during the Archean Eon.

Evolutionary significance of the Archean Eon

It marks the initial establishment and early diversification of prokaryotic life on Earth.

Proterozoic Eon

It is the eon spanning from 2.5 billion years ago (Ga) to 538.8 million years ago (Ma) characterized by major evolutionary transitions toward complex eukaryotic and multicellular life.

Notable events of the Proterozoic Eon

They are the following: 1. Great Oxidation Event starting around 2.4 Ga 2. Increase in atmospheric oxygen due to oxygenic photosynthesis 3. Rise of aerobic metabolism 4. Evolution of the first eukaryotic organisms 5. Appearance of multicellular organisms 6. Evolution of sexual reproduction in eukaryotes 7. Global glaciations including Snowball Earth 8. Appearance of Ediacaran biota in late Proterozoic oceans.

Great Oxidation Event

It is a major environmental transition beginning approximately 2.4 Ga during the Proterozoic Eon where atmospheric oxygen increased substantially due to oxygenic photosynthesis, enabling aerobic metabolism.

Ediacaran biota

They are among the earliest large and complex multicellular organisms that inhabited late Proterozoic oceans.

Evolutionary significance of the Proterozoic Eon

It witnessed the critical transitions from prokaryotic to eukaryotic life and from unicellular organisms to complex multicellular forms.

Phanerozoic Eon

It is the eon spanning from 538.8 million years ago (Ma) to the present, characterized by abundant fossil evidence of complex life.

Eras of the Phanerozoic Eon

They are the following: 1. Paleozoic Era (538.8–251.9 Ma) 2. Mesozoic Era (251.9–66.0 Ma) 3. Cenozoic Era (66.0 Ma–Present).

Paleozoic Era

It is the era spanning from 538.8 to 251.9 million years ago (Ma) characterized by rapid animal diversification, land colonization, and the establishment of major plant and vertebrate groups.

Cambrian Period

It is the initial period of the Paleozoic Era, spanning from 538.8 to 485.4 million years ago (Ma).

Notable events of the Cambrian Period

They are the following: 1. Cambrian Explosion causing rapid diversification of major animal groups 2. Widespread development of hard shells and skeletons 3. Appearance of early chordates and vertebrate relatives 4. Establishment of major animal body plans.

Cambrian Explosion

It refers to the relatively rapid evolutionary diversification during the Cambrian Period that gave rise to most major animal body plans in the fossil record.

Ordovician Period

It is the period spanning from 485.4 to 443.8 million years ago (Ma) marked by marine diversification and early land colonization by plants.

Notable events of the Ordovician Period

They are the following: 1. Major diversification of marine organisms including jawless fishes, corals, brachiopods, and bryozoans 2. Appearance of the earliest evidence of land plants 3. Conclusion with the Ordovician–Silurian mass extinction caused by climate and sea-level shifts.

Silurian Period

It is the period spanning from 443.8 to 419.2 million years ago (Ma) highlighting increased terrestrial colonization.

Notable events of the Silurian Period

They are the following: 1. Emergence of the first well-established vascular plants on land 2. Establishment of arthropods in terrestrial habitats 3. Expansion of coral reef ecosystems.

Devonian Period

It is the period spanning from 419.2 to 358.9 million years ago (Ma), commonly known as the Age of Fishes.

Notable events of the Devonian Period

They are the following: 1. Major diversification of jawed fishes, sharks, and bony fishes 2. Development of the first extensive forests 3. Diversification of terrestrial arthropods and insects 4. Evolution of tetrapods from lobe-finned fish ancestors 5. Conclusion with the Late Devonian extinction series.

Carboniferous Period

It is the period spanning from 358.9 to 298.9 million years ago (Ma) famous for extensive swamp forests, high atmospheric oxygen, and the evolution of amniotes.

Notable events of the Carboniferous Period

They are the following: 1. Extensive tropical swamp forests that accumulated to form major coal deposits 2. High atmospheric oxygen concentrations supporting large arthropods 3. Diversification of amphibians 4. Evolution of amniotes.

Amniotic egg

It is an evolutionary adaptation that evolved during the Carboniferous Period, allowing vertebrates to reproduce independently of water bodies.

Permian Period

It is the final period of the Paleozoic Era, spanning from 298.9 to 251.9 million years ago (Ma).

Notable events of the Permian Period

They are the following: 1. Assembly of the supercontinent Pangaea 2. Development of dry continental interiors 3. Diversification of amniotes, including synapsids 4. Rise of gymnosperms as dominant terrestrial plants 5. Termination with the Permian–Triassic mass extinction.

Synapsids

They are a lineage of amniotes that diversified during the Permian Period and eventually gave rise to modern mammals.

Permian–Triassic Extinction

It is the largest mass extinction in Earth's history, occurring approximately 252 million years ago (Ma) at the end of the Permian Period, commonly known as the Great Dying.

Causes and impacts of the Permian–Triassic Extinction

They are the following: 1. Driven by massive volcanism, rapid global warming, ocean acidification, and ocean deoxygenation 2. Eliminated a very large proportion of marine species 3. Severely affected terrestrial ecosystems and reorganized global biodiversity.

Mesozoic Era

It is the era spanning from 251.9 to 66.0 million years ago (Ma), commonly known as the Age of Reptiles.

Triassic Period

It is the first period of the Mesozoic Era, spanning from 251.9 to 201.4 million years ago (Ma).

Notable events of the Triassic Period

They are the following: 1. Ecosystem recovery following the Permian mass extinction 2. First appearance of dinosaurs 3. Evolution of true mammals from mammaliaform ancestors 4. Initial breakup of Pangaea late in the period 5. Conclusion with the Triassic–Jurassic mass extinction.

Jurassic Period

It is the middle period of the Mesozoic Era, spanning from 201.4 to 145.0 million years ago (Ma).

Notable events of the Jurassic Period

They are the following: 1. Dominance of dinosaurs as large land vertebrates 2. Abundance of marine reptiles and aerial pterosaurs 3. Evolution of early birds from small theropod dinosaurs 4. Continued dominance of gymnosperms.

Archaeopteryx

It is an important transitional fossil from the Jurassic Period that displays a combination of avian and non-avian dinosaur features.

Cretaceous Period

It is the final period of the Mesozoic Era, spanning from 145.0 to 66.0 million years ago (Ma).

Notable events of the Cretaceous Period

They are the following: 1. High diversification of dinosaurs 2. Dramatic radiation of flowering plants (angiosperms) 3. Important ecological interactions and co-diversification between insects and angiosperms 4. Continued diversification of birds and mammals 5. Conclusion with the Cretaceous–Paleogene mass extinction.

Cretaceous–Paleogene Mass Extinction

It is the mass extinction occurring approximately 66 million years ago (Ma), strongly associated with the Chicxulub asteroid impact, that ended the reign of non-avian dinosaurs.

Impacts of the Cretaceous–Paleogene Mass Extinction

They are the following: 1. Extinction of non-avian dinosaurs and many marine and terrestrial organisms 2. Survival of birds as the sole living dinosaur lineage 3. Creation of ecological opportunities for surviving groups, including mammals.

Cenozoic Era

It is the current era spanning from 66.0 million years ago (Ma) to the present, commonly known as the Age of Mammals.

Paleogene Period

It is the first period of the Cenozoic Era, spanning from 66.0 to 23.03 million years ago (Ma), consisting of the Paleocene, Eocene, and Oligocene Epochs.

Paleocene Epoch

It is the epoch spanning from 66 to 56 million years ago (Ma) marked by ecological recovery and rapid radiation of mammals, birds, and early primate relatives following the K–Pg extinction.

Eocene Epoch

It is the epoch spanning from 56 to 33.9 million years ago (Ma) featuring warm global temperatures, appearance of modern mammalian groups, evolution of early whales from land ancestors, and primate diversification.

Oligocene Epoch

It is the epoch spanning from 33.9 to 23.03 million years ago (Ma) characterized by global cooling, establishment of Antarctic ice sheets, expansion of grasslands, and anthropoid primate diversification.

Neogene Period

It is the second period of the Cenozoic Era, spanning from 23.03 to 2.58 million years ago (Ma), comprising the Miocene and Pliocene Epochs.

Miocene Epoch

It is the epoch spanning from 23.03 to 5.33 million years ago (Ma) characterized by substantial grassland expansion, diversification of grazing mammals, radiation of apes, and marine mammal diversification.

Pliocene Epoch

It is the epoch spanning from 5.33 to 2.58 million years ago (Ma) marked by cooler and drier climates, presence of Australopithecus and other hominins in Africa, and the formation of the Isthmus of Panama.

Great American Biotic Interchange

It is the exchange of land species between North and South America made possible by the formation of the Isthmus of Panama during the Pliocene Epoch.

Quaternary Period

It is the period spanning from 2.58 million years ago (Ma) to the present, characterized by repeated glacial cycles and human evolution.

Pleistocene Epoch

It is the epoch spanning from 2.58 Ma to 11,700 years ago, commonly called the Ice Age, characterized by continental ice sheet movements, megafauna, diversification of Homo, and evolution of Homo sapiens in Africa.

Holocene Epoch

It is the current epoch spanning from 11,700 years ago to the present, featuring a stable climate, rise of agriculture, animal domestication, civilizations, and significant human modification of Earth's ecosystems.

Five Major Mass Extinctions

They are the following: 1. End-Ordovician (~444 Ma) caused by rapid climate and sea-level changes 2. Late Devonian (~372–359 Ma) affecting marine life 3. End-Permian (~252 Ma) known as the Great Dying 4. End-Triassic (~201 Ma) paving the way for dinosaur diversification 5. End-Cretaceous / K–Pg (~66 Ma) caused by asteroid impact.