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- Guide fossil: Stromatolites
- Geological events: Formation of the lithosphere, hydrosphere, and atmosphere. Origin and early evolution of the Earth. Grouping and alternative drifts of the first continental nuclei. First orogenies
- Climate events: Primitive atmosphere. Atmosphere with oxygen levels. Oxidizing atmosphere with an ozone layer. First ice age
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This leap in complexity from single-celled organisms occurred approximately 2.1 to 1.2 billion years ago in Earth's ancient oceans. The origin of the first multicellular organisms is theorized to have arisen from colonies of individual eukaryotic cells that, over time, developed mutual interdependence and a division of labor, leading to greater efficiency and adaptability. -
The earliest signs of life on Earth are revealed through fossil and geochemical evidence dating back approximately 3.5 billion years. This evidence suggests the existence of single-celled prokaryotic microorganisms—simple life forms without a defined nucleus, similar to modern bacteria and archaea—that inhabited the early oceans in an environment devoid of free oxygen. -
Guide fossils: Trilobites
Geological events: Orogenies, intense volcanism and Formation of Pangaea
Climate events: Ice ages and global warming -
Both the first arthropods (with exoskeletons and jointed appendages, such as trilobites) and the first cephalopods (mollusks with shells that control buoyancy) emerged in the oceans during the Cambrian period (541–485 million years ago).
Arthropods evolved to inhabit the seabed and, later, the land; while cephalopods became the ocean's first active swimming predators, diversifying into the forms we know today as octopuses, squid, and nautiluses. -
The first jawless fish were the ostracoderms. Were characterized by the absence of jaws and paired fins. Many of them had their bodies, especially their heads, protected by bony plates, which served as a defense. They lived in aquatic environments and fed by filtering particles or small organisms. These fish are very important because they represent one of the earliest forms of vertebrates and are the ancestors of modern jawless fish, such as lampreys. -
The first vertebrates were primitive, jawless fish that emerged in the oceans during the Cambrian period (530–500 million years ago).
They possessed a notochord (a primitive backbone) and a skull, but lacked jaws and paired fins. They fed by filter-feeding. This innovation in body support allowed for the evolution of more complex life forms, from jawed fish to, eventually, all terrestrial vertebrates. -
The Ordovician–Silurian extinction was the first mass extinction in Earth’s history. It happened about 444 million years ago and mainly affected marine life. It is thought to have been caused by a global cooling and the formation of large glaciers, which lowered sea levels and destroyed many shallow-water habitats. As a result, about 85% of marine species became extinct, especially trilobites, brachiopods, and coral species. -
The diversification of invertebrates was an explosive evolutionary event, known as the "Cambrian explosion," which transformed the early oceans beginning approximately 541 million years ago. During this relatively short period, most of the animal phyla (basic body plans) that exist today arose and diversified. -
A fish's age is primarily determined by counting the annual growth rings, or annuli, that form in its hard structures. The most accurate method is to examine the otoliths (ear bones), which accumulate an opaque layer in summer (rapid growth) and a translucent layer in winter (slow growth) each year. Other less invasive, but less accurate, methods include analyzing scales, fin rays, or vertebrae. -
The emergence of jawed fish (gnathostomes) was a crucial evolutionary event that occurred during the Silurian period, approximately 439 million years ago.
Jaws evolved from the gill arches of primitive jawless fish, granting them an immense advantage: the ability to bite and become active predators. This led to rapid diversification and the dominance of marine ecosystems by early groups such as the placoderms (extinct armored fish). -
The second mass extinction of marine life was the Late Devonian extinction. It occurred about 375–360 million years ago and mainly affected marine organisms, especially reef builders, armored fish, and trilobites. The extinction happened in several phases and is thought to have been caused by climate changes, low oxygen levels in the oceans, and possibly volcanic activity. This event led to a major decline in marine biodiversity and changed the structure of ocean ecosystems. -
Around 370 million years ago, amphibians (vertebrates) appeared, evolving from lobe-finned fish. These were the first vertebrates with limbs for moving on land, although their life cycle (eggs and aquatic larvae) kept them tied to water. Insects, by establishing terrestrial ecosystems, created a crucial food source for the amphibians that would later arrive. -
The year 459 BC was a period of significant political and military upheaval in the Mediterranean, with no events related to the fern forests. In Greece, Athens' alliance with Megara sparked the first hostilities of the First Peloponnesian War against Corinth and Sparta, resulting in mixed victories on land and sea. -
The first reptiles emerged more than 310 million years ago, during the Carboniferous period, long before 299 BC. Their success was due to the evolution of the amniotic egg, which allowed reproduction on land. -
The third and most devastating extinction was the Permian–Triassic extinction, also known as the Great Dying. It occurred about 252 million years ago and caused the extinction of around 90–96% of marine species and many land organisms. It is believed to have been caused by massive volcanic eruptions, extreme climate warming, ocean acidification, and a lack of oxygen in the seas. This extinction drastically changed life on Earth and marked the end of the Paleozoic era. -
The extinction of trilobites occurred at the end of the Permian period, approximately 251 million years ago, coinciding with the "Great Dying," the most devastating mass extinction in Earth's history.
After a long decline, the last groups of these successful marine arthropods were wiped out by catastrophic environmental changes. The primary cause was massive volcanism in Siberia, which triggered extreme global warming, ocean acidification, and deoxygenation. -
Guide fossils: Ammonites
Geological events: Fracture of Pangaea. Meteorite impact
Climate events: There were no ice ages -
The first mammals emerged at the end of the Triassic period, evolving from advanced reptiles.
These small, nocturnal, shrew-like animals were characterized by key innovations such as an advanced jaw joint, improved hearing, specialized teeth, and the presence of fur. Mammals coexisted with dinosaurs. -
The first dinosaurs appeared during the Triassic period, approximately 245 to 233 million years ago, evolving from archosaur reptiles.
Initially, they were relatively small, carnivorous, bipedal creatures, like Eoraptor. Their upright posture gave them a key advantage in movement. Following a mass extinction event at the end of the Triassic, they disappeared. -
The first flowering plants appeared during the Early Cretaceous period, approximately 130 to 140 million years ago. They evolved from gymnosperm ancestors, introducing key innovations such as the flower for insect-assisted reproduction and the fruit for efficient seed dispersal.
Fossils such as those of Archaefructus and Montsechia show the primitive forms of these plants, which rapidly diversified and became the dominant group of terrestrial flora -
The first reptiles emerged around 315 million years ago (Carboniferous period), becoming the first truly terrestrial vertebrates thanks to their key innovation: the amniotic egg.
Much later, the first birds evolved directly from a group of feathered theropod dinosaurs, appearing approximately 165-150 million years ago. Transitional fossils like Archaeopteryx show a blend of reptile (teeth, bony tail) and bird (feathers) traits. -
Summarize it in text
The fourth mass extinction, or end-Triassic extinction, occurred approximately 201 million years ago, wiping out between 70% and 80% of terrestrial and marine species. This event was caused by massive volcanism that led to severe global warming and ocean acidification. The resulting extinction eliminated many competitors of the dinosaurs, allowing them to become the dominant terrestrial vertebrates for the remainder of the Mesozoic Era. -
The fifth mass extinction, or Cretaceous-Paleogene (K-Pg) extinction, occurred approximately 66 million years ago and is the most well-known event for wiping out the non-avian dinosaurs.
The primary cause was the impact of a massive asteroid, which triggered immediate global catastrophes and a prolonged by blocking sunlight. This event eliminated 75% of species, but allowed mammals and birds to survive, diversify, and become the dominant groups in the present era. -
Guide fossils: Nummulites
Geological events: Completion of continental drift. Orogeny
Climate events: Last ice ages (Ice Age) -
The dominance of angiosperms (flowering plants) began during the Middle and Late Cretaceous (100 to 80 million years ago), transforming terrestrial ecosystems.
Their success was based on key evolutionary innovations: flowers enabled efficient pollination by co-evolved insects, and fruits facilitated superior seed dispersal. These advantages, combined with faster growth, allowed angiosperms to outcompete gymnosperms and ferns, becoming the dominant plant group worldwide. -
The dominance of birds and mammals as the dominant biological groups on Earth began 66 million years ago, after the mass extinction that wiped out the non-avian dinosaurs, not in 65 BC.
65 BC was a period of political intrigue and expansion within the late Roman Republic. Key events included Lucius Sergius Catilina's failed attempts to obtain the consulship and Pompey's continued consolidation of Roman power in the Near East. -
The first hominins (the human lineage) appeared in Africa approximately 7 to 5 million years ago, after diverging from common ancestors with chimpanzees.
Early fossils such as Sahelanthropus and Orrorin show the key innovation that defined this group: primitive bipedalism (walking on two legs). These early hominins still had small, ape-sized brains and retained traits adapted for living in trees, marking the beginning of human evolutionary history. -
The first Homo sapiens (anatomically modern humans) emerged in Africa at least 300,000 years ago, according to fossil evidence found at Jebel Irhoud, Morocco. These early humans had a mix of modern and archaic traits and developed sophisticated stone tools. They remained in Africa for hundreds of thousands of years, and the major migratory waves that colonized the rest of the world began between 70,000 and 100,000 years ago, marking the start of our global expansion.