Earth’s Geological Timeline: From Hadean to Today
Earth’s Geological Timeline: 4.65 Billion Years of History
Geological Time Divisions
Earth’s existence spans approximately 4,650 million years (Ma). The largest unit of geological time is the eon. There are two great eons:
- Precambrian: Comprises 90% of Earth’s history.
- Phanerozoic: Divided into three eras: Paleozoic, Mesozoic, and Cenozoic.
These eras are further divided into periods. The Cenozoic era, for instance, is divided into the Tertiary and Quaternary periods.
Hadean Period: The First 500 Million Years
The first 500 Ma of Earth’s history are known as the Hadean period. The evidence from this period is limited to a few zircon crystals. The first 10 Ma saw intense bombardment by planetesimals. Thanks to the Moon, we know that this bombardment lasted until about 3,900 Ma ago.
Differentiation into Layers
- Accretion: Earth was bombarded by protoplanets and planetesimals, integrating their materials and generating heat.
- Differentiation: The Earth was almost completely liquid, causing heavy materials to sink to the interior and gases to be released through outgassing.
- Cooling: Impacts became less frequent. A metallic core, mantle, and crust formed. The first atmosphere was created, different from the current one.
- Ocean Formation: The cooling of the surface allowed water vapor from outgassing to condense, flooding the land. Only some volcanic islands emerged.
The Primitive Atmosphere and the Rise of Eukaryotes
The primitive atmosphere contained large quantities of water vapor, CO2, nitrogen (N2), and smaller quantities of other gases. To achieve the current composition, two major changes occurred:
- Large quantities of oxygen were incorporated due to the appearance of photosynthetic organisms.
- CO2 decreased dramatically, dissolving in water and being used by cyanobacteria.
Through bacterial integration, new types of eukaryotic cells originated, according to the endosymbiotic theory. The existence of these cells made it possible to exploit new resources, and multicellular organisms appeared.
Ediacaran Fauna and the Cambrian Explosion
Animals appeared in the last 15% of Earth’s history. Ediacaran Fauna fossils, dating back 680 Ma, show impressions of organisms shaped like leaves or discs.
Around 100 Ma later, the first mineralized skeletons appeared in the fossil record, marking the Cambrian explosion. The exoskeleton was an evolutionary success, notably in trilobites. More than 125 animal species, many soft-bodied, are found in the Burgess Shale fossils. Vertebrates also appeared during this time.
Land Plants, Arthropods, and the Permian Extinction
Around 450 Ma, land plants appeared, adapting to the environment by developing rigid cell walls, xylem, and seeds. This allowed colonization by invertebrate animals (arthropods) and vertebrates (the first amniotes, reptiles).
The Great Permian Extinction (approximately 250 Ma ago) saw the disappearance of 80% of marine and 70% of terrestrial species. Possible causes include:
- Sea level drop
- Reduction of atmospheric oxygen
- Climate change due to intense volcanic activity
In 2006, a 250 Ma old crater was found under the Antarctic ice. If confirmed, it could be the cause of this extinction.
Pangaea’s Breakup and Vertebrate Adaptations
Pangaea’s Division: Pangaea separated into two supercontinents. North America split from the other continents (Africa, India, South America, Antarctica, and Australia). Subduction zones led to the formation of large mountain ranges like the Himalayas, Andes, and Pyrenees.
Adaptation of the Last Vertebrates: During the Mesozoic, some vertebrates managed to control their body temperature. While reptiles were ectotherms, needing external heat sources, mammals and birds were endotherms. Mammals also nourished embryos through the placenta and provided milk for their young.
The K-T Boundary and the End of the Dinosaurs
K-T Boundary: Walter and Luis Alvarez found a layer of iridium, a rare metal, at different sites. This layer, dating back 65 Ma, marks the boundary between the Cretaceous and Tertiary periods and is believed to be due to a meteorite impact. The energy of this meteorite would have been a thousand million times that of the Hiroshima bomb, opening a crater 150-200 km in diameter. This event led to the disappearance of 50% of species, including dinosaurs and ammonites. Some small mammals survived, leading to an explosion of diversity that continues to this day.
Climate Changes
Climate changes can be attributed to:
- External Causes: Changes in solar activity, variations in Earth’s orbit, and meteorite impacts.
- Internal Causes: Changes in albedo, atmospheric composition, presence of aerosols, and changes in ocean currents.