A History of the Earth (Geologic Time)

The first three eons (Hadean, Archean, and Proterozoic) are often combined into the Precambrian supereon covering the first four billion years of the Earth’s history – around 4 ½ billion years ago (Gya) up to 539 million years ago (Mya).

• Hadean eon (4.6 to 4.0 Gya) is when the Earth first formed. It was a hot molten sphere during a time when the Moon also formed, and the Late Heavy Bombardment rained asteroids down on the planet.
• Archean eon (4.0 to 2.5 Gya) is when the Earth cooled forming the first rocks, and life emerged some 3.8 to 3.5 billion years ago – although some studies suggest it began earlier. The Sun’s intensity was around 70% of today’s levels, but greenhouse gases kept the Earth warm. Cyanobacteria began performing photosynthesis, which eventually produced the first oxygen.
• Proterozoic eon (2.5 to .539 Gya) saw the Great Oxidation event (GOE) when aerobic (oxygen breathing life) began to dominate the planet. Around 700 million years ago, the planet froze during the Cryogenian ice age, creating a Snowball Earth. This led to the Ediacaran (EE-dee-A-care-an) period and possibly the first simple multi-cellular animal life.

The Paleozoic era saw the greatest expansion of life and the greatest disappearance of life in the Earth’s history. It was during this time that life diversified from simple structures to many complex forms similar to what we see today.

• Cambrian (Kam-bree-an) period (541 to 485 Mya) began with the Cambrian Explosion. This was the most significant increase in the diversity of life. Many invertebrates including hard shelled organisms such as Trilobites appeared (and fossilized). The first vertebrates (jawless fish) also appeared. Life was found primarily in the seas, while the land was mostly barren.
• Ordovician (Or-doh-vish-an) period (485 to 444 Mya) began with the Great Ordovician Biodiversification Event (aka the Ordovician Radiation). This period ended with the Andean-Saharan ice age that resulted in the Ordovician-Silurian extinction.
• Silurian (Sih-lyoor-ee-an) period (444 to 419 Mya) saw placoderms (armored fish with jaws) first appear. Fish then diversified with the appearance of cartilaginous and bony fish late in the period.
• Devonian (Dah-voh-knee-an) period (419 to 359 Mya) is called the “age of fishes”. Placoderms, such as Dunkleosteus, dominated the oceans but vanished by the end of the period. Bony fish split into two groups. There were the ray-finned fish (with fins attached with a ray of bones) similar to many fish of today. However, it was the lobe-finned fish (with fins attached by a single bone) that led to the first land dwelling tetrapods (four legged amphibious vertebrates). The Devonian ended with the beginning of the Karoo ice age and the Devonian-Carboniferous extinction.
• Carboniferous (Carbon-if-er-us) period (359 to 299 Mya) saw mossy swamps containing trees, which decayed to form coal deposits. Amphibians led to land-dwelling amniotes (tetrapods having eggs with a membrane). Amniotes diverged into mammal ancestors (synapsids) and reptile ancestors (sauropsids/diapsids). Oxygen levels were higher than today, which supported large insects such as giant 2 ½ foot dragonflies (Meganeura) and 10-foot millipedes (Arthropleura).
• Permian (Per-me-an) period (299 to 252 Mya) saw the Carboniferous Rainforest Collapse bringing a more arid climate. Seed-bearing plants and larger mammal ancestors (pelycosaurs and therapsids) became prevalent. The supercontinent of Pangaea formed during this time.

The Paleozoic era ended with the Permian-Triassic (P-T) extinction (aka the Great Dying) that wiped out around 90% of Earth’s marine species and 70% of the land species. This was the greatest mass extinction in Earth’s history. According to Michael Benton in When Life Nearly Died and Andrew Knoll in A Brief History of Earth, the P-T extinction was most likely caused by volcanic eruptions that created the Siberian Traps in northern Russia. The eruptions produced a general increase in CO₂ levels, which resulted in a catastrophic effect on the planet including a runaway Greenhouse effect with reduced oceanic oxygen levels, acid rain, and global warming.

The Mesozoic era brought the age of reptiles and particularly the dinosaurs. The Mesozoic is divided into three periods (Triassic, Jurassic, and Cretaceous).

• Triassic period (252 to 201 Mya) saw life slowly recover from the Great Dying. Early on, the last big therapsid, the two-tusked Lystrosaurus, spread across Pangaea before dying off. Archosaurs (ruling reptiles) eventually dominated the Triassic, while the first small mammals and dinosaurs appeared. The Earth’s land mass started to split in two during the latter part of the Triassic. The Triassic period ended with the Triassic-Jurassic extinction.
• Jurassic period (201 to 145 Mya) saw large plant eating sauropods (e.g., Apatosaurus/Brontosaurus) and the first of the large two-legged meat eating theropods (e.g., Allosaurus) roam the Earth. Large dinosaur like reptiles (ichthyosaurs, plesiosaurs, and pliosaurs) swam the depths of the seas. The first of the Pterosaurs, Pterodactyl, circled overhead. The supercontinent of Pangaea continued to split apart into Laurasia (in the north) and Gondwana (in the south).
• Cretaceous period (145 to 66 Mya) saw the giant theropods, such as T. rex, rule the Earth. Large sauropods called titanosaurs continued through the Cretaceous. Mosasaurs replaced the earlier giant marine reptiles of the Jurassic. Larger Pterosaurs such as Pteranodon and Quetzalcoatlus were found in the skies. Laurasia and Gondwana each continued to break up into continents somewhat similar to those we find today.

The Mesozoic era ended 66 million years ago with the Cretaceous-Paleogene (K-Pg) extinction that wiped out around 70% of the Earth’s species including the dinosaurs. The K-Pg extinction is thought to be the result of a 6- to 9-mile-wide asteroid that smashed into the Earth near the present-day town of Chicxulub, Mexico on the Yucatan Peninsula.

The Cenozoic (Sign-oh-zo-ik) era brought the age of mammals. The modern divisions include the Paleogene, Neogene, and Quaternary periods.

• Paleogene period (66 to 23 Mya) saw a warm climate with jungles and the first expansion of mammals including early primates. The Paleogene is further divided into the Paleocene, Eocene, and Oligocene epochs.
• Neogene period (23 to 2.6 Mya) brought a cooling climate and more grasslands. Hominids, including the ancestors of humans, orangutans, gorillas, and chimpanzees, appeared roughly 15 Mya during the Miocene epoch (23 to 5.2 Mya). The human tree, hominins, branched off from the others around 7 Mya. Australopithecus appeared around 4 Mya near the beginning of the Pliocene epoch (5.3 to 2.6 Mya). Lurking under the water during this time was the bus sized shark, Megalodon.

The Quaternary (Kwah-tur-nah-ree) period is divided into two epochs and continues to this day. This period experienced the last great ice age (Quaternary/Pleistocene ice age) with periods of “glacials” and “interglacials”.

• Pleistocene (Ply-stoh-seen) epoch (2.6 Mya to 11.7 Kya) was the age of saber-tooth tigers, woolly mammoths, and mastodons. The human genus Homo appeared around 2 Mya, and the species Homo sapiens around 300 thousand years ago (Kya). The most recent period of glaciation (i.e., THE ice age) began around 115,000 years ago.
• Holocene epoch (11.7 Kya to now) is the “age of humans” with a warming and stable climate as the last period of glaciation ended and humans came to dominate the planet.