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The Hadean Eon (4.6 - 3.8 Billion Years Ago)[]

Time after the oldest known meteors, but before the earliest known Earth rocks

The Earth formed with Sun and the rest of the Solar System. Rocky grains orbiting the Sun coalesced into the Earth. Icy grains could not survive in the region the Earth formed at, because it was too hot, so the initial Earth had no water. The Earth also lacked the building blocks of life or the capacity for any such building blocks to advance to the next stage of complexity.

There are no surviving rocks on the Earth from this long ago, but radioactive Earth's Uranium deposits formed about 4.5 Billion years ago. Knowing how old Earth Uranium is helps one determine the age younger rocks that contain Uranium, since it decays at a predictable rate, and the amount of decay can be measured.

News article: Scientists Refine Earth's Clock: Some Events in Planet's History Happened More Recently Than Previously Thought (Mar 29, 2012)

Meanwhile, in the outer solar system, it was cold enough for icy grains to form. There, comets coalesced. These comets also contained simple organic ices. When these ices are exposed to ultraviolet radiation from nearby stars (the Sun likely formed in a cluster and had many nearby neighbors in its early days), they begin to form animo acids, nucleobases, and amphiphiles, which make up the building blocks of proteins, and thus are considered the building blocks of life.

News article: Organics Probably Formed Easily in Early Solar System (Mar 30, 2012)

About 3.8 Billion years ago, something caused many of the comets that had formed to sent inwards towards the Earth, Moon, and other inner planets, in an event called the Late Heavy Bombardment. This event completely obliterated the Earth's surface, so that there are no known rocks prior to this time period. The comets delivered the water needed for Earth's ocean. They also delivered the building blocks of life, Amino Acids. These building blocks survived the impact to the surface. In addition, the energy caused by this impact caused the animo acids to form "peptide bonds", which link amino acids into proteins.

News article: New Evidence That Comets Deposited Building Blocks of Life On Primordial Earth (Mar 27, 2012)

The amino acids delivered to the Earth had a left-handed orientation. Through chemical reactions, right handed sugars was formed. DNA would later be created from these sugars and life as we know it would come into existence.

News article: Could 'Advanced' Dinosaurs Rule Other Planets? (Apr 11, 2012)

The Archeon Eon (3.8 - 2.5 BYA)[]

Time of the earliest life in an un-oxygenated atmosphere

The origins of how life's building blocks evolved into something that we would call life today is a mystery. It is possible that the first life existed utilizing some sort of unknown precursor to the RNA molecule that was capable of self-reproducing and passing on information from one generation to the next. This would give rise to RNA, which in turn gave rise to the more complex DNA, the basis of today's life.

News article: Strange Cousins: Molecular Alternatives to DNA, RNA Offer New Insight Into Life’s Origins (Apr 19 2012)

Bacteria are the earliest known forms of life we have physical evidence of. These single cellular organisms multiply by replicating their genes and dividing to form a new organism, creating an exact duplicate. Random mutations in this gene splitting process drive their evolution. They are able to protect their most valuable genes from random mutations. Bacteria also have the means to evolve by swapping beneficial DNA traits amongst themselves.

News article: Study Shows Unified Process of Evolution in Bacteria and Sexual Eukaryotes (Apr 5 2012)
News article: Bacteria Evolved Way to Safeguard Crucial Genetic Material (Apr 23 2012)

The Sun was somewhat cooler in this period of time. However, instead of being encased in ice, the Earth was not substantially cooler than it is today, as evidence of rivers and ocean sediments from this time period attest to. This is thanks to a large amount of greenhouse gasses in its atmosphere, especially Methane. The atmosphere also had about the same pressure as it does today.

News article: Fossil Raindrop Impressions Imply Greenhouse Gases Loaded Early Atmosphere (Mar 28 2012)

As photosynthesis had not yet developed, the Earth also had no free oxygen molecules in its atmosphere or in the oceans. The first form of life that could perform photosynthesis did so with a primitive kind that did not produce oxygen. These life forms would evolve into ones capable of splitting oxygen molecules (Cyanobacteria and algae), which released oxygen into the environment.

News article: New Light Shone On Photosynthesis (Apr 3 2012)

At first, the oxygen released by photosynthesizing life forms did not go into the atmosphere. Instead, it was absorbed by iron in the Earth's crust, causing it to rust. The atmosphere at this time alternated between stable hydrocarbon haze states (somewhat resembling Saturn's moon Titan), and haze free states. This was caused by varying amounts of methane production by microbes.

News article: Hazy Shades of Life On Early Earth (Mar 18 2012)

Proterozoic Eon (2.5 Billion - 542 Million Years Ago)[]

Early life in an oxygenated atmosphere

Eventually, the iron and other rocks that were absorbing the oxygen produced by photosynthesis became saturated. The oxygen was released into the sea and atmosphere in an event known as the Great Oxygenation Event. Oxygen was poisonous to the majority of life forms living at this time, sparking a great extinction event. The Oxygen also reacted with the Methane in the atmosphere that was keeping the Earth warm, which caused the first "snowball Earth" period and more hardships for life forms.

The oxygen in the atmosphere and oceans provided an abundant energy source for more advanced life to form. The Earth eventually warmed again, cooled again in a second "snowball Earth" period, and then warmed again. Once it warmed, a complex multi-cellular animals started to evolve. These early animals were made up of only soft-tissue, so fossil evidence of them is scarce.

Phanerozoic Eon (542 Million Years Ago to present)[]

Time of complex "visible" life.

Palozoic Era (542 - 241 Million Years Ago)[]

Early Animal life

During the Cambrian Period, the ocean levels repeatedly rose to form shallow seas over areas that were once covered by land, and then retreated, exposing the land again. This weathering process caused a lot of loose ions from rocks, such as Calcium, Potassium, Iron, to be stripped from the Earth's surface and enter the ocean, greatly affecting its chemistry. In order for complex life to survive, they must maintain a precise amount of these substances. In order to maintain the necessary balance, some of these animals mineralized the excess substances, which formed the first shells, bones (early vertebrates), exoskeletons (like in Trilobites, early Arthropods), and teeth. This resulted in what is known as the "Cambrian Explosion", when many of today's animal forms first appeared. Their hard bodied remnants became fossils which can be studied today.

Article: Evidence for a Geologic Trigger of the Cambrian Explosion (Apr 18 2012)

Animals used these hard body parts as teeth to eat other animals, or as armor to protect themselves, starting an evolutionary arms race between predator and prey. While some animals would later evolve large teeth with strong jaws to eat their prey, Conodonts, early eel-like vertebrates developed microscopically small teeth that could bring down a lot of pressure to grind their food with relatively weak muscles without jaws. These animals lacked a skeleton in the majority of their long bodies, reserving all of it for their their mouths.

Article: Tiny Teeth of Long-Extinct Vertebrate – With Tips Only Two Micrometers Across -- Are Sharpest Dental Structures Ever (Mar 14 2012)

The Cambrian Period ended with a mass extinction, which was followed by the Ordovician Period. Much of the life at this time existed in shallow seas, where sunlight could reach the sea floor. Towards the end of the Ordovician, much of the Earth's land masses became concentrated at the South Pole. Glaciers formed on top of it, which caused the overall temperature of the planet to cool significantly and also lower the sea level. This caused the second greatest mass extinction event in Earth's history, the Late Ordovician mass extinction. Many of the animals lost their habitats as the shallow seas they were living in became exposed to the air. Others could not tolerate the lower temperatures.

Article: What Triggers a Mass Extinction? Habitat Loss and Tropical Cooling Were Once to Blame (Apr 10 2012)

The Silurian Period came after this mass extinction, followed by the Devonian Period (age of fish), the Carboniferous Period (early land plants and animals), and Permian.

The Permian Period was drier than the previous Carboniferous. Reptiles were better able to survive under these conditions and overtook Amphibians on land. Some of these reptiles returned to the sea, such as Mesosaurs, which resembled tiny alligators to a degree. The oldest reptile embryo was recently found, which belonged to a Mesosaur. These Mesosaurs may have given birth, rather than laid eggs. It is also possible that their embryos were almost completely formed before their eggs were laid, and then they hatched shortly thereafter.

Article: Oldest-Ever Reptile Embryos Unearthed

The Permian and Proterozoic ended with the largest mass extinction event ever, the Permian Extinction event.

Mesozoic Era (250 - 65 Million Years Ago)[]

"Middle" animal life

It took a while for life to recover from the last extinction event. Dinosaurs would evolve during the Triassic period to fill some of the niche's left open by earlier creatures.

Some rodent-sized mammals called Multituberculates evolved during this time and would thrive alongside Dinosaurs. They had specialized teeth that allowed them to eat the new flowering plants that appeared at this time.

Article: Some Mammals Used Highly Complex Teeth to Compete With Dinosaurs

Another extinction event separated the Triassic period from the Jurassic Period. In the sea, dolphin-like reptiles called Ichthyosaurs. These reptiles were completely adapted to water and lacked the ability to crawl on the land to lay their eggs, so they gave birth.

Article: Exploding Dinosaur Hypothesis Implodes

The supercontinent Pangea broke up during the Jurassic period. North America would be divided into two separate land masses by an inland sea. The western part would become connected to Asia via a land bridge that formed during the colder era of the Early Cretaceous. Animals in western North America were more closely related to animals found in China than they were to those in eastern North America.

Feathers may have developed to keep dinosaurs warm during the colder Early Cretaceous. A feathered "Yutyrannus Huali" lived in China, which was somewhat smaller than its later relative Tyrannus Rex.

Article: Newly Discovered Close Relative of T. Rex Is Largest Known Feathered Dinosaur

The land bridge between Asia and North America closed as the climate got hotter again in the Late Cretaceous. Yutyrannus's close relative Tyrannosaurus Rex became the apex predator of western North America.

Cenozoic Era (65 Million Years Ago to Present)[]

Modern animal life