Timeline of the Different Stages of Earth's Development

Dust left over from the formation of the sun clumped together to form planet Earth. The other planets in our solar system were also formed in this way near the same time. The atmosphere contained carbon dioxide, carbon monoxide, nitrogen, ammonia, methane gases, as well as water vapor given off during volcanic eruptions. The surface was covered with magma oceans due to the Earth's intense heat at the time.

Dense metals sank to the center of the Earth and formed the core, while the outside layer of magma cooled and solidified to form the Earth’s crust. 

Water vapor was released into the Earth’s atmosphere by volcanism. Volcanism is the eruption of magma from volcanoes; the most abundant gas released during this process was water vapor. The vapor then cooled, fell back down as rain, and formed the Earth’s first oceans. Some water may also have been brought to Earth by comets and asteroids. 

Fossils discovered and analyzed by scientists have proved that single-celled prokaryotes were the first living organisms to exist on Earth. The prokaryotes began to release oxygen into the atmosphere by conducting the process of photosynthesis.

The Great Oxidation Event occurred around 2.4 billion years ago due to the prokaryotes in the ocean producing oxygen. This caused the oxygen level in Earth's atmosphere to increase dramatically. The event also caused Earth's first mass extinction - oxygen was toxic to anaerobic bacteria, so most of them died off (except those that escaped to low-oxygen areas, like the bottom of the ocean).

Scientists have found fossils of red algae that were living 1.6 billion years ago. This algae served as the ancestor of several other types of organisms, including plants, fungi, and even animals!

The first ever vertebrates evolved, these being the Myllokunmingia fish, 500 million years ago. Invertebrates, such as trilobites, crinoids, brachiopids, and cephalopods, are common in the oceans by this point in history.

Fossils of arthropods give proof that they were the first organisms to live on land. At the time, our atmosphere offered no protection from the sun's harmful radiation. Arthropods were able to colonize the land under the cover of darkness to avoid radiation.

Unlike arthropods, plants had to wait for land to become habitable for them to survive. Thankfully for them, oxygen in the atmosphere reacted with the sun's radiation to form the ozone layer. This layer shielded organisms from harmful radiation, so plants were able to prosper on land. These first plants on land comprised of moss and liverworts; these were non-vascular plants that did not have deep roots.

With plants present on the land to provide a food source, animals rapidly followed. The first to venture onto the land were primitive amphibians, and reptiles evolved soon afterwards. The first major groups of amphibians evolved from lobe-finned fish, which were similar to the modern coelacanth. These ancient fish had evolved multi-jointed leg-like fins with digits that enabled them to crawl along the sea bottom, and eventually, onto land.

Towards the end of the Permian period, all landmasses were brought together to form the supercontinent, Pangea. This formation caused the basin of the ocean, known as Panthalassa at the time, to become deeper. This brought about the extinction of swallow water organisms due to the lowering of sea level.

The Permian Extinction was a series of extinction pulses that contributed to the greatest mass extinction in Earth's history. Over 90% of marine life and 70% of terrestrial life went extinct because of this event. The warming of the Earth's climate led to global warming; animals were unable to breathe under this condition. Therapsids and reptilian Archosaurs were among the survivors of this extinction event.

During the Mesozoic, or "Middle Life" era, life diversified rapidly and giant reptiles, dinosaurs and other monstrous beasts roamed the Earth. The most well-known dinosaurs evolved and existed during this time like the Tyrannosaurus and the Triceratops. On the other hand, the first mammals evolved from a population of vertebrates called therapsids and coexisted with the dinosaurs during this period. Morganucodontids were tiny creatures that possessed skeletal features that marked them as the first true mammal species.

The most famous of all the mass extinction events is the Cretaceous-Tertiary extinction, better known as the day the dinosaurs died. An asteroid crashed into modern-day Yucatan, Mexico, scorching everything within 900 miles, thus ending the reign of the dinosaurs. Months of blackened sky filled with dust and debris killed off the plants, since they could not photosynthesize without access to sunlight. This broke down the food chain, causing even more animals to die out. The only species who survived were those with diets that did not require sustenance from plants.

On November 24, 1974, fossils of one of the oldest known human ancestors, an Australopithecus afarensis specimen nicknamed “Lucy,” were discovered in Hadar, Ethiopia. Australopithecus afarensis is an extinct species of australopithecine which lived from about 4–2.9 million years ago. They are considered to be direct ancestors of us humans.

Most scientists think that people who look like us - anatomically modern Homo sapiens - evolved by at least 130,000 years ago from ancestors who had remained in Africa. They evolved from their ancestors, Homo erectus. The main change that occurred was that the Homo sapiens had larger brains and were more intelligent than their predecessors.