“Fight for her.” -Bulletin board poster of Earth
Earth, also known as Sol III, known to prehistoric humans and Forerunners as "Erde-Tyrene," is the third planet and one of the four terrestrial worlds in the Sol System. It is the presumptive homeworld of humanity, though this is disputed by some Ancient Humans due to the loss of records to a technological dark age and the presence of human ruins on other worlds. It is the current capital world of the United Earth Government and home to the headquarters of the UNSC. Earth has been at the forefront of galactic history, from encounters with the Precursors to the end of the Covenant Empire. Inhabited long ago by a prehistoric interstellar human government conquered and dissolved by the Forerunner.
With a population of around ten billion, Earth was possibly the most populated planet in UEG territory prior to the Covenant invasion. Despite being dramatically reduced during the Covenant Attack in 2552 due to both casualties inflicted by the Covenant and a major evacuation effort, the population has again raised to several billion by the end of 2553
It is believed that Earth first formed around 4.6 billion years ago, and gave rise to the first single-celled organisms about 3.9 billion years ago. Before the discovery of records of ancient spacefaring humans and it was believed the ancestors of modern humans, "Homo Sapiens", first evolved in Africa around 200,000 years ago. This date has recently been called into question largely thanks to Bornstellar Relation, the remains of a deceased Catalog unit, and a damaged monitor, which revealed that humanity achieved its earliest known spacefaring technological state prior to 1,100,000 BCE and that Forerunners believed humans had "moved their interstellar civilization outward along the galactic arm" by 150,000 BCE. Not only is the dating for humanities origin less sure now, but the very idea of Earth has humanity's birthplace has also been revealed to have been a matter of debate for the civilization that preceded and witnessed the Human-Forerunner Wars. That ancient empire had lost many of the records that could have proven or disproven the issue, and human ruins had also been discovered on multiple worlds besides Earth with little in the way of explanation. Regardless of when a and how humanity first evolved and developed, one thing is clear, and it is that humans were present on multiple planets over a million years ago, and it was not until thousands of years into this civilization that Earth was even proposed as their planet of origin.
Spreading across the Orion Arm of the Milky Way Galaxy, humanity underwent several periods of technological and social regression, losing most of their records in the process. Earth was one of the first human's worlds to fall to the Forerunners in the Human-Forerunner Wars. After being defeated by the Forerunner civilization, most humans were exiled to Earth, with their civilization dismantled, populations splintered amongst the many human species and their culture degrading back to a primitive state for millennia. Earth and humanity came under the charge of the Forerunner lifeshaper, the Librarian, who helped the humans slowly work towards rebuilding a primitive civilization. Remnants of their former technology, such as steam power and hot-air balloon flight, were re-learned under her care but later forgotten after the firing of the Halo Array.
Near the end of their war with the Flood, the Forerunners, specifically the Lifeworkers under the direction of the Librarian, indexed many of Earth's species - including humans - as part of their Conservation Measure to preserve life in the galaxy after the activation of the Halo Array. Prior to the activation, after surveying the planet, beacons were activated informing humans imprinted with the Librarian's geas to gather to evacuate the planet. Lifeworkers only had time to evacuate one out of a thousand of the planet's large species; countless species would be left behind. Several days into the evacuation the Librarian and the IsoDidact arrived in the system to speak with the Catalog. The evacuation was halted ahead of schedule as the Flood had broken through the local Forerunner defenses and were entering the system.
After the Ur-Didact composed most of the human population on Omega Halo, the Librarian realized the remaining population would not be enough to repopulate the species. She sent Chant-to-Green to see how much the Flood had taken in the system and save whatever humans she could. The Flood had left Earth untouched after the Forerunners evacuated. Chant was only able to gather a few hundred humans to send to Installation 00. When Precursor star roads began to appear in the system, she told Chant she would stay on Earth and gave Chant the title of Lifeshaper. Later, a Gargantua-Class Transport arrived to be disassembled and reconfigured, and after gathering the necessary materials formed a portal to the Ark.
After the Array's activation, human life was planted back on Earth, where it successfully re-developed.