Human

Summary
The human race is an Early-Development space faring civilization whose original home world is Earth, the third planet of the Sol System. Throughout their existence, they have developed their civilization by engaging in food production on a larger scale, developed language, developed religion and political philosophies and exhibited telescoping technological and infrastructural development. After succeeding in space colonization, the human race was able to terraform and inhabit other planets besides Earth, but the human race is under pressure due to the discovery of the caste-driven Covenant after several centuries of space travel. By the time the war ends, there are approximately 200 million members of the species living in parts of the Milky Way Galaxy.

Early development on Earth
The Homo sapiens were believed to have appeared in Earth's fossil record between approximately 100,000 and 200,000 years ago (there was another subspecies of Homo sapiens, the Homo sapiens idaltu; this subspecies is extinct, but was believed to have lived as long as 160,000 years previous). The current human race is not the only humanoid species known to live on Earth, but the Homo sapiens sapiens is believed to have either displaced or merely outlasted all other branches of the genus, including Homo neanderthalensis and Homo sapiens idaltu. Homo sapiens usually stands several feet tall, though this varies depending on nutrition, genetics and other factors. Members of the species, like some Covenant species, are four-limbed and bipedal; they walk upright on two legs and use their arms and hands (complete with opposable thumbs) to hold and manipulate objects such as tools.

The development of human civilization as humans recognize it today started from the ability to harness the growth cycle of plants, producing edible food-energy in a systemic fashion. This time is referred to as the Neolithic Revolution, which occurred first on Earth over 10,000 years ago and soon spread to disparate human groups. The stability that came from systemic agriculture allowed for more permanent settlements and the development of ever-more-complex tools to further benefit the members of the species. Innervated toolmaking and the domestication of other species on Earth gave human populations greater and greater power to expand their populations, and trade ensured the diffusion of new technologies to adjacent groups. These exploding populations led to the formation of genuine civilizations over 8,000 years ago. The first such sites are believed to have formed in Southwest Asia and Asia Minor, appearing elsewhere soon after. The infrastructure of civilizations continued to expand and grow in complexity as innovative technologies and larger populations allowed for increasing food-energy availability. Economic, political, scientific and religious advancement continued to quicken as this species was able to automate many tasks in the Industrial Revolution. This automation gave birth to the Digital Revolution, in which computing machines allowed scientific discovery and technological development to accelerate with explosive speed. Larger urban centers and tools for civilian and military uses were able to develop, as were more modern economic and political systems. As these systems developed further, the species was able to enjoy longer, more pro-creative, and more comfortable lives, virtually impervious to other creatures from whose midst they sprang. Thus-unchecked human population growth eventually led to the presence of several billions more humans on Earth than could be sustained for any long period of time...

Colonization and War
The species was able to explore the space immediately surrounding their home planet in the 20th century, as the population began to advance beyond the capacity of the planet to sustain their civilization. The subsequent conflicts over resources, economics, religion and politics on Earth put great pressure on the species in the beginning of the twenty-third century, and the transnational organization that became the United Nations Space Command engaged in more frequent space colonization and sought to represent the entire species, even though no one state ever previously ruled the entire human race. The development of Slipspace travel in 2291 helped the UNSC engage in spreading the human race beyond their home solar system, releasing the serious ecological pressure on the home world.

Generally the exploring and peace-making variety of species after, approximately several centuries of spreading throughout many solar systems and inhabiting hundreds of worlds, they discovered the violent, theocratic Covenant and were forced into the war covered by the Halo games. As the decades-long war continued, the human species has gained the reputation as a worthy adversary against the Covenant. Humans have been able to show great bravery when facing great odds. Some of the members of the Covenant wonder why the Hierarchs had refused to allow the humans join them. As they have been able to hold steadfast even when outmatched in almost every way. However, despite great courage and brilliance, the future is dark for the survival of the human race as the Covenant continue to wipe out their colonies.

March 3rd, 2553, saw the end of the Covenant-UNSC War, as a victory for humanity and their newfound Sangheili allies - the majority of the Covenant having been slaughtered at Delta Halo by the Elite Fleet, and the rest annihilated by the joint Elite-UNSC forces at the Ark. This victory came in time to save the human homeworld, Earth, which remains humanity's only world with "Any significant population", thanks chiefly to the actions of the Arbiter and the Master Chief.

Speculation: Similarities Between The Forerunners and Humans
A common speculation by fans is that the Forerunners may be ancient ancestors of Humanity. Examples of possible similarities include comments made by 343 Guilty Spark during Halo: Combat Evolved, Halo 3 and the proposed Forerunner hand (see section in the Forerunner page for more information on this subject)

However due to the recent web comic that included the character N'chala, this theory becomes less likely. It became impossible due to the Server 05 message in the Iris viral campaign stating that the Forerunners discovered humanity and built the Ark to protect humanity from the activation of the Halos and the Flood. As shown with the Bestiarum, the Forerunners worked to catagorize species that they found. Humanity then, was NOT created by the forerunner.

Another theory (one supported by the Halo 3 Terminals) is that the humans became the inheritors of the Forerunner empire and technology because the Forerunners passed the Mantle of Guardianship down to humanity, as it was passed to them, presumably from the Precursors. (see Reclaimer for more)