Terra, or Earth, is the planet of origin of humanity.
The earliest known civilizations on Earth can be traced back to approximately 19000 BG (c. 2800 BCE). The First Empire was an era started by Aleksandr (Alexander III of Macedon) roughly around 16500 BG (circa 300 BCE), a millennia-long period that saw humanity's advancement from the use of bronze tools to an iron-based civilization.
The "Imperial Seat" was considered to be the geopolitical center of power belonging to the most influential nation-state on the planet during a given time period, and changed on many occasions, such as during the ascendancy of Rome around 16400 BG (c. 200 BCE). The Roman Empire ended in 16000 BG (c. 200 AD), but the First Empire persisted under Byzantium, followed by a brief interval in the Century Without an Emperor (14700 BG-14608 BG, or c. 1500-1608 AD) until discoveries in America brought about by primitive naval exploratory missions allowed Madrid to attain the historical status of the Imperial Seat.
Expansion of humanity
Approximately around the year 14100 BG (c. 2100 AD), the colonization of the solar system commenced from Old Earth following the formation of a coalition involving the northern, southern, and western political blocs, with at least one high-ranking member of the Bene Gesserrette (ancient forerunner of the Bene Gesserit) serving as chief director of off-Terran exploration for the coalition. Eventually, the population of Earth would be outnumbered by those living offworld in the greater solar system by a factor of 20 to 1. This great expansion later became known as the Little Diaspora.
In 13402 BG (2798 AD), a strike by a planetoid (possibly an asteroid) devastated Earth, and the asteroid Ceres became the new Imperial Seat/political capitol of humanity. Starting the following year, the Rescue of the Treasures salvaged many relics of human history, and spread them across the Known Universe.
In 13360 BG (2840 AD), Terra was re-seeded with both plant and animal life, and set aside as a natural park. Earth (and its Hawaiian islands) is also where the Commission of Ecumenical Translators later converged to assemble the OCB not long after the Butlerian Jihad (c. 108 BG).
By 2800 AG (c. 19,000 AD) Old Earth was now a Barony, an Imperial fief under the rule of House Corrino. Terrans, inhabiting the revered cradle of human life, enjoyed exemption from Imperial draft for the purposes of compulsory military service or interstellar colonization efforts. Even at this late period, Terra's deserts were also populated by Zensunni nomads (who inhabited the Sahara Desert).
However, upon the discovery of the planet Poritrin by Imperial scouts, Padishah Emperor Elrood V decided to demand a tradition-shattering levee of colonists from Old Earth in order to populate the new world. Siridar Baron Charles Mikarrol, rightly fearing a massive revolt among his subjects over the news, instead sent some two million Zensunni nomads to Poritrin against their will, beginning the Zensunni Migration.