From the perspective of those residing in Thet now, Homo sapiens
left Earth more than three hundred thousand years ago, some time in the fifth millennium CE. While this is an immense amount of time from the perspective of cultural memory, and far too long an interval for any group but the Lyrisclensiae
to remember more than a small portion of, this era in particular, when humanity prepared to step out of its cradle and reached for the stars, is well-remembered and well-studied, for it was the last time when the human family still resided under one roof.
The date of humanity's departure cannot be given precisely, as many ships began departing Earth following the first day of January, 4000. Over the next thousand years the planet's health began to fail, as the best and brightest minds emigrated. Contact with Earth was a regular feature of many space-faring nations' cultures well into the sixth millennium, but it is not known when the last communications were made, nor if the planet and its few remaining inhabitants ever recovered from the political, agricultural, and environmental declines it was then facing. Following the invention of interbrane jump technology in 5704, and the subsequent discovery that many smaller branes were far more rich in alien life than our own visible universe, knowledge of how to return to Earth (and even our own galaxy) rapidly faded. It is estimated that the progenitors of the Telai
were homeless within five thousand years of their departure.
But that being said, the history of the recorded human-dominated period on Earth is relatively well-studied and curated for something so old. The biographies, works, and cultural contexts of many historical figures are preserved, and have even been translated into modern and alien languages to facilitate understanding. (The Pesenese
fondness for quoting mid-Holocene astronomers like Carl Sagan and Johannes Kepler is particularly stereotyped.) Video and other primary sources are not as well-preserved, but a rich catalogue of media can still be assembled with enough research.