When someone says that a process happens ad infinitum, it tends to imply that the process happens again and again, carrying on into the future. But how do you talk about something that has been happening again and again, back into the past, i.e. an infinite regression? Is there a Latin phrase which encapsulates this meaning?
Consider recurrent, perennial, and perpetual.
Recurrent has senses including “Recurring time after time” and “Running back toward its origin” that taken together might suffice.
Although the etymology of perennial includes an element annus, or “year”, perennial also has (since 1750) a sense of enduring or permanent, and wiktionary shows figurative senses that include “Continuing without cessation or intermission; perpetual; permanent; unceasing; never failing” and “Recurrent; appearing or recurring again and again”.
Macmillandictionary.com shows senses of perpetual including “happening so often that you become annoyed as a result [eg] He soon grew tired of her perpetual demands for money” and “literary, continuing all the time [eg] Many people live in perpetual fear of losing their jobs”.
Also note: With its sense of “Endlessly; for ever; neverendingly”, ad infinitum perhaps can refer to a perpetual series of cycles in the past as well as in the future.
How about ab initio?
Wikipedia has it as:
L., lit. "from the beginning", from ablative case of initium "entrance, beginning", related to verb inire "to go into, enter upon, begin".
It's often used to signal that a certain activity has to start again from the top, but perhaps it matches what you're after.