Ok so 3rd Earth is planet Earth deep in the future…so what time periods would have been 1st and 2nd Earth? At one time I thought the planet was being called ‘3rd Earth’ because its the 3rd planet from the sun, But I saw an episode where Mumm-Ra says something like “Ever since this was 1st first Earth…” http://thundercatslair.org/forums/images/icons/icon5.gif
The episode you refer to, Bengali, is “The Unholy Alliance”, the second episode of the “ThunderCats” cartoon. It’s where Mumm-Ra and the Mutants meet for the first time. Mumm-Ra says he knows the Mutants seek to possess the Eye of Thundera. “You know of the Eye of Thundera? Yess?” asks an amazed Slithe. “I have known of its power for a thousand years,” Mumm-Ra replies, “from a time when this was still First Earth.” Therefore, one can safely conclude that there was a First Earth and a Second Earth and that “Third Earth” means “the third planet out from the Sun” and “Third-Age Earth”. :bengali:
Unfortunately, the series never said how long it was between the three epochs, and I’ve never been able to figure that out myself. So, I’m sorry that I can’t help you there. 
Chris here…
My own theory is that the version of Earth that we’re living in now is “First Earth” (which is an impression I also got from some of the TCats stuff I’ve seen and read over the years), the early post-apocalyptic version would be “Second Earth”, and then when life began to re-establish itself on the planet, that was the start of “Third Earth”. However, like I say, that’s just my theory! 
Take care…
Without information nobody can do anything more than just theorise on what he was referring to. He could be referring to the life cycle of a galaxy, as in when a sun performs a super nova, the entire galaxy is destoryed and when the sun calms down again, the floating molten liquid of a planet begins to cool and go hard again.
Or he could, like Chris was saying, be talking about the number of collapses of major civilisations of the planet, so the destruction of ours, the destruction of the next and then finally the new world.
Chris here…
Well, in “The Unholy Alliance”, Mumm-Ra states that he’s known of the Eye of Thundera’s existence for “a thousand years, from a time when this was still First Earth” - so, assuming that the galaxy of Third Earth is “our” galaxy (which certain shots in “Exodus” would seem to support), I would have thought that a thousand years was quite a short space of time for two different super novas to occur - however, stranger things have happened! 
However, it’s from this perspective that I talk about the “First”, “Second” and “Third” prefixes tying somehow into an apocalypse - which does also explain how you have a variety of humanoid life living on Third Earth, from the regular human Warrior Maidens to sub-humans like the Wollos (and their donkeys! ;)) and the Snowman of Hook Mountain. You could theorise that these species are all basically the next stage of evolution from the humans of today, veering off and evolving into different races, with the differences in those races occuring due to the different ecological circumstances surrounding them.
Following that theory, it also tells us that in the event of a nuclear holocaust, the walrus will become one of the dominant species of this planet, rising up and naming themselves the Tuskas!
Take care…
I just noticed that in the “Lost Intro” video, when they introduce the mutants, the narrator says “They form an unholy alliance with the ageless devil priest of FIRST Earth, Mummra.”
So IMHO, the Earth (Planet wise) has always been the same, and Mummra has seen the rise and fall of the Dinosaurs plus humanity (First Earth, our time), Second Earth (A time of little to no life, perhaps an Ice Age), and then ultimately Third Earth (The settling down of the Earths environment to support life).
In two episodes they make references that give you a glimpse of the planets past (Third Earth wise).
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In the episode that introduces Hammer Hand, The Unicorn keepers say “I am afraid this world is no longer safe for Unicorns to roam free anymore” (or something to that affect).
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In the Ghost Warrior episode, the Bulkans state two things. First when they are considering robbing the tomb, they say “These are hard times we are living in” (Eluding to not much in the form of currency or valuable items), and also, they later tell the Thundercats that mining for Thundranium has been forbidden for thousands of years.
As far as the Unicorns, I think the keepers meant the land was no longer safe for them due to the arrival of the Mutants and the Pirates. But it also eludes to a past where Unicorns could roam free with out fear of these types of characters. Probably a very early time period in Third Earths history.
The Bulkans statements lead me to believe that early Third Earth life was good and prosperous, and perhaps there was even a governing body of sorts to watch over things. Once the evil characters started appearing though, espescially Grune, things started to deteriorate and life on Third Earth slowly got a lot harsher.
Its kinda funny, because its possible that Jaga is responsible for Third Earths bad living conditions since he is the one that exiled Grune, whom inadvertently landed on Third Earth.
Also, the fact that Queen Willa and her tribe are of human form can indicate evolution having taken place, or that somehow a small number of humans survived throughout Second Earth. (Supporting He-Fans theory)
Or perhaps I am just really bored at work today!