The Alterra are an advanced stage of Human evolution, or rather Humans are a primitive form of Alterrans. Both share the same basic appearance, but there the similarities end. Alterrans are what present day Human culture would refer to as 'Superhumans,' and what past Human culture would refer to as 'gods.'
The Alterra were wiped out by a plague several million years ago, but due to an ingenious plan to circumvent their doom, their civilization was restarted using a Repository of Knowledge along with the creation of many Seed Species before the Alterra's eventual demise. The combination of these two projects resulted in the creation of a new Alterra, Ryan Stevenson, from the seed species of the Avalona galaxy. Through him, the Alterran civilization is once again on the rise.
Discovered after the fact, the plague that resulted in the death of the Alterra was in fact a bioweapon created for their destruction by the Ascended Empire. The Empire feared the Alterrans' power, and it remains to be seen how long the rebirth of the Alterran civilization will be tolerated before the Ascended Empire chooses to intervene once again.