Nega Earth is the name of an alternate dimension where the 7 Element Hunters travel as they strive to rescue the earth. It is primarily composed of nega elements (similar to anti matter). They go there to retrieve posi elements which were transported there via element dematerialization.
Nega earth is populated by a variety of living creatures, many of which share similarities to Earth creatures. While it is in many respects, very similar (having oceans), it is revealed that unlike Earth where life originated in the sea, life here originated in magma.
The biological balance of Nega Earth is altered by elements that travel back in time from earth, so that when the Element Hunters travel there in present times, the posi elements removed from earth have had millions of years to incorporate themselves in the evolutionary biology of the life forms there. These creatures somehow return to normal examples when these elements are removed.
This explanation is sometimes ignored in the story, such as when NeoQ is mated to a non-NeoQ even though they should be incompatible for breeding purposes or coupling due to their radically different biologies.