Our race is not dying. We, the Zetas, have been incarnating and reproducing for almost 8.5 times the length of time that intelligent humans have been doing so. And there is no end in sight. Yet where we can continue, we desire change. Humans have many characteristics that we, the Zetas, previously had in richness - sex drive, rage, longing, hunger. We perceived these to be a root cause of the steps we took to ruin our planet. We genetically engineered ourselves to rid ourselves of these characteristics. In doing so we threw the baby out with the bath water, as you say. We went too far. The richness is gone, and our life is more intellectual than physical. We wish to incorporate into our incarnation experience the depth of sexual desire and emotional commitment that humans are capable of feeling. This we had in the past, and we wish to re-introduce it again. Our genetic engineering did not result in the confusion that all the many passes at Homo Sapiens have caused for your race, as we were originally a single transplant from another world. Our unique shape has come not so much from our original model as from what we have done to ourselves. Being focused on war, and success in the battlefield, early genetic engineering was focused at making a lean, mean Zeta. Strong, requiring little nourishment and no sleep, and forthright.
In our past we poisoned our planet as a result of nuclear and biological warfare. Biological elements proved to be more tenacious than radiation poisoning, and it was for this reason that genetic engineering was undertaken aggressively when we were forced underground. At first genetic engineering was to create a more impervious soldier, but later these skills were put to other uses - creating a pacific Zeta. Genetic engineering did not take place on a universal level among the Zetas. Where some groups were hard at work trying to create a Zeta that could live above ground, among the poisons we had unleashed, other groups were trying to remove what they viewed to be the source of the problem, our warlike nature. Over time, through interbreeding, these traits spread throughout the Zetas, just as the human race, through interbreeding, is tending to become homogeneous. In some cases the scientists trying to create a better soldier had a change of heart, and switched over to the group trying to pacify the Zetas. The Zetas resulting from these joint efforts proved to be most viable, and over time that model became dominant.
There are, of course, many variations on the Zeta form, and many of these are due to the genetic engineering efforts. Line them all up, and you might be able to say that this one is the original Zeta, this one the better soldier, this one the pacific Zeta, and in like manner proceed through the genetic engineering experiments. Some variations in Zetas come from different environments, from living on different planets, but most come from this source. The amphibious water Zeta, for instance, was the result of an attempt to move the Zetas into the water, for protection from the poisoned air.