Where carbon and silicon serve as the base upon which life is built, throughout the Universe that we, the Zetas, are cognizant of, there are other variants. Many elements you, on Earth, consider to be trace elements are common on other worlds. An example is the element lithium, which is so remarkably soothing to those humans diagnosed as bipolar. It is no accident that these individuals are found to have a higher incidence of creativity, energy, and intelligence. It was for those qualities that a genetic contribution was made from worlds where lithium happened to be a common element. Doctors are not curing a disease by medicating with lithium, they are correcting an imbalance.
On some worlds iron is a trace element, and if carbon based, the blood does not rely on iron in the manner that creatures on your Earth do. There, they do not develop iron deficiency anemia, but develop a similar reaction to a lack of silicon, the most common substitute for iron in this matter. Of course, the manner in which blood chemistry works in these instances has more differences than parallels. Humans are sometimes shocked to learn that aliens have green or clear blood, and correctly assume that the blood chemistry is radically different. One reason that we, the Zetas, were chosen to create a hybrid race with humans was our relative compatibility in this area. Our home planets had iron in abundance. Should one doubt that elements vary in their proportion throughout the Universe, one has but to look to meteors, where nickel is sought. Beyond nickel, look to what variance is found between the substances in meteors and your average hunk of Earth, or for that matter, what variances are found when comparing one meteor against another.
Perhaps, to humans, the most surprising difference would be that oxygen is not the breath of life to all, as it is on Earth and to those life forms commonly visiting Earth. Why else would some of your ancient visitors have worn helmets? On their worlds, a helium component is key, and on the surface of your Earth they are drowning for lack of this as surely as you drown when under water, away from free oxygen in the proportion you require it. We have mentioned that some worlds would be considered radioactive by you, yet life lives there. How has their life adapted to the bombardment that disrupts your processes. Their chemistry, in fact, requires this bombardment, in the same manner that you require sunlight in order to manufacture vitamin D, without which your bones go soft. Life evolved on those planets while this situation existed, and DNA that learned to utilize the environment, rather than sicken from it, was the survivor that propagated. Just so many variations developed, on many worlds, and their differences are greater than their similarities.