"Earthly" Characteristics
Earthlings / the original population of the planet. Matter / Heavy as soil. "I felt the earth as part of myself." They survive only if they are close to the earth. They do not understand aliens.
Characteristics: They are afraid. They are comprised of "strong" material and base. They are fast, clever, witty. These types are very rare now, and almost none are completely pure earthly beings. They adapt to their geographic location and can only reside there. Well-versed in only their own specific climate, location, geography, etc. Aggressive. 
"Alien" Characteristics
Within the Earth's material
Creatures from other universes, planets. The majority of the population is of extraterrestrial origin. Millions of these types, and all carry different levels of information. Different set times for life longevity (some live longer, some less). Identify with Earthly material on the surface to blend in. Different cosmic backgrounds (planets, cosmos, etc).
Higher: Beings who have matched their cosmic energies with the earthly energies help with the evolution of the Earth, their material. Creators.

Not all beings can exist in matter on earth. Many species are currently learning how to align their matter and energies with the matter of the bodies of the Earth.


Why they are strong / their structure, their tasks, how long they will be strong

Creatures with the body is a robot. They can be added to circuit boards or a type of material. They are created to complete missions or complex missions that require characteristics that cannot exist, etc., or cannot so quickly.

Cannot create nor destroy- no effect on the Earth's material.
Observers. Collect information. Military, educators, psychologists, geologists, etc. Example: Putin

Imitators of all Earthly and Alien characteristics- do not have their own. Tend to be strong beings with special, tough, Earthly missions, or with the easiest missions. Can never replace the true "Aliens" with Earthly material, as they rely on other beings. Therefore cannot be creative.
Artificial intelligence / Robots
"Aliens" without Earthly form
Without matter that can live on earth Many species that are around the earth that fulfill their missions through the beings that can exist on Earth