An evolutionary timeline would not be instructive in identifying alienness, because then all life would be alien. (or no life would be alien) There was a time before which there was no life, according to mainstream biological thought.
This leads to the point that alienness is a human construct. Alienness is not recognized by nature. New species, possessing never-before-experienced DNA may enter an environment in many ways, evolution, mutation, various modes of locomotion. It may exert a variety of effects, again evolution, mutation, extinction, or even a combination of changes. But nature does not discriminate among causes, only among the results.
"This is alien, but this is not," is also a distinction that humans make. Animals, like dogs, may be curious about a never-before-experienced odor or life form, but once their curiosity is satisfied, it is back to life as usual.
Modern humans obsess over alienness. Sometimes, if it can be monetized, alienness can be marketed as a new and prestige product. Japan is a fine example of this. The Japanese ability, on a large scale, to import previously unexperienced foods began after WWII, during their spectacular economic rise. Japan began importing fruits from other countries. Once inaccessible, tropical delicacies, such as bananas, became commonplace. Bananas are not grown domestically, and once very expensive, are now the cheapest fruit available at most markets.
Kiwi fruit is another example of a fruit that was imported and began as an expensive treat, but now, in addition to being imported, kiwis are easily grown domestically. Similar to the distinction people make between crops and weeds, gai rai shu is never used for a crop, like kiwi, only for weeds.
When it comes to people, the distinction of being an alien becomes a tricky one. In the feature film "Men in Black," we were introduced to an Earth where people are not as they seem. Clerks in shops and even Hollywood stars and politicians are exposed to be aliens, not from other countries, but of the extraterrestrial variety.
We never gave a thought to their alienness. They were entertainment brands and local fixtures. No one knew, and then after they were exposed to us in this fictional portrayal, it brought a laugh, not condemnation or fear.
People are alienized by other people, based on characteristics that are multitude, and for reasons that are equally varied, but making people into aliens endangers more than their images. Humans who are alienized based on an association with the geographical location of their birth, most often referred to as nationality, and are characterized differently than plants. If plants serve a purpose, as in the case of the kiwi fruit from above, they are not referred to as invasive species, or alien. People are alien until they petition and receive acceptance from a government. This is often referred to as naturalization. A bizarre term in itself, as it is a biological term that means,
a process by which a non-native organism spreads into the wild and its reproduction is sufficient to maintain its population.
People are labeled alien regardless of their contributions to society where they exist. This nationality is an arbitrary label, controlled by nation states for the purpose of controlling people.
People are necessarily harmed by this distinction, as was shown by Howard Saul Becker in his book, Outsiders, published in 1963. Becker's labeling theory holds that,
Deviance is not inherent to an act, but instead focuses on the tendency of majorities to negatively label minorities or those seen as deviant from standard cultural norms.and that
A stigma is defined as a powerfully negative label that changes a person's self-concept and social identity.Alien is a powerfully negative label, and changes a person's self-concept and social identity.