if we found aliens would they get their own kingdom? Or would we put them in the existing ones?
Wouldn't that depend on their biology and degree of connection to us? If panspermia is correct we might find the aliens to be closer cousins than otherwise. It's hard to imagine that convergent evolution, on the other hand, would reproduce all the details we use to define our own categories.
I believe that life in other nvironments would be simaler to life here on earth, and we'd probably have a classification that fits them.
add: only that people out there might not have looks like you and me
Assuming the aliens shared no common ancestors with any Earth life, it seems to me that they would need their own kingdom.
If you think about it, life here had to develop under certain conditions, and all abiotic factors lead to the different kingdoms. Because to be able to live this long and adapt, evolution had to occur at some point. So if there's a habitable place somewhere else in the galaxy, they'd have to follow the same evolutionary basis, and therefore, follow the same biological rules.
Our evolutionary "tree" (with the kingdoms at the foundation) is a representation of ancestry and common descent. Unless, as SelfAdjoint said, panspermia is a factor, then the alien species would have a separate biogenesis and evolutionary history. Therefore, it would have a separate tree.
However, I suspect astrobiologists would use similar classifications/terms for alien species (bacteria-like species, plant-like species, animal-like species, etc.) in order to facilitate our understanding of them.
I assume something totally new (e.g., silicon-based life form?) would get a new kingdom.
I guess it would all depend on their origin, and how/who we would choose to classify them. It's possible that the conditions of Life on Earth are the only way for life to start, it is also equally possible that under different conditions life could form. Carbon is the essential atom on Earth, but there is nothing preventing another planet from being based off of some other element, known or unknown.
You can find people to argue about classification of species on Earth, let alone what would happen regarding life from another origin.
We could even find a different type of "life" one that follows different biological rules.
They would get a totally new independent tree of evolution.
Sicilon life would work but has great disadvantages compared to carbon based life.
But alien carbon based life can still be totally different from life on earth in very many ways. How different or how similar is hard to say. And this goes for all kinds of levels of organisation.
Also, we might get boxes with trees. Every time of enviroment or planet type could get its own box with several independent trees. Or maybe organisation based on structure would work better. These things are hard to tell. We cannot look into the future or guess right for a 100%.
Aliens would NOT be classified into an existing kingdom. The biomes would not be similar enough outside of earth for aliens to grouped thus. Remember, if there were aliens, it is not guaranteed that they would even be able to survive by breathing oxygen. They would have their own genes and traits suited to their specific home environment.
I think first we need to define what we mean by "alien".
In the case of "panspermia" or "exogenesis", then there would be some "common" ancestry, however I would agree, not enough similarity to be grouped into an exisiting kingdom, as you, selfadjoint and others mentioned. Of course there are some that say we are a breeding experiment of aliens.
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