Understanding the diversity of life has always been of interest to scientists. Finding order in this diversity can help make sense of the world. Biological classification involves ordering life-forms by identifying different traits in species, and grouping together those species with common traits. But there is a huge range of traits which distinguish one species from another - some more meaningful than others. Taxonomists of all species must decide which is the best for their own particular circumstances. We could classify all animals by numbers of legs. This would put crocodiles together with cats, octopuses with spiders, and humans with ostriches. (This depends on recognising what is a leg in the first place - as distinct from an arm or wing or tentacle).
Or, we could try classifying according to presence or absence of fur or feathers - but this could end up putting humans in with the octopuses.
Another way would be to divide all animals into carnivores and herbivores. But this would split humanity into at least two camps - depending how many sub-species of vegetarian and vegan we cared to recognise. This may not be the most useful primary division of life on Earth, except perhaps at mealtimes.
It seems that it is not enough to simply recognise similarities and differences, but somehow to organise these in some sort of hierarchy, so that the most important distinctions form the main structure for classification. For example, whether one eats plants or animals would come somewhat down the pecking order compared with whether one is a plant or an animal.
Fortunately, we do have a systematic way of organising all life on the planet. This is based on how different species are related to each other in an evolutionary sense: the genealogy of all living things. This puts the crocodile in with the ostrich, and the human with the cat.
So, any living species, however bizarre, can be categorised precisely because all are related in a systematic way, through common ancestry. This taxonomy excludes mythical creatures such as chimeras and griffons, but ensures that catfish, horseflies, elephant seals and rhinoceros beetles are all correctly placed in the grand scheme of things. Taxonomy according to evolutionary ancestry has become a standard means of classifying all forms of life on Earth. However, this does not mean it would be a universally useful method.
Elsewhere in the universe, life-forms may not all be conveniently related, as they are on Earth. Indeed, if the life on Earth is peculiar to this planet, then any extraterrestrial life-forms must be unrelated and in a separate category, for a start.
In alien worlds, there may not be a single 'tree of life' in which everything living is related to each other. Perhaps on some alien world the raw ingredients of life may be promiscuously configured in many possible arrangements, giving rise to multiple 'births' and 'extinctions' of different kinds of life. Perhaps a multitude of species of 'trees of life' coexist in an exotic forest of unrelated lineages.
When we put together all possible kinds of worlds, each with their separate biologies, then classification strictly by lineage would generate a fragmented archipelago of unrelated systems: a taxonomical muddle of biological loose ends. We would not be able to relate some real extraterrestrial to our Earthly biology with any more credibility than a mythological terrestrial one. All life-systems would effectively be separate, and each might end up having to be classified by astronomy - with all Earth dwellers lumped together as a single planetary family.
In the circumstances, an alien taxonomist may feel that counting legs is, after all, as good a way as any of usefully distinguishing similar 'species' across the light years.
And so, while taxonomy by genealogy may be clear and purposeful for Earthly biology, it is not the only one 'true' way of doing it. The scientist will use different systems of classification for different purposes, whether this means using genealogy or astronomy - or, for that matter, gastronomy.
The cod and squid may be less closely related to each other than the cod and human. But when it comes to mealtime, the hard-pressed alien scientist going to the supermarket on the way home from the lab will be pleased to find all 'seafood' conveniently located in the same corner of the store, just as 'vegetables' like chilli peppers (fruits) are not far from the mushrooms (fungi), and all species of tempura are placed together in the snack section. There is more to taxonomy than arranging chickens next to eggs.
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