Science in Fiction: Humanoid Aliens – A Case of Convergent Evolution

Either out of laziness or convenience, it is very common for science fiction writers to create aliens who are very similar in appearance to humans.  In science, this similarity is called Convergent Evolution.

What is Convergent Evolution?

Convergent Evolution is when similar form or function evolve completely separately.  On Earth, a commonly used example are marsupial and placental mammals (marsupials are animals like kangaroos).  There are a lot of examples of convergent evolution in the wikipedia link above.  See also some examples in the photographs at the end of this document.There are some animals that clearly evolved separately because their young develop very differently, but look almost identical.

The Devil’s In the Details

Marsupial Mouse

When writing the alien species in The Alien’s Pet, I focused on the differences, specifically how those differences might have evolved.  In the story, the species has purple skin.  In this instance, the purple skin is likely caused by one of two different background elements in my story: natural selection for a camouflage advantage (tree bark and some leaves are purple on this planet) and plant life stores a lot of a purple byproduct from one of the resources commonly available on the planet (sometimes these unusable materials can be stored in the cells of animals and plants rather than excreting them).


In general, when planning out traits for a species, assume that the trait would create some sort of advantage for the species.  Common advantages include:

  • Camouflage/Hiding
  • Defense (e.g. spines on a cactus)
  • Offense/Hunting (e.g. claws, speed, strength)
  • Intelligence/Tool use

On an ice planet, white skin or fur would have advantages.  Appendages like those in octopi can be surprisingly agile and alteratives for opposable thumbs, with the ability to get in small places in ways that joint-based (think elbows) cannot.  Sadly, I can’t find the video my dad showed me a few years back of a robotic hydraulic arm that worked like an octopus tentacle (there are a few examples online but that video was especially demonstrative).

Don’t Fall into These Traps

I read a lot of SciFi, a lot of SciFi romance, and after a while, it all looks very similar.  All the alien men are tall, muscled, and dominant as hell.  Most of them look just like humans with only minor variations, usually associated with whatever fascination the author has (e.g. horns, wings, etc.). Hell, most of them seem to enjoy bringing their female over their knee and spanking them. Which is odd, because BDSM is a minority fetish in sexuality but it spells the majority of SciFi romance.


  • Give yourself a good reason for each trait your aliens have—physical, psychological, and societal
  • Develop more than just that alien species.  What is their environment?  What did they have to battle/survive to evolve to the point they are at today?
  • Look at examples in nature.  There are wonderful ideas out there.
    • Just the other day, I was annoyed that all aliens in books seemed to have the same “package,” so I searched different types of animal penises on Google.  Well, truthfully, I didn’t include the animal part the first time, and I truly regret that.  I really didn’t want to see that…
  • Ask a child (if one is available).  Nothing tops the imagination of a child as they’re not limited by the constraints we build for ourselves.
  • Read other authors in your genre.
    • What are they doing right?
    • What are they doing wrong?
    • How would you fix their books?
    • What new spin can you put on what they’ve created?
    • Learn from their mistakes and example.


  • Copy what everyone else is doing.  They may be successful at it, but at the end of the day, the reader won’t remember your name.
    • I recently read a 23 novella anthology of Alien “Alphas.”
    • The only story I can really remember is one where an alien race infiltrated the Earth, invading by manipulating the Earth’s reaction to them so everyone thought it was okay.  The aliens could control minds and had shapeshifting dicks with tentacles around it.
    • I wrote down the name of the novella and the author’s name and immediately tried to find if it was part of a series. Sadly, it wasn’t.
    • The story was called “Strangeways by Addison Cain.

Are you working on an alien species?  What are you doing that’s unique?

Discover more from Danielle Forrest | Sci-Fi Romance Author

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