The first thing to remember is that aliens are alien. That may sound obvious but it is amazing how often it’s forgotten. Often aliens are just humans but with different skin. Their senses will be different (e.g. could see a different part of the spectrum to us), their morals and social structures will almost certainly be different. We may not even recognise the way they communicate as language. But the biggest thing to be aware of is biochemistry.
Aliens will have a different biochemistry unless life on one planet was seeded from life from another. (Quite likely within the one solar system, if we ever find life on Europa or Mars it will be interesting for lots of reasons not least to see how related they to life on Earth).
Consider the reasons why aliens could differ.
Let’s start off by getting rid of the more extreme possibilities first and assume that they are carbon based and use water as medium, (and not something like ammonia).
All life on Earth uses DNA as genetic material which is transcribed into RNA (similar but slightly different) which is then transcribed into proteins.
First of all there is no reason why all of the above should be true, and even if alien life does use DNA, RNA, and proteins. The Amino acids used would almost certainly be different and would not match the twenty-two used by Earth’s life. And then if they were the same, the genetic code that translates RNA to proteins would differ.
Then there is the fact that amino acids and sugars can have to two forms. In biology most amino acids are of the L form and sugars are D. There is no reason why other life could not use the opposites.
What I am trying to point out is if life evolved somewhere else then it would almost certainly be incompatible with Earthly life so it would be very unlikely that we would be able to share an ecosystem. One species’ food could be deadly poisonous to the other.
Even if all the above problems were overcome then we would certainly not produce viable hybrids. Mr. Spock could not exist.
Of course you can ignore all of the above for a good story, just as long as you’re aware of it.
Tom Greenwood is a married father of two living in Edinburgh. He has a Zoology degree and had what he thought was a good idea for a book which he has written in his spare time. He has not done all that much interesting in his life, perhaps now he has written a book that will change.