Everything we know about life – what it is, what it looks like and how it works – has come from studying life on our planet.

This creates a problem for those of us who look for signals from extraterrestrials.

 How can we plan to communicate with an extraterrestrial intelligence, when we’ve not even found evidence for the existence of any form of alien life at all?

You might think that alien life will be very different from life on Earth – very alien. But that may be a little pessimistic.

While it’s good to be cautious and to remember that our experiences on Earth may not prepare us fully for all the weird and wonderful possibilities that exist across the Galaxy, it’s also true that not everything is possible.

Just as the universal laws of physics constrain the nature of planets and stars, so the laws of biology constrain what kind of life is possible, and in particular, what intelligent life must be like.

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