sentient aliens are probably terrestrial

Almost three years ago, during the summer of 2013, I wondered whether it was a coincidence that the best tool-builders ever were terrestrial creatures. I wanted to know whether others had thought about it so I asked the question on Quora. If we ran thousands of monte carlo simulations of evolution on Earth-like planets for billions of earth-years would we almost always end up with terrestrial creatures being the best at invention?

Defining the initial conditions for such a monte carlo simulation is probably not trivial but as a first approximation I would define the scenario where everything on these planets is a constant except the spatial distribution of  hydrothermal vents. Having said that, we can’t currently create such a complex monte carlo simulation, so I’ll focus on my reasons for believing that sentient aliens, if they should exist, are probably terrestrial:

  1. locomotion:
    Under water, it’s very difficult to move quickly between any two points without a tubular body. This makes it difficult to simultaneously develop limbs for grasping and move around quickly as the limbs would impede motion. This would compromise the utility of a social organism as a mobile sensor for a variety of raw data that is also capable of computation.
  2. data storage:
    It’s very difficult to store data by any means under water due to currents. And here we have a serious problem. In order for an organism to be able to store data it must be a creature that lives on the ocean floor and it’s more likely that this organism should try to store data in shallow waters as visibility is much greater there compared to deeper waters. But, as everybody knows, currents are very strong around beaches so this would make any form of data storage in shallow waters much less practical than data storage on land.But, what about lakes or lagoons? Well, it’s certainly a possibility but due to the fact that weather patterns are quite variable it’s unlikely that intelligent life would have as much time to develop as terrestrial life developing at similar or greater rates. It’s very likely that the lakes or lagoons would dry up and they would go extinct unless they developed a pair of legs and eventually became terrestrial.Data storage is important as this is what makes the strength of any human civilization. If it were not for knowledge transmitted across generations, the current batch of humans would probably be no more able at reasoning than cave men.
  3. fire:
    Fire is essential for kick-starting an industrial society which depends on the transformation of readily-available materials in order to suit our needs. Whether these happen to be metals, plastics, ceramics…great heat is necessary in order to encourage deformation of these materials.Granted, there are hydrothermal sources in the ocean but it’s practically impossible for a very clever organism endowed with limbs, even an octopus, to ‘carry’ hydrothermal vents with them.
  4. computing:
    There are many ways to create practical computing machines that would somehow help for data-processing. The earliest computers were mechanical like Pascal’s calculator and not electronic. But, even if a very clever species of aquatic alien should somehow succeed in building mechanical computers I don’t see how they would succeed in building much more powerful digital computers.Moreover, I believe the probability that its terrestrial analogues should be far more advanced in computation technology is almost certainly 1.
  5. space exploration:
    Resistance to aerial motion is much greater in aquatic environments than terrestrial environments on earth-like planets. This would make it very difficult for even a very intelligent aquatic species to start exploring space even if they should somehow overcome the four previous obstacles. For this last reason, an aquatic civilisation, if it should exist and remain aquatic, would have a strict upper-bound on its ability to learn about the universe.

I must say that an implicit assumption I made in my list of points is that sentient aliens will probably be social organisms. My short argument is that it’s much more likely for an intelligent species to survive if its members work on problems using different approaches(i.e. algorithms) in parallel in order to find a solution to that problem in time. I can say more about this, and I might, in a future blog post.

Meanwhile, I’d be interested to hear what an astrobiologist might think of my arguments.


  1. The last point was added after a brief exchange with an astrobiologist at Edinburgh,Charles Cockell, who went through the points on my blog.
  2. You may verify that the Quora question was added three years ago by checking this link.