The reason why some aliens are more likely to conquer Earth than others could come down to just one thing, an expert has said.
UFO researcher Philip Mantle has suggested that while depictions of alien lifeforms in film and TV took humanoid form because of the limitations of special effects, it may not be that far off reality due to one thing.
Humanoids are frequently seen on the big and small screen, as for a long time extra terrestrials were portrayed by "men in suits wearing plastic heads", said Philip, as production teams were restricted by the technology available at the time.
As the world celebrates Alien Day, Philip acknowledges that while aliens could take many forms, one species of alien could be more likely to colonize other planets over other alien species for the same reason humans are on top of the food chain here on Earth - thumbs .
Without our thumbs, we'd be useless", Philip said, adding that thumbs have enabled humans to create agriculture and build constructions.
He suggests thumbs set humans apart from other highly intelligent animals who are limited by their physicality – and it may not be that different for aliens.
Philip said: “There's an argument that when it comes to mother nature, it's the same irrespective of where you are in the universe, so maybe humanoid form is the perfect form for conquering another planet, manipulating it and then leaving the planet, like we have done.”