What is the likelihood of actual rubber forehead aliens?

[pilight]

It's likely that the vast majority of life possessing worlds out there have nothing with greater than animal intelligence. It's certainly not an inevitable result of evolution. Dinosaurs were the dominant life form for 200,000,000 years here, on a planet we know has conditions suitable for the development of intelligent life, and never made a bit of of progress in that direction.

[clearspira]

It's likely that the vast majority of life possessing worlds out there have nothing with greater than animal intelligence. It's certainly not an inevitable result of evolution. Dinosaurs were the dominant life form for 200,000,000 years here, on a planet we know has conditions suitable for the development of intelligent life, and never made a bit of of progress in that direction.

Very true. That is one of the reasons for my list. I think that the OPPORTUNITY to become intelligent is far more important than the ABILITY to become intelligent. Tool using appendages is an opportunity to shape the world around you and learn for example.

I think it was Chuck who pointed out in the ''Are We Alone'' vid that intelligence is by no means the end point of evolution as it conveys very few obvious survival advantages. If we look at bacteria for example, they are older than anything on Earth and will most likely be the last thing living on Earth.

[Riedquat]

I think it was Chuck who pointed out in the ''Are We Alone'' vid that intelligence is by no means the end point of evolution as it conveys very few obvious survival advantages. If we look at bacteria for example, they are older than anything on Earth and will most likely be the last thing living on Earth.

If you take a very broad sweep... The bacteria today are different species to the bacteria from millions of years ago. But in any case the ability to think your way out of a problem or work out ways to get to additional food sources very much does have survival advantages. "Be incredibly numerous and fit in to lots of environments" also works, but intelligence certainly helps.

[Artabax]

For sapient life, OP's list is close: language is necessary, speech less so. Sign Language is a thing. Deep Sea fish do flashing lights, Morse Code is a thing.

Most biophorous planets are just bacteria and blobs, few have space-rabbits and space-crocodiles and interesting animals, even less have space-people even fewer have space-gods. It is known.

POV Science: Watson Bacteria planets are MOST of the Life we will evah find.

POV Story: Doyle Earth-people meet Space-people, that is the easiest and commonest story; Earth-people versus dangerous Space-rabbits and Space-Gods are rarer. Earthers versus something truly alien is difficult to make a good story.

Extra arms, legs, wings, tails, willies would be better, but each would require more brain and we need sapience which requires lots of brain.

[Al-1701]

I suspect complex animal life would favor tetrapods (or perhaps hexapods). The more limbs, the more brainpower is relegated to controlling the body. I agree the humanoid form is simple which gives it a natural advantage in evolution. So, if we did find intelligent life, it would probably have a similar form.

Of course, it depends on the planet. A warmer, drier planet would yield a race more like our reptiles. A colder planet would lead to a stouter life form covered in hair or having an insulating layer of fat. On a wetter world, amphibians or something like lungfish might be dominant.

However, I think what's important for intelligence to develop is the need to see things differently. Some suspect early humans were scavengers that would go to carcasses picked over and break open the bones to get at the nutrient rich marrow. Most other predators and scavengers ignore the bones, so knowing there is something to eat in them was an advantage. Having to break open the bones might have required the development of tools to break open larger bones. Communicating when the other predators and scavengers have left would lead to language. Also, this is during the periodic ice ages when weather patterns could shift and adaptability within a generation would be key.

As for intelligent herbivores, the closest we probably have is the beaver. They build structures, plan for the future, teach skills to their young, and can adapt their behavior to accommodate the environment and resources available. And all of this is to frustrate animals that prey on them. They aren't nearly on our level, but you could take them to their logical conclusion to have an herbivore with human level intelligence. You just need plant material rich enough in energy to power their brain.

[Darth Wedgius]

A somewhat germane video from Isaac Arthur about technology developing without fire:

https://www.youtube.com/watch?v=q8uJ2int43Y&t=1072s