Scientists would have you believe that life can only exist on worlds that are in some way similar to ours. There needs to be an atmosphere and water, apparently, for there to be any chance at all.
I’ve never agreed with that point of view. Maybe our type of life requires those things but the universe is a vast place and to narrow down the possibilities based on the restrictions placed on us is just crazy.
Even on Earth there are some crazy life-forms that scientists thought impossible until they went and discovered them. For example there are whole ecosystems surrounding deep sea hydrothermal vents. Yes, all those creatures require water and oxygen in one form or another, but my point is that nothing should be ruled out.
And those very same scientists, having told us that no type of life can exist without the things that make it possible for our lives to exist, then go and say that the odds of finding humanoid life on other planets is so slim as to be impossible.
I’m not sure I agree with that either.
So, I’m assuming we’re talking about Earth-like worlds here, because once we move away from them all bets are off. It’s completely possibly we wouldn’t even recognize life if that was the case. And I’m also assuming the aliens still have to do things physically. If they’ve developed telekinesis or something similar then, probably, anything goes.
And having made those two big assumptions, I think humanoid life isn’t all that unlikely at all. Let’s think about what it takes to gain self awareness and stay alive long enough to make use of it.
Firstly you need to be able to get around. All the brains in the world won’t help if any old predator can catch you.
This makes me think you probably need limbs of some kind. I know snakes get around just fine without any but, let’s face it, the odds are against them climbing the evolutionary ladder.
So, how many limbs? 1? That gives you a snake trying to stand up and I’ve already scratched snakes from the fight to rule the worlds. 2? A snake with an arm? Two legs and no hands? Two hands and no legs? How about one wing and one leg? Or a flipper and a leg?
So, I’d reckon 3 is the minimum umber of limbs you need but even that would be difficult.
If you start off with 3 legs and stick with that, you won’t be King of the World any time soon. So, a three legged creature evolves arms and hands of some kind, taking over from legs.
One leg and two arms makes getting around difficult. It’s possible but hopping around is awfully slow and awkward. In this situation using one or both of your hands to help with walking isn’t a good option either. How do you hold a weapon and your lunch all at the same time? Sliding like a snake has limitations as well. How do you spot predators above the grass and undergrowth (or trees, if you’re that big– which, of course comes with a whole new set of problems)? Or imagine trying to carry a bag of food while you’re slithering.
So, two legs and one arm? But that doesn’t work either. It’s all very well being able to hold something (opposable thumbs are an important development there as well), but making it interact with other things is important too. Flint is no good without steel. A long stick is no good without a nice piece of stone to sharpen the end. If you pick berries you need a basket to hold them. Or another arm to hold your baby while you’re doing it. Or do you need to stop your child from running away from the tiger and hold your spear at the same time?
Three limbs could work– there are a lot of people in the world right now who only have one arm and get along fine– but it would be awkward back in the stone-age when life was just a smidgen tougher and a little bit more physical than it is in the 21 century.
I’m no expert (perhaps I should have mentioned that earlier) but four limbs seems like the minimum optimum number to me.
Four legs give you a nice solid base to start with before you get your hands organized and then obviously gives you two legs and two arms which are all good as stated above.
How about 5 limbs? Or 6? It’s possible they would all work but (with reference back to the “I’m not an expert” comment) it seems to me that each extra limb brings an increase in weight, which means you need more muscle mass, which adds more weight which means… you see what I mean. I might be barking up completely the wrong tree (or making some other noise up what isn’t even a tree) there but I don’t think it’s a coincidence that pretty much (give or take the odd one like octopi– which live in the ocean and so isn’t as effected by weight as land based creatures) the only creatures on earth with more than 4 limbs are insects. We have animals as diverse as elephants and mice, crocodiles and koalas, birds and humans, and they all have 4 limbs.
So, humour me and agree that 4 limbs– resulting in two legs two arms– is the best option.
This results in an upright stature– that whole thing about carrying stuff and walking at the same time. It also gets away from the problem the snakes were having about being able to see predators above the grass.
Too small and you get squished. Too big and you need to find food. Or, more importantly, grow food once you move out of the hunter gatherer stage.
Two legs, two arms, walking upright. What does that sound like? Opposable digits are a given or it’s all just very awkward.
As I said, I’m not an expert. In fact I admit that I don’t know much at all about evolution. But all the stuff above seems pretty logical to me. I’m not saying I’d lay money on finding humanoid life forms on every earth like planet out there, I’m just saying I wouldn’t rule it out.