in most science-fiction stories that include alien races, said aliens have a roughly humanoid appearance (being an erect bipeds). now while it's likely that the vast majority of sapient life (beings with human level intelligence) wouldn't resemble us that got me thinking on a earth a like planet does the humanoid body plan necessitate sapience to be viable?
does the humanoid body plan necessitate sapience?
I think it does for a successful species, or at least that it strongly favor developing it.
Humanoid body plans have
- no strong fangs
- no strong claws
- poor bite strength
- poor peak velocity. Usain Bolt can be taken over by almost any ruminant in the savanna
- poor insulation from outside environment, due to the lack of fur
The only way for such a "miserable" design to not be trashed in the evolutionary competition is to be sapient. The ability of making tool and master fire has allowed these naked apes to spread all around the globe. Else they'd be just an inglorious part of the fossil record.
Walking upright is strictly inferior except when you're smart enough for tools
The lack of killing power (strength/claws/fangs/bites) L.Dutch identifies would indeed prevent a solitary modern human from living very long in the wild if they were not sapient. But lack of fangs and claws is not what makes a the humanoid body plan unique, nor would a human with claws and fangs be very successful without sapience.
To determine whether a stronger human with fangs could be successful, we can compare with monkeys and apes in particular. Apes like chimpanzees, gorillas and even baboons have a lot of killing power with large fangs and enough strength to tear off your arms. But that clearly hasn't allowed them to compete effectively away from forest environments. Baboons have made it the furthest, having made their homes in the Savannah, but they need large groups, tree sanctuaries and a decent top speed (45 km/h / 28 mp/h) to get away from lions, hyenas and the like. The baboons main defense is the threat/bluff of inflicting serious injuries with their huge fangs, but actually taking on a large predator is suicide.
Besides sapience, the main difference is that humans are built to walk upright. This would clearly make the human body plan inferior to apes in the forest. Humans could do well for themselves in times of abundance, but would have trouble when food gets scarce since everything can move through the trees more easily than humans and humans couldn't effectively challenge apes.
Away from the forest, walking on two legs being more advantages. Humans can look over the tall grass and have very good vision compared to most animals. They also have endurance on their side. But walking upright means running slower, being less agile and having a harder time actually making contact with the prey.
On the hunting side, Husain Bolt could probably overtake a baboon. I seriously doubt that he could turn as nimbly on two feet with his high center-of-mass as a baboon could on four limbs being much closer to the ground, so getting within grabbing distance would be much harder than the speed comparison indicates. (Compare videos of a chase by any predator with a sprint race to see this for yourself)
Finally, our hypothetical clawed and fanged Husain Bolt has another problem: Those claws and fangs are in a bad position to actually strike with some force. Swiping at the prey hoping to unbalance them would probably be the easiest, but requires bending the upper body forward or sideways, unbalancing the human as well. Even a headlong all-or-nothing dive forward is more difficult since the body needs to be moved into the jumping position first, unlike any four legged predator whose body is in the correct position by default.
All non-Husain-Bolt humans of course wouldn't stand a chance. A week old gazelle probably runs faster than most humans. That leaves scavenging for food which brings the next problem: Keeping that food and not becoming food.
Not being eaten
On the defensive side, a human can't hope to outrun most predators, not even Husain Bolt. Again, not being able to turn quickly is a bigger problem even than speed. Having four legs is simply better.
So, bring in the claws and fangs and stand your ground. Our clawed human makes like a baboon and lets any predator know that it'll bear the scars if it tries to eat them. Finally there's an advantage: Both arms are always free and have good reach to an opponent's head/eyes. The arms can also be used to block and grapple. Unfortunately, standing upright also means humans expose their vulnerable bellies when facing an enemy and lower legs are much harder to defend. Compare this to any four legged animal where only the head and front legs are exposed and you see that without armor and weapons the humanoid body plan doesn't work well in a fight.
To summarize, a non-sapient humanoid walking on two legs could perhaps find a niche like baboons, scavenging and using numbers for defense, if they had fangs/claws to threaten injury to predators. They would however be inferior to baboons and all four-legged animals and likely go extinct if adverse conditions increase competition.
In any other niche, individual humans would be easy prey in a world with lions, wolves and other predators.
On Earth, there are many species of mammals with bodies large enough to have large brains.
A subset of those large enough mammals actually have large brains.
There are about a hundred species of mammals with large brains, including Homo sapiens, at least four species of apes, three species of proboscideans, and about 90 species of cetaceans.
The intelligence levels of various species among them are rather uncertain, but it is possible that at least some of the non humanoid proboscideans and cetaceans are equal enough in intelligence to Homo sapiens to count as intelligent beings and people.
Thus it seems uncertain, from the evidence on planet Earth, whether a humanoid body plan is necessary for intelligence.