Comments

You must log in or register to comment.

jlpulice t1_jdtvhaa wrote

Because the trade off is extremely rare. Humans became bipedal to travel great distances, other animals either can do that on four legs, or were evolved to live in trees. Our evolutionary history is quite unique, we are descended from climbers/tree dwellers, and then went back to land.

131

Nikkolai_the_Kol t1_jdu3qol wrote

True bipedalism is pretty rare. Frankly, toddling around half-unbalanced is a good way to get eaten in the wild.

In all seriousness, in a tetrapod body plan (four limbs), to get bipedalism, one needs an adaptive change for the other two limbs. If the two front limbs aren't doing anything useful, generally speaking, evolution favors keeping four legs, rather than withering perfectly useful limbs. Obviously, that's a generalized statement, but let's talk about specialized forelimbs.

In humans, they are for fine manipulation. This is also true in all the great apes, bears, raccoons, otters, and the like. Hominins are the only ones, apparently, to get full bipedalism for this reason, and that is likely because we were the only ones with the right evolutionary pressure.

In badgers and pangolins, it's digging. (Badgers have only partial bipedalism.)

In birds, it's flight. (Yeah, all birds are bipedal!)

In penguins, their wings adapted for swimming control.

For other flightless birds (emus, cassowaries, etc.), current thinking is that they first evolved flight, then evolved to no longer have flight (say, when evolutionary pressures and genetic happenstance favored them being big enough to fight back, instead of flying away).

Now, imagine the evolutionary pressures that led to snakes losing all four limbs!

So, basically, the four-legged form just needs a genetic mutation and a complementary evolutionary pressure to encourage bipedalism, and there just aren't very many reasons to pressure for full bipedalism.

129

CyclicDombo t1_jduanij wrote

You mentioned penguins but forgot all the other birds. We all (those of us with spines at least) evolved from 4 legged things. The only reason to get up on two legs is if you have a really great use for those two front legs that makes it worth the mobility and stability sacrifice.

15

senorali t1_jduc191 wrote

Bipedal animals that primarily move with their legs are efficient long-distance runners but less nimble than comparable four-legged animals. They do well on open plains, typically. Humans, ostriches, and kangaroos all fall into this category.

These types of open environments can't support as much biomass or biodiversity as, say, forests. So even if there was an equal distribution of forests and plains across the world, the plains could support far fewer species overall and thus there would be fewer species optimized for this type of long-distance running.

And honestly, it's not a terribly effective body plan. A lot of flightless birds go extinct when they come in contact with quadrapedal mammals, from the terror birds of old to modern species that are currently being wiped out by invasive rats and cats in isolated island habitats. Kangaroos survive because they live on the only continent without mammalian megafauna. The only things big enough to regularly threaten them are slow-moving reptiles like monitors and crocs.

Humans are kind of a fluke. We developed tool use before we were fully bipedal, and even with that advantage, our ancestors were preyed upon by big cats and other quadrapedal mammals. The loss of two functional limbs for locomotion is a huge risk, even with tools.

13

envybelmont t1_jduc1jm wrote

Replying to michaelrohansmith (#2,385,834)

The great apes seem to have a pretty dominating spot too. Not sure if they count as a proper biped though since they will often drop to all fours, for example when they’re charging at a person with death in their eyes and let out a furious roar that would soil the pants of even the bravest of men.

9

ebinWaitee t1_jduedxq wrote

Replying to [deleted] (#2,386,079)

> all dinosaurs were bipedal

Pretty sure there were tons of dinosaur species that weren't. Triceratops for example

Edit: wow, mind blown that they've all believed to have a common ancestor that was bipedal!

Edit2: hmm, turns out that's probably not the case after all

12

hal2k1 t1_jdufmsp wrote

Replying to michaelrohansmith (#2,386,467)

There are only a hundred or so species of kangaroos, wallaroos, wallabies and kangaroo rats but, if you count birds capable of flying as well as walking, there are many thousands of species of bipedal birds.

6

MrRoundtree17 t1_jdugj8e wrote

Not really an answer to your question, but a fun fact. In order to stand upright, human legs have to connect directly downward from the pelvis. This means the opening which baby’s pass through during childbirth is smaller than in 4-legged animals where the legs connect at an angle to the pelvis. In order to fit the baby through the narrower passage, we evolved to give birth earlier when the baby is smaller. See, most mammals have a similar gestation period relevant to their life span. The human gestation period should be around 14 months. So that’s why newborn babies are such helpless potatoes when they’re born at 9 months.

Edit: I kept saying tailbone when I meant pelvis.

40

hal2k1 t1_jdum41b wrote

Replying to michaelrohansmith (#2,387,296)

How Fast Can a Kangaroo Run?

In terms of covering ground kangaroos have some impressive statistics. Interestingly kangaroos don't run, they jump. Kangaroos are recognized in the jumping category as the best jumpers in the world. They can spend more than 30 minutes jumping as they have excellent resistance and know how to manage their energy well.

4

jlpulice t1_jdus9q8 wrote

Replying to Gtronns (#2,388,016)

I’m a PhD candidate in biomedical sciences, but people I’ve met in human evolutionary biology feel extremely confident the switch to bipedalism was about energetics of long distance travel. May be due to that too but it’s a walking thing!

62

Gtronns t1_jdutbga wrote

Replying to jlpulice (#2,388,095)

Yeah, the idea was that once there were no more (significantly less) trees, we had a lot of walking to do. My point though, is that the trees left us, not us leaving the trees.

26

Peter_deT t1_jduykza wrote

Replying to senorali (#2,386,157)

There are other factors involved. The dinosauria had very efficient lungs and light bones, and seems to have dominated over most types of terrain. Kangaroos have evolved connective tissues between the diaphragm and the legs, so use their leaps to power their breathing, and are very efficient over long distances. They do fine against wild dogs (dingos).

8

Ok-Championship-2036 t1_jduz7xd wrote

Just want to add that bipedalism isn't even an efficient evolution within humans. We basically just shifted our hips and torso to an upright position, but everything else stayed the same. From a skeletal or evolutionary standpoint, only the legs/hips really evolved to the bipedal part. The rest of us is catching up.

6

senorali t1_jdv0ctn wrote

Replying to Peter_deT (#2,388,966)

Oh, definitely. All the extant bipedal animals are pretty decent at what they do in various ways (the ostrich uses its wings as rudders to steer at high speeds, which is neat). It just seems that large cats are their greatest enemy, and probably a much bigger obstacle to their success than other carnivorans like dogs and bears. Big cats have successfully hunted both apes and flightless birds for millions of years, and would probably give kangaroos a lot of trouble as ambush predators if they ever found their way to Australia.

5

senorali t1_jdv2bqm wrote

Replying to hal2k1 (#2,389,379)

I should've been more specific, I was thinking more along the lines of carnivorous megafauna that would directly antagonize kangaroos. The camels are an interesting case, but haven't been there long enough to seriously disrupt the ecosystem. Given a few million years, they likely will push the kangaroos out of some prime real estate if left unchecked. Every other part of the world has large hoofed mammals and large cats, and nothing quite like a kangaroo.

4

MrRoundtree17 t1_jdvcxl5 wrote

Replying to vsmack (#2,390,777)

Yeah, they’re definitely still helpless. But anyone who’s been around a newborn can attest to how shockingly fragile and vulnerable they are. At 6-7 months babies can crawl, sleep through the night, have more weight on their bones, etc.

6

vsmack t1_jdve42t wrote

Replying to MrRoundtree17 (#2,391,348)

Totally, like many of them can't even hold their necks up. We're expecting our second in a few months, and I can't even imagine how much easier it would be (lol as a parent, not as a pregnant mother) if the baby was 5 months older when it was born. You'd avoid the first sleep regression - I do wonder if the first sleep regression happening around 4 months means they're "supposed" to have gone through it before they're born.

7

MrRoundtree17 t1_jdvhy35 wrote

Replying to vsmack (#2,391,565)

Haha, yeah not for mothers lol. Those first 3 months are rough with a newborn. I have two myself (second is now 8 months) and it was almost like clockwork that at 3 months old they started sleeping through the night and it felt like we got past the super vulnerable phase. Best of luck to you and family.

3

Rolldal t1_jdvpgmo wrote

Replying to senorali (#2,389,541)

Australia did have some megafauna. Thylacoleo was a masupial lion (101- 130 kg) roughly comparable to the weight of a lioness. They died out during the pleistocene and were Australia's largest known carnivorous mammal. There were also grazers such as Diprotodon (a kind of giant wombat), Palorchestes, plus a few others nearly all of which died out in the pleistocene

5

razovor t1_jdvwixm wrote

Walking on two legs lets you;

- Walk further

- Use more tools

- Cool down easier

- See predators coming from further away

The disadvantages are

- Lower back pain and similar problems.

- Poor climbing ability

- Lower sprint speed.

it seems our niche, where we rely on tools and walking long distances, is a lot rarer than the niche that relies on sprinting and climbing

7

Lonely-Description85 t1_jdw2vlj wrote

Replying to jlpulice (#2,384,200)

Slight correction if I may. Being bipedal allows higher endurance levels (distance and speed /time). Quadrapeds are faster, but do not have the long distance endurance we do. That's why our ancestors were able to hunt gazelles successfully: ambush or outrun them literally. You can chase a deer to exhaustion if you have enough endurance to run about 3-4 miles.

5

ackillesBAC t1_jdw36fq wrote

I like the water ape theory. We evolved in trees then spent a lot of time in a shallow water environment where walking on 2 legs is more effective and easier. Then moved to land and became the most efficient runners on the planet able to chase animals till they can't run away anymore.

This explains some other odd human traits, why new borns can swim and why our hands and feet wrinkle in water (to increase surface area and traction)

3

dataphile t1_jdw3r83 wrote

Replying to razovor (#2,394,301)

It seems this is a better answer than many—there is rarely a ‘smoking gun’ single reason that a bodily design is selected for. There is usually a constellation of various bodily features that mutually reinforce a ‘successful’ species design. Also there are multiple reasons why a feature is selected — it could be that walking long distances, seeing further on the savannah, and using our hands are all contributors.

Humans are in several ways a constellation of unusual features that work together. We are odd in being relatively hairless mammals. It’s unusual we eat such a broadly omnivorous diet. Our heavy focus on intelligence is weird.

What seems to be the précis for our design is adaptability and sociability. We’re somewhat like orangutans (who also have comparatively long childhoods) in that we range over large distances, eat a lot foods, and have the intelligence to know how to adapt to these varying environments and foods. However, we’re more social than orangutans and range over wider areas.

3

Gtronns t1_jdw5plw wrote

Replying to Lonely-Description85 (#2,395,301)

Exactly. There are some great theories out there by evolutionary biologists.

So the idea is that we adapted a long time into trees, then the trees receded, and we adapted to the ground.

Now that we had gripping hands, and feet that could walk us places, we started being able to shape our environment and our tools, which led to greater intelligence.

Combine all of that, leads to us spreading out. Humans that went borth to Europe adapted to the lack of sunlight evolved. The humans that went on to asia evolved differently, to their climate, and they were the ones that were the ones that kept migrating, until they got to the americas (land bridge from russia to alaska that was exposed when ocean water receded into the ice caps - ocean water evaporates, clouds flow over the continents, it snows/rains, that precipitation turns to ice and stays as ice, ie a glacier, thus leading to the exposed land bridge). There they kept migrating south. The evidence for this is found in similarities that asians and native americans share genetically.

TL;DR We evolved in trees, trees stopped growing where we were, we evolved for the ground, ground got dry, we evolved to travel. We traveled far while the oceans were low. Go humans!!

3

Lonely-Description85 t1_jdw7lyh wrote

Replying to Gtronns (#2,395,551)

There is biological evidence, can't find link atm, that when our species first started to eat saltwater fish was when our brains' frontal and temporal lobes really started to develop. Our occipitals were already highly evolved from our ape ancestors. So as we started to initially travel along ocean and river lines, eating omega rich foods skyrocketed our cognitive development even more.

5

Gtronns t1_jdw86h9 wrote

Replying to Lonely-Description85 (#2,395,866)

Yeah, ive heard some theories like that, basically we are hairless water apes.

Ive also heard that we made leaps and bounds intelligence wise once when we started cooking our food with fire. Some theories say that we started eating cooked food after scavenging the remains after a forest fire, and finding cooked meat. Hard to say what actually happened, but i find that one to be a fun one.

5

OneShotHelpful t1_jdwo8iz wrote

Replying to ebinWaitee (#2,397,708)

Archosaurs are a common ancestor of everything we consider a dinosaur and a lot of things we don't, like crocodiles. If you only want to go back as far as the ancestor of all dinosaurs, then Ornithodera is what you're looking for and it's also a biped.

Birds are theropods, which are one of the three big groups of dinosaurs. They are the ones that survived to today and kept the bipedal bodyplan all the way through.

Ornithicians are things like triceratops. Many became quadripetal, but things like the duck billed dinosaur didn't.

Sauropods are things like brontosaurus. They probably became quadripetal early after the split because I don't know of any bipedal examples.

If you look at quadripetal dinosaurs you might notice a tendency for the front limbs to be short and underdeveloped or have odd rounded shoulders. That could be called a remnant of the bipedal ancestry.

1

banestyrelsen t1_jdwph4s wrote

Bipedalism is actually not that rare among animals like us, ie primates. Lemurs and gibbons walk on two legs on the ground (though lemurs tend to skip more than walk). 20 species of gibbons and they all do this. For some tree climbers it just seems to be the most comfortable way to move around on the ground.

It seems to have been the same with our ancestors because full bipedalism was already present right at the start of human evolution with Australopithecus, which still had a brain not much larger than a chimp. So maybe bipedalism is not something we evolved for any particular reason, maybe it was just how we started out as a byproduct to how we moved in the trees, and when we started living on the ground more we had to work with it.

2

atomfullerene t1_jdwspcc wrote

It's less about adaptive value or not and more about body plan.

Your basic tetrapod body plan is pretty lizard-like (in turn, it's very basically a fish with four legs stuck on). The key thing about this body plan is that the main axis is horizontal. Makes sense, after all the ground is horizontal and that's what you are moving across.

Most tetrapods, heck, most vertebrates keep this basic orientation (for exceptions, see seahorses). Swimming, flying, walking, the body tends to be held horizontally. And this goes for bipedal movement too, which is actually pretty common. If the hind legs are longer than the front legs, and the animals has a big tail (like most tetrapods), it's pretty easy to get up on two legs. Tends to happen in lizards when they run, they are essentially just popping wheelies. Loads of dinosaurs also went around on two legs, with the body horizontal and the tail out behind for balance.

But this doesn't really work with most mammals, which tend to have piddly little tails and long front limbs. Which is why you rarely see any sort of bipedal mammal.

Now like I said, most vertebrates go around horizontally. What's an exception? Stuff in trees. Climbing, hanging from branches, that often puts the body vertical. And requires good balance too. Various primates will go vertical and even bipedal in trees, running along branches and doing that sort of thing, or swinging and leaping around upright.

When we get to apes, you have no tail at all, so there's no hope of going around horizontal-bipedal like a normal tetrapod. But Gibbons like to spring around in trees, run along branches, and walk upright quite a bit when forced onto the ground. Apes that spend less time in trees need to get around more efficiently. Chimps and gorillas seem to have adopted knuckle walking, hominids seem to have improved the original upright stance for efficient use on the ground.

But it's only the odd confluence of lack of a significant tail and preexisting history of upright orientation that makes upright movement plausible in hominids.

As for penguins, you'll note they also lack significant tails and (unlike most birds) have the feet at the very rear of the body to improve swimming efficiency. Which also leaves them stuck with no other options.

2

Ok-Championship-2036 t1_jdwtgn5 wrote

Replying to 3arrowsdown (#2,394,313)

Depends how we adapt. It's dangerous to assume the end goal, since being "completely" bipedal isn't actually something biology would conform to. For all intents and purposes, we work fine as we are. That means not being 100% designed for any one thing. Except survival!

2

jqbr t1_jdxkzf4 wrote

Replying to ebinWaitee (#2,398,680)

That's not believed--OneShotHelpful is wrong. Read their Wikipedia link carefully--there's no support for the claim. See also https://en.wikipedia.org/wiki/Ornithischia

>Ornithischia (/ˌɔːrnəˈθɪski.ə/) is an extinct order of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds.

The key word here being "superficially".

>However, birds are only distantly related to this group as birds are theropod dinosaurs.[3] Ornithischians with well known anatomical adaptations include the ceratopsians or "horn-faced" dinosaurs (e.g. Triceratops), the pachycephalosaurs or "thick-headed" dinosaurs, the armored dinosaurs (Thyreophora) such as stegosaurs and ankylosaurs, and the ornithopods.

Only the last of those was bipedal. And of course all of these bipeds had quadrupedal ancestors--we're all tetrapods.

2

FlattopMaker t1_jdywcqn wrote

Replying to razovor (#2,394,301)

No other creature in the fossil record or recorded alive at any point in time can throw like a human and fashion tools to throw like a human with that curved and fragile radius and fragile shoulder held together by ligaments and tendons. It's not just the standing and the walking, the back pain, and the twisting foetus for birthing - the tool use outweighed the disadvantages. Cave guys standing up and bumping in to the stone shelf to grab the good cave drawing soot cursed but kept the verticality.

3

Krail t1_je2qpqb wrote

Replying to Jonah_the_Whale (#2,393,415)

Those herds of migrating quadrupeds evolved from land-based quadruped ancestors, like most mammals. They already had an effective mode of locomotion for their environment that could develop and become more refined.

Our most direct ancestors were tree dwellers, with hands for hands and hands for feet, with hands and feet that were both built for gripping branches, and shoulders and hips that were both built for climbing and swinging. So as our ancestor's environment became less tree-dense, we came from a very different starting point that animals that were already quadrupeds.

For whatever reason, it was more advantageous for our ancestors to develop bipedalism rather than returning to being quadrupeds. There are lots of factors here, and we don't know all of them. One very likely factor is that, we already had limbs adapted for grabbing stuff, and having two limbs free for holding and carrying things has proven to be extremely advantageous for us.

4

Allfunandgaymes t1_je5cxbw wrote

They're not. All birds are bipedal and are descended from bipedal dinosaurs that existed long before bipedal mammals. Those dinosaurs and their bird descendants - which are technically still dinosaurs by the way - are both classified as "theropods" due to their hip structure, which is what enabled them to become bipedal in the first place, as opposed to the "ornithischian", mostly quadropedal dinosaurs which are completely extinct.

In dinosaurs and birds, bipedalism evolved as adjustments to the hips and leg orientation. In humans, the spine also underwent significant changes to support a fully upright posture.

2

CompetitiveYou2034 t1_je8pgtb wrote

Replying to duc4rm3 (#2,407,400)

Horses have large heads, but do not have comparably large brains. They do have strong necks.

Humans around the globe are social pack hunters. Language skills help coordinate. Accident with hyoid bone placement in our neck helps us produce many diverse sounds.

1