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I was wondering what kind of planet would lead to aliens being significantly larger and stronger than humans.

From my understanding, a planet with lighter gravity would lead to larger creatures, but they would most likely be weaker, as it takes less effort to move stuff. But a planet with heavier gravity would lead to an alien which is shorter, but most likely significantly stronger, as it takes more effort to move itself and things around it.

Is my understanding faulty, or is there a way for a planet to produce these kinds of creatures? What are some things they would need in order to get around this size? They should be bipedal with two arms and legs, but no other restrictions besides those.

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  • $\begingroup$ Do you mean 15-20% larger - like Wookies or that wrestler dude from Guardians of the Galaxy? Or 200-300% larger; giants? $\endgroup$ – Willk Oct 27 '17 at 23:45
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    $\begingroup$ Probably more around 50 percent larger, maybe around 8 feet tall. $\endgroup$ – OneSurvivor Oct 28 '17 at 2:00
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    $\begingroup$ Consider adding science-based tag. Nice example of fundamental question possibly usable for many authors. $\endgroup$ – Mołot Oct 28 '17 at 9:58

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You can do it on a planet identical to earth for all intents and purposes. They are aliens not humans, just something with a vaguely human body plan, you have an entirely different evolutionary history to work with and their are plenty of organisms much bigger than humans, even intelligent ones.

Just come up with a different starting point, heck just look at gigantopithecus. If the hominid line had started from something like that, or even just something gorilla sized, instead of something close to the body size of a chimp, humans might fit your description. And that is just an example you can make up an evolutionary history that has nothing to do with primates or terran-like life.

enter image description here

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  • $\begingroup$ I hate it when someone's already given what would have been my answer... :) $\endgroup$ – RonJohn Oct 28 '17 at 11:25
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    $\begingroup$ Sweet! Thanks for the tip about the gigantopithecus. He's almost the exact size I had in mind. $\endgroup$ – OneSurvivor Oct 30 '17 at 18:31
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    $\begingroup$ Note that gigantopithecus were probably knuckle walkers like gorillas and or fist walkers like orangutans rather than true bipeds. $\endgroup$ – Arcanist Lupus Dec 11 '17 at 6:14
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    $\begingroup$ @ArcanistLupus of course so were our ancestors. $\endgroup$ – John Dec 11 '17 at 21:46
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UPDATE - after re-reading the question, this answer is an acceptable answer to a totally different question, which was "Can I have aliens significantly larger and stronger than a human, but still in human shape?". I don't know where I pulled those two qualifiers from.

aliens not unreasonably larger than humans (new answer)...

Yes, it is possible. To be as close as possible to the human shape, just check the largest primates on Wikipedia.

With some adaptations from the "human shape" section below - a stronger heart, thicker blood vessel walls, valves and possibly even a secondary heart in the abdomen, you can easily get a 3 m tall human weighing around 250 kg.

The bone structure only needs minor tweaking to be able to bear up to twice the weight, so while you will need thicker bones, especially in the pelvis, this added thickness might not be too visible if we stay under 300 kg (the rhino bones can bear a load up to one hundred tons but that comes at a cost) and slightly different pelvic and especially knee and foot joints.

Feet will need to be larger no matter what, but not too disproportionately so. It would be good if we could accommodate a less mobile and fragile foot.

From an evolutionary point of view these aliens would probably never have climbed trees, so a great neck mobility isn't necessary. The skull will probably be larger than expected and provide more protection from falls (a larger, more complex meningeal layer with more powerful shock absorbers, and "plated" interlocking skull bones). The eyes will need to balance the advantages of being deeply inset with the advantages from being outside with a wider field of view; in the end they might be retractable, like Cu Chulainn's riastradh in Irish mythology.

The minor surface-to-area ratio will mean that the aliens will need to get rid of more heat for square inch, which calls for less hairy bodies than Earth's large primates (there is still debate on why exactly humans lost most body hair, though).

Abandoning a full-human blueprint, an anthropoid bear - all the way to Megatherium - is possible. The gait will not be human-like, and the creature will require a tail to help balance. It is totally possible for a Megatherium-analogue to get rid of its claws and have more civilized tread and table manners; they'll still have a low metabolism, though, because otherwise the fuel requirements for their mass would be prohibitive. On the other hand, this might be a powerful incentive to developing intelligence and manipulatory skills to improve foraging chances.

...with giant human shape (old answer)

I'm afraid there aren't many ways.

They would be like humans in almost every other way, for convenience.

Consider a frame twice as large in every direction than a human being. It would have a volume eight times larger (2x2x2), and therefore weigh reasonably at least five or six times as much, possibly the full eight times as much if its density remains the same as a human's.

But the maximum weight that an animal frame can carry is related to the section of its bones, and to move it the same applies to muscles; so you would need a section six (maybe eight!) times larger. This translates to a size factor of SQRT(6) or 2.45, not just 2. This creature would need to be appreciably disproportionate in his feet, legs and lower body.

This is why the largest known primate, the extinct Giganthopitecus Blacki, is thought to have been a preferential quadruped rather than a biped: from Wikipedia,

The majority view is that the weight of such a large, heavy animal would put enormous stress on the creature's legs, ankles, and feet if it walked bipedally; while if it walked on all four limbs, like gorillas, its weight would be better distributed over each limb.

Similar considerations apply to the heart and circulatory system, with the added difficulty that pressure is proportional to height and gravitational acceleration; a larger being and a heavier world require a slightly different material with which to build veins and arteries, or they will burst. This already happens in some terrestrial mammals - the giraffe has specialized muscles and valves to keep its arteries intact when it lowers its head, and even so it has 280/180 blood pressure.

Then you have metabolism and pulmonary volume. You can fiddle a bit with the air's density and oxygen contents - with enough oxygen you can get bugs as large as a dinner plate - but as any SCUBA diver will tell you, this requires further fiddling with biochemistry.

In the end, the larger a human being gets, the less human it can afford to look.

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  • $\begingroup$ Your skull and eye bits are very interesting. Will keep that in mind. $\endgroup$ – OneSurvivor Oct 30 '17 at 18:37
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Gravity & Bio-mechanics

The principal contributing factors to physical size of fauna is: gravity and bio-mechanics. According to Jonathan Payne, a paleobiologist at Stanford University who has done extensive research on how body size has evolved over the history of life.

If you took an animal and blew it up in size, mathematics of bio-mechanics dictates that the creature’s mass would increase cubically, or by a power of three. However, by the same ratio of size increase, the width of the creature’s body, and thus its bones and muscles, would increase only by a power of two. As the alien creature gets bigger it needs to dedicate more and more of its body mass to its bones to support itself.

Atmospheric Oxygen

On Earth, a big factor that has historically (over the length of life on Earth) driven animal size is the percentage of oxygen in the air. More oxygen lead to bigger fauna, less oxygen lead to smaller fauna. It isn't entirely clear if this is mostly due to animal respiration or mostly due to availability of food.

Metabolism

The Earth’s biggest terrestrial animals, elephants, for instance, can reach about 13 feet tall and weigh up to 7.5 tons. In the past, however, life got far larger: Dinosaurs like the Titanosaur weighed in at nearly 80 tons—10 times larger than the African elephants of today. The reason has to do with the fact that dinosaurs were reptiles. To maintain their higher body temperatures, warm-blooded mammals spend about 10 times more energy than cold-blooded reptiles do on their metabolisms. This is energy that a mammal can’t devote to increasing its body size. Consequently, terrestrial mammals aren't very large in size.

Foraging

Following from the previous point, aquatic fauna, the Blue Whale, is a mammal and larger than any terrestrial fauna ever. Of course buoyancy and gravity trade-off explain this disparity. An important distinction though is the foraging habit of Blue Whales. Blue whales swim across ranges of thousands of miles to find krill to eat. Even the African elephants can cover up to 80 miles in a day looking for vegetation. The large fauna require food to supply their metabolism, growth and reproduction requirements. An alien would need to move large distances to acquire all the food it needed unless the alien planet had surplus vegetation that somehow regrew quickly such that the gigantic alien did not need to cover thousands of miles on land to forage.

Evolutionary pressure

The largest animals through history on Earth are understandably herbivorous. Its a question of energy efficiency or economy of metabolism; how much metabolism is invested in acquisition of food? For comparison, consider carnivorous animals like lions and tigers on a hunt. They are physically strong animals but spend a lot of energy on hunting their prey. This energy could alternatively have been spent on growing bigger and reproduction, instead evolutionary forces drive these animals to work in groups (reproduction priority) rather than have larger individuals. On the other hand, herbivorous animals don't fight with their food, so physical strength beyond that required to support their own weight is not an evolutionary priority.

Speculative Gigantic Alien

Lets get our ingredient for gigantic alien

Gravity: Low

Oxygen: High

Metabolism: cold-blooded

Foraging: Cornucopia of rapidly regenerating food source

Evolutionary pressure: Preference for physical size

A likely candidate alien would be a hybrid of flora and fauna. Unlike herbivorous animals that spend energy on eating plants that acquire and store energy from photosynthesis, the candidate alien has its own energy producing capability. This saves it time and energy on foraging. enter image description here

The alien plant could have perennial incident solar energy, say there are multiple stars in the planetary system. Then this alien could be acquiring energy through some manner of photosynthetic process in a continuous manner.

This savings in metabolic energy could be spend on growth and building strength, which leads to gigantic and strong alien.

Answer: I AM GROOT

A flora-fauna hybrid alien. Groot

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Thanks to a comment, I did a bit more research on warm and cold blooded animals. I've mentioned some interesting points of distinction that would shed light on the kind of alien planet and alien creature. If we have full creative control then a synergy between planetary conditions and alien could be achieved.

Warm-blooded vs Cold-blooded metabolism and survival pros and cons:

There are many advantages to being warm-blooded. Warm-blooded animals can remain active in cold environments in which cold-blooded animals can hardly move. Warm-blooded animals can live in almost any surface environment on Earth, like in arctic regions or on high mountains where most cold-blooded animals would have difficulty surviving. Warm-blooded animals can remain active, seek food, and defend themselves in a wide range of outdoor temperatures. Cold-blooded animals can only do this when they are warm enough. A cold-blooded animal's level of activity depends upon the temperature of its surroundings. A reptile, for example, will increase its body temperature before hunting and is better able to escape predators when it is warm. Cold-blooded animals also need to be warm and active to find a mate and reproduce.

Being cold-blooded, however, has its advantages. Cold-blooded animals require much less energy to survive than warm-blooded animals do. Mammals and birds require much more food and energy than do cold-blooded animals of the same weight. This is because in warm-blooded animals, the heat loss from their bodies is proportional to the surface area of their bodies, while the heat created by their bodies is proportional to their mass. The ratio of a body's surface area to its mass is less the larger the animal is. This means that larger warm-blooded animals can generate more heat than they loose and more easily keep their body temperatures stable. Smaller warm-blooded animals loose heat more quickly. So, it is easier to stay warm by being larger. Warm-blooded animals cannot be too small or else they will loose heat faster than they can produce it.

Being large requires a greater food supply, but for mammals, being small also requires a lot of food to generate more heat. Small mammals need to eat very often to survive because they need more energy to keep a constant body temperature. They also need to eat high energy foods such as fruit, seed, and insects and even other small mammals. Larger mammals can get by with eating lower energy foods or eating less often. In an environment where food is scarce, such as in deserts, reptiles have an advantage. Since cold-blooded animals do not have to burn a lot of food to maintain a constant body temperature, they are more energy efficient and can survive longer periods without food. Many cold-blooded animals will try to keep their body temperatures as low as possible when food is scarce.

Another disadvantage to being warm-blooded is that warm-blooded bodies provide an nice warm environment for viruses, bacteria and parasites to live in. Mammals and birds tend to have more problems with these infections than do reptiles, whose constantly changing body temperatures make life more difficult for these invaders. However, an advantage of this is that mammals and birds have developed a stronger immune system than cold-blooded animals. A reptile's immune system is more efficient when the animals is warmer, however, since bacteria probably grow more slowly in lower temperatures, reptiles sometimes lower their body temperatures when they have an infection.

Finally, I haven't considered the possibility of gigantic size for extremophiles, which are creatures that live in extreme conditions, like hyperthermophiles. If the alien planet was something like Venus, and these aliens could absorb the energy in the high temperature, high chemical environment, they could solve their metabolism needs and possibly grow very large.

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    $\begingroup$ "The reason has to do with the fact that dinosaurs were reptiles." This is disputed in the sense you mean. Many dinosaurs were more like featherless birds than reptiles. And we're not sure about the featherless part. Anyway, not necessarily cold blooded. $\endgroup$ – Brythan Oct 28 '17 at 23:59
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Make the planet colder.

One of the most important facts governing body size is the "square-cube" law - that the volume to surface area ratio of an object changes as an object changes in size. LSerni touched on this regarding body strength and weight, but the other critical place this law matters is in heat generation and dissipation. Heat generation scales with body volume, but heat dissipation scales with surface area.

As a consequence, large animals have problems with being too warm, while small animals have problems with being too cold. That's why you find many large animals in Arctic environments but few in deserts, and why the last Ice Age featured many species that were larger than their cousins who are alive today.

A planet that is colder than Earth would therefore encourage the development of larger hominids.

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The light build of humans has more to do with terrain than with gravity or climate: it optimises endurance and minimises energy consumption, good when the species is evolving in a savannah. In such an environment, you can only afford to be heavy by being exceptionally strong (elephants, lions), but in that case you don't need to be particularly smart, so that's not too plausible for an intelligent species. Intelligence did give human ancestors an edge in hunting technique and avoiding enemies, but physical strength wasn't so crucial.

To evolve intelligent ape-like creatures without losing strength, put them in an environment where they need it despite the intelligence, and can afford the extra energy. I'd say your best bet is denser vegetation, such as apes generally inhabit.

Somewhat weaker gravity will certainly be helpful, so that bipedal locomotion is easy despite the high stature and mass. A cold climate, like Arcanist Lupus suggests, is also a good idea because it encourages both sturdy build and intelligence. Fat and fire, IOW.

So: make your planet humid, cold and with strong seasons, so that much of the land mass is occupied by dense boreal forests.

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I picture your hominid giganticus resembling the Hulk somewhat (except perhaps not with the green skin). The creature would have to have thicker and heavier bones to support their greater weight, and the might be more dense overall.

Gravity is not the absolute indicator for height. Consider the various different feline species out there - large felines like lions and tigers are very similar in appearance and build to smaller ones like lynxes. They have evolved to fit fit certain niches, and probably also to the size of their prey.

Your planet could simply have larger versions of predators, herbivores, plants, etc.

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Although what you have described appears to make sense, we really do not know what effects a planet's greater or lesser gravity have on its indigenous life. Currently, life has only evolved on one planet that we know of, so our available sample size for studying such effects is... one ...which is not a valid sample size for this kind of conjecture.

It could just as easily be theorized that omnivorous (prey) primate size is determined by the appetites of the carnivores which hunt them. Tribes of apes, which are individually big enough to fill a tiger's stomach, loose only one member each time the tiger successfully hunts. Tribes of smaller apes loose multiple members during that same time frame. The bigger apes therefore have a survival advantage (at the tribal level) over the smaller apes. Even bigger apes however have no additional survival advantage because one tribe member dies for each tiger meal, even if half of the corpse is left uneaten. So if you want a bigger evolutionary starting point for your intelligent primates, give your tigers bigger stomachs.

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The planet wouldn't necessarily have to be larger than Earth to have stronger gravity. Maybe it consists of denser matter than Earth has... a planet with a core of solid gold might make for an interesting situation.

The greater the gravity, the more dense the atmosphere that the residents have evolved into. So your super strong big planet dwellers might find Earth's atmosphere too thin to support their needs.

Why would they have to be bipedal? Life on another planet may evolve in a different direction, due to slightly different circumstances, or just the element of random chance. It would make for a less convenient, but seemingly more authentic story, if life encountered on other planets didn't look like life as we know it.

On Planet Minotaur, they could have four legs and two arms, for speed and dexterity. Planet Millipede... oh, we've lost count already. And there's Planet Pogo, where they have only one leg, and the Earth visitor has to leave before they laugh themselves silly from seeing an entire population hopping around. The possibilities of non bipedal intelligent life are limited only by the imagination.

'ugly bags of mostly water'

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Your planet doesn't necessarily need to be very different from earth... It needs to have a different history. We once had big animals, but earth's history and evolution negatively selected them. Big animals are usually slower, easier targets, and a huge body mass has a lot of constrains. Big animals require big territories and open enought for them to coup with they life needs... Take for example the larger animals we still have... They are not in the very dense and closed forests, but in wide open savannas... Or in the sea. Even in forests, we once had bigger animals than we have now (not bigger then savanna's, though). They were hunted down to extiction many ears ago

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A planet identical to earth could produce aliens 50% larger than humans (mass not height). That's a bilaterally symmetrical biped of 93kg (human average is 62kg). If you want noticeably larger you could go 2.4m tall but you'd probably need to double the mass to 125kg or heavier.

Homo sapiens is a recent evolutionary result. This planet has had at least 5 Mass Extinction Events and all life on earth has evolved from the survivors of each of those.

The Cretaceous-Tertiary mass extinction saw the end of the dinosaurs. They had many top predators that were significantly bigger than humans. There's an infographic How do humans size up to the dinosaurs in Jurasic Park

Aquatic predators like sharks, orca, dolphins and extinct species can also be bigger than homo sapiens. Megalodon?

I'm using predators as the prelude for intelligence because "How much intelligence does it take to sneak up on a blade of grass?". But that ignores the role of cooperative community and omega-3 fatty acids in human evolution (if any).

This also ignores the potential for elephants or hippos to evolve enough intelligence to become the apex species in the absence of homo sapiens. But for a few environmental die rolls maybe the pachyderms could have out-competed us.

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