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The krogan evolved in a hostile and vicious environment. Until the invention of gunpowder weapons, "eaten by predators" was still the number one cause of krogan fatalities. Afterwards, it was "death by gunshot".
If your means are limited, you have to show resourcefulness. This is especially true if the environment one works and lives within is unforgiving and dangerous. Simply surviving in places like a Death World can make people stronger and tougher, and those who live in such places can be expected to be more than a match for those who don't have to spend every day simply fighting to stay alive. If the world is dangerous enough and generations of people have survived in such a place, natural selection ensures that the toughest and most resourceful will live through, and over time those who live in this environment will change to adapt.
Enter this trope, which deals with people, whether individuals, societies, or entire species, that are meaner, tougher, stronger, and/or smarter than others because their home is the kind of place that breeds out those not fit to survive. Often the cause for a proud warrior race/species. If this is applied to humanity it might imply that Humans Are Warriors. If someone survived to old age in such an environment, they are most likely a Badass Grandpa or Old Master. When this is created artificially it is The Spartan Way. Expect Everyone Is Armed to be a result — or occasionally the CAUSE — of this trope.
Weaver Nine: The Society is comprised of criminals, malcontents, misfits, mutants, refugees, and capes from third-world countries. They are constantly besieged by every other power on the planet who want to kill, enslave, brainwash, or study them. They are also considered (rightly) the most powerful villain organization on planet earth and have a fearsome reputation to match.
Girl Genius Jägers. They are super soldiers who like to kick ass, laugh loudly and drink, not necessarily in this order — no one expects a Jägerkin to notice everything and get non-obvious conclusions. But then there are those who wandered for many years on their own both in lands where the people remembers them as ex-conquerors and in wastelands where they are not nearly the toughest creatures...
Dimo: Haff all dose guys vit de baron gone soft over de years — or haff ve become... sharper because ve leave de group und haff to tink better?
Mass Effect: This is krogan evolution in a nutshell; their planet was so dangerous that for centuries, the most common cause of death was 'eaten by predators'. This explains why they have secondary and even tertiary organs, and why they are Explosive Breeders.
Spelljammer got Scro. Orcs and other goblinoids in the First Unhuman War were cornered and pushed back by elves until they ended up in a barren Death World. When they finally emerged... uh oh. They see common orcs as backward louts for a reason.
Underdark inhabitants in Forgotten Realms. The world below the world is full of creatures incredibly tough just because the Death World requires this... which in turn frequently contributes to the Death World's nastiness. While the Drow prefer cheapskate solutions, they are still packed with magic to the eartips and are highly lethal whether they fight with blades or brains (their cities are about as dangerous as the wild caverns around). And the Swirfneblin are barely comparable with surface gnomes (except tinkers, who are just that crazy).
Regional Feats introduced in Forgotten Realms 3 ed. reflect this. Those who live in the jungles learn to survive in the wild, people raised in The Magocracy tend to know how to talk their way around problems and have an eye for magic stuff, and so on.
The Drow in Eberron are possibly the toughest humanoid inhabitants of the setting, since they live in the jungles of Xen'Drik and have to tangle with giants for resources regularly.
Friedrich Nietzsche said "What doesn't kill you only makes you stronger." Whether or not that's an example depends on how literally you interpret "stronger."
Proverbs. "Necessity is the mother of invention". Or an old Russian variant, "the need will force you to eat pies" (instead of plain rye bread, that is).
Ernest Rutherford said "We don't have the money, so we have to think." The beginnings of physics saw a lot of creativity funnelled into making tools and experimental devices in a barn — sometimes literally. The early scientists used to improvise in experiments simply because there were no established methods in their area. Even Lord Rayleigh made his own equipment out of near-junk, despite being a lord in his land — and many, many others. On the way they made a lot of great inventions. Robert Williams Wood was not only the Patron Saint of Education Through Pyrotechnics but, as one of the pioneers of physical optics, made state-of-the-art spectroscopes using materials like sewer pipes or a tombstone, and cleaned one of these by shoving a cat into the pipe and closing it so that the cat had to exit from the other end. Oh, and among other things he was a co-inventor of film animation and cartoon as we know these (you might call him the grandfather of Mickey Mouse), color photography and ultrasonic echolocation.
This trope is the reason why, in Warhammer 40,000, one of the acceptable tariffs the Imperium of Mankind imposes on entire planetary governments is... Manpower. Living in a Death World tends to create the sort of humans perfect for cannon fodder in a galaxy where there is only war.
Fenris is an ice planet whose ecosystem seems to have been designed by a Norse Mythology buff, inhabited by continually warring pseudo-Vikings. And if that weren't enough the Space Wolves only recruit young warriors who fall in glorious battle.
Atilla is pretty much a planet-sized Mongol steppe, as is Chogris, the home of the White Scars.
Cadia is a nice, earth-like planet. Oh, and it has the Eye of Terror for a neighbor just a spit away which spews out Ax-CrazyChaos warriors. That Cadia's militarization and Imperial Guard troops are a model for planets all around the Imperium is no coincidence.
The majority of Imperial Guard regiments actually come from Hive Worlds. Most often underhive gangs are simply rounded up, given a flak jacket and lasrifle, than dropped onto the front lines with no additional training. In many cases, they don't need it.
In a way, this is true of Humanity as a race. When you are constantly battling omnivorous monsters that outnumber the stars, invincible omnicidal Terminators, Ax-CrazyReality Warper fungus aliens, psychopathic space elves, mind-controlling Communist Japanese aliens, and The Legions of Hell, all at the same time, and surviving, you have to be pretty damn Badass.
Everyone is scared into obedience by the Emperor's Sardaukar, recruited on a prison planet and trained into elite troops. Then they are thrown against Fremen, natives of Arrakis who had just enough industry to stay alive. The results are highly demoralizing. Fremen, upon being informed whom they faced, comment along the line of "ah, we wondered why these were challenging..."
A detachment of Sardukar in three troop transports was sent to attack a Fremen settlement while the warriors were away attacking somewhere else. A handful escaped in one transport (by using the attitude thrusters as weapons) with a prisoner from a defense force that consisted of "women, children, and old men". The kicker? The prisoner, a small child, allowed herself to be captured, because she knew she would be brought before the Emperor, which is precisely where the next phase of the war plan required her to be.
The Reynard Cycle: A combination of brutal winters, recurrent famine, and the resultant decree that its citizens are only entitled to have half as many children as one has personally slain in combat has transformed the Calvarians into a Proud Warrior Race. Want to be the mother to a bouncing baby boy? Better become a Blood Knight . . .
Aiel in The Wheel of Time are a desert-dwelling Proud Warrior Race known and feared everywhere for being extreme badasses: in the years prior to the start of the books, a coalition of all the western countries was needed to stop them, and actually failed (the Aiel returned to the desert only when they did what they had set out to). It is also implied that this was their destiny, and that they had been sent to live in the desert, 3000 years before, precisely so that they would become a reliable and powerful army for the Dragon Reborn when the time came.
The War Against the Chtorr. Chtorran fauna and flora is naturally more aggressive and adaptive than its human equivalent; scientists speculate that Chtorr is a Death World where all life has to be either aggressive or lunch.
In Discworld, this trope is used to justify Instant Messenger Pigeon - Ankh Morpork rooftops are full of gargoyles, so the local pigeons have to be smarter than average.
This is the background of the Barbarian class in Diablo. Though they specifically reject technology and magic because they think it will make them weaker in the long run.
In The Silmarillion the three tribes of the Edain which arrived in elven lands, had arrived their after spending generations marching through Morgoth's territory.
Worm is set in the city of Brockton Bay — a city which experiences a number of severe incidents over a short period of time. Compared to the people who evacuated town and only returned after the worst of the disasters had resolved themselves, the people who stayed are a lot tougher and a lot more willing to engage in violence.
The Sishrak in the Space Empires series are, by the standards of most of the other species, a Proud Warrior Race who have a reputation for being warmongers. Since they are known to be natives of another dimension who invaded the universe, it is speculated that in their own dimension they are at the bottom of the food chain and must constantly fight everything else in order to survive.
Barrayarans in Vorkosigan Saga. After being confined on a Deathworld with no access to technology and subjected to generations of eugenic infanticide and civil wars and they are invaded by the Cetagandians who were centuries ahead of them in technology. Miles is a special example of this as everyone considers him a "mutie" even if he is a prince and he is driven to succeed. As Barrayar is patterned after Tsarist Russia it makes sense.
Borderlands and Borderlands 2: Anyone who can survive on Pandora for any length of time without dying or going insane must be a degree of badass. Doubly so for the Vault Hunters, who instead of holing themselves up in the few livable cities, go exploring around the planet, which is chock full of vicious animals, insane bandits, and minions of the local Mega Corp..
Subverted in H. G. Wells' short story The Land Ironclads, one side is from a rural environment and the other live in cities. The rurals think that they are going to slaughter the soft city-folk because they've been hunting for as long as they could hold a gun. But it turns out that human strength means nothing against superior firepower.
Eighteenth and nineteenth century regimes much preferred mercenaries from extremely nasty wilderness areas. A large part of this was based on this theory.
Greek philosophers bragged that Greeks were this because of having to tend farms on their own. Persians were stereotyped as soft and wussy.
Likewise, the Spartans believed they were stronger than the other Greeks because they spent their entire lives fighting. That attitude eventually led to their conquest by the other city-states, which Rome used as a pretext for conquering Greece.
Venetians and Dutchmen spent generations not only living from the ocean and fighting off all competitors but actually living on the ocean(by way of building "fortifications" against flooding). The result was that Venice managed to take over the Mediterranean for a time. And Dutchmen for a short time managed to Take Over the World.
It is sometimes said that carnivores are unusually intelligent among mammals because they have to catch their food, and sometimes, fight over it among each other. While grass eaters just bow their head and eat.
It's animals that have to use their brains (memory/lateral thinking/both) to get their sustenance (food/water/both), not just their senses (hunting requires sight, tracking scent).
By the above theory, the ideal place for intellect to grow is in the middle of the food chain, where creatures both have to hunt for their food and have to avoid other predators. Add to that that these creatures are often not that much stronger than their prey (think wolf vs. wild buffalo), and so have to resort to teamwork and tactics to get their meal. Now guess where early humans stood.
Mongols. Remember those guys who beat the Chinese and chased the Kumans into the Adriatic Sea? Not even distracted much by dealing the same way with several Russian princedoms, Hungary and most of Muslim Middle East alike? This mostly was converted tribal militia without benefits of any non-conventional training methods. Their success stems indeed largely from learning the lessons of the steppes: endurance of great hardship making them tough as nails and good at cooperation, nomadic life made everyone a good rider, hunting and warring in open field gave obvious incentives to develop the best long-range weapons around, territorial clashes of tribes and later a civil war gave them plenty of experience. The one major change Ghenghis Khan did introduce was a total replacement of the former social hierarchy with the united militarized Mongolian nation — their men no longer paid much attention to clan affiliation, but only achievements and command structure. Subodai, his greatest general, an example of having been a 'commoner' who rose to the top. Between the hero cult on par with Greeks and a chance to beat traditional enemies, they tried to be great warriors pretty hard.
Finns, living in the Arctic in some of the most beautiful and most rigorous terrain in the world. It was no accident that the Russians and Swedes preferred other people to pick on.
The human hand. It is able to open and perform delicate and precise movements... or it can be balled up into a remarkably effective built-in blunt force weapon (commonly known as a "fist"). Chimpanzee hands cannot ball up into fists, and a tightly-balled fist surprisingly can weather forces that'd cause serious injury to it otherwise when used for blunt force.
The Ancient Romans became a major military power due to this. In the beginning it was that they lived in the best commercial position of Latium, and had to defend themselves from neighbouring peoples trying to subdue them. When they had subdued all of their neighbours the Gauls showed up, sacked Rome and promised they would return, so Rome prepared to defeat them. After repelling their successive attack and expanding even more to get the manpower to defeat them once and for all they had to deal with the Samnites, who were a major power in Italy and wanted to force Rome in their League. After breaking their power and that of the Gauls they had to deal with the Greeks (not just those in Italy but their mercenary armies from mainland Greece) who were helped by the Samnites, then with the Carthaginians (and again the Samnites and the Gauls), then with the successors of Alexander the Great, then with the Germans, then with all of Italy (led again by the Samnites. This was the last time, as Sulla exterminated them as a people and the survivors never found new allies), and so on. Note that the list includes only the wars where Rome actually risked to lose, ends with the Republic (the Empire would collect even more enemies), and doesn't include civil wars. In the end the Romans were that good because they needed to be, and most of their wars during the Republic were defensive in nature (Rome's concept of "defensive war" included conquering or exterminating the enemy to prevent another attack, resulting in almost accidental expansion). It has been put thus: Romans are the only people who ever conquered the world in self defense.
My understanding of ancient Rome was that they had enough strength to fight a civil war while also fighting two foreign wars at the same time.
The Gurkhas, "Sardaukar" of the British Empire. To quote British Field Marshal William Slim: “If a man says he is not afraid to die, he is either a liar or a Gurkha.” Growing up in terrain that would give the Mongols pause is certainly good practice for some of the places they've gone to war.
In both world wars Britain sent units from Canada and Australia to the front lines, partially because it was believed that living in frozen tundra or a desert filled with poisonous animals made them tougher, and partially because colonials were considered more expendable. The Germans learned to fear them.
This is still true to some extent. Australia and Canada have some of about the best trained and equipped desert and arctic warfare specialists respectively in the world, simply because that's where so much of their territory is.
It's sometimes jokingly suggested that the British forged themselves a mighty world-spanning Empire because our weather is just that bad.
Evolution works in the manner because if something wasn't tough enough it didn't survive to pass on its genes. This is especially apparent in Africa and Australia where limited resources created serious competition resulting in an evolutionary arms race with everything having to compete with and survive everything else