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This is question is related to the medieval world from my earlier question in which several types of intelligent hominids evolved in isolation, until they were rediscovered, conquered, and bred into a biological caste system by a dominant species.

BACKGROUND

To go with my farm-laborers I'd like a warrior hominid that can run incredibly fast. Ideally, they might end up in a military role comparable to light cavalry; moving rapidly over the battlefield, skirmishing where needed, and then breaking off. (Edit: They would prioritize speed over endurance, while still being able to run and fight long enough to be useful before tagging out.)

But first there are some obvious problems with this that need working out, which is why unlike the last time I'm devoting this entire question just to the issue of speed. First, bipedalism and a faster-than-human running speed isn't a common evolutionary strategy, so I need to justify that. Second, they also still need to be viable as close combat soldiers, so I'd like them to be tall and strong. Depending on the physique, this may not be conducive to speed.

I don't have a good grasp of the mechanics, but I anticipate one solution might be for them to drop to all fours while running. If so, that would be an workable but imperfect fix, because it creates all kinds of problems for holding weapons, especially large ones like spears. Happily, there are two large, fast, and 100% bipedal runners I can think of from the animal kingdom: the ostrich and the kangaroo. But I'm not sure what lessons there can be applied to a recognizably hominid physique.

QUESTION

  • How fast could a human sized or greater intelligent hominid evolve to run? How would its physiology differ from our own? As a bonus: In what circumstances might such a creature evolve? Could they become fast enough to be a feasible competitor in a world with cavalry, or would I have to remove fast-moving mounts like horses from the world to make them more effective?
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    $\begingroup$ Generally speaking, there's two types of speed that you might want to develop; sprinting and endurance. Both have their uses, and both are mostly incompatible with each other as they use muscles in different ways. Could you please clarify which you were thinking of, short range speed or the ability to run long distance? It will help with the physiology. $\endgroup$ – Tim B II Jan 3 '18 at 5:16
  • $\begingroup$ @TimB: That's a great question. Where it comes down to a choice, sprinting. The caveat is they need to have enough endurance to be useful in warfare. (Though that doesn't mean as much as human soldiers -- and needing to cycle out teams of runners and guard the weary ones while they rest might even make an interesting facet of strategy.) $\endgroup$ – Era Jan 3 '18 at 5:20
  • $\begingroup$ Have a think about Patas Monkeys ~ 35 mph and also Gibbons. Gibbons reach similar speeds through the trees by swinging. It could be that for non-open terrain warfare these guys, with an unusual for of locomotion would be an idea candidate. $\endgroup$ – josh Jan 3 '18 at 12:15
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Ever wondered what the difference between red meat and white meat is?

Animals like cows and sheep are herd animals. Their survival strategy for attacks by predators is to be able to out-run them. Not run faster, just run for longer than the predator. This means that their muscles can be saturated with hemoglobin for longer; they can't fill the muscle with blood as quickly as an ambush predator so they have to build up to their top speed, but neither does the blood leave the muscle quickly, so they can maintain that speed for longer.

Predators and some other animals on the other hand don't need endurance, they need the ability to sprint from a standing start. These animals have muscles that are designed to be flooded with oxygenating blood very fast, providing massive amounts of energy quickly, but they drain just as fast meaning that the speed cannot be maintained. Very much a 'light that burns twice as bright lasts half as long' kind of thing.

So; sprinters are more likely to be predatory in their nature so that's a start. The first thing that would be needed for their adaptation is longer legs.

Longer legs mean larger muscles, but also larger 'levers' in the form of the bones. That means for every contraction the muscles make (especially quads and hamstrings), the further you go distance wise with more or less the same energy. The tradeoff here is that your hominid would lose a little agility in this so they can charge an enemy line, but they're less likely to be able to dodge fast spears or swords. They would be taller though, so perhaps they'd be able to see further into the enemy ranks and anticipate attacks better, making up for the reduced capacity to react. Balance would be another weakness in this case because of the long legs and lack of agility when running.

If their arms were also longer and they had strong upper body strength, they could wield larger swords and maces, making them quite dangerous in melee situations against smaller foes.

Your hominids would have evolved from ambush predators who work in groups to take down larger prey. They would have larger reserves of blood in their system to maintain a powerful sprint even for a short distance, and that would mean larger lungs to fuel the energy conversions. This probably means faster metabolisms (they'd have to eat more than a normal human, probably by a lot) and they'd tire easily after their short stint in combat.

They'd probably live a shorter lifespan than a conventional human. Faster metabolism is one thing, but using all that energy in a single burst would put a massive strain on their systems, just as it does with Cheetahs for instance. You can also see the longer, more spindly legs on these creatures by comparison to other cats.

For the record, horses are endurance runners, especially when they are fully mature (say around 6 years old). That means that they have good speed, but not the acceleration that a Cheetah is capable of (although they can run for a very long time).

So; I'd be using your hominids as cavalry hunters. Ambush them, kill the horses (or preferably the rider) and then get out. They would make great neutralisers of enemy cavalry ranks if deployed correctly, whereas against a line of pikemen and other infantry, they would be less effective.

In short, you use your cavalry against infantry, and your hominids against cavalry, and your infantry against hominids. Very much a scissors, paper, rock scenario in my humble opinion.

How would these hominids evolve? They're ambush hunters, pure and simple. They band together to take down larger prey, but use speed and weapons instead of strategy. They rely on overwhelming their prey rather than herding it into a difficult terrain or trap like the neoliths. I'd imagine that these hominids would evolve in large sweeping savannas, where the terrain can't be used to advantage and where stealth and teamwork are the answer to hunting. Much like modern lions.

Obviously I don't have hard data available, but with the right metabolism, blood and hemoglobin volumes, lung capacity, limb length, etc. you should be able to get your hominid up to around 80 Kph; fast enough to just outrun a horse over a short distance (again based on the Cheetah). This though would inhibit many of the other traits that you want for melee combat, which is why I suspect they would only fill niche tactical roles.

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    $\begingroup$ Hi, thanks for this awesome answer! I wanted to ask about the speed. I agree that nailing an exact speed down seems very unlikely, but was wondering if your estimate of 80 kph was based on anything in particular? That would be an amazing speed, even in short bursts, but enough to leave many well known quadrupeds in the dust ... so I wanted to double-check it before I let myself get too optimistic. $\endgroup$ – Era Jan 3 '18 at 13:06
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    $\begingroup$ Hi Era. It's based on the Cheetah and it's specific speed based adaptation, as well as some math on the proportions of leg sizes for someone like Usain Bolt by comparison to other sprinters. I have to admit though, it's less of an 'estimate' and more of an educated guess IMHO. Fast twitch muscles will only get you so far without an oxygenation system that can support it, which is in part a limitation of the size a cheetah can grow to, so I'm not expecting to reach those speeds. $\endgroup$ – Tim B II Jan 3 '18 at 21:19
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Let's talk about Usain Bolt

...Who ran the 100 meter dash in 9.58 seconds. Or, as we Americans like to say, 23.35mph. That's whomping fast for a human. What are some of the reasons?

From this source:

  • A top sprinter takes longer and more powerful strides.
  • They spend 60% of their time in the air.
  • They naturally have more "fast-twitch" muscles.

But what's really cool is this statement:

"Bolt is a genetic freak because being 6ft 5ins tall means he shouldn't be able to accelerate at the speed he does given the length of his legs," says former Great Britain sprinter Craig Pickering. "At the beginning of a race you want to take short steps in order to accelerate, but because he's so tall he can't do that. But then when he reaches top speed he has a massive advantage over everyone else because he's taking far fewer steps."

And from this source

This raises the much-discussed phenomenon of fast-twitch muscle fibres, which are boosted by possession of the ‘sprinting gene’ - ACTN3. Studies have found that 75% of Jamaicans carry this gene, compared to 70% of US international-standard athletes. It has also been suggested that the aluminum-rich soil of Jamaica increases the activity of this gene; Bolt’s ability could be attributed to the very earth upon which he was raised.

  • Sprinters have longer muscle fascicles (bundles of muscle fibres)
  • Sprinters have shorter Achilles tendon ‘moment arms’ (the perpendicular distance from the tendon to the centre of the ankle joint).
  • Sprinters have been shown to have longer toes, allowing them to apply force to the ground for longer.

So, if you want to have a true hominid (not something that drops to all fours to run like mad), you want to genetically breed your warriors for:

  • Height
  • Long legs
  • A high percentage of fast-twitch muscles
  • Long muscle fascicles
  • The presence of the ACTN3 gene
  • Short Achilles tendons
  • Long toes

And always remember, in the words of the Great Sage, "It's not what you look like, when you're doin' what you're doin', It's what you're doin' when you're doin' what you look like you're doin'!"

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    $\begingroup$ Hi, thanks for this excellent answer! This gives me a lot of information about what makes a great runner, but it's not yet wholly clear to me what the end result would be for speed. I don't suppose you have a way to (approx.) estimate how fast this runner could get if all of the above were taken beyond human? $\endgroup$ – Era Jan 3 '18 at 13:00
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    $\begingroup$ I don't know, and neither does anyone else. When you read those articles about Bolt you'll discover that the list I gave you is common to great sprinters - but science doesn't actually know why. And if they don't know, I certainly won't know. Gratefully, that doesn't matter at all. What I've given you is information that allows you to rationalize whatever speed you need for your story. $\endgroup$ – JBH Jan 3 '18 at 14:29
  • $\begingroup$ Thanks. Also relevant is I just found this, which I may use for a measure: wired.com/2010/02/40-mph-human $\endgroup$ – Era Jan 3 '18 at 14:30
  • $\begingroup$ Also, a diet heavy in sweet potatoes and chicken :o) [serious here, though] $\endgroup$ – Will Crawford Jan 3 '18 at 21:19
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I would imagine for a human to achieve Kangaroo like locomotion there would be several changes.

Firstly, their feet would be huge and pretty long. This would allow them to act as springs similar to the way a kangaroo moves. However, we would be incapable of balancing ourselves with a tail. Instead the human would probably have longer arms which it would use to stop itself from tipping over and face planting into the ground.

I would imagine it as people with rabbit like legs, and really long creepy arms. They would move by pushing out with both legs, support themselves momentarily with their arms, which keep their body horizontal before pushing out with their legs again. For slower motion, I imagine they would sort of skip or bounce around, as their large feet would make moving small amounts difficult, and with a lack of tail, the would need to be upright and move slowly.

To evolve this way, they would probably have to travel very long distances to get foot and water, so the more efficient method of bouncing, rather than running is used. As for speed and distance/time I would use this. They might end up faster since there is a clear lack of tail and they use arms to run, but that should also translate into less distance as its more of a mix between hopping and 4 legged running.

For the fighting part, They would probably carry their weapon on their back, and if there is a need to use it. For a spear like weapon, They would leap up and use it, attempting to end the fight in one blow, but for normal weapons they would need to slow down. or have it attached closed to their forearms.

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About 40 mph (64 km/h), based on research published in 2010.

The main limit is to how fast the muscles can react: if you could increase the response speed of muscles, you allow them to put out force in briefer period of time while contact with the ground.

If you can get your fast-twitch muscles to be faster, however, then at around the above speed you hit another limit, namely how much force the human leg can take before bad things start happening.

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