From Shalamov’s “Rain”:
I could see our horses getting worn out and dying—that’s the only way I can put it, there are no other verbs to apply to the horses’
existence. The horses were no different from the human beings. The north, the unbearable workload, the bad food, the beatings were killing them, and although they suffered only a thousandth of what the human beings suffered, they died first. I also understood the main thing: man was human not because he was God’s creation, or because he had an amazing thumb on both hands, but because he was physically stronger, more enduring than any other animal and, eventually, because he succeeded in making his spiritual side the effective servant of his physical side.
Vaclav Smil has a related but different idea in Energy and Civilization:
The extraordinary human ability to thermoregulate rests on very high rates of sweating. Horses lose water at an hourly rate of 100 g/m2 of their skin, and camels lose up to 250 g/m2, but people lose more than 500 g/m2, With peak rates of more than 2 kg/hour (Torii 1995; Taylor and Machado-Moreira 2013). Perspiration rate translates to heat loss of 550—625 W, enough to regulate temperature even during extremely hard work. People can also drink less than they perspire, and make up for any temporary partial dehydration hours later.
Running turned humans into diurnal, high-temperature predators that could chase animals to exhaustion (Heinrich 2001; Liebenberg 2006).