Since the most dramatic instances of speciation seem to have happened in the aftermath of mass extinctions, this essay will survey extinction first. A corollary to is that if any critical nutrient falls low enough, the nutrient deficiency will not only limit growth, but the organism will be stressed. If the nutrient level falls far enough, the organism will die. A human can generally survive between one and two months without food, ten days without water, and about three minutes without oxygen. For nearly all animals, all the food and water in the world are meaningless without oxygen. Some microbes can switch between aerobic respiration and fermentation, depending on the environment (which might be a very old talent), but complex life generally does not have that ability; nearly all aerobic complex life is oxygen dependent. The only exceptions are marine life which has adapted to . Birds can go where mammals cannot, , for instance, or being , due to their . If oxygen levels rise or fall very fast, many organisms will not be able to adapt, and will die.
In today’s hunter-gatherer societies, the EROI for killing large animals dwarfs all other food sources. The EROI, of calories produced divided by those burned during the hours of labor invested, for large game (a deer, for example), is more than 100, and on average four times that of small game, fifteen times that of birds, about eight times that of roots and tubers, and 10-15 times that of seeds and nuts. The hunter-gatherer EROI for seeds, nuts, and birds is around ten-to-one. An average-sized adult African elephant carcass provides about 13 million calories, which would sustain a band of 12 people for a year if they could eat it all before it rotted and did not die of protein poisoning. The EROI for those easily killed proboscideans when humans invaded the Western Hemisphere could have been in the hundreds and even more than one thousand. Large animals have always been the mother lode of hunter-gatherer peoples, and the consensus among anthropologists is that no instincts urge a hunter to kill only what is needed, but a hunter will kill whatever he can. That finding partly derives from studying modern hunter-gatherers. There is no doubt that when early humans intruded into environments that never before encountered humans, where animals would have had no intrinsic fear of humans, people would have had an exceptionally easy time killing all large animals encountered. Animals without experience around humans, such as Antarctic penguins, are easily approached and killed. As happened innumerable times in the historical era, intruding humans killed all the naïve animals that they could. The only animals that survived developed a healthy fear of humans and avoided them, but how many could develop that fear before they were all killed? From the very beginning of the , . More than 500 million years later, a new kind of animal appeared that turned that advantage into a fatal disadvantage, as it found a way to mine that energy stored in large animals, and it quickly plundered it to exhaustion whenever it could.
We all share the art of language in one form or another
So far in this essay, mammals have received scant attention, but the mammals’ development before the Cenozoic is important for understanding their rise to dominance. The , called , first , about 260 mya, and they had key mammalian characteristics. Their jaws and teeth were markedly different from those of other reptiles; their teeth were specialized for more thorough chewing, which extracts more energy from food, and that was likely a key aspect of success more than 100 million years later. Cynodonts also developed a secondary palate so that they could chew and breathe at the same time, which was more energy efficient. Cynodonts eventually ceased the reptilian practice of continually growing and shedding teeth, and their specialized and precisely fitted teeth rarely changed. Mammals replace their teeth a . Along with tooth changes, jawbones changed roles. Fewer and stronger bones anchored the jaw, which allowed for stronger jaw musculature and led to the mammalian (clench your teeth and you can feel your masseter muscle). Bones previously anchoring the jaw were no longer needed and . The jaw’s rearrangement led to the most auspicious proto-mammalian development: . Mammals had relatively large brains from the very beginning and it was probably initially . Mammals are the only animals with a , which eventually led to human intelligence. As dinosaurian dominance drove mammals to the margins, where they lived underground and emerged to feed at night, mammals needed improved senses to survive, and auditory and olfactory senses heightened, as did the mammalian sense of touch. Increased processing of stimuli required a larger brain, and . In humans, only livers use more energy than brains. Cynodonts also had , which suggest that they were warm-blooded. Soon after the Permian extinction, a cynodont appeared that may have ; it was another respiratory innovation that served it well in those low-oxygen times, functioning like pump gills in aquatic environments.