"The Lowest Animal" by Mark Twain

Here is a brief summary of this essay. Ever since more than three billion years ago and about a billion years after the Sun and Earth formed, organisms have continually invented more effective methods to acquire, preserve, and use energy. after three billion years of evolution and, pound-for-pound, it used energy . The story of life on Earth has been one of , and in turn influencing them. During the eon of complex life that began more than 500 million years ago, there have been many brief for some fortunate species, soon followed by increased energy competition, a relatively stable struggle for energy, and then cleared biomes and set the stage for another golden age by organisms adapted to the new environments. Those newly dominant organisms were often marginal or unremarkable members of their ecosystems before the mass extinction. That pattern has characterized the journey of complex life over the past several hundred million years. among some animals, which provided them with a competitive advantage.

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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.

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