200 million years ago
The first mammals evolved while reptiles were taking to the seas and skies. Reptiles strut the stage of life as actors reading the part given them by the sun and the weather. They are ectotherms, which means that they cannot generate heat internally. As a result, their pattern of activity is determined by the sun. We sometimes mistakenly call this cold-bloodedness, but reptiles can be as warm as you. Wheat makes you different is that your body automatically--whatever the weather--accurately keeps much the same temperature. Mammals have hair, sweat glands, insulating fat under the skin, and cells densely packed with power-generating mitochondria. The temperature differs among mammal species (wolves' body temperature is the highest at 105 degrees F while echidnas' is the lowest at 73.7) but these mammalian traits stabilize internal temperature. Inner temperature control was necessary for mammals to roam under cover of the night. In the dark they could not be seen and in the cold reptiles were sluggish. Under cover of the dark mammals could hunt invertebrates and steal eggs from the stronger but stupider rulers of the day, reptiles. The reptiles' inability to control their temperature does not make them less successful than mammals. There may be no polar iguanas or Arctic sea snakes but there are an estimated ten billion lizards in Italy alone. Reptiles have a big advantage: keeping one's blood warm within narrow limits requires eating vast amounts of food. Even at an ideal air temparature, a mouse uses up to thirty times more energy than a similar-sized foraging lizaard. But cold-bloodedness is a dead end for the evolution of intelligence. And here mammals had an edge. That edge became crucial sixty-five million years ago when an asteroid or comet smashed into the Yucatan areal off the Mexico coast. Not only did its impact kick vast amounts of dust into the upper atmosphere, but it started worldwide fires that belched out smoke clouds of Armageddon proportions. As a result, the Earth was lightless for several years, vegetation died, and there were mass extinctions, including the vanishing of the last of the dinosaurs. While living at night could not save those living in the Yucatan peninsula, it did help others. At such a dark moment in the world's history, it paid to be able thrive in the night, in the cold, and to possess an intelligent brain. Learn More Take me to the Late Mesozoic
© 2005 by Sciencewriters
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