Threshold 6—What Makes Humans Different? Lecture 20
[Big history] encourages us to think seriously about questions such as the meaning of being human. And it encourages us to think that they are not just metaphysical or philosophical questions, but they are questions to which there may be good, rigorous, evidence-based scienti¿c answers.
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hat does it mean to be human? The previous lectures described the history of life on Earth and the evolution of our own ancestors through the adaptive mechanism of natural selection. The next group of lectures takes us across a new threshold, describing the creation of our own species and the earliest stages of human history. But before we can determine when our species appeared we need some clear ideas about the features that distinguish us from other hominines. The differences, we will see, are fundamental. We have seen how similar we are to other living organisms. Now we must ask: What makes us so different that our evolution counts as a fundamental turning point in the history of our planet? One distinctive feature is the amount of energy we control. Eric Chaisson has calculated that about 20,000 ergs per second per gram Àow through large-bodied animals such as apes. He calculates that modern humans use on average 25 times as much energy (500,000 ergs/sec/gram; calculated by dividing total energy consumption by the number and mass of human beings; Chaisson, Cosmic Evolution, pp. 136–39). Though approximate, these ¿gures clearly point to a profound difference between us and all other animal species. Human control of energy increased slowly at ¿rst, then accelerated. In the Paleolithic era, more than 10,000 years ago, humans probably used enough energy to stay alive with a small surplus, perhaps 3,000 to 5,000 kilocalories a day. Early agriculturalists may have used up to 12,000 kilocalories a day. Today, each of us uses on average 230,000 kilocalories a day. In contrast, chimp use of energy, like that of most other species, has remained stable. More energy allowed humans to multiply. Today, there are a few hundred 89
