Threshold 6—What Makes Humans Different? ................................

LECTURE

[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.

W

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 À ow 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