planetesimals: Objects formed by accretion during the formation of the solar
system; protoplanets.
planets: Chemically complex objects orbiting stars; it is now known planets
orbit a majority of stars.
plate tectonics: The central idea (paradigm) of modern Earth sciences since
the 1960s; based on the notion that the Earth’s crust is broken into separate
plates that are in constant motion.
power: Power relations in human societies can usefully be analyzed into two
fundamental forms: “power from below” is power granted by followers to
a leader to ensure the successful achievement of group tasks (such as the
election of captains in sports teams); “power from above” is power that
depends, in addition, on the ability of rulers to impose their will by force;
in the history of human societies, power from below preceded power from
above for the simple reason that to pay for a body of retainers that could
impose one’s will by force it was necessary already to have the ability to
mobilize signi¿ cant resources.
prestige goods: Goods such as silk or precious metals that combine high
value and relatively low bulk or weight; before the Modern era, such goods
were more often traded over large distances than goods of greater bulk or
lesser value, such as grains.
primary producers: Those groups of people (such as peasants or foragers)
who produce resources from the natural environment; elite groups exact
resources from primary producers.
primates: An order of mammals that appeared about 70 million years
ago, characterized by relatively large brain size, manipulative hands, and
stereoscopic vision; all these features may be the result of dwelling in trees;
apes (and therefore hominines and humans) belong within this order.
prime movers: Used in these lectures to refer to the most important forces
driving innovation and growth in human history; they include commerce,
collective learning, and population growth.