red shift: In the 1920s, Edwin Hubble observed that the light from many
distant galaxies appeared to be shifted toward the red end of the spectrum; he
interpreted this as the result of a Doppler effect, which implied the galaxies
emitting such light were moving rapidly away from us; the ¿ rst piece of
evidence that our Universe was expanding.
regimes: Complex structures such as stars or living organisms or entire
ecosystems that achieve a certain stability but eventually break down.
relative dates: Dates that can determine the order in which events occurred
(such as the order in which different geological epochs occurred) but not the
time periods between them (see absolute dates).
reproduction: One of the three fundamental features of living organisms; the
ability of all living organisms to make almost perfect copies of themselves;
the occasional imperfections provide the variety from which natural selection
constructs new species (see also adaptation, metabolism).
retinues: Armed retainers of political leaders; a critical step toward the
creation of states capable of coercive power (tribute-taking states), or “power
from above.”
RNA: Ribonucleic acid; similar to DNA, but it comes in single strands so
it can fold like a protein and engage in metabolic activity, yet it can also
carry genetic information; RNA, with its ability both to encode genetic
information and engage in metabolism, may have played a crucial role in the
early evolution of life on Earth.
secondary products revolution: A concept developed by the late Andrew
Sherratt to describe a series of technological innovations from about 6,000
years ago that made it possible to exploit domesticated animals more
ef¿ ciently by using products such as their wool, their milk, and their traction
power, all of which could be used without ¿ rst killing the animals; these
innovations revolutionized transportation, made possible plow agriculture,
and led to the emergence of pastoralist lifeways.