nucleotides: Basic chemical constituents of the genetic material of all
living organisms.
obsidian: A hard, glass-like substance formed during volcanic eruptions and
widely used in prehistory for the making of sharp and durable blades; widely
traded in the Neolithic era.
Olbers’ paradox: The observation that if the Universe were in¿ nite in size,
there ought to be an in¿ nite amount of light and heat so that the night sky
should not be dark.
oxygen: A highly reactive chemical element with eight protons (atomic
number 8), normally a gas on Earth; because oxygen is so reactive, the
oxygen in the Earth’s atmosphere has to be constantly renewed by the
activity of photosynthesizing organisms, so free oxygen did not appear in
the atmosphere until about halfway through the Earth’s history; as James
Lovelock pointed out, any planet with free oxygen in its atmosphere must
have some chemical mechanism (perhaps associated with living organisms?)
that constantly replenishes the supply of oxygen.
ozone layer: Ozone is a molecule consisting of three oxygen atoms, in
contrast to the more common form consisting of just two atoms; a thin layer
of ozone high in the atmosphere shields the Earth’s surface from harmful
forms of ultraviolet radiation; in the 1980s it was found that the use of CFCs
(chloroÀ uorocarbons) was breaking up the ozone layer; international treaties
have led to the banning of most production and use of CFCs.
Paci¿ c zone: The largest but least populous of the four world zones of
human history; created within the last 3,000–4,000 years as migrants from
the western edge of the Paci¿ c Ocean slowly settled Paci¿ c islands, bringing
with them technologies of farming.
Paleolithic era: One of the three great eras of human history; literally, the
“Old Stone Age,” as most surviving evidence consists of stone tools; in this
course, refers to the era of human history from the origins of our species
(perhaps 200,000 years ago) to the appearance of agriculture about 10,000
years ago.