Glossary
sedentism: Living in one place for most of the year; sedentism was rare in
foraging societies but became widespread with the adoption of agriculture
because agriculture made it possible to produce more resources from a given
area and encouraged farmers to stay in one place to protect their crops.sexual reproduction: A form of reproduction that emerged about 1 billion
years ago, in which two organisms exchange genetic material before
reproduction so that their offspring are not clones of the parents; sexual
reproduction increased variations between individuals and thereby sped up
the pace of biological evolution.spectrometer: A prism-like device that can split light into its different
wavelengths; fundamental tool in the study of stars.steady-state theory: An alternative theory to big bang cosmology, holding
that the “red shift” was an illusion created by the constant creation of new
matter in the Universe; the theory lost credibility after the discovery of the
cosmic background radiation, which it could not explain.supernova: The explosion of a large star at the end of its life; most chemical
elements can only be manufactured in supernova explosions.swidden agriculture: A form of agriculture in which woodlands are
burned down, crops are planted in the ashy soil, and then, when the fertility
of the newly cleared ¿ elds declines, new regions are cleared; because
it is seminomadic, swidden agriculture is possible only in regions of low
population density, such as the Amazon basin.symbiosis: Relations of interdependence between different species, such as
those between humans and domesticated plants and animals, which offer
bene¿ ts (of different degrees) to each species; such relations are extremely
common in the natural world.symbolic language: A form of communication unique to human beings,
using symbols and grammar; much more powerful and precise than
the forms of communication used by all other animals; the basis for
“collective learning.”