metals) in the core, lighter materials in the mantle, the lightest elements at
the surface, and gassy materials in the atmosphere.
diffusion: The spread of ideas or innovations or people from a single center.
dissipative structures: Technical term used by Nobel Prize–winning
chemist Ilya Prigogine to describe complex structures (such as stars or living
organisms) that exist far from equilibrium and require À ows of energy in
order to survive; so-called because, by using free energy, they “dissipate” it,
thereby increasing entropy.
divergent margins: Geological regions where tectonic plates are driven
apart by upwelling magma; a modern example is the center of the Atlantic
Ocean.
division of labor: Differentiation in human occupations so that different
professions appear and individuals have to exchange goods and services in
order to support themselves. There was very little division of labor in the
Paleolithic era, but as populations and population densities increased in
the Agrarian era the division of labor developed rapidly, generating greater
social, economic, and political complexity.
DNA: Deoxyribonucleic acid, the complex, double-stranded molecule that
carries genetic information in all living organisms on Earth.
domestication: Genetic modi¿ cation of species by humans to make them
more docile, more productive, and more amenable to human control; a form
of symbiosis, in which domesticated species bene¿ t from human protection.
Doppler effect: The stretching out or contraction of wavelengths because
of the relative movement of two bodies; the Doppler effect explains why
an ambulance siren seems higher when the ambulance is traveling toward
you than when it is moving away; it also explains why the light from
distant galaxies is displaced toward the red end of the spectrum if they
are moving away from us. This is crucial evidence for the idea that our
Universe is expanding.