the genus that includes us because they made simple stone tools, they
are now thought to have been much closer to australopithecines than
to modern humans.
Homo sapiens: Human beings, our own species; probably evolved in
Africa between 200,000 and 300,000 years ago; the only species on Earth
(or anywhere else as far as we know at the moment) that can adapt by
“collective learning.”
hub region: A geographical region characterized by an exceptional number
of exchanges of people, ideas, and goods taking place; Mesopotamia was an
ancient hub region; after 1500, the Atlantic regions became a signi¿ cant hub
region through Europe’s control of the major international sea routes.
Hubble constant: The rate of expansion of the Universe; calculating this
constant precisely has been extremely dif¿ cult; modern estimates range from
55 to 75 km per second per megaparsec, implying that the Universe must be
between 10 and 16 billion years old.
hunting and gathering: See foraging.
hydrogen: The simplest of all atomic elements, hydrogen was produced
soon after the big bang; each hydrogen atom consists of one proton and one
electron; deuterium is an isotope (or form) of hydrogen whose nucleus also
contains a single neutron; hydrogen made up about 76% of all the matter in
the early Universe and still makes up about 71% today.
intensi¿ cation: The type of growth or innovation characteristic of the
Agrarian and Modern eras, in which innovation allows the support of more
people from a given area and therefore generates larger and denser human
communities; contrast with extensi¿ cation.
isotopes: Atoms of a given element that have varying numbers of neutrons in
their nucleus and therefore varying atomic weights; carbon dating techniques
depend on measuring changes in the ratio of different isotopes of carbon as
carbon 14 (the only radioactive isotope of carbon) breaks down over time.