early in the history of life. Furthermore, no new life-forms are being created
today either because the present chemical environment is not conducive to
the formation of life or living organisms prey upon the newly created organ-
isms before they have any chance of evolving.
Since life appeared within the first half-billion years of Earth’s existence,
it must have evolved into complex organisms from simple materials rather
quickly. Primitive bacteria, which descended from the earliest known form of
life, remain by far the most abundant living beings. Evidence that life began
early in Earth’s history when the planet was still quite hot exists today as ther-
mophilic (heat-loving) bacteria, found in thermal springs and other hot-water
environments throughout the world (Fig. 14) as well as deep underground or
far below the ocean floor.
The existence of these organisms is evidence that thermophiles were the
common ancestors of all life.The early conditions on Earth would have been
ripe for the evolution of thermophilic organisms. Most of these have a sulfur-
based energy metabolism, and sulfur compounds would have been plentiful on
the hot, volcanically active planet.
Fortunately for the early Earth, it had an abundance of sulfur, which
spewed out of a profusion of volcanoes. As long as surface temperaturesFigure 13An active
hydrothermal vent and
sulfide mineral deposits at
the East Pacific Rise.
(Photo courtesy USGS)
Historical Geology