(where atoms are usually too far apart to react) or in solids (where atoms are
locked so tightly together that there can be little change). He added energy in
the form of heat and electric charges. Within days a dark red sludge appeared
containing amino acids, nucleotides, and phospholipids. Later versions of the
experiment, using different gases, have shown that all life’s basic chemicals
could have formed spontaneously on the early Earth.
Second, we must explain the evolution of the much larger and more complex
molecules found in living cells. The Urey-Miller experiment generated
simple molecules with just a few atoms. Yet even the simplest viruses
contain billions of atoms in complex con¿ gurations, many arranged in huge
chains. How could such huge and complex molecules have formed? As Fred
Hoyle argued, it was unlikely that such molecules would be assembled by
pure chance. Yet there is an answer to Hoyle’s riddle, and it involves natural
selection. Though random changes are unlikely to create living organisms,
if each successful step toward life can be locked into place, then the odds
improve drastically. This is precisely how natural selection works, by
locking into place random variations that create viable life forms. The idea
that chemicals can “evolve” through a chemical version of natural selection
(“chemical evolution”) underlies modern theories of the origins of life.
In the 1950s and 1960s, Sydney Fox (1912–) showed how “chemical
evolution” might work. Under certain conditions, organic molecules
spontaneously form long chains similar to those in living organisms.
Some of these molecules naturally curl up to form cell-like spheres with
semipermeable membranes through which they can ingest chemicals from
outside (eating?). They can also divide (reproduction?). With a metabolism
and the ability to reproduce, they can also adapt over time, giving them all
the “emergent properties” of life. Where might such reactions have occurred
in the early Earth? Darwin assumed they might have occurred in a “warm
pond,” perhaps on the edge of the seas. Yet early in the Earth’s history,
its surface would have been extremely dangerous, so today it seems more
likely that life evolved under the seas, near mid-oceanic vents. Here there
was energy, a rich mix of organic chemicals, and protection from ultraviolet
radiation. Today, rich colonies of chemical-eating bacteria thrive in
such environments.