40 ■ CHAPTER 03 Chemistry of Life
CELLS
up of positively charged protons and electrically
neutral neutrons. A cloud of negatively charged
electrons surrounds the nucleus (Figure 3.1).
Electrons have significantly less mass than
protons and neutrons have: if an electron weighed
as much as a 1-liter bottle of water, a proton or
neutron would be as heavy as a car.
The number of protons in an atom’s nucleus
is called its atomic number and is unique to
that element. Isotopes of an element have the
same number of protons but different numbers
of neutrons. The sum of the number of protons
and the number of neutrons is the atomic
mass number of an isotope. The atomic mass
number is how much mass is in an element or,
in other words, how much it weighs. For exam-
ple, the most common isotope of carbon has 6
protons and 6 neutrons, giving it an atomic mass
number of 12, and we call it carbon-12 (^12 C). The
isotope of carbon with 6 protons and 8 neutrons
is carbon-14 (^14 C).
Atoms interact with other atoms via electrons;
they can donate electrons, accept electrons, and
even share electrons. When two atoms share
electrons, they form a covalent bond. Atoms
linked by covalent bonds form molecules. Mole-
cules that include at least one carbon atom are
referred to as organic molecules. Urey and
others suspected that gas molecules in early
Earth’s atmosphere combined to form the earli-
est organic molecules, which later assembled
into the first living organism.
Urey proposed that Earth’s early atmosphere
resembled that of other planets in our solar
system, including Jupiter, Saturn, and Uranus.
He suggested that, like the atmospheres of those
planets, our atmosphere was once rich with
important chemical compounds, molecules
that contain atoms from two or more elements.
Earth’s early atmosphere, he hypothesized, was
made up of mainly four compounds: the gases
methane (CH 4 ), ammonia (NH 3 ), hydrogen (H 2 ),
and water vapor (H 2 O).
The young Miller was intrigued by Urey’s
hypothesis, and he asked Urey if he could
perform an experiment to test it, to see whether
such a combination of gases could be used to
create other simple compounds. Urey dismissed
the idea, saying that it was too difficult for a
student’s thesis and that, if it didn’t work, Miller
would have nothing to show in order to gradu-
ate. But Miller pestered him until at last UreyJ
im Cleaves hauled yet another box to the
dumpster. The young, dark-haired researcher
at UC San Diego had the unenviable task of
cleaning out the laboratory of his PhD adviser,
the recently retired chemist Stanley Miller. It
was 2003. Cleaves tossed box after box—20
years’ worth of chemical samples from experi-
ments done by Miller and his former students.
As the lab emptied, a small cardboard box high
on a shelf caught Cleaves’s eye. It was labeled
“Electric Discharge Samples.”
“That box looked like something I’d really
regret throwing out,” says Cleaves. So instead of
making a trip to the trash, Cleaves gave the box
to another former student and friend of Miller’s,
marine chemist Jeffrey Bada at the Scripps Insti-
tution of Oceanography. Once again, the box
was relegated to a shelf, where it sat, unopened,
for years. Neither Bada nor Cleaves had any
idea there was a scientific treasure trove inside,
preserved for them by Miller.
In 1952, Miller was a thin, bespectacled grad-
uate student at the University of Chicago, look-
ing for a thesis idea. He approached Harold Urey,
a Nobel Prize–winning chemist who, a year and
a half earlier, had proposed a radical hypothesis
about Earth’s early atmosphere and the origins
of life. Scientists knew that several key types of
matter (anything that has mass and occupies a
volume of space) existed on the early Earth, but
they debated which types were necessary for
the emergence of life 3–4 billion years ago. One
type of matter is an element, a pure substance
that has distinct physical and chemical prop-
erties, and that cannot be broken down into
other substances by ordinary chemical methods.
There are 98 natural elements known to us, and
another 20 have been created in laboratories.
An atom is the smallest unit of an element
that retains the element’s distinctive properties.
Atoms make up all common materials, includ-
ing this book, the air, and you. Every atom has a
dense core called a nucleus (plural “nuclei”) madeStanley Miller was an American chemist who
designed the first experiment to mimic Earth’s early
atmosphere and pioneered the study of the origins
of life. He died in 2007 at the age of 77.STANLEY MILLER