Ingredients for Life ■ 47gas as the cause of the awful smell released by
rotten eggs, but it is also released by volcanoes
and so may have been present in Earth’s early
atmosphere (Figure 3.7). Miller’s lab notes show
that he performed the experiment and isolated
the amino acids, but then never analyzed them.
“Why he never analyzed it, I don’t know,” says
Bada. Bada’s team finished the job, identifying
22 amino acids in Miller’s test tube, including
six that contained sulfur. One of those sulfur-
containing amino acids, methionine, initiates
the construction of all proteins in cells.
Miller’s experiments support the hypothe-
sis that volcanoes—a major source of hydrogennetwork of hydrogen bonds emerges. The mole-
cules become spaced farther apart, locked into
an orderly pattern known as a crystal lattice.
That spacing is the reason ice occupies more
space than liquid water. Normal ice is 9 percent
less dense than liquid water (density is mass
divided by volume), which explains why ice
floats on water. This property of water is quite
unusual—most substances are more dense in
the solid state than in the liquid state—but it
has helped shape life as we know it. If ice did
not float on water, then each time a lake or river
froze in the winter, the frozen top layer would
sink to the bottom and the new, liquid top
layer would freeze. This process would repeat
until the entire lake or river was frozen solid,
no matter the depth. All the aquatic creatures
and plants would be killed in the process, and
terrestrial plants and animals would no longer
have access to the freshwater.
To boil water, as Miller did to inject steam
into his experiments, requires the addition of a
significant amount of energy to snap the water
molecules’ network of hydrogen bonds before
the molecules can move fast enough to escape
as steam (Figure 3.6, bottom). That transition
from the liquid to the gas state is a phenom-
enon known as evaporation. By the opposite
reaction, as water vapor cools, molecules slowly
re-form hydrogen bonds and return to the
liquid state—a process known as condensation.
Without evaporation and condensation, both of
which are essential parts of the water cycle, life
on Earth could not exist (see Figure 18.12 for
more details).
The Smell of Success
Water was key to the success of Miller’s spark
discharge experiments, but another compound
would turn out to be almost as important. In
2011, three years after reanalyzing the first of
Miller’s samples, Bada, Cleaves, and a group of
scientists teamed up again to analyze another
set of vials in the cardboard box.
In 1958, Miller had performed spark discharge
experiments that included a new gas: hydrogen
sulfide (H 2 S). You may know hydrogen sulfide
Figure 3.7
Miller’s experiments were like volcanic eruptions
An erupting volcano releases all of the gases included in Miller’s mixtures,
and the volcanic ash contains iron and other metals. Steam is produced
when the magma (superheated rock) comes in contact with groundwater, and
lightning bolts lance through the cloud of gas and ash.Q1: Suppose you were going to repeat Miller’s experiments. How
would you decide how much of each gas to include in the chamber?Q2: Why did the addition of steam to the gases in Miller’s second set
of experiments increase the yield of amino acids?Q3: Miller used electrical energy in his experiment. What other forms
of energy were present in the early atmosphere of Earth that could
have led to the formation of complex molecules?