The Solar System

588 PART 5^ |^ LIFE

Th e Miller experiment is important because it shows that

complex organic molecules form naturally in a wide variety of

circumstances. Lightning, sunlight, and hot lava are just some of

the energy sources that can naturally rearrange common simple

molecules into the complex molecules that make life possible. If

you could travel back in time, you would expect to fi nd Earth’s

early oceans fi lled with a rich mixture of organic compounds

called the primordial soup.

Many of these organic compounds would have been able to

link up to form larger molecules. Amino acids, for example, can

link together to form proteins by joining ends and releasing a

water molecule (■ Figure 26-4). Th at reaction, however, does not

Scientists are professionally skeptical about scientifi c fi nd-
ings (see “How Do We Know?” 19-2), and they have reevalu-
ated the Miller–Urey experiment in light of new information.
According to updated models of the formation of the solar
system and Earth (see Chapters 19 and 20), Earth’s early atmo-
sphere probably consisted mostly of carbon dioxide, nitrogen,
and water vapor instead of the mix of hydrogen, ammonia,
methane, and water vapor assumed by Miller and Urey. When
gases corresponding to the newer understanding of the early
Earth atmosphere are processed in a Miller apparatus, lesser,
but still signifi cant, amounts of organic molecules are
produced.

⎫

⎪

⎬

⎪

⎭

Spark

discharge

Electrodes

Water droplets

Water containing

organic compounds

Condenser

Cooling water out

Cooling water in

CH 4

NH 3

H 2 O

H 2

To

vacuum

pump

Boiling water

a Liquid water in trap

Gases

b

■ Figure 26-3

(a) The Miller experiment circulated gases through water in the presence of an electric arc. This simulation of primitive conditions on Earth produced
many complex organic molecules, including amino acids, the building blocks of proteins. (b) Stanley Miller with a Miller apparatus. (Stanley Miller)

CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3

HH HH HH HH HH HH H

C

OOOOO

O

OO

NCCC NC CNCNC

HH

C N C C OH H C

H

NCOH

H

C

CH

3

Amino acid

N

H

H

C

OH

Amino acid Amino acid O

Water

Growing carbon-chain molecule

N C

■ Figure 26-4

Amino acids can link together via the release of a water molecule to form long carbon-chain molecules. The amino acid in this hypothetical example
is alanine, one of the simplest.