CHAPTER 26 | ASTROBIOLOGY: LIFE ON OTHER WORLDS 587
you can imagine there probably were earlier, simpler organisms.
Where did those fi rst simplest organisms come from?
An important experiment performed by Stanley Miller and
Harold Urey in 1952 sought to re-create the conditions in which
life on Earth began. Th e Miller experiment consisted of a sterile,
sealed glass container holding water, hydrogen, ammonia, and
methane. An electric arc inside the apparatus made sparks to
simulate the eff ects of lightning in Earth’s early atmosphere
(■ Figure 26-3).
Miller and Urey let the experiment run for a week and then
analyzed the material inside. Th ey found that the interaction
between the electric arc and the simulated atmosphere had pro-
duced many organic molecules from the raw material of the
experiment, including such important building blocks of life as
amino acids. (Recall that an organic molecule is simply a molecule
with a carbon-chain structure and need not be derived from a
living thing: “Organic” does not necessarily imply “biological.”)
When the experiment was run again using diff erent energy
sources such as hot silica to represent molten lava spilling into
the ocean, similar molecules were produced. Even a source of
ultraviolet radiation representing the small amount of UV in
sunlight was suffi cient to produce complex organic molecules.this is a scientifi c hypothesis for which you can seek evidence.
What evidence exists regarding the origin of life on Earth?
Th e oldest fossils are the remains of sea creatures, and this
indicates that life began in the sea. Identifying the oldest fossils
is not easy, however. Ancient rocks from western Australia that
are at least 3.4 billion years old contain features that biologists
identify as stromatolites, fossilized remains of colonies of single-
celled organisms (■ Figure 26-2). Fossils this old are diffi cult to
recognize because the earliest living things did not contain easily
preserved hard parts like bones or shells and because the indi-
vidual organisms were microscopic. Th us the evidence, though
scarce, indicates that simple organisms lived in Earth’s oceans less
than 1.2 billion years after Earth formed. Stromatolite colonies
of microorganisms are more complex than individual cells, so
■ Figure 26-2
(a) A fossil stromatolite from western Australia that is more than 3 bil-
lion years old, constituting some of the oldest evidence of life on Earth.
Stromatolites are formed, layer upon layer, by mats of bacteria living in
shallow water where they are covered repeatedly by sediments. (Chip Clark,
National Museum of Natural History) (b) Artist’s conception of a scene on the
young Earth, 3 billion years ago, with stromatolite bacterial mats growing
near the shores of an ocean. (Mural by Peter Sawyer)
ab