600 PART 5^ |^ LIFE
expected, given the wide variety of living things now found in extreme
environments on Earth. Tidally heated moons orbiting large planets
could have liquid water at any distance from a star.
▶ Because of distance, speed, and fuel, travel between the stars seems
almost impossible for humans or for aliens who might visit Earth.
▶ Communication between planetary systems using electromagnetic
signals such as radio waves or laser beams may be possible, but a real
conversation would be diffi cult because of long travel times for such
signals.
▶ Broadcasting a radio (or light) beacon of pulses would distinguish
the signal from naturally occurring emission and identify the source
as a technological civilization. The signal can be anticoded (p. 595)
in the hope it would be possible for another civilization to decode.
▶ One good part of the radio spectrum for communication is called the
water hole (p. 595), the wavelength range from the 21-cm spectral
line of hydrogen to the 18-cm line of OH. Even so, millions of radio
wavelengths need to be tested to fully survey the water hole for a
given target star.
▶ Sophisticated searches are now underway to detect radio transmis-
sions from civilizations on other worlds, but such SETI (Search for
Extra-Terrestrial Intelligence) (p. 595) programs are hampered
by limited computer power and radio noise pollution from human
civilization.
▶ The number of civilizations in our galaxy that are at a technologi-
cal level and able to communicate while humans are listening can be
estimated by the Drake equation (p. 596). This number is limited
primarily by the lifetimes of their and our civilizations.Review Questions
- If life is based on information, what is that information?
- How does the DNA molecule produce a copy of itself?
- What would happen to a life form if the genetic information handed
down to offspring was copied extremely inaccurately? How would that
endanger the future of the life form? - What would happen to a life form if the information handed down to
offspring was always the same? How would that endanger the future of
the life form? - Give an example of natural selection acting on new DNA patterns to
select the most advantageous characteristics. - What evidence do scientists have that life on Earth began in the sea?
- Why do scientists generally think that liquid water is necessary for the
origin of life? - What is the difference between chemical evolution and biological
evolution? - What is the signifi cance of the Miller experiment?
- How does intelligence make a creature more likely to survive?
- Why are upper-main-sequence (high-luminosity) stars unlikely sites for
intelligent civilizations? - Why is it reasonable to suspect that travel between stars is nearly
impossible? - How does the stability of technological civilizations affect the
probability that Earth can communicate with them? - What is the water hole, and why is it a good “place” to search for
extraterrestrial civilizations? - Why is it diffi cult to anticode a message? In other words, why is
it hard to make a message that potentially can be understood by
completely unknown recipients? - How Do We Know? How do science and religion have complementary
explanations of the world? - How Do We Know? Why are scientists sure Earth has never been
visited by aliens?
▶ The unit of heredity is a gene (p. 585), a piece or several pieces of
DNA that in most cases specifi es the construction of one particular
type of protein molecule. Genes are connected together in structures
called chromosomes (p. 585), that are essentially single long DNA
molecules.
▶ When a cell divides, the chromosomes split lengthwise and duplicate
themselves so that each of the new cells can receive a copy of the
genetic information.
▶ Biological evolution (p. 586) is the process by which life adjusts
itself to changing environments.
▶ Errors in duplication or damage to the DNA molecule can produce
mutants (p. 586), organisms that contain new DNA information and
therefore have new properties. Variation in genetic codes can become
widespread among individuals in a species. Natural selection (p. 586)
determines which of these variations are best suited to survive, and the
species evolves to fi t its environment.
▶ Evolution is not random. Genetic variation is random, but natural selec-
tion is controlled by the environment.
▶ The oldest defi nitely identifi ed fossils on Earth, structures called stro-
matolites (p. 587) that are composed of stacks of bacterial mats and
sediment layers, are at least 3.4 billion years old. Those fossils provide
evidence that life began in the oceans.
▶ Fossil evidence indicates that life began on Earth as simple single-
celled organisms like bacteria and much later evolved into more com-
plex, multicellular (p. 589) creatures.
▶ The Miller experiment (p. 587) shows that the chemical building
blocks of life form naturally under a wide range of circumstances.
▶ Scientists hypothesize that chemical evolution (p. 589) occurred
before biological evolution. Chemical evolution concentrated simple
molecules into a diversity of larger stable organic molecules dissolved in
the young Earth’s oceans, but those molecules did not reproduce copies
of themselves. The hypothetical organic-rich water is sometimes referred
to as the primordial soup (p. 588). Biological evolution began when
molecules developed the ability to make copies of themselves.
▶ (^) Life forms did not become large and complex until about 0.5 billion
years ago, during what is called the Cambrian explosion (p. 589).
▶ (^) Life emerged from the oceans only about 0.4 billion years ago, and
human intelligence developed over the last 4 million (0.004 billion)
years.
▶ (^) Life as it is known on Earth requires liquid water and thus a specifi c
range of temperatures.
▶ (^) Organisms that thrive in extreme environments are called extremo-
philes (p. 591). Genetic evidence indicates that the common ancestor
of all Earth life was a thermophile (p. 591), a heat-tolerant version of
present-day single-celled organisms called archaea (p. 591).
▶ (^) No other planet in our solar system appears to harbor life at present.
Most are too hot or too cold, although life might have begun on Mars
before the planet became too cold and dry. If so, life conceivably could
persist today on Mars in limited hospitable environments.
▶ (^) Liquid water exists, and therefore Earth-like life is at least possible,
under the surfaces of Jupiter’s moons Europa and Ganymede and
Saturn’s moon Enceladus. Saturn’s moon Titan has abundant organic
compounds but does not have liquid water.
▶ (^) Because the origin of life and its evolution into intelligent creatures
took so long on Earth, scientists do not consider middle- and upper-
main-sequence stars, which shine for astronomically short time spans,
as likely hosts for life-bearing planets.
▶ (^) Main-sequence G and K stars are thought to be likely candidates to
host planets with life. Scientists are not sure whether the M stars are
also good candidates.
▶ (^) The habitable zone (p. 594) around a star, within which planets can
have liquid water on their surfaces, may be larger than scientists had