582 PART 5^ |^ LIFE
defi nition of what living things do, distinguishing them from
nonliving things, might be: Life is a process by which an organism
extracts energy from the surroundings, maintains itself, and
modifi es the surroundings to foster its own survival and
reproduction.
One very important observation is that all living things on
Earth, no matter how apparently diff erent, share certain charac-
teristics in how they perform the process of life.The Physical Basis of Life
Th e physical basis of life on Earth is the element carbon
(■ Figure 26-1). Because of the way carbon atoms bond to each
other and to other atoms, they can join into long, complex,
stable chains that are capable, among many other feats, of stor-
ing and transmitting information. A large amount of informa-
tion is necessary to control the activities and maintain the forms
of living things.
It is possible that life on other worlds could use silicon
instead of carbon. Silicon is right below carbon in the periodic
table (Appendix Table A-16), which means that it shares many of
carbon’s chemical properties. But life based on silicon rather than
carbon seems unlikely because silicon chains are harder to assem-
ble and disassemble than their carbon counterparts and can’t be
as lengthy. Science fi ction has proposed even stranger life forms
based on, for example, electromagnetic fi elds and ionized gas,
and none of these possibilities can be ruled out. Th ose hypotheti-
cal life forms make for fascinating speculation, but for now they
can’t be studied systematically in the way that life on Earth can.Did I solicit thee from darkness to promote
me?
— ADAM, TO GOD, IN JOHN MILTON’S PARADISE LOSTA
s a living thing, you have been promoted from dark-
ness. Th e atoms of carbon, oxygen, and other heavy
elements that are necessary components of your body
did not exist at the beginning of the universe but were built up
by successive generations of stars.
Th e elements from which you are made are common every-
where in the observable universe, so it is possible that life began
on other worlds and evolved to intelligence. If so, perhaps those
other civilizations will be detected from Earth. Future astrono-
mers may discover distant alien species completely diff erent from
any life on Earth.
Your goal in this chapter is to try to understand truly
intriguing puzzles—the origin and evolution of life on Earth and
what that tells you about whether there is life on other worlds
(How Do We Know? 26-1). Th is new hybrid fi eld of study
is called astrobiology.
The Nature of Life
What is life? Philosophers have struggled with that question for
thousands of years, and it is not possible to answer it completely
in one chapter or even one book. An attempt at a general
26-1
■ Figure 26-1
All living things on Earth are based
on carbon chemistry. Even the
long molecules that carry genetic
information, DNA and RNA, have
a framework defi ned by chains of
carbon atoms. (a) Ana, a complex
mammal, contains about 60 AU of
DNA. (Jamie Backman) (b) Each rod-
like tobacco mosaic virus contains
a single spiral strand of RNA about
0.01 mm long as its genetic mate-
rial. (L. D. Simon)
a b