586 PART 5^ |^ LIFE
Life in the Universe
Life as we know it consists of just the single example of life on
Earth. It is OK to think of all life on Earth as being just a single
type of life, because, as you learned in the previous section, all
living things on Earth have the same physical basis: the same
chemistry and the same genetic code alphabet. How life began
on Earth and then developed and evolved into its present variety
is the only solid information you have to work with, when con-
sidering what might be possible on other worlds.
Everything currently known about life on Earth indicates
that the same natural processes should lead to the origin of life
on some fraction of other planets with liquid water. If there is life
on other worlds, does it use DNA and RNA to carry the infor-
mation for life processes, or diff erent molecules playing the same
role, or some radically diff erent scheme? Th ere is no way to know
unless another example of life is found on another world. If and
when that day comes, even if the non-Earthly life is a simple one-
celled organism, the discovery will be one of the most important
in the history of science. It will complete the journey of human
understanding, begun in the Copernican revolution, of progres-
sive realizations that Earth is not unique.Origin of Life on Earth
It is obvious that the 4.5 billion chemical bases that make up
human DNA did not come together in the right order just by
chance. Th e key to understanding the origin of life lies in pictur-
ing the processes of evolution running “backward.” Th e complex
interplay of environmental factors with the DNA of generation
after generation of organisms drove some life forms to become
more sophisticated over time, until they became the unique and
specialized creatures alive today. Imagining this process in reverse
leads to the idea that life on Earth began with very simple
forms.
Biologists hypothesize that the fi rst living things would have
been carbon-chain molecules able to copy themselves. Of course,26-2
Modifying the Information
Earth’s environment changes continuously. To survive, species
must change as their food supply, climate, or home terrain
changes. If the information stored in DNA could not change,
then life would go extinct quickly. Th e process by which life
adjusts itself to changing environments is called biological
evolution.
When an organism reproduces, its off spring receive a copy
of its DNA. Sometimes external eff ects such as natural radiation
alter the DNA during the parent organism’s lifetime, and some-
times mistakes are made in the copying process, so that occasion-
ally the copy is slightly diff erent from the original. Off spring
born with random alterations to their DNA are called mutants.
Most mutations make no diff erence, but some mutations are
fatal, killing the affl icted organisms before they can reproduce. In
rare but vitally important cases, a mutation can actually help an
organism survive.
Th ese changes produce variation among the members of a
species. All of the squirrels in the park may look the same, but
they carry a range of genetic variation. Some may have slightly
longer tails or faster-growing claws. Th ese variations make almost
no diff erence until the environment changes. For example, if the
environment becomes colder, a squirrel with a heavier coat of fur
will, on average, survive longer and produce more off spring than
its normal contemporaries. Likewise, the off spring that inherit
this benefi cial variation will also live longer and have more off -
spring of their own. In contrast, squirrels containing DNA reci-
pes for thin fur coats will gradually decrease in number. Th ese
diff ering rates of survival and reproduction are examples of natu-
ral selection. Over time, the benefi cial variation increases in
frequency, and a species can evolve until the entire population
shares the trait. In this way, natural selection adapts species to
their changing environments by selecting, from the huge array of
random variations, those that would most benefi t the survival of
the species.
It is a Common Misconception that evolution is
random, but that is not true. Th e underlying mechanisms creat-
ing variation within each species may be random, but natural
selection is not random because progressive changes in a species
are directed by changes in the environment.
SCIENTIFIC ARGUMENT
Why is it important that errors occur in copying DNA?
Sometimes the most valuable scientifi c arguments are those that
challenge what seems like common sense. It appears obvious that
mistakes shouldn’t be made in copying DNA, but in fact variation
is necessary for long-term survival of a species. For example, the
DNA in a starfi sh contains all the information the starfi sh needs
to grow, develop, survive, and reproduce. The information must
be passed on to the starfi sh’s offspring for them to survive. Thatinformation must change, however, if the environment changes. A
change in the ocean’s temperature may kill the specifi c shellfi sh
that the starfish eat. If none of the starfish are able to digest
another kind of food—if all the starfish have exactly the same
DNA—they all will die. But if a few starfi sh are born with the abil-
ity to make enzymes that can digest a different kind of shellfi sh,
the species may be able to carry on.
Variations in DNA are caused both by external factors such as
natural radiation and by occasional mistakes in the copying pro-
cess. The survival of life depends on this delicate balance between
mostly reliable reproduction and the introduction of small varia-
tions in DNA. Now build a new argument. Why does the DNA copy-
ing process need to be mostly reliable?