CHAPTER 26 | ASTROBIOLOGY: LIFE ON OTHER WORLDS 589
it is for complex organic molecules to form naturally from sim-
pler compounds, so it is not surprising to fi nd them in space.
Although speculation is fun, the hypothesis that life arrived on
Earth from space is presently more diffi cult to test than the
hypothesis that Earth’s life originated on Earth.
Whether the fi rst reproducing molecules formed here on
Earth or in space, the important thing is that they could have
formed by natural processes. Scientists know enough about those
processes to feel confi dent about them, even though some of the
steps remain unknown.
Th e details of the origin of the fi rst cells are unknown. Th e
structure of cells may have arisen automatically because of the way
molecules interact during chemical evolution. If a dry mixture of
amino acids is heated, the acids form long, proteinlike molecules
that, when poured into water, collect to form microscopic spheres
that behave in ways similar to cells (pictured in the image on the
opening page to this chapter). Th ey have a thin membrane sur-
face, they absorb material from their surroundings, they grow in
size, and they divide and bud just as cells do. However, they con-
tain no large molecule that copies itself, so they are not alive. Th e
fi rst reproducing molecule to be surrounded by a protective mem-
brane, resulting in the fi rst cell, would have gained an important
survival advantage over other reproducing molecules.Geologic Time and the Evolution of Life
Biologists infer that the fi rst cells must have been simple single-
celled organisms similar to modern bacteria. As you learned
earlier, these kinds of cells are preserved in stromatolites
(Figure 26-2), mineral formations produced by layers of bacteria
and shallow ocean sediments. Stromatolite fossils are found in
rocks with radioactive ages of 3.4 billion years, and living stro-
matolites still form in some places today.
Stromatolites and other photosynthetic organisms would
have begun adding oxygen, a product of photosynthesis, to
Earth’s early atmosphere. An oxygen abundance of only 0.1 per-
cent would have created an ozone screen, protecting organisms
from the sun’s ultraviolet radiation and later allowing life to colo-
nize the land.
Over the course of eons, the natural processes of evolution
gave rise to stunningly complex multicellular life forms with
their own widely diff ering ways of life. It is a Common
Misconception to imagine that life is too complex to have
evolved from such simple beginnings. It is possible because small
variations can accumulate, although that accumulation requires
great amounts of time.
Th ere is little evidence of anything more than simple organ-
isms on Earth until about 540 million years ago, almost 3 billion
years after the earliest signs of life, at which time fossils indicate
life suddenly developing into a wide variety of complex forms
such as the trilobites (■ Figure 26-6). Th is sudden increase in
complexity is known as the Cambrian explosion and marks the
beginning of the Cambrian period.proceed easily in a water solution. Scientists hypothesize that this
step may have been more likely to happen on shorelines or in
sun-warmed tidal pools where organic molecules from the pri-
mordial soup could have been concentrated by water evapora-
tion. Th e production of large organic molecules may have been
aided in such semidry environments by clay crystals acting as
templates to hold the organic subunits close together.
Th ese complex organic molecules were still not living things.
Even though some proteins may have contained hundreds of
amino acids, they did not reproduce but rather linked and broke
apart at random. Because some molecules are more stable than
others, and some bond together more easily than others, scientists
hypothesize that a process of chemical evolution eventually con-
centrated the various smaller molecules into the most stable larger
forms. Eventually, according to the hypothesis, somewhere in the
oceans, after suffi cient time, a molecule formed that could copy
itself. At that point, the natural selection and chemical evolution
of molecules became the biological evolution of living things.
An alternate theory for the origin of life holds that reproduc-
ing molecules may have arrived here from space. Astronomers
have found a wide variety of organic molecules in the interstellar
medium, and similar compounds have been found inside mete-
orites (■ Figure 26-5). Th e Miller experiment showed how easy
■ Figure 26-5
A piece of the Murchison meteorite, a carbonaceous chondrite (see Chapter
25) that fell near Murchison, Australia, in 1969. Analysis of the interior of
the meteorite revealed the presence of amino acids. Whether the fi rst chemi-
cal building blocks of life on Earth originated in space is a matter of debate,
but the amino acids found in meteorites illustrate how commonly amino
acids and other complex organic molecules occur in the universe, even in the
absence of living things. (Chip Clark, National Museum of Natural History)