Encyclopedia of Evolution.pdf - Online Reading Center

Sahelanthropus
• Nuclear winter. Sagan was one of the authors of the
famous 1983 “TTAPS Paper,” named after the initials
of the five authors, which predicted that nuclear winter
might follow a major nuclear exchange. Previous scholars
who investigated the possible consequences of nuclear war
focused on the destruction of human life and civilization.
Sagan and coauthors claimed that the billows of black
smoke would block sunshine and cause worldwide freezing,
and a collapse of food production, in countries not
bombed by nuclear weapons. Once a considerable portion
of the Earth was covered by ice, the reflection of sunlight
would prevent it from melting and maintain a permanent
frozen state. The TTAPS paper was controversial, as was
the paper that followed it in the journal of the American
Association for the Advancement of Science, which was
coauthored by more than 20 scientists and claimed that
a nuclear winter might result in the extinction of all life
on Earth. Subsequent research has shown that a major
nuclear exchange was more likely to result in a “nuclear
autumn” than a nuclear winter. Nuclear winter would be
similar to what happened in the Cretaceous extinction,
which was at the time a still-controversial theory, and in
Snowball Earth, which had not yet been publicized and
remains controversial today. Sagan’s advocacy of nuclear
winter theory was one of the main reasons that he spoke
out and performed acts of civil disobedience against the
development of outer-space nuclear weapons during the
presidency of Ronald Reagan.
• Global warming. Sagan had studied the way carbon dioxide
causes an intense greenhouse effect on Venus, and
he was familiar with the evidence that an excessive greenhouse
effect, caused by human activity, was beginning on
Earth.
Carl Sagan contributed to the public understanding of
science by bringing together insights from different fields of
study, and by his zeal for the importance of science. Sagan
died December 20, 1996.
Further Reading
Davidson, Keay. Carl Sagan: A Life. New York: John Wiley and
Sons, 2000.
Sagan, Carl. Billions & Billions: Thoughts on Life and Death at the
Brink of the Millennium. New York: Random House, 1997.
———. Broca’s Brain: Reflections on the Romance of Science. New
York: Random House, 1979.
———. Contact. New York: Simon and Schuster, 1985.
———. Cosmos. New York: Random House, 1980.
———. The Demon-Haunted World: Science as a Candle in the
Dark. New York: Ballantine Books, 1997.
———. The Dragons of Eden: Speculations on the Evolution of
Human Intelligence. New York: Bantam, 1977.
———. Pale Blue Dot: A Vision of the Human Future in Space. New
York: Ballantine, 1997.
Turco, R. P., O. B. Toon, T. P. Ackerman, J. B. Pollack, and Carl
Sagan. “Nuclear winter: Global consequences of multiple nuclear
explosions.” Science 222 (1983): 1,283–1,292.
Sahelanthropus See australopithecines.
scala naturae Also known as The Great Chain of Being,
the scala naturae (scale of nature) was an ancient and medieval
classification of all created objects that arranges them
from lower to higher and makes connections among all of
them. Inanimate objects (such as the four elements air, water,
earth, and fire) are lowest on the scale, organisms without
consciousness (such as plants) are a little higher, animals a
little higher, humans yet higher, and angelic beings highest.
The scala naturae was often interpreted to show connections
among the created objects. From this viewpoint, sponges were
between animals and rocks, and scholars expected that animals
intermediate between humans and animals should exist.
Among the earliest sources of the scala naturae are the
Greek philosopher Plato and the writings of the fourth century
c.e. theologian and philosopher Augustine. Augustine’s
two major reasons for laying the foundation of the scala
naturae were the principle of plenitude and a defense of
theodicy. Plenitude refers to the completeness of creation: It
contains all levels of complexity and all intermediates among
types. The world, in order to operate, requires all these levels
and intermediates. While scholars in the Augustinian tradition
considered plants below animals and animals below
people, they knew that animals could not exist without plants
or people without a world of plants and animals. Theodicy
is a subdiscipline of theology which investigates the reasons
why a good and all-powerful God would allow suffering and
death. Although the scala naturae never offered a complete
explanation of human suffering and death, it did explain why
animals and plants had to die: The world would not operate
without having them as food.
Some observers have noted a similarity between the scala
naturae and the ecological understanding of nature, with food
chains and complex webs of interaction, which began late
in the 19th century. If this similarity is real, it did not lead
ancient or medieval scholars to understand actual ecological
processes. Other observers have noted a similarity between
the scala naturae and evolution, especially when they find
out that the scala naturae included animal-human intermediates.
This similarity is accidental. First, ancient and medieval
scholars had no concept of the development of one form
of organism from another, or of organisms from inanimate
beginnings. Second, the evolutionary process has produced a
tree of life with branches that, except for symbiogenesis,
separate from one another rather than forming a network as
depicted in the scala naturae.
Further Reading
Lovejoy, A. B. The Great Chain of Being. Cambridge, Mass.: Harvard
University Press, 1936.
scientific method Darwin’s friend Thomas Henry Huxley
(see Huxley, Thomas Henry) called the scientific method
“organized common sense” or “simply common sense at its
best,” which is almost completely true. There is no established
list of steps or rules in the scientific method, but cer-