Encyclopedia of Evolution.pdf - Online Reading Center

0 Smith, William
———. Tempo and Mode in Evolution. New York: Columbia University
Press, 1944.
———. This View of Life: The World of an Evolutionist. New York:
Harcourt, Brace and World, 1964.
Smith, William (1769–1839) British Geologist, Engineer William
Smith was the geologist who figured out that fossils could
be used to correlate sedimentary rock layers (see fossils and
fossilization). If a sedimentary layer in one location contained
an assemblage of fossils, and a sedimentary layer in another
location contained the same or similar assemblage of fossils, the
layers in both locations (even if separated by hundreds of miles)
were almost certainly formed at the same time. In this way, the
relative ages of fossil deposits can be determined, although their
absolute ages cannot. Smith’s insight allowed geologists in the
19th century to reconstruct the geological record of the Earth’s
entire history, although they had no idea of how many millions
of years the geological record spanned (see age of Earth;
radiometric dating).
Born March 23, 1769, William Smith received little formal
education but spent much time collecting fossils as he
grew up. He studied mapping and became a surveyor. Excelling
in this skill, Smith spent six years supervising the digging
of the Somerset Canal in southwestern England. Smith
noticed that the fossils within a vertical section of sedimentary
rocks were always in the same order from the bottom
to the top of the section, and the sedimentary rock sections
on one side of England matched those on the other side. Not
every species was useful for correlating the strata of different
locations; species that had very widespread distributions
or that persisted for a long time were not suitable for precise
correlation.
Smith was not the first to construct geological maps but
was the first to do so by fossils and not by the mineral composition
of the rocks. When Smith finally secured money from
private investors in 1812, he began a fossil-based geological
map of all of England and Wales, a task he finished in 1815.
Even though this map was initially disregarded by scientists,
it is difficult to overestimate its importance. It was an essential
base of information that allowed uniformitarianism to
replace catastrophism and that allowed the insights of later
geologists (see Lyell, Charles) and, eventually, evolutionary
scientists (see Darwin, Charles). Smith died August 28,
1839.
Further Reading
Winchester, Simon, and Soun Vannithone. The Map That Changed
the World: William Smith and the Birth of Modern Geology. New
York: Perennial, 2002.
Snowball Earth Periods of ice formation, global in extent,
occurred during Precambrian time. The Sturtian Glaciation
ended about 700 million years ago, the Marinoan Glaciation
about 635 million years ago, and the Gaskiers Glaciation
about 580 million years ago. There was an earlier period of
ice formation, about 2.4 billion years ago, about which much
less is known, because there are far fewer deposits that have
survived from that distant time. The ice formed on both land
and sea, not only in polar and temperate latitudes but even
in the tropics, which would have made the Earth look very
much like a snowball. Because there may have been little
open ocean, there would have been very little evaporation,
hence very little rain or snow, once the snowball conditions
formed.
Calculations by Russian scientist Mikhail Budyko in the
1960s suggested that the Earth could never have had such a
period of cold temperatures, because if this had occurred,
the Earth would have remained frozen forever. This would
occur because of the albedo effect: The sunlight would
have reflected from the white snow and ice, directly back
into outer space, without being absorbed; the only sunlight
that creates warmth is the sunlight that is absorbed. (This
is why a stick lying on top of the snow on a sunny day will
melt its way into the snow; the stick absorbs sunlight and
becomes warm, while the snow does not.) Budyko’s calculations
did not include the effects of volcanic eruptions.
Eruptions would have created numerous unfrozen spots in
the ocean and would have released carbon dioxide into the
atmosphere. Carbon dioxide contributes to the greenhouse
effect by absorbing infrared photons that are emitted by
the Earth. Very little infrared light would have been emitted
by a Snowball Earth, but as carbon dioxide levels built up
over millions of years, enough heat might have accumulated
to begin melting the ice. Two major processes by which carbon
dioxide is removed from the air are the photosynthesis
of organisms and the weathering of rocks. With so few
organisms, and with the rocks and oceans covered by ice,
there would have been almost nothing to remove carbon
dioxide from the air, thus allowing it to accumulate. These
processes would have allowed the Earth to emerge from its
snowball condition.
Snowball Earth is an example of a truly radical theory
that has become widely accepted, in various forms, by the
scientific community. Its principal proponents are geologists
Paul Hoffman and Daniel Schrag of Harvard University, basing
their studies on earlier work by geologists Joseph Kirschvink
and Brian Harland.
• One line of evidence suggesting that there was a Snowball
Earth is the massive accumulation of tillite dropstones in
late Precambrian deposits. Dropstones are a diverse mixture
of rocks that have been transported far from their
places of origin and dropped onto the bottom of the ocean.
It is generally agreed that only icebergs can carry dropstones
for such distances. The large deposits of dropstones
indicate that there must have been a lot of icebergs in the
late Precambrian. These deposits are found from all over
the world: Spitsbergen, China, Australia, Russia, Norway,
Namibia, Newfoundland, and the Rocky Mountains.
Therefore, the glaciation was global.
• Another line of evidence is the presence of late Precambrian
ironstones. When oxygen first began to be produced
by photosynthesis of microscopic organisms, it reacted
with iron in the ocean water and precipitated to the bottom
of the sea, forming iron deposits. This stopped happening
when the iron was removed from the ocean water,