Encyclopedia of Evolution.pdf - Online Reading Center

sissippi River is getting shorter each year because new river
courses cut off oxbows. From this, he said, scientists may
conclude that a million years ago the Mississippi River stuck
out over the Caribbean like a fishing rod, and a million years
from now, the Mississippi River will shorten, causing New
Orleans and St. Louis to be in the same place and have a joint
board of aldermen.
Uniformitarianism played a valuable role in allowing
geological science to develop the tools of investigating the
past using knowledge from the present. Only recently, however,
has the strict grip of uniformitarianism loosened enough
to allow exceptions to be understood.
Further Reading
Benton, Michael. “The Death of Catastrophism.” Chap. 3 in When
Life Nearly Died. London: Thames and Hudson, 2003.
universe, origin of Although the universe includes everything,
this discussion is limited to the origin of galaxies, stars,
and planets. Although some scholars use the term evolution
for the processes and changes in the history of the universe,
the processes of origin and change in the universe have no
counterparts to genetics or natural selection.
The modern understanding of the origin of the universe
would have been practically unthinkable to people even a
century ago. For one thing, they had no concept of the size
of the universe. Astronomers knew that the universe was, for
them, incalculably vast. It might have been possible to use triangulation
to calculate the distance of a star, using the diameter
of Earth’s orbit around the Sun as the base of the triangle,
but the stars are so far away that the angle opposite the base
was effectively zero. Before large modern telescopes, it was
not clear whether the nebulae were clouds of gas or clusters
of stars (there are many of each). It was not until 1923 that
astronomer Edwin Hubble was able to focus on the Andromeda
galaxy well enough to discern individual stars. If those
stars appeared so small, then the galaxy must be incredibly
distant. There was a suspicion that nebulae and stars went
through the equivalent of life cycles, but there was no direct
observation that showed that anything changed in the universe,
except for occasional supernova explosions.
With improved techniques, it became possible to estimate,
though not directly measure, the distance of galaxies.
Astronomers Henrietta Swan Leavitt and Harlow Shapley, at
the Harvard College Observatory, determined that Cepheid
variable stars had a very reliable correlation between the
periodicity of their variation and their absolute luminosity.
The intensity of light decreases with the square of its distance
from the observer. When Edwin Hubble looked at the
individual stars in the Andromeda galaxy, he found some
that changed their luminosity in the same way as a Cepheid
variable star. From the correlation calculated by Leavitt and
Shapley, Hubble calculated the absolute luminosity; and by
comparing the absolute with the observed luminosity, he
calculated the distance of the galaxy. The resulting distance
could be meaningfully expressed only in light-years, the distance
that light can travel in a year, as it travels at 186,000
universe, origin of 0
miles (almost 300,000 km) per second. The Andromeda galaxy
was 900,000 light-years away.
Astounding as this discovery was, Hubble’s main breakthrough
was his discovery that the universe is expanding.
Ever since German chemist Joseph Frauenhofer discovered
that chemical elements block certain wavelengths of light,
astronomers had been able to analyze the chemical composition
of stars and planets by studying these lines of darkness
in the spectrum of light from a star or planet. Hubble
discovered that the Frauenhofer lines displayed a marked
redshift—the absorbance lines were further toward the red
(long wavelength) end of the spectrum than they should
be—and that different stars had different degrees of redshift.
When an object is coming toward the observer, waves emitted
from the object are shortened; when an object is moving
away from the observer, waves emitted from the object
are lengthened. This Doppler effect explains why the whistle
of a train coming toward an observer has a higher pitch
than when the train has passed the observer: Sound consists
of waves of air molecules. The redshift of stars and galaxies
suggested that they are moving away from the Earth (or,
more properly, that the space between them and the Earth is
expanding). This suggested an expanding universe.
Hubble and other astronomers noticed something else.
The more distant galaxies had a greater redshift than the
closer galaxies. There was a constant relationship between
speed and distance, a constant now called the Hubble constant.
This meant:
• For galaxies too distant for Cepheid variable stars to be
observed, the Hubble constant allowed a calculation of distance,
once the redshift is measured. The redshift of some
galaxies was huge: In some cases, the hydrogen band that
should be at 0.000005 inch (122 nm, in the ultraviolet
part of the spectrum) is shifted all the way over to 0.00003
inch (720 nm, in the infrared part of the spectrum). But the
relationship between redshift and distance had to be reliable,
unless the laws of nature were not reliable. Some of
the galaxies observed by the orbiting telescope named after
Edwin Hubble are so distant that they actually appear red
in the photographs.
• All the galaxies began at a single point at a single time.
This was the origin of the modern big bang theory. An
explosive origin of the universe had been proposed by the
Belgian priest and physicist Georges Lemaître earlier in the
century, but the redshift was the first evidence for it. The
universe had to have begun from a gigantic explosion that
produced literally everything. Finally, the inverse of the
Hubble constant even provided an estimate of when this
happened, sometime less than 15 billion years ago. One
recent estimate by the National Aeronautics and Space
Administration (NASA) indicates that the universe began
13.7 ± 1 billion years ago.
The very idea that the universe even had a beginning
was not automatically accepted by all astronomers. Sir Fred
Hoyle, a famously free-thinking British astronomer, defended
an alternative view, which he called the steady state model.