Encyclopedia of Evolution.pdf - Online Reading Center

ecapitulation
would cause some of the lead to diffuse out. In 2004 a new
method was published in which acid-pressure treatment of
zircon crystals is used to remove the weathered portions of
zircon crystals. Radiometric dating of geological deposits can
only be used on volcanic rocks. Therefore sedimentary deposits
can only be dated by determining the ages of the volcanic
rocks above and below them.
The uranium-lead method was developed by chemist
Clair Patterson, starting in 1948. One of the major problems
he encountered was environmental lead contamination. Patterson
had to design a special clean laboratory and methods for
obtaining specimens that were not contaminated. He proved
from an examination of ice cores from glaciers that the lead
contamination began about 1923, which was the year that tetraethyl
lead began to be added to gasoline. In this way, Patterson
not only developed a valuable scientific technique but also
brought a major problem of environmental pollution to light.
The radiocarbon method can be used on organic material.
Organisms incorporate 14C along with other carbon isotopes
( 13C and 12C) into their bodies. This process stops when
the organism dies. Because 14C is only about 0.1 percent of
the total carbon in the original specimen, this method cannot
be easily used on specimens older than 50,000 years.
The radiocarbon method was invented by chemist Willard
Libby, who received the Nobel Prize in 1960 for this discovery.
It has proven immensely valuable but there are several
problems that have to be taken into account:
• Due to an inaccuracy in the formula that was originally
used, all raw radiocarbon dates are too young by three percent.
This is why radiocarbon dates are reported in scientific
literature as raw or as corrected.
• Another problem is that older organic specimens may be
contaminated by carbon from decomposing litter, making
the determined ages too young. Alternatively, 14C can be
leached out of a specimen. Only a slight amount of leaching
of 14C in or out of a specimen can lead to inaccurate
age determination. A one percent increase in 14C can make
a 67,000-year-old bone appear to be 37,000 years old.
• 14C is produced by cosmic radiation as a neutron enters the
nucleus of a nitrogen atom. The amount of 14C in the atmosphere
is influenced by the Earth’s magnetic field, which
varies and reverses (see paleomagnetism). This is why
radiocarbon dates must be calibrated. The standard against
which this calibration is performed, called IntCal98, uses
tree rings, which are reliable to within 30 years, to calibrate
specimens up to 11,800 years before the present, then uses
data from marine sediments to calibrate older specimens.
Fluctuations in the ratio of 12C to 14C make some periods
of history difficult to calibrate, such as the period 750 to
400 bpe, and the Younger Dryas period following the most
recent of the ice ages, 13,000 to 11,900 years ago.
• Even a diet high in fish can make radiocarbon dates of
bones too old.
Uranium 234 ( 234 U) decays into thorium 230 ( 230 Th)
relatively quickly and is therefore not very useful for radiometric
dating of rocks. Corals, however, incorporate a small
amount of uranium into their shells, and some of this is 234 U.
The decay of 234 U into 230 Th can be used to determine the
age of old coral specimens, in the range of a few thousand to
about 300,000 years. In 2004 this method was used to determine
the ages of Hawaiian archaeological deposits in which
coral was used as a building material.
Particles released by radioactive decay can leave fission
tracks in surrounding material. The older the material is, the
more fission tracks will be found in it. Fission tracks have
also been used as a method of radiometric dating.
Buried objects, such as bones too old for radiocarbon
dating, can absorb radioactive material from the deposits
that surround them. There are three techniques (thermoluminescence,
optically stimulated luminescence, and electron spin
resonance) by which the amount of absorbed radioactivity
can be measured.
Further Reading
Bard, Edouard, Franke Rostek, and Guillamette Ménot. “A better
radiocarbon clock.” Science 303 (2004): 178–179.
Mundil, Roland, et al. “Age and timing of the Permian mass extinctions:
U/Pb dating of closed-system zircons.” Science 305 (2004):
1,760–1,763.
Robbins, Michael W. “How we know Earth’s age.” Discover, March
2006, 22–23.
recapitulation Biologists who studied the development of
animal embryos in the 18th and 19th centuries noticed the
similarities between the stages of embryonic development
and the range of animal diversity. In this view, embryonic
development recapitulates, or retells the story of, animal history.
In 1816 the German anatomist Friedrich Tiedemann
published evidence that the human brain, during embryonic
development, passed through stages that resembled those of
simpler vertebrates. He wrote, “We therefore cannot doubt
that nature follows a uniform plan in the creation and development
of the brain in both the human fetus and in the
sequence of vertebrate animals.”
Once evolution had been accepted (see Darwin,
Charles; origin of species [book]; Descent of man
[book]), some scientists translated Tiedemann’s sequence of
divine pattern into the belief that embryonic development
retraced the evolutionary history of vertebrates. The most
famous of these evolutionary scientists (see Haeckel, Ernst)
claimed that humans go through fish, amphibian, and reptile
stages during embryonic development. His famous slogan
for this hypothesis was “Ontogeny recapitulates phylogeny.”
Haeckel published drawings of embryonic development in a
fish, a salamander, a tortoise, a chick, a hog, a calf, a rabbit,
and a human. The early embryos of each were virtually
identical in structure, while the later embryos showed the distinctive
features of each animal. Haeckel was so convinced of
recapitulation that his drawings were idealized rather than
realistic. In particular, early vertebrate embryos are not as
similar to one another as Haeckel depicted. Haeckel’s drawings
were immediately recognized as inaccurate, and some
scientists declared them fraudulent.
Recapitulation is no longer considered an essential evolutionary
hypothesis. Evolutionary scientists do not claim