Encyclopedia of Evolution.pdf - Online Reading Center

Precambrian time
at least some genetic variability due to mutation. Genetic variability
in populations can be measured on phenotypic traits,
or on proteins. The Hardy-Weinberg equilibrium expresses
the mixture of genes that will result in the absence of natural
selection. Recessive lethal mutations can be eliminated only
slowly from populations; lethal mutations may be selected if
they provide a benefit to heterozygotes; and lethal mutations
can show up more frequently in small populations, often of
endangered species, than in large populations.
Further Reading
Avise, John C. Population Genetics in the Wild. Washington, D.C.:
Smithsonian, 2002.
Ayala, Francisco J. Population and Evolutionary Genetics: A Primer.
Menlo Park, Calif.: Benjamin-Cummings, 1982.
Freeman, Scott, and Jon C. Herron. “Mechanisms of evolutionary
change.” Part 2 of Evolutionary Analysis, 3rd ed. Upper Saddle
River, N.J.: Pearson Prentice Hall, 2004.
LeRoi, Armand. Mutants: On Genetic Variety and the Human Body.
New York: Viking, 2003.
Lewontin, Richard. The Genetic Basis of Evolutionary Change. New
York: Columbia University Press, 1974.
Ruder, Kate. “Genomics in Amish country.” Genome News Network.
Available online. URL: http://www.genomenewsnetwork.org/articles/
2004/07/23/sids.php. Accessed October 5, 2005.
Stebbins, G. Ledyard. Processes of Organic Evolution. Englewood
Cliffs, N.J.: Prentice Hall, 1971.
Precambrian time Precambrian time refers to the history
of the Earth before the Cambrian period of the Paleozoic
era (see geological time scale). Precambrian time makes
up about almost 90 percent of Earth history. Geologists have
divided Precambrian time into:
• Hadean Eon (4.5 billion–3.8 billion years ago), when the
Earth was forming from solar system dust (see universe,
origin of). The Earth experienced massive bombardments
(see asteroids and comets). Oceans had not yet
formed.
• Archaean Eon (3.8 billion–2.5 billion years ago), when the
Earth was mostly covered by oceans, and the atmosphere
contained almost no oxygen gas. Bacteria evolved early in
the Archaean Eon (see archaebacteria; bacteria, evolution
of; origin of life). A tremendous amount of biochemical
evolution occurred during this time, which was
invisible in the fossils.
• Proterozoic Eon (2.5 billion–0.5 billion years ago), when
continents began to form, and oxygen began to accumulate
in the atmosphere (see photosynthesis, evolution
of). Although oxygen buildup began in the atmosphere
and shallow oceans at the beginning of the Proterozoic,
the deep oceans may have remained anoxic until less than
a billion years ago. In the middle of the Proterozoic Eon,
the first complex cells formed (see eukaryotes, evolution
of).
Toward the end of the Proterozoic Eon, multicellular
organisms existed: seaweeds, animal embryos, and Ediacaran
organisms. The Ediacaran period has been recently
defined as lasting from about 600 million to about 540 million
years ago.
Further Reading
Kennedy, Martin, et al. “Late Precambrian oxygenation: Inception of
the clay mineral factory.” Science 311 (2006): 1,446–1,449.
Kerr, Richard A. “A shot of oxygen to unleash the evolution of animals.”
Science 314 (2006): 1529.
primates Primates constitute the class of mammals (see
mammals, evolution of) that includes lemurs, monkeys,
and apes. Humans are apes, therefore primates. The term
primate, used by scientists for several centuries, suggests that
this group, which includes humans, is the most important
among mammalian classes. While scientists no longer rank
classes by their importance relative to humanity, the term primate
continues in use.
Biologists classify primates into approximately 230 living
species of prosimians, tarsiers, and anthropoids. Primates,
in general, share these characteristics:
• They have the largest brains among mammals, after adjustment
for body size (see allometry).
• They often live in large, complex social groups. Large
brains aid survival but are considered to be most important
for social interactions (see intelligence, evolution of).
• They take a longer time to reach maturity than most
other mammals, a characteristic related to the evolution
of intelligence.
The first primates lived in trees, and most still do, except
for a few groups such as baboons and humans that have
(independently from one another) evolved the ability to live
on the ground. This adaptation is complete enough that in
humans, uniquely among primates, the lower limbs (legs) are
relatively long. Some, like galagos, have remarkable leaping
abilities: Though less than a foot and a half (50 cm) in length,
galagos can leap over six feet (2 m). Leaping from branch to
branch in trees has encouraged the evolution of important
characteristics that primates possess more than most other
mammals:
• The ability to grasp. This results primarily from an opposable
thumb, rare in other mammals. Primates not only
have the ability to grasp branches in trees, but the juvenile
primates can efficiently cling to the fur of their mothers.
Most primates also have opposable big toes, although primates
that have evolved a ground-based existence have lost
this characteristic. Nonhuman apes have opposable big
toes; humans do not; the australopithecine ancestors of
humans had big toes that were intermediate in their adaptation
for grasping branches.
• Excellent vision. Nocturnal primates have big eyes. For
example, each of the tarsier’s eyes is larger than its brain.
Diurnal primates have inherited big eyes from nocturnal
ancestors. Visual acuity is important not only for night
vision but for jumping around in trees. For prosimians,
smell remains an important sense, while most anthropoids
rely primarily on vision.