Encyclopedia of Evolution.pdf - Online Reading Center

pials successfully moved northward, the opossum being the
lone North American marsupial success story.
• When modern humans arrived in Australia thousands of
years ago, they brought dogs, which became the wild dingos;
Europeans brought many placental mammals such
as dogs, cats, and rodents a couple of hundred years ago.
Competition with placentals has driven most Australian
marsupials (except kangaroos) near or into extinction.
This should not, however, be taken as evidence of inherent
inferiority of the marsupial reproductive system. Some
observers think that marsupials lost in competition because
they had not been, in southern continents, subjected during
their evolution to the climatic extremes that had occurred in
the northern continents. However, the marsupial reproductive
system seems well suited to variable and occasionally harsh
conditions. In placentals, short of spontaneous abortion, the
mother has to carry the fetuses to full term, even under stressful
conditions such as famine. The infants may die soon after
birth, but not until after the placental mother has made a tremendous
prenatal investment in them. In marsupials, however,
the fetus is born at a very early stage, and when food is
scarce this is the stage at which they can be abandoned. Marsupials,
in addition, have a lower metabolic rate during rest,
which may help them conserve scarce energy. Because marsupials
appear superior under the stressful conditions that all
lineages have experienced at some time or other, the reasons
why the placentals have largely prevailed over the marsupials
are not clear.
Further Reading
Benton, Michael. “Four Feet on the Ground.” Chap. 4 in Gould,
Stephen Jay, ed., The Book of Life: An Illustrated History of the
Evolution of Life on Earth. New York: Norton, 1993.
Bininda-Emonds, Olaf R. P., et al. “The delayed rise of present-day
mammals.” Nature 446 (2007): 507–512.
Gould, Stephen Jay. “Sticking up for marsupials.” Chap. 28 in The
Panda’s Thumb. New York: Norton, 1980.
Hopson, J. A. “The mammal-like reptiles: A study of transitional fossils.”
American Biology Teacher 49 (1987): 16–26.
Kielan-Jaworowska, Zofia, Richard L. Cifelli, and Zhe-Xi Luo.
Mammals from the Age of Dinosaurs: Origins, Evolution, and
Structure. New York: Cambridge University Press, 2004.
Rich, Thomas H., et al. “Independent origins of middle ear bones in
monotremes and therians.” Science 307 (2005): 910–914.
Margulis, Lynn (1938– ) American Evolutionary biologist
Lynn Margulis (see photo) is one of the few scientists
alive today who has changed some of the basic assumptions
of a whole field of study. She persistently defended the endosymbiotic
theory of the origin of mitochondria and chloroplasts
that had been proposed decades earlier by Russian
botanists and by American biologist Ivan Wallin. She gathered
evidence from many studies that finally established this
theory. Mitochondria and chloroplasts are the evolutionary
descendants of bacteria. She is currently testing the theory
that eukaryotic cilia, flagella, and microtubules are also the
evolutionary descendants of bacteria (see symbiogenesis).
Margulis, Lynn
Lynn Margulis is most famous for her research into symbiogenesis.
(Courtesy of University of Massachusetts at Amherst)
She has presented evidence that symbiogenesis is an important,
perhaps the most important, source of evolutionary
novelty and diversification. Symbiosis (for example, with
microorganisms living inside the roots of plants) made plant
life on land possible, and plants then created the habitats in
which animals lived. Margulis says, “Symbiogenesis was the
moon that pulled the tide of life from its oceanic depths to
dry land and up into the air.” She also explains that, because
all life-forms interact so closely, they produce a global living
system that regulates some physical conditions (such as
temperature and atmospheric gas composition) of the entire
planet (see Gaia hypothesis). To Margulis, symbiosis is pervasive
throughout the living world, from the smallest cellular
components to the global ecosystem. She says, “Symbiosis is
not a marginal or rare phenomenon. It is natural and common.
We abide in a symbiotic world.”
Margulis has always been a creative thinker, willing
to take bold steps to pursue new theories. As Lynn Alexander,
she was an early entrant, at age 14, into the University
of Chicago. She pursued a general liberal arts degree that
emphasized independent study and the reading of original
publications rather than textbooks. While at Chicago she met