Encyclopedia of Evolution.pdf - Online Reading Center

A scanning electron micrograph from meteorite ALH 00 shows structures
that researchers initially thought were bacteria-like life-forms, although
they are much smaller than bacteria on Earth. (Courtesy of NASA)
fossilized structures are a hundred times smaller than the
smallest known bacteria on Earth, being more similar in size
to viruses. Viruses cannot live outside of cells. Is it possible
that such small organisms could have lived in a shallow Martian
pond billions of years ago? Every piece of evidence for
life in the Martian meteorite can be explained in other ways.
Most scientists remain unconvinced that the organic chemicals
were produced by Martian structures. It is often said that
extraordinary claims (such as life on Mars) require extraordinary
evidence. This evidence is good, but not good enough
for most scientists.
Further Reading
Jet Propulsion Laboratory, National Aeronautics and Space Administration,
Pasadena, Calif. “Mars Meteorites.” Available online. URL:
http://www2.jpl.nasa.gov/snc/index.html. Accessed July 10, 2006.
Kerr, Richard A. “New signs of ancient life in another Martian meteorite?”
Science 311 (2006): 1,858–1,859.
Malin, Michael C. et al. “Present-day impact cratering rate and contemporary
gully activity on Mars.” Science 314 (2006): 1,573–1,577.
McKay, D. S., et al. “Search for past life on Mars: Possible relic biogenic
activity in Martian meteorite ALH84001.” Science 273
(1996): 924–930. Available online. URL: http://www-curator.jsc.
nasa.gov/curator/antmet/marsmets/SearchForLife/SearchForLife.
htm. Accessed April 25, 2005.
Morton, Oliver. “Mars: Planet Ice.” National Geographic, January
2004, 2–31.
mass extinctions Mass extinction occurs when many species
become extinct at roughly the same time. extinction
may result if populations do not have the genes that allow
them to adapt to environmental changes, that is, from “bad
genes”; but extinction may also occur suddenly as a result
of a worldwide disaster, for which no populations possess
genetic resistance, that is, from “bad luck,” to use paleontologist
David Raup’s terminology. Global mass extinctions
are usually considered to have resulted from “bad luck.”
Therefore true mass extinction events may result from rare
catastrophic events rather than an acceleration of normal
extinction patterns.
Most of the geological periods (see geological time
scale) ended with global changes in climate and the origin of
recognizably different sets of species. Five of these were mass
extinctions (see biodiversity) (see table). Two of the mass
extinction events brought the first two eras of the Phanerozoic
Eon: the Permian extinction ended the Paleozoic
era, and the Cretaceous extinction ended the Mesozoic
era. A mass extinction, especially of the Ediacaran organisms,
may have occurred at the end of the Precambrian
time. A new analysis in 2005 suggests that the Ordovician,
Permian, and Cretaceous mass extinctions occurred against
a background of rapid evolution and therefore were caused
by global catastrophes, whereas the Devonian and Triassic
mass extinctions occurred against a background of ongoing
extinction and may have been caused by an amplification of
climatic trends that were already occurring.
Many scientists describe the extinctions that are now
occurring throughout the world as a result of human activities,
such as the destruction of natural habitats, as the “sixth
mass extinction.” Extinctions are in fact now occurring at a
rate that would justify this description. Before the appearance
of humans, the global extinction rate was about one per million
species per year; now the global extinction rate is one per
thousand per year.
The most famous example of a mass extinction event is
the Cretaceous extinction event 65 million years ago. At that
time, many species of organisms, most famously the dinosaurs,
became extinct. Exceptionally high levels of the element
iridium in the geological deposits 65 million years ago
suggest that the Earth was hit by an asteroid (see asteroids
and comets); asteroids usually contain higher levels of this
element than do the rocks of the Earth’s crust. The greatly
eroded remnants of this impact can still be seen in the Chicxulub
crater of Yucatán.
Even the Cretaceous extinction event, however, had more
than one cause. By the time the asteroid hit the Earth, populations
of many dinosaur and other species had already been
in decline for millions of years. Moreover, a massive volcanic
eruption occurred about the same time in what is now India,
Major Extinctions in the History of Life
mass extinctions
Millions Percent Percent Percent
Geological era: of of families of genera of species
End of years ago dying dying dying
Ordovician 439 26 60 85
Devonian 367 22 57 83
Permian 251 51 82 95
Triassic 208 22 53 80
Cretaceous 65 16 47 76