Encyclopedia of Evolution.pdf - Online Reading Center

Ordovician period The Ordovician period (510 million to
440 million years ago) was the second period of the Paleozoic
era (see geological time scale). It followed the
Cambrian period and preceded the Silurian period.
Climate. Oceans were warm around equatorial continents,
but cool around the mass of southern continents. There
was, however, little if any multicellular life on the continents
to be affected by terrestrial climate.
Continents. Oceans separated the continents of Laurentia,
Baltica, Siberia (these three continents today dominating
the Northern Hemisphere) and Gondwana (today, the southern
continents).
Marine life. Almost all organisms lived in the oceans
and freshwaters at this time. The diversity of marine animals
more than tripled over that of the Cambrian period. This was
partly due to an increasing specialization in the ways that the
animals obtained food. It was also partly due to the formation
of the first reefs, though the corals that formed them
were of classes that are now extinct, and many reefs were
formed by sponges. When the reefs formed, they created
many new habitats for other swimming and crawling and
sessile animals. Most animal species were invertebrates (see
invertebrates, evolution of). The most abundant group
of arthropods were the trilobites; modern forms of arthropods
such as crabs were rare. The only vertebrates were fishes
(see fishes, evolution of), jawless descendants of the early
fishes of the Cambrian period. Astrapsis had no jaws or fins;
only its tail protruded outside a bony armor that covered its
body. This four-inch- (10-cm-) long fish presumably used its
simple tubelike mouth to filter food from the ocean floor.
Seaweeds underwent evolutionary diversification.
Life on land. Terrestrial life-forms have not been clearly
confirmed from the Ordovician period. Spores characteristic
of primitive land plants have been found from the later part
of this period.
Extinctions. One of the great mass extinctions
occurred at the end of the Ordovician period. It is estimated
O
that 26 percent of families, representing 83 percent of species,
became extinct by the end of the Ordovician. While
the continent that later became North America and Europe
was equatorial, and surrounded by tropical reefs, the large
Gondwanan continent (which today exists as South America,
Africa, Australia, Antarctica, and fragments of other landmasses)
was at the South Pole. Not only did Gondwanan
weather become colder but ocean levels dropped as glaciers
formed around the pole, and this devastated tropical reefs.
The return of warm weather, and the melting of glaciers,
toward the end of the Ordovician may also have caused stagnation
of deep-sea waters.
Further Reading
Museum of Paleontology, University of California, Berkeley. “The
Ordovician.” Available online. URL: http://www.ucmp.berkeley.
edu/ordovician/ordovician.html. Accessed May 1, 2005.
origin of life One of the greatest challenges to evolutionary
science has been to explain how life began. To approach this
question, one must consider what life is, and when, where,
and how it began.
What
What is life? This question has come up in the discussion
of whether the chemicals and structures in ALH84001, the
meteorite from Mars (see Mars, life on), did or did not constitute
evidence that there was life on Mars billions of years
ago. Unfortunately, this question has proven very difficult to
answer. Is artificial intelligence a form of life? If robots can
construct copies of themselves, does that mean they are alive?
Is the Earth alive?
Ancient writings, such as the Bible, make numerous references
to life. It is on this basis that creationists (see creationism)
have launched numerous attacks on the scientific
hypotheses of the origin of life. However, these ancient documents
did not mean the same thing as biologists mean when