Encyclopedia of Evolution.pdf - Online Reading Center

Pole, were cold. Some researchers have suggested that the
oxygen content of the atmosphere of the Earth began to rise
during the Devonian period, reaching 35 percent by the subsequent
Carboniferous period, in contrast to today’s 21
percent. This would have resulted from the extensive growth
of plants, which produced oxygen (see photosynthesis,
evolution of), and which accumulated into thick deposits
of peat (later to become coal) rather than decomposing.
The high concentration of oxygen would explain how some
animals, particularly insects, could grow so large during the
Devonian and Carboniferous periods. Other researchers disagree,
saying that such high levels of atmospheric oxygen
would have promoted spontaneous combustion of the plant
materials, producing extensive fires for which there is no
clear evidence.
Continents. During the Devonian, there were three major
continental areas. What is now North America and Europe
formed a single landmass near the equator, much of it covered
by shallow oceans. A portion of what is now Siberia lay
to the north. Gondwanaland, a continent composed of what
is now South America, Africa, Antarctica, India, and Australia,
formed a large mass in the Southern Hemisphere.
Marine life. Many kinds of invertebrate animals, such
as trilobites, were abundant in the oceans of the Devonian
period (see invertebrates, evolution of). In addition,
many kinds of fishes, in groups that had evolved earlier, proliferated
into many species (see fishes, evolution of). So
great was their diversity that the Devonian period has been
called the Age of Fishes, even though there were also many
species of plants and animals on land. These included jawless
agnathan fishes; the heavily armored placederms (“plateskins”),
a group now extinct; sharks; ray-finned fishes, which
are the most abundant group of fishes today; and the lobefinned
fishes, which had finger-like bony reinforcements in
their fins. One group of lobe-finned fishes was the lungfishes.
A few lungfishes still live in tropical rivers today. Their lungs
allow them to survive long periods in mud when ponds
become seasonally dry.
Life on land. At the beginning of the Devonian period,
there were only small plants (about 3 feet [1 m] in height),
and a few arthropods, in terrestrial wetlands. By the end of
the Devonian period, the first trees and terrestrial vertebrates
had evolved.
• Plants. Early Devonian Rhynia was not much bigger than
the late Silurian Cooksonia. These plants had xylem cells,
which carry water from the ground up to the tips of the
stems. Xylem allowed the evolutionary descendants of
these plants to grow to the size of trees. By the end of the
Devonian, some trees were about 30 feet (10 m) tall. Their
descendants became much larger trees during the subsequent
Carboniferous period. Silurian plants evolved into
Devonian club mosses, horsetails, and ferns, which reproduced
by spores rather than by seeds (see seedless plants,
evolution of). Silurian plants had only simple branching
stems; but during the Devonian, the first leaves evolved:
Some of the branches were united by layers of green tissue,
which became the veins and blades of the leaves. “Progym-
DeVries, Hugo
nosperms” such as Archaeopteris had primitive forms of
seeds (see gymnosperms, evolution of).
• Animals. Living among the small plants in the early Devonian
mud were the first land animals, relatives of modern
arthropods such as scorpions, mites, and spiders. Shortly
thereafter, the ancestors of insects, centipedes, and millipedes
were living among the small plants of early Devonian
wetlands. Also during the Devonian period, amphibians
evolved from ancestral lobe-finned fishes. These amphibians
still closely resembled their fish ancestors. The Devonian
fish Eusthenopteron had bones in its paired fins that
corresponded to the leg bones of terrestrial vertebrates
(see amphibians, evolution of). The Devonian amphibian
Ichthyostega retained many of the features of its fish
ancestors. The shape of the body and head, and the tail fin,
made Ichthyostega look very much like a walking fish.
Extinctions. One of the mass extinction events in Earth
history occurred at the end of the Devonian, at which time an
estimated 22 percent of families, representing 83 percent of
species, became extinct (see mass extinction). The causes of
this extinction event are unclear.
Further Reading
Dudley, Robert. “Atmospheric oxygen, giant Paleozoic insects and
the evolution of aerial locomotor performance.” Journal of
Experimental Biology 201 (1998): 1,043–1,050.
Museum of Paleontology, University of California, Berkeley. “The
Devonian.” Available online. URL: http://www.ucmp.berkeley.
edu/devonian/devonian.html. Accessed March 23, 2005.
DeVries, Hugo (1848–1935) Dutch Botanist Hugo DeVries,
born February 16, 1848, studied the genetics of plants in
the late 19th and early 20th centuries. In 1889 DeVries suggested
that the heritable characteristics of organisms were
transmitted from one generation to another by “pangenes,”
now called genes. He also argued that new species were
produced by sudden large changes or mutations in the pangenes
of the ancestral species, producing the new species
instantly. This is known as the mutationist theory. DeVries
maintained that environmental stress could produce
these changes simultaneously within multiple individuals
within the ancestral species. While he like most other scientists
of his day accepted Darwin’s proof that evolution
had occurred (see Darwin, Charles; origin of species
[book]), he did not accept Darwin’s proposal of natural
selection as the mechanism by which evolution worked.
He believed that natural selection operated within species,
to produce local varieties, and selected among the species
that had been produced by sudden mutation. Today, geneticists
believe that most mutations are small and that the
same mutation does not occur simultaneously in more than
one individual.
DeVries had experimental evidence that he believed
confirmed the mutationist theory. He did crossbreeding
experiments with the evening primrose, Oenothera
lamarckiana. He found that strikingly new varieties arose
within a single generation. Later geneticists showed that