DEFINITION OF PANAMA ISTHMUS

A narrow body of land that connects

Central and South America. Most of the isthmus is occupied

by the country of Panama, with Costa Rica occupying the

western part of the isthmus, and Colombia located to the

west. The isthmus has a wet tropical climate and has rugged

volcanic mountains in the west, reaching 11,410 feet (3,478 m)

on Chiriqui. The center of the isthmus is characterized by low

hills, and there is a low mountain range in the east. The most

famous feature of the isthmus is the Panama Canal. The canal

was built by the United States in 1904–14 to connect the Pacific

Ocean to the Caribbean and Atlantic Oceans, and to avoid

the need to ship materials along the long difficult route around

southern South America. The canal is 40 miles (64 km) long

from coast to coast, but 51 miles (82 km) long between channel

entrances. Ships must pass through a series of locks and

sail across Lake Gatun to cross the continental divide, and

emerge on the other side of the canal seven–eight hours later.

Approximately 240 million cubic yards (184 million m3) were

excavated from the canal by the Americans, at a cost of $337

million. The canal and canal zone bordering the canal reverted

from U.S. control to Panamanian control in 2000.

The formation of the Isthmus of Panama had profound

effects on global ocean circulation models and climate. Prior

to its formation in the Pliocene (about 5 million years ago),

Caribbean Ocean and Pacific Ocean waters were able to flow

through the open passageway between North and South

America, and there was greater communication between

waters and organisms in the equatorial oceans. When the

isthmus formed with the movement of the Caribbean plate to

the east, and the Central American subduction-related arc

forming on the western side of the Caribbean plate, this passage

was blocked. Warm waters that formed in the shallow

Caribbean Sea were deflected into the north-flowing Gulf

Stream and moved into the North Atlantic. This dramatic

change in ocean circulation produced dramatic effects on

global climate and may even have indirectly triggered the

Pleistocene ice ages in the Northern Hemisphere. Warm

water in the North Atlantic increased humidity and snowfall

at high latitudes, which then increased albedo (surface

reflectance of solar energy), leading to temperature decreases.

The formation of continental glaciers reduced sea levels,

reduced the size of the North Atlantic basin, and forced the

Gulf Stream closer to Europe.

Many species of plants and animals were also strongly

affected by the formation of the isthmus. The isthmus served

as a new land bridge that allowed mammals to migrate

between North and South America. South America was inhabited

by a diverse population of marsupial mammals that developed

in isolation from other landmasses after the breakup of

Pangea in the Late Mesozoic, so the mammal population was

unique and different from those of Australia, Antarctica, and

Africa. Some of the marsupials included giant land sloths and

armadillos, as well as opossums. Some of these marsupials

migrated north, including the armadillos, porcupines, opossums,

anteaters, and monkeys. However, most of the species

that migrated across the isthmus moved in the opposite direction,

invading the south from the north. Among these animals

were the pig, deer, horse, elephant, camel, rhinoceros, squirrel,

raccoon, rabbit, rat, bear, dog, and cat.

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