Land bridges

The theory that the earth's continents were once connected by land bridges that accounted for the migration of flora and fauna was considered viable by many leading geologists during the last half of the nineteenth century and for the first three decades of the twentieth century.

Changing Earth

For geologists, the earth has long been a huge, complicated jigsaw puzzle waiting to be reconstructed. The difference between this jigsaw puzzle and those with which most people are familiar is that this one constantly changes. The Earth changes in small ways—and sometimes in more significant ways, such as when a volcano erupts—daily. It changes more drastically over longer periods. It is now thought to be changing as global climate change slowly melts the polar ice caps, with a resulting increase in the volume of ocean waters and a corresponding decrease in the shorelines adjacent to the rising oceans.

It has long been acknowledged that drastic climatic changes have occurred on the Earth over millions of years. The Arctic and Antarctic, now solidly frozen, once had tropical and subtropical vegetation. This is evident from impressions of plants found in mineral deposits in these regions and from other paleontological evidence. Now-moderate regions were much colder during the Ice Age. Glaciation pushed debris unrelentingly from one region to another as the glaciers moved toward the equator.

Long baffled by the existence of similar flora and fauna in areas seemingly unrelated to each other climatically—Africa, Australia, and Antarctica, for example—geologists arrived at various conclusions regarding the phenomenon. It must be remembered that geologists talk in terms of hundreds of millions of years more often than they do in terms of a century or a millennium. Geological change often occurs so slowly that it is virtually unnoticeable within geologically abbreviated periods of thousands of years.

Land-Bridge Theories

Among the theories promulgated to explain the presence of similar flora and fauna in widely disparate areas was that of land bridges. Some land bridges obviously existed, such as the one across the Bering Strait that linked Siberia to what is now Alaska. It is apparent that two continents, Asia and North America, were at one time linked by this broad bridge. Similarly, some geological conjectures also show that land bridges existed between present-day Ireland and Scotland and between present-day Gibraltar and North Africa.

A Viennese geologist, Eduard Suess, proposed an extensive land-bridge theory in his influential book The Face of the Earth (1885). Suess coined the term “Gondwanaland,” named after Gondwana in east-central India, an area with rocks that revealed unique fossil plants and showed indisputable signs of glaciation dating from the late Paleozoic to the early Mesozoic eras.

As Suess pieced together the discrete elements of his geological puzzle, he found that separate continents had the sorts of fossil plants and glacial rocks he found in Gondwana. He attributed these occurrences in widely separated areas to land bridges rather than to continental drift. Suess and the scores of notable geologists who accepted his thesis contended that the land bridges connecting the continents sank beneath the ocean at some time in prehistory.

Other geologists were trying to solve the same riddles with which Suess was confronted. In 1908, American geologist F. B. Taylor published a privately printed pamphlet suggesting present mountain ranges occurred when enormous landslides advanced slowly and steadily from the polar regions toward the equator. He conceived of the world's great continents as originally consisting of huge sheets of rock torn apart by glaciation as gigantic fields of ice moved toward zero latitude.

Taylor advanced two fundamental ideas, both of which have been opposed by other geologists to the point that these two hypotheses have been discredited. First, he speculated that mountain ranges were products of thousands of miles of lateral movement; second, he believed that the moon became associated gravitationally with Earth during the Cretaceous period, about 100 million years ago. In this period, there was a considerable increase in the variety of flora and fauna found on the earth. Taylor suggested that in the earliest times, the moon created phenomenal tidal forces that slowed the rate at which Earth rotated and pulled the continents away from the poles. Both of these theories were subsequently disproved.

H. H. Baker, an American geologist, considered the supercontinent of the Earth's earliest years to have been split along the Atlantic and Arctic Oceans as the Miocene period neared its end some 7 million to 8 million years ago. He asserted that great tidal action had torn a huge piece from Earth. The resulting void, according to Baker, became the Pacific Ocean. He speculated that the portion catapulted into space became the moon.

In his landmark 1915 book Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans, 1924), Alfred L. Wegener proposed the Earth had at one time consisted of a single land mass that he called “Pangaea,” the Greek term for “all Earth.” He concluded that during the Mesozoic era, the single landmass was torn apart, with the southern continents (Africa, Australia, South America, Antarctica) being pulled away first. Looking at the globe as though it were a jigsaw puzzle, one can imagine that the east coast of South America would fit easily into the declivities on the west coast of Africa.

Evidence for Land Bridges

As this investigation into the Earth's origins was ongoing, the fossil record uncovered during the 1870s and 1880s led to the inevitable conclusion that land links, either land bridges or isthmian links, had existed between Africa, India, Australia, South America, and Antarctica. These areas of vastly differing climates in modern times (geologically speaking) appeared to share a fossil plant record that suggested a close affinity among them.

F. H. Knowlton, speaking in 1918 about Mesozoic floral relations, supported the notion that land bridges from Antarctica once linked most of the southern landmasses. In 1947, however, after the land-bridge theory had been largely discredited, Theodor Just suggested that “the needs of animal and plant geographers… vary sufficiently with their respective interests and so do the land bridges assumed by them.” Just discounted land links as deciding factors in the migration of flora.

During the Eocene epoch, the southeastern United States experienced an influx of tropical flora, suggesting to some that a land bridge or isthmian link had once existed between South America and what is now the United States, possibly between South America and southern Florida. Modern researchers suggest that this tropical foliage actually came to the southeastern United States from an intermediate region such as Central America or the Antilles.

During the Oligocene epoch, a considerable exchange of plant life appears to have taken place between present-day Panama and the Antilles. Petrified wood found in Antigua reflects several forms of flora found in both Panama and southeastern America. While the land-bridge or isthmian-link theory might explain this coincidence, other, more viable theories exist to explain it. Among these is the theory that certain flora simply drifted on the open sea, perhaps hitchhiking on debris that eventually washed ashore where the climate could support its germination and growth.

During the Pleistocene epoch, North and South America were connected. The Isthmus of Panama did not exist, and the region of what is now the Antilles was considerably higher than it is today, possibly with a land bridge to South America, less than a hundred miles away, and certainly with isthmian links to that continent. Vegetation common to the rain forests of the Amazon and Orinoco basins occurred in southern Florida. Still, most geologists consider it improbable that a land bridge existed between that region and the Antilles or South America.

By comparison, those who dispute the existence of a land bridge between North and South America are on shakier ground than those who dispute a land bridge between Florida or the Antilles and South America. Vertebrate paleontologists have made a convincing case for animal migration (including human) in this area, as they have for animal migration between Asia and North America by way of the land bridge known to have existed between Siberia and present-day Alaska. In 1917, Edward W. Berry postulated that the Antilles were once a part of South America but that continental drift separated the region from that continent. Such a theory would explain much of the coincident flora and fauna in both places.

Numerous catastrophic theories regarding continental drift have been used to explain the separation of the Earth's landmass into continents. Most of these theories have to do with the separation of the moon from Earth, accompanied by the violent tidal action that followed this separation. As late as 1932, Charles Schuchert supported the notion that land bridges connected parts of an enormous supercontinent and that these bridges subsequently sank beneath the deep waters of the Atlantic, Pacific, and Indian Oceans, as suggested several decades earlier by Eduard Suess.

The major portions of the jigsaw puzzle geologists have worked on for the past century are now firmly in place. Other portions do not yet fit into any reasonable pattern. Although it explains certain animal migrations in a limited geographical range, the land-bridge theory has been displaced by other theories relating to diverse flora and fauna in such widely separated areas as India, Antarctica, Africa, and South America. The land-bridge hypothesis is important because it demands testing, and this testing has led to valuable insights into the Earth's earliest history.

In the twenty-first century, however, scientists continued to explore land-bridge theories. In October 2023, scientists dated footprints found in New Mexico far earlier than it was previously thought humans lived in North America. It is believed the human footprints are betweeen 23,000 and 21,000 years old. This discovery brought challenges to the widely accepted theory that humans migrated to North America 14,000 years ago over the Bering Strait land bridge and suggested a possible alternative route. Scientists have begun to explore the possibility that multiple migration routes occurred, and this has revived some land-bridge theories. 

Principal Terms

Cenozoic era: the geologic era dating from the present to about 65 million years ago

continental drift: the theory that landmasses separated and drifted apart in prehistoric times

Cretaceous era: a geologic period ending some 65 million years ago, during which seas covered much of North America and the Rocky Mountains were formed

Eocene epoch: part of the Cenozoic era, dating to about 37 million years ago

glaciation: the process of being covered by an ice sheet or glacier

isthmian links: chains of islands between substantial landmasses

land bridges: narrow land formations that connect landmasses

Mesozoic era: the geologic era spanning the period from about 245 million years ago to about 65 million years ago

Bibliography

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