Coasts as a bridge between sea and land
Fish of the sturgeon family also exhibit an amphibious adaptation. Sturgeons live primarily in the sea, but seek out freshwater areas to spawn. Interestingly, in addition to the gills typical for fish, sturgeons also have lung-like organs, small cavities in the skull. With a gulping action they fill these with air and can extract oxygen from it – presumably as an adaptation to possible arid conditions. Thanks to the ability to breathe air a sturgeon can survive these dry periods, for example, when a stream or lake shore dries up or carries less water for a short time.
But coasts have also played a role in the opposite direction by facilitating the return of life forms from land back into the sea. Today there are numerous animals whose ancestors lived on the land that have now readapt-
ed to the marine habitat. Whales, for example, derive from four-legged land animals, but their two rear appendages have regressed to rudimentary stumps of bone. Their mode of swimming, however, is similar to the motion of some four-legged animals on land whose lower body moves up and down at a fast run. The fluke, or tail fin, of the whale moves in a similar way because the spine and skeleton are still much the same as those of the land mammals. By contrast, fish move their tail fins horizontally back and forth.
Some turtle species have also made the return from the coast back into the water, although they had originally evolved as four-legged land animals. Sea turtles have developed an amphibious habit, living between the land and sea. Many of these species search out a beach to lay their eggs at spring tide when the water reaches especially high levels. They can thus bury the eggs in the sand high up on the beach where they are protected from flooding. Later the hatchlings also break out during a spring tide, when the water is high again and the arduous and dangerous journey back across the beach into the sea is shortest.
Highs and lows through the millennia
Not only do coasts change their shape at a scale of millions of years, significant changes also occur over much shorter time periods. In cycles with magnitudes of several tens of thousands of years, alternating warm periods and ice ages, with the accompanying sea-level changes, play a significant role.
During the ice ages large areas of the land masses freeze. Precipitation in the form of snow forms glaciers thousands of metres thick. Because large volumes of water are bound up in ice on the land, and river flow into the sea is diminished, sea level falls gradually during an ice age.The most recent ice age ended around 12,000 years ago.The last period of heavy ice cover on the Earth was from 26,000 to 20,000 years ago. Sea level then was about 125 metres lower than today. Broad regions of the northern hemisphere were covered with glaciers, to as far as the Netherlands in central Europe. In warmer regions of the Earth the coastline looked completely different than today.
Around 15,000 years ago temperatures on the Earth began to rise rapidly again. This warm phase is still continuing today. The last warm phase before this one to see temperatures comparable with today’s occurred between 130,000 and 118,000 years ago. Sea level at that time was about four to six metres higher than it is today.
The big melt
Sea level rose again with the melting of glaciers after the last ice age. This rise generally proceeded steadily but there were occasional periods of accelerated rise triggered by events called meltwater pulses. These involved large amounts of meltwater that were released within a relatively short time. One significant event was a meltwater pulse that began about 14,700 years ago and lasted 500 years. The cause of this, presumably, was calving of the large glacial masses in the Antarctic, or in the Arctic between Greenland and Canada. With the melting of glaciers, sea level rose globally during this time by around 20 metres. Other large events included the runoff of immense dammed lakes that had formed from the meltwaters of retreating inland glaciers. According to scientific estimates, Lake Agassiz in North America had a maximum area of around 440,000 square kilometres, making it even larger than today’s Great Lakes.
- 1.9 > At the peak of the last ice age sea level was around 125 metres lower than today. The total global land mass protruding out of the water was about 20 million square kilometres greater.
- It broke through the surrounding glaciers multiple times, pouring large amounts of fresh water into the ocean, with one especially significant episode around 8200 years ago. This one meltwater pulse alone is believed to have raised sea level by several metres within just a few months.
The magnitude of sea-level change since the last ice age can be reconstructed based on various lines of evidence, for example, by studies of coral reefs or sediments on the sea floor. Tropical coral banks on the slopes of South Pacific islands have been growing slowly upward along with sea-level rise over recent years and decades. They can only grow in shallow water that is flooded by sufficient light. When sea level rises, the zone in which corals can thrive also shifts slowly upward. By drilling deep into the coral banks, older dead corals are encountered whose age can be determined by special analytical methods. Sea-level elevation at different times can thus be estimated.
- 1.10 > Sea level has not risen at a constant rate over the years. It has been punctuated by surges resulting from events such as meltwater pulses.
- The second method involves detailed study of sediments on the sea floor. By examining microfossils found in the sea-floor sediments, including the remains of single-celled organisms or fossilized fish bones and teeth, it is possible to determine when the bottom was part of the exposed land area, whether it was covered by fresh water from the melting glaciers, and when it was finally flooded by salt water from rising sea level. Depending on environmental conditions, different organisms are present and their organic remains are concentrated there. A sediment layer that derived from land plants can thus be clearly distinguished from one in which the remains of marine algae are found.
1.11 > The Earth changes its precession, the rotation motion, over a period of about 23,000 years. This is comparable to a gyroscope that gradually begins to wobble. It continues to rotate but the axis makes increasingly large circles.
The sun – a climate engineThe cause for alternating warm and cold phases, with the associated rise and fall of sea level, is related to natural climate fluctuations at regular intervals. Milankovich Cycles, postulated by the mathematician Milutin Milankovi´c in the 1930s, could have had an influence on the warm and cold periods. His theory maintains that the position of the Earth relative to the sun changes regularly, causing variations in the amount of incoming solar radiation received by the Earth. These variations particularly affect the northern hemisphere. According to Milankovi´c there are three primary causes:
- Change in the precession of the Earth’s axis, which varies on a cycle of around 23,000 years. Precession can be best explained by a spinning top that has been disturbed by a gentle push. The top continues to rotate, but the axis direction defines a larger circle. The cyclical change in direction of the axis is called precession.
- Change in the tilt angle (inclination) of the Earth’s axis, with a cycle duration of around 40,000 years.
- Change in the eccentricity of the path of the Earth around the sun. The shape of the elliptical orbit of the Earth varies. The change occurs in cycles of around 100,000 years and 400,000 years.
- It is known today that the Milankovitch Cycles alone cannot explain the large temperature differences between warm phases and ice ages. But it is very probable that they contribute greatly to the change. There is also an amplifying effect that contributes to the origin of ice ages: the ice-albedo feedback. Ice and snow strongly reflect sunlight (the ratio of reflection is called albedo). The thermal radiation of the sun is thus also reflected, which results in further cooling. The growth of glaciers is thus enhanced.
Changing sea level –
the pulse of human evolution
The rise and fall of sea level changed the available land area significantly with each cycle. Many areas that are flooded today were dry at the peak of the last ice age when sea level was about 125 metres lower. The land area in Europe was almost 40 per cent greater than it is today, and worldwide it was about 20 million square kilometres larger, which is approximately equal to the area of Russia. People thus had more extensive areas available that could be used for fishing, hunting and settlements. Experts believe that humans were already practising navigation then. At that time many land bridges between present-day islands and the mainland were still above sea level. Pathways that no longer exist today were available to people for exploiting new areas. These include the northern connection between America and Asia, which is cut off today by the Bering Strait. Another example is the 500-kilometre wide Arafura Sea, the marine region between Australia and the island of New Guinea to the north, which is an important fishing area today but was dry land at the peak of the last ice age.
Out of Africa
Today, it is widely accepted that humans originated in East Africa. The following important epochs of their dispersal are recognized. The first was around two million years ago. At this time the early man Homo ergaster/Homo ercetus spread, presumably by land, to Europe, China and down to southern Africa. Whether Homo ergaster and Homo erectus are related, and to what extent, is an object of ongoing research. It is conclusive, however, that both became extinct and were not direct ancestors of modern humans, Homo sapiens.
- 1.12 > The early man Homo ergaster had many of the skills of modern humans. He made tools. This could have helped in his migration two million years ago from Africa to the north and east.
- The second epoch involves Homo sapiens, who had a significantly wide range almost 200,000 years ago. Around 50,000 years ago they migrated to New Guinea from present-day Indonesia and finally to what would become the continent of Australia. New Guinea, which belongs half to Indonesia and half to Papua New Guinea, was separated from the rest of Indonesia by the sea, like today. But by that time, according to experts, the people already had simple boats and basic nautical skills. During this phase navigation on the water, from coast to coast over large distances, already played a role. America, however, was reached and colonized by crossing the land bridge in northern Asia about 15,000 years ago. Much of the evidence of these early human migrations is covered by water today, so there is often an absence of relics or prehistoric indicators of settlements. It is presumed, however, that people spread primarily along the coasts. Inland forests would have made migration difficult over the land, so the coastal pathways were simpler. In addition, fish and seafood were a reliable source of food. With the end of the last ice age, the conquest of new areas by Homo sapiens also received a boost. As the glaciers thawed they made room for modern humans, who were now able to spread northward as far as the arctic regions.
Modern technology reveals old cluesTo better reconstruct the spread of humans and to evaluate the importance of coasts, specialists from various disciplines have been collaborating intensively for several years. Teams comprising geologists, archaeologists and climatologists have joined forces to search for the traces of early settlements and, using modern submersible vehicles and high-resolution echo-sounder technology, to reveal structures in the sea floor in great detail. Underwater archaeology is important in this endeavour because areas on land have been continuously altered by people over thousands of years, while some evidence on the sea floor – including stone-age – has been covered and protected by sediment layers. Near the coasts, scientists now search systematically for underwater caves that were above sea level and dry during the ice age. These caves were used in the past as living areas and could hold interesting clues.
New knowledge is now being obtained from many areas of the world, for instance of the settlement pathways between Africa and Europe in the Mediterranean region. It was long believed that modern humans from Africa advanced to the north by land, along the eastern margin of the Mediterranean. But new finds indicate that migration over the sea from coast to coast must also be considered as a possibility. At present, there are ongoing intensive studies of the role that Malta, an island archipelago between Tunisia and the Italian island of Sicily, could have played. It may have been an important bridge between the two continents. At the peak of the last ice age Malta was significantly larger and was connected to present-day Sicily over a 90-kilometre long land bridge called the Malta-Ragusa Platform, so that the distance northward from Africa across the Mediterranean was much shorter than it is today.
The sea floor around Malta has been mapped in detail in recent years with the help of modern underwater technology. Bottom samples have also been taken. Ancient land structures on the sea floor that have hardly changed over thousands of years became visible: old river valleys, sand banks, stone-age shore lines and possibly even old lakes. In the past, the three present-day islands of the Malta archipelago were connected and there were evidently large fertile areas that would have been of great interest for settlers from Africa. According to the researchers, the trip would have been possible with simple boats. Efforts to find concrete evidence of early settlements are continuing.
Evidence of early settlements is also being sought on the sea floor 200 kilometres to the northwest. There lies the small island of Pantelleria, directly upon the shortest line between Tunisia and Sicily. It is known for its occurrences of obsidian, a black, glassy volcanic rock that was used by stone-age people. Scientists searched a small area for chipped obsidian and were successful. The flaked stones appear to be concentrated at an ancient shoreline that lies below 20 metres of water today. Closer investigation should be able to determine whether it is a stone age find. The scientists believe this is probable. >