Humans on the Brink of Extinction

We humans are an egocentric lot. We tend to look at the world and all of its history and prehistory as leading to that great moment when humans would reign supreme, the ultimate goal of creation.

As the humorist Douglas Adams once observed: "This is rather as if you imagine a puddle waking up one morning and thinking, 'This is an interesting world I find myself in—an interesting hole I find myself in—fits me rather neatly, doesn't it?…Must have been made to have me in it!'"

The truth, as hard as it may be for us to accept, is that our preeminence on the planet was not preordained. Our human ancestors could well have gone the way of the dinosaurs, and in fact, they nearly did.

Since the beginning of life on earth, there have been periods of mass extinction, bringing down a once-dominant species and making way for a new life-form to have its moment in the sun. The last such mass extinction, sixty-five million years ago, destroyed the dinosaurs and allowed the mammals to take over. Geologists and archaeologists spend their entire careers exploring the causes of mass extinctions. The reigning theory at present is that many can be blamed on extreme weather conditions created by natural disasters.

The dinosaurs were most likely the victims of a wayward meteor that struck the Yucatán Peninsula of Mexico and bored a twenty-five-mile crater into its surface. The explosion was the equivalent of the detonation of one hundred million hydrogen bombs—the heat vaporized seawater and saturated the atmosphere. The superheated air swept outward. Dust from the blast traveled so far that it blanketed what is now Kansas and flew into the atmosphere, where it encircled the globe and blotted out the sun, cooling the planet. Plants could not photosynthesize, so they died, and the creatures that depended on them soon followed. It is called the Great Dying. Nearly 90 percent of all life that then existed was wiped out forever. Fortunately for us, one of the survivors was the cynodont, the ancestor of modern mammals.

Our own very similar brush with extinction came about seventy thousand years ago. DNA studies point to a population crisis, sometimes called a population bottleneck. Scientists sought to understand why there was so little genetic variation among humans. There is more genetic variation in a single group of chimpanzees or a clan of gorillas than there is in the entire six-billion-member human population. This points to a time when there were only a few procreating females around. One study suggests the number dropped to as few as five hundred; it would take another twenty thousand years for the human population to fully recover and regain its previous numbers.

The cause of the bottleneck was one of the largest volcanic eruptions in four hundred and fifty million years. The explosion of the Toba volcano on what is now the Indonesian island of Sumatra produced a crater that measured 100 km (60 mi) across and a plume that was at least 30 km (19 mi) high, scattering rock and ash as far away as Greenland. It tossed about 2,800 cu km (684 cu mi) of molten rock into the atmosphere. That's enough to build more than a million Great Pyramids of Egypt. The blanket of ash blocked out the sun, and global temperatures dropped by up to 12°C (22°F). The volcanic winter lasted for six years. The increased snow cover that accumulated in this period further reflected the sun's rays, preventing the ground from absorbing heat, which made the world still colder. It was the beginning of a thousand-year ice age.

Some researchers speculate that an ice age that was already in progress was the cause, not the effect, of the eruption. The ice age may have lowered sea levels, relieving pressure on the volcano and allowing it to blow, like a cork being removed from a bottle of champagne. The effects of the eruption then sped the glaciation in progress, providing the trigger that changed the climate system from warm to cold, and as glaciers formed, the sea level dropped further. The exposed soils were carried away by the wind. Dust storms raged for days, killing plants and animals.

Homo sapiens was very close to going the way of the Neanderthal and other extinct human species, but a few hardy individuals survived in isolated pockets in Africa, Europe, and Asia. As a result, our population has only a small sample of the genetic diversity we once had.

Could such an eruption happen again? Absolutely. Not only could it happen, it is almost inevitable that it will. The most likely site of the next supereruption is Yellowstone National Park. The geysers, hot springs, and mountains that bring tourists to the area are caused by a large underground magma chamber that extends about 20 km (12.5 mi) across and 2,900 km (1,802 mi) down—nearly halfway to the center of the Earth. Yellowstone has already exploded three times. It blows every six hundred thousand years or so. The last six hundred thousand-year mark was reached four hundred thousand years ago. Yellowstone is only one of forty supervolcano sites, but most are extinct, and none is as close to heavily populated areas.

When the Yellowstone volcano blows, scientists say it will unleash a force that is larger than the entire planet's nuclear arsenal. The blast would be heard as far away as England. About one hundred thousand people would die immediately. Toxic gases and ash would be thrown into the atmosphere, and it would fall across the entire western United States within hours. It would continue to spread across the globe on the winds, creating a volcanic winter. It could happen next week, or two hundred thousand years from now.

Of all the meteorological events that supposedly changed history, the greatest would have to be the flood that wiped out all of creation except for Noah, his family, and all but two of every type of animal. But did the biblical flood actually happen?

William Ryan and Walter Pitman, the authors of the book Noah's Flood, believe that there was indeed a great flood that inspired the Noah story and many of the flood legends recorded throughout the world. More than two hundred similar flood myths have been recorded, in the cultures of the Greeks, Egyptians, and Babylonians. The Sumerian epic Gilgamesh, inscribed on clay tablets around 2000 BC, says that Gilgamesh was warned by a god to build a great ship to protect all the living things on earth from a forthcoming flood.

All of these stories may well have a common source, a deluge so great that it seemed as if the world was immersed: a sudden disaster that survived in folklore and was passed along from generation to generation, becoming even more dramatic with each retelling. The researchers believe they have found exactly such an event. It happened about seventy-five thousand years ago in the land surrounding the Black Sea.

Fossil evidence shows that until that time the Black Sea was a modest freshwater lake fed by meltwater from ice age glaciers. Along its shore were numerous settlements, and a small ridge divided this lake from the Sea of Marmara. All at once, however, the fossils in the Black Sea shifted from freshwater mollusks to saltwater mollusks. Research by explorer Robert Ballard, using sonar and a remote-controlled underwater camera, shows an ancient shoreline 167 m (550 ft) from the current coast of Turkey. Stone tools, mud-and-wattle walls, and shards of pottery were buried beneath the sea for millennia.

Some scholars have suggested that the shores of this body of water—we'll it call the "Black Lake"—may have been the cradle of civilization. The land would have been more fertile than Mesopotamia in the arid Middle East, and artifacts, language patterns, and ethnic relationships could be explained by the movement of people from the Black Sea region following a flood. This theory remains controversial. Still, there is little doubt that there were settlements around the Black Lake that were washed away by a great flood.

What happened? During the last ice age, glaciers extended down from the North Pole as far as Chicago and New York City. Much of the world's water was locked up in solid form, so the oceans were about four hundred feet lower than they are today. As the Ice Age drew to a close and the glaciers started to melt, the waters of the Mediterranean rose. They began to flow into the Sea of Marmara. As they rose, the pressure on the ridge dividing it from the lake became greater and greater. Finally, the natural dam was unable to hold.

It released a torrent of saltwater with the force of two hundred thousand Niagara Falls. The roar of the water could be heard at least one hundred miles away. Each day another 42 km (10 cu mi) of seawater poured in. As it did, it inundated villages in what are now Turkey, Bulgaria, Moldova, Ukraine, Russia, and Georgia. The influx of water and its evaporation caused torrential rain across the region. Villagers ran for high ground, but the water kept coming. Every day it would advance another half a mile (.8 km). When it finally leveled off, the lake was a saltwater sea, 150 m (500 ft) higher than it had been before.

Was this event the origin of the story of Noah's flood? It is impossible to know. If it was, it would have been handed down as oral history for one hundred thousand generations, which could easily account for some of the variation in the details—you know how different your family stories get after only one generation.

Researchers Valentina Yanko-Hombach at the Avalon Institute of Applied Science in Winnipeg, Manitoba, and Andrey Tchepalyga of the Institute of Geography in Moscow believe that the flood would have been much less dramatic than Ryan and Pitman envisioned. Their analysis of sediment and seismic data from the Black Sea shows that water from the Caspian Sea flooded into the area about fourteen thousand years ago and that the Black Sea began to rise gradually over the course of one thousand years, driving the inhabitants out of the basin. After the Caspian overflow stopped, the level of the Black Sea fell, to be flooded by seawater a few years later. Yanko-Hombach believes this second flood raised the sea only forty meters and did so more gradually than the American team suggested. The various theories are still the subject of archaeological speculation.

Copyright © 2006 by Laura Lee.