Introduction to How the Ocean Came to Be
Soon after the solar system formed out of a cosmic cloud of gas and dust more than 4.5 billion years ago, Earth was a dry, cratered ball barren of life. It had no ocean and no continents. As the eons rolled by, our planet was transformed into a water world teeming with myriad forms of life. How did such a dramatic transformation occur?
Scientists believe that Earth in its earliest years was a horribly hot and violent place. Asteroids, comets, and other chunks of space debris left over from the solar system's formation continually bombarded the young planet, releasing huge amounts of heat. The decay of radioactive elements inside the Earth also generated great quantities of heat. At the same time, frequent volcanic eruptions may have covered much of the planet's surface in red-hot flows of lava. According to scientists, the early Earth's surface was hot enough to turn any liquid water instantly into steam. Nonetheless, the planet somehow obtained enough water to eventually form a vast ocean.
Scientists have long sought to explain where this water came from. Some researchers have theorized that comets, which contain large amounts of water ice, delivered water to Earth during the millions of years that comets pummeled the young planet. Other investigators, however, dispute this view. They point out that in the 1980's and 1990's, scientists used radio telescopes to analyze the chemical makeup of three comets as they came close to Earth. These studies revealed that comets contain 2 to 20 times more “heavy water” (relative to “normal” water) than is found in the ocean. Normal water molecules are composed of one oxygen atom and two hydrogen atoms. Heavy-water molecules contain at least one atom of deuterium, an isotope (variant form) of hydrogen that has twice the mass of ordinary hydrogen. The higher proportion of deuterium in comet ice suggests that comets could not have been a major source of seawater.
Most scientists now believe that the ocean originated from water molecules in the cloud of gas and dust that gave rise to the solar system. According to the current theory, as Earth formed, these water molecules first became trapped in porous rock deep inside the hot planet, but they later boiled out as steam. For hundreds of millions of years, volcanoes ejected the water vapor along with other hot gases into the Earth's atmosphere. The volcanoes spewed out enough water vapor to wrap the entire planet in a dense blanket.
After the meteorite bombardments and volcanic eruptions subsided, Earth finally cooled enough to permit the water vapor to condense into clouds and fall as rain. An almost continuous rain, fed by evaporation and volcanic outgassing, may have drenched the planet for some 10 million years. As the deluge persisted, ponds formed in shallow depressions and then grew into lakes. Eventually, the lakes merged to form an ocean. Many researchers now believe that the first permanent ocean was likely in place sometime between 4.3 billion and 3.8 billion years ago.
The early sea may have covered the entire planet to an unknown depth as rain water collected everywhere on the nearly featureless landscape. Raised continents and deep ocean basins had not yet formed. These features would take shape after geological processes began altering the landscape.
The Ocean Takes Shape
Plate tectonics played a major role in reshaping Earth's surface. According to this theory, Earth's outer shell, or crust, broke into a number of rigid pieces, called tectonic plates, more than 2 billion years ago. These plates began to move relative to each other over a layer of partially molten rock. In areas where two tectonic plates collided, layers of rock rose up, forming mountains and other features high enough to be above the water. At sites where plates moved apart, gaps were created that spread farther and farther apart. Magma (liquefied rock) oozed up through cracks at these sites and hardened to form new floors on the bottoms of the widening basins. As water filled these basins, the planet looked more like the world of today. However, plate tectonics continued to change the shapes of the ocean basins and shift the positions of continents.
About 360 million years ago, tectonic plates carried a number of land masses together to form a single continent, called Pangaea, which was surrounded by an ocean called Panthalassa. Then, beginning about 200 million years ago, Pangaea broke apart into the northern continent of Laurasia and the southern continent of Gondwanaland. The modern basins of the Pacific, Atlantic, and Indian oceans began to take shape about 120 million years ago, when Laurasia broke apart into Eurasia and North America, while Gondwanaland split into Africa, Antarctica, Australia, India, and South America. By 60 million years ago, the global ocean closely resembled the seas of today.
The process by which the ocean's waters evolved their modern characteristics is only partially understood. For example, exactly how salty the original sea was remains a mystery. Nonetheless, most scientists agree on a scenario of where the ocean's salt came from.
According to the accepted theory, volcanoes released large amounts of the element chlorine into the early atmosphere, along with water vapor. The chlorine combined with the water molecules to make hydrochloric acid. After falling to the ground with rain water and flowing over the surface, the acid gnawed at rocks, stripping them of their salts, particularly sodium. Some of the sodium atoms combined with chlorine atoms to form sodium chloride—more commonly known as table salt—the primary salt in the sea.
Life Evolves
Eventually, organisms appeared in the sea. Fossil evidence indicates that life originated in the sea a few hundred million years after the first ocean formed. These first life forms were probably unicellular (one-celled) organisms. In 1993, paleobiologist J. William Schopf of the University of California at Los Angeles discovered evidence of the earliest-known unicellular life. He found fossils of microorganisms in Australian rocks that dating methods have shown are approximately 3.5 billion years old. The microbes were similar in appearance to modern cyanobacteria, or blue-green algae. These are single-celled organisms that carry out photosynthesis, the use of sunlight to make energy-rich compounds (food) from carbon dioxide and water.
The ability to harness the sun's energy is a sophisticated biological process that scientists believe took many millions of years to develop. This fact suggests that simpler forms of life were drifting in the sea even earlier than 3.5 billion years ago. Minerals discovered in Greenland rocks in 1996 by a team of researchers led by oceanographer Gustaf Arrhenius of the Scripps Institution of Oceanography in La Jolla, California, indicated that life may have started as early as 3.8 billion years ago. The team dated grains of apatite, a calcium-phosphate substance that can be a by-product of life processes, to that time. Based on this and other chemical evidence from the rocks, the researchers concluded that the apatite grains may have been produced by a living microorganism.
Precisely how and where the first living cells appeared remains one of science's most profound unanswered questions. Some scientists believe that the first primitive cells formed in shallow pools of seawater, when the “molecules of life”—proteins and nucleic acids (substances that encode and transmit genetic information)—became enveloped in fatlike substances called lipids.
An alternative theory proposed by some scientists is that the first cells evolved away from sunlight, in the dark depths of the ocean along chimneylike structures in the sea floor called hydrothermal vents. Water heated by magma shoots upward through hydrothermal vents, mixing various minerals and nutrients. Today, microorganisms, tube-dwelling worms, giant clams, and other organisms live around hydrothermal vents, using the minerals and nutrients as an energy source independent of the sun.
However life began, it was, for the vast majority of Earth's history, exclusively unicellular. Fossil evidence indicates that simple animals and other multicellular life forms did not exist in the ocean until sometime between 1 billion and 600 million years ago.
Approximately 540 million years ago, in a biological phenomenon called the Cambrian explosion, most major groups of animals made their first appearance in the sea. Fossils from this time in shale rocks found in British Columbia, Canada, show an ocean suddenly abundant with life forms, including trilobites (now extinct shelled animals with segmented bodies), various other shelled creatures, sponges, and wormlike animals. By the end of the Devonian Period 360 million years ago, the first bony fish and sharks swam in the sea, and the ocean was becoming more and more like the body of water we know today.