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.
