The Human Cell / The Human Cell

The story of how cells evolved remains an open and actively investigated question in science.

The combined expertise of physicists, geologists, chemists, and evolutionary biologists has been required to shed light on the evolution of cells from the nonliving matter of early Earth. The planet formed about 4.5 billion years ago, and for millions of years, violent volcanic eruptions blasted substances such as carbon dioxide, nitrogen, water, and other small molecules into the air. These small molecules, bombarded by ultraviolet radiation and lightning from intense storms, collided to form the stable chemical bonds of larger molecules, such as amino acids and nucleotides—the building blocks of proteins and nucleic acids. Experiments indicate that these larger molecules form spontaneously under laboratory conditions that simulate the probable early environment of Earth.

Scientists speculate that rain may have carried these molecules into lakes to create a primordial soup—a breeding ground for the assembly of proteins, the nucleic acid RNA, and lipids. Some scientists postulate that these more complex molecules formed in hydrothermal vents rather than in lakes. Other scientists propose that these key substances may have reached Earth on meteorites from outer space. Regardless of the origin or environment, however, scientists do agree that proteins, nucleic acids, and lipids provided the raw materials for the first cells. In the laboratory, scientists have observed lipid molecules joining to form spheres that resemble a cell's plasma membrane. As a result of these observations, scientists postulate that millions of years of molecular collisions resulted in lipid spheres enclosing RNA, the simplest molecule capable of self-replication. These primitive aggregations would have been the ancestors of the first prokaryotic cells.

Fossil studies indicate that cyanobacteria, bacteria capable of photosynthesis, were among the earliest bacteria to evolve, an estimated 3.4 billion to 3.5 billion years ago. In the environment of the early Earth, there was no oxygen, and cyanobacteria probably used fermentation to produce ATP. Over the eons, cyanobacteria performed photosynthesis, which produces oxygen as a byproduct; the result was the gradual accumulation of oxygen in the atmosphere. The presence of oxygen set the stage for the evolution of bacteria that used oxygen in aerobic respiration, a more efficient ATP-producing process than fermentation. Some molecular studies of the evolution of genes in archaebacteria suggest that these organisms may have evolved in the hot waters of hydrothermal vents or hot springs slightly earlier than cyanobacteria, around 3.5 billion years ago. Like cyanobacteria, archaebacteria probably relied on fermentation to synthesize ATP.

Eukaryotic cells may have evolved from primitive prokaryotes about 2 billion years ago. One hypothesis suggests that some prokaryotic cells lost their cell walls, permitting the cell's plasma membrane to expand and fold. These folds, ultimately, may have given rise to separate compartments within the cell—the forerunners of the nucleus and other organelles now found in eukaryotic cells. Another key hypothesis is known as endosymbiosis. Molecular studies of the bacteria-like DNA and ribosomes in mitochondria and chloroplasts indicate that mitochondrion and chloroplast ancestors were once free-living bacteria. Scientists propose that these free-living bacteria were engulfed and maintained by other prokaryotic cells for their ability to produce ATP efficiently and to provide a steady supply of glucose. Over generations, eukaryotic cells complete with mitochondria—the ancestors of animals—or with both mitochondria and chloroplasts—the ancestors of plants—evolved

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