The relative absence of oxygen from the atmosphere of the young earth meant that no ozone layer existed to screen out ultraviolet radiation and no oxygen was available for aerobic respiration. Therefore, the first cells were probably photosynthetic and used ultraviolet light. Because photosynthesis generates oxygen, the oxygen content of the atmosphere gradually increased. As a result, cells that could use this oxygen to generate energy, and photosynthetic cells that could use light other than ultraviolet, eventually became predominant.
Eukaryotes may have evolved from prokaryotes. This idea comes from speculation about the origin of mitochondria and chloroplasts. These organelles may be the degenerate descendants of aerobic and photosynthetic prokaryotes that were engulfed by larger prokaryotes but remained alive within them (endosymbiosis). Over the years the host cell became dependent on the endosymbionts for energy (ATP), while they in turn became dependent on the host for most other cell functions. The fact that mitochondria and chloroplasts are surrounded by two membranes, as if they had originally entered the cell by phagocytosis, supports this theory. In addition, these organelles contain their own DNA and ribosomes, which resemble the DNA and ribosomes of bacteria more than those of eukaryotes. It is possible that other eukaryotic organelles originated similarly.
History
Cells were first described in 1665 by the English scientist Robert Hooke, who studied the dead cells of cork with a crude microscope. Living cells were first described in detail in the 1670s by the Dutch scientist Anton van Leeuwenhoek. These early descriptions were not improved on until the early 19th century, when better-quality microscope lenses were developed. In 1839 the German botanist Matthias Schleiden and the German zoologist Theodor Schwann formulated the basic cell theory of today. Struck by the underlying similarity between plant and animal cells, they stated that all living organisms consist of cells and cell products. Thus, a whole organism could be understood through the study of its cellular parts. In 1858 the German pathologist Rudolf Virchow’s theory, that all cells come from preexisting cells, led to the development of ideas about cell division and cell differentiation.
The development in the late 19th century of techniques for staining cell parts enabled scientists to detect tiny cell structures that were not actually seen in detail until the advent of the electron microscope in the 1940s. The development of various advanced optical techniques in the 20th century also increased the detection power of the light microscope for observations of living cells.
The study of cells (cytology) is not limited to describing structures (morphology). A central concept in modern cytology is that each structure has a function that may be understood as a series of biochemical reactions. The understanding of these functions has been greatly aided by the development of cell fractionation techniques, using an ultracentrifuge to separate specific intracellular structures from the rest of the cell. Another technique is tissue culture, by which specific kinds of cells can be isolated and grown for stuDY.