Eukaryotes

All the commonly seen organisms on the earth are eukaryotes, or organisms built up of eukaryotic cells. These organisms evolved from a prokaryotic ancestor and developed over the last 1.4 billion years into extremely diverse and successful groups of invertebrates, fish, amphibians, reptiles, and mammals. Today, eukaryotes live in a vast array of different environments in almost all areas of the earth.

Characteristics

Eukaryotes are organisms whose constituent cells are characterized by the presence of a nucleus and other membrane-bounded organelles such as mitochondria and, in plants, chloroplasts. Prokaryotic cells do not have a nucleus or membrane-bounded organelles. Biologists now recognize that the greatest discontinuity in life is not between animals and plants but between the prokaryotes and the eukaryotes. All organisms on the earth except viruses, bacteria, and cyanobacteria (blue-green algae) are eukaryotes, and the vast majority of fossil species of the last 700 million years have also been eukaryotes. The fossil record of the Precambrian was dominated by the prokaryotes, and the transition from prokaryotic to eukaryotic cells represents one of the greatest steps in the development of life on the earth. This transition probably occurred between 2 and 1.4 billion years ago, and it led to the evolution of complex multicellular plants and animals. The evolution of highly successful groups such as fish, reptiles, and mammals was dependent upon the formation and elaboration of the eukaryotic cell.

The major distinctive trait of the eukaryotic cell is the nucleus, an organelle that houses the deoxyribonucleic acid (DNA). In prokaryotic cells, the DNA molecules are arranged in a single loose strand within the cytoplasm of the cell. Prokaryotes generally reproduce by simple splitting (binary fission), whereas the reproduction of the cells and organisms is much more highly organized in eukaryotes. Eukaryotic cells reproduce by the complicated processes of mitosis and meiosis. In mitosis, the DNA is copied, and each new cell receives an exact copy of the original DNA. In meiosis, however, a second division of the genetic information occurs, forming sperm and egg cells. These sex cells from separate organisms can then fuse to form an offspring that has a combination of genetic information from each parent. This sexual reproduction, which is not found in prokaryotes, has led to a high level of diversity among the eukaryotes.

Eukaryotic cells also contain other organelles that perform specific metabolic functions in a more tightly organized manner than they occur in prokaryotes. Mitochondria are small organelles found in almost all eukaryotic cells and are the sites of aerobic respiration. In aerobic respiration, carbon-rich molecules are broken down into smaller molecules, and chemical energy is released and captured by the cell. This process requires oxygen. The amount of energy released in aerobic respiration is much greater than the energy released by fermentation, a process that occurs in prokaryotes.

Algae and green-plant cells also have chloroplasts, or organelles, which are the sites of photosynthesis. During photosynthesis, sugars are produced by the combination of carbon dioxide and water, with the release of oxygen (from the water molecule). The energy to drive this reaction is in sunlight captured by pigments located in the chloroplasts. Some prokaryotes are photosynthetic, but the process is more highly organized on the membranes within the chloroplasts of eukaryotes. Eukaryotic photosynthesis is efficient enough to supply most of the energy for all other eukaryotes on the earth. Many other organelles are found within the eukaryotic cell, and they perform such functions as waste removal, transport of materials, and movement of the cells.

The advantage given to eukaryotes by these organelles is the spatially ordered arrangement of sequential biochemical reactions that allows very efficient metabolic processes. This increased efficiency permitted the development of larger and more complex multicellular organisms. Although a few prokaryotes are loosely congregated into multicellular organisms, it is in the eukaryotes that well-defined separation of functions between cells is found.

Eukaryote Fossils

The origins of eukaryotes are largely unknown as a result of the notorious selectivity of the fossil record. Advanced eukaryotes such as clams, fish, birds, and mammals have shells, bones, or teeth that are commonly preserved in sedimentary rocks. These eukaryotes have left behind a decipherable, if sporadic, fossil record over the last 700 million years. The older single-cell eukaryotes were rarely fossilized, and the evidence for the first eukaryotes probably will never be found. It is clear, however, that the eukaryotes arose after the first accumulation of significant amounts of oxygen in the atmosphere at about 2 billion years ago and the appearance of the first multicellular animals at about 680 million years ago.

However rare an occurrence, single cells are occasionally fossilized in the rocks. The oldest fossils are from sedimentary rocks in the Pilbara Shield of Australia and are dated at about 3.5 billion years old. All these fossils and all the other Archean and early Proterozoic fossils are prokaryotes, showing that only prokaryotes lived during the first two-thirds of the earth’s history.

Fossil eukaryotic cells are rarely preserved with intact organelles, but they may be distinguished from prokaryotes by size. Eukaryotic cells range in size from 5 microns to 1 millimeter (one micron equals 0.001 millimeter), whereas prokaryotes are generally 1 to 20 microns in diameter. Relatively large fossilized cells have been found in the Precambrian rocks in the Death Valley region of California and in Australia, and these rocks have been dated at about 1.4 billion years old. Although some paleontologists are not convinced that these fossils are eukaryotes, a statistical analysis of more than eight thousand fossil cell sizes from Precambrian rocks throughout the world shows that the eukaryotic stage of evolution had probably been achieved by about 1.75 billion years ago. Large sheets of algae have been found in some of the sedimentary rocks of the Northwest Territory of Canada and have been dated at about 1 billion years old, and almost certainly eukaryotic cells have been found in the 900-million-year-old rocks of Australia. Therefore, scientists believe that eukaryotes developed in the interval between 2 billion years and the appearance of definite eukaryotes at about 1 billion years ago, or about 1.75 billion years ago. The manner in which complex eukaryotic cells arose from a prokaryotic ancestor could not be recorded in the fossil record, and theories attempting to describe the evolution of the early eukaryotes are based upon the biochemical relationships between present-day organisms.

Evolution Theories

Two general theories have been proposed to explain the evolution of eukaryotes. The traditional view, direct filiation, suggests that the nucleus, mitochondria, chloroplasts, and other organelles arose by mutations with the prokaryotes. Although most mutations are deleterious, some may have been beneficial to the ancestral prokaryotes and were retained within the organisms. Through sequential accumulation of mutations, each of the organelles developed over a long interval in the Precambrian eon. This view is supported by the very complex interrelationships between the organelles within the eukaryotic cell.

A competing theory was proposed in the early twentieth century by the Russian biologist R. C. Mereschkowsky and has been revived and revised by some modern biologists, particularly Lynn Margulis of Boston University. This theory, known as the endosymbiotic theory, suggests that mitochondria, undulipodia (an organelle for mobility), and chloroplasts were sequentially incorporated into symbiotic relationships within the ancestral prokaryotes. The mitochondria were, in this theory, originally free-living aerobic bacteria that arose after the initial oxygenation of the atmosphere. These bacteria were capable of normal living processes, but they invaded a large prokaryotic cell and continued to live and respire within the host cell. Both the invader and the host cell benefited from this arrangement. The host provided food to the invader in exchange for some of the energy derived from that food. In addition, the invader may have supplied some protection from oxygen to the host cell. The two continued to live together and, over a long period of time in the later Precambrian, became closely related until they grew completely dependent upon each other. The same general scenario is used to explain the origin of the undulipodia from spirochetes and chloroplasts from cyanobacteria.

There is biochemical evidence to support the endosymbiotic theory as interpreted by Margulis and others. First, separate and different DNA has been found in mitochondria, chloroplasts, and the connecting sites for the undulipodia. This DNA is not coated with proteins as is the nuclear DNA, and it is usually found in a single strand as it is in the bacterial cells. Second, some mitochondria can replicate independently of the main cell. Finally, the ribonucleic acid (RNA) in some mitochondria is more closely related to bacterial RNA than to the RNA of the main cell. All these data suggest an endosymbiotic origin of some of the organelles in eukaryotic cells.

However the earliest eukaryotic cells arose, they rapidly diversified. This diversification in the eukaryotes is in the arrangement and the organization of the cells, not in the internal biochemistry of the cells, which remains remarkably consistent throughout all the eukaryotes. The eukaryotes joined together and differentiated into tissues, organs, and systems to form multicellular organisms. Soft-bodied metazoans were present by 680 million years ago, and all the major phyla of animals were present by 500 million years ago. The eukaryotes have developed different forms, abilities, and behaviors over the last 500 million years and are able to live in most of the environments of the world. Eukaryotes are found in polar to tropical regions, from the depths of the oceans to the tops of mountain ranges, and from rain forests to deserts. It is estimated that there are between 3 and 10 million living species of eukaryotes today and perhaps one hundred to one thousand times as many eukaryotic species that have lived in the geologic past. All those organisms are based upon the eukaryotic cell that developed about 1.75 billion years ago from some prokaryotic ancestor.

Principal Terms

chloroplast: the organelle in eukaryotes in which photosynthesis is performed by algae and green plants

deoxyribonucleic acid (DNA): a stable molecule that contains most of the genetic information of a cell

endosymbiotic theory: the concept that eukaryotes arose from prokaryotes by incorporating free living microbes into symbiotic relationships

eukaryotic cell: a cell that contains a nucleus and other membrane-bounded organelles

mitochondrion: the eukaryotic organelle in which energy is generated by aerobic respiration

organelles: subcellular membrane-bounded units that perform specific functions within the eukaryotic cell

Precambrian: the interval of geologic time from the formation of the earth (4.6 billion years ago) to the beginning of the Cambrian period (544 million years ago)

prokaryotic cell: a cell that does not contain a nucleus or other membrane-bounded organelles

Bibliography

Cloud, Preston C. Oasis in Space. New York: W. W. Norton, 1988.

Cooper, G.M. “The Origin and Evolution of Cells.” The Cell: A Molecular Approach. 2nd ed. Sunderland (MA): Sinauer Associates, 2000. National Library of Medicine, www.ncbi.nlm.nih.gov/books/NBK9841/. Web. 27 July 2024.

“From Prokaryotes to Eukaryotes.” Understanding Evolution, evolution.berkeley.edu/it-takes-teamwork-how-endosymbiosis-changed-life-on-earth/from-prokaryotes-to-eukaryotes/. Accessed 27 July 2024.

Gunde-Cimerman, Nina, Aharon Oren, and Ana Plemenita. Adaptation to Life at High Salt Concentrations in Archaea, Bacteria, and Eukarya. New York: Springer, 2011.

Margulis, Lynn, and Michael J. Chapman. Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth. 4th ed. Boston: Elsevier, 2009.

Margulis, Lynn, and Michael F. Dolan. Early Life. 2d ed. Boston: Jones and Bartlett, 2002.

Margulis, Lynn, and Michael F. Dolan. Symbiosis in Cell Evolution: Life and Its Environment on the Early Earth. 2d ed. New York: W. H. Freeman, 1992.

McMenamin, Mark. Discovering the First Complex Life: The Garden of the Ediacara. New York: Columbia University Press, 1998.

Scheckenbach, Frank, et al. “Large-Scale Patterns in Biodiversity of Microbial Eukaryotes from the Abyssal Sea Floor.” Proceedings of the National Academy of Sciences of the United States of America 107 (2010): 115–120.

Schopf, J. William, ed. Major Events in the History of Life. Boston: Jones and Bartlett, 1992.

Starr, Cecie, et al. Biology: The Unity and Diversity of Life. 12th ed. Belmont, Calif.: Wadsworth, 2008.

Yaacov, Davidov, and Eduard Jurkevitch. “Predation Between Prokaryotes and the Origin of Eukaryotes.” BioEssays 31 (2009): 738–757.