There are said to be tens of millions of species on Earth, and their forms・sizes・and ways of life are truly diverse. At the same time, all organisms have surprisingly many common features. Understanding both diversity and commonality is the starting point of biology.
Basic Knowledge
Five fundamental principles common to all organisms:
① They are made of cells (cell theory, Schleiden・Schwann).
② They possess DNA as genetic material and transmit information to offspring.
③ They use energy to maintain life activities (ATP is the common energy currency).
④ They maintain a constant internal environment through homeostasis.
⑤ They evolve (change over generations through variation and natural selection).
These common features show that all organisms evolved from a common ancestor about 4 billion years ago, called LUCA (Last Universal Common Ancestor). Organisms are broadly divided into three domains: Bacteria・Archaea・and Eukarya.
Cell (The basic unit of structure and function in organisms)
Cell theory (All organisms consist of cells. Schleiden 1838・Schwann 1839)
DNA (Deoxyribonucleic acid. A macromolecule carrying genetic information)
ATP (Adenosine triphosphate. The common energy currency of organisms)
Homeostasis (The function of maintaining a constant internal environment)
Evolution (The phenomenon in which traits of organisms change across generations)
Three domains (The highest-level classification of Bacteria・Archaea・and Eukarya)
In Depth
The relationship between diversity and commonality in organisms is beautifully represented by a phylogenetic tree. Phylogenetic trees are drawn by comparing the DNA base sequences of existing organisms, based on the molecular-biological fact that more closely related species have more similar sequences.
For example, human and chimpanzee DNA is about 98.8% identical, and even humans and yeast share remarkably conserved basic genes (cell division・respiration). This means that the basic mechanisms of life were established in a common ancestor and have changed very little since then.
In 1977, Carl Woese discovered Archaea by comparing ribosomal RNA sequences, revising the traditional division of “prokaryotes vs. eukaryotes” into the “three-domain theory.” Archaea often live in extreme environments such as high temperatures・high salt concentrations・and oxygen-free conditions, thriving around deep-sea hydrothermal vents and strongly acidic hot springs.
- Five common features of all organisms = cells・DNA・ATP・homeostasis・and evolution
- Cell theory = Schleiden (plants)・Schwann (animals)
- Common ancestor LUCA = about 4 billion years ago
- Three domains = Bacteria・Archaea・and Eukarya
- Archaea were discovered by Woese in 1977
- Human and chimpanzee DNA identity is 98.8%
- Phylogenetic trees are drawn by comparing DNA sequences
Points to Note
① Cell theory was established by two scientists: Schleiden (plants・1838) and Schwann (animals・1839). Do not confuse their order or fields. ② Archaea are not “ancient bacteria”; they are an independent domain on a lineage separate from Bacteria. In some respects, Archaea are actually more closely related to eukaryotes. ③ Homeostasis is also seen in unicellular organisms, but it is more advanced in multicellular animals. Plants also possess it in different forms. ④ Evolution means “change,” not “progress.” Degeneration and extinction are also forms of evolution.
Practice
- What compound serves as the common energy currency of all organisms?
- Give all three names used in the three-domain classification of organisms.
- Which scientists proposed cell theory for plants and animals, respectively?