Members of a group of bacteria that differ in certain morphological, physiological, and genetic features from the eubacteria. One of the three major divisions of living organisms. They do have some features in common with the eubacteria, suggesting to some researchers that the two groups shared a common ancestor early in their evolutionary history. The two groups of bacteria are as different from each other as they are from eukaryotes (plants and animals). Archaea are microbes and most live in extreme environments requiring excesses of heat or salt for survival, and neither sunlight nor oxygen. They absorb carbon dioxide, nitrogen, or hydrogen sulfide and give off methane gas as a waste product the same way humans breathe in oxygen and breathe out carbon dioxide. Archaea could survive in environments such as those on Venus and in the past on Jupiter, Mars, and Saturn, and Jupiter's moon, Io.活體有機體三大分類的其中一類。古菌屬於細菌中的一類；，其形態、生理、以及基因特徵與真細菌不同，但與真細菌仍有某些相同之處。基於這點，部份研究員認為早期的進化過程中，古菌與真細菌具有共同祖先。古菌與真細菌這兩種細菌群之間的差異與古菌和真核生物 (植物與動物)間的差異相同。古菌是微生物，大多存活在條件極端的環境裡，不需要陽光或氧氣，而是仰賴大量的熱度或鹽生存，他們吸收二氧化碳、氮、或是氫硫化物，並且排放甲烷，如同人類吸入氧氣且呼出二氧化碳。古菌可以在像金星這樣的環境中生存。過去則可在木星、火星、土星以及木衛一埃歐上(Io)生存。