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Many scientists now suspect that those two groups diverged from a common ancestor relatively soon after life began. Archaea and some bacteria evolved in these conditions, and are able to live in similar harsh conditions today. There was little if any oxygen in the atmosphere. The early Earth was hot, with a lot of extremely active volcanoes and an atmosphere composed mostly of nitrogen, methane, ammonia, carbon dioxide, and water. The ability of some archaea to live in environmental conditions similar to the early Earth gives an indication of the ancient heritage of the domain. But archaeans also share genes with eukaryotes, as well as having many genes that are completely unique.Īrchaea are so named because they are believed to be the least evolved forms of life on Earth (‘archae’ meaning ‘ancient’). Archaea and bacteria also share certain genes, so they function similarly in some ways. This difference led microbiologist Carl Woese of the University of Illinois to propose reorganizing the Tree of Life into three separate Domains: Eukarya, Eubacteria (true bacteria), and Archaea.Īrchaea look like bacteria – that’s why they were classified as bacteria in the first place: the unicellular organisms have the same sort of rod, spiral, and marble-like shapes as bacteria. A type of prokaryotic organism that had long been categorized as bacteria turned out to have DNA that is very different from bacterial DNA. However, new insight into molecular biology changed this view of life. Instead, the DNA is part of a protein-nucleic acid structure called the nucleoid. The cells of prokaryotes, on the other hand, lack this nuclear membrane. Animals, plants, protists and fungi are all eukaryotes because they all have a DNA-holding nuclear membrane within their cells. Eukaryotes are organisms whose cells have a nucleus - a sort of sack that holds the cell’s DNA. Eukaryotes represent four of the five Kingdoms (animals, plants, fungi and protists). The five Kingdoms were generally grouped into two categories called Eukarya and Prokarya. There eventually came to be five Kingdoms in all – Animalia, Plantae, Fungi, Protista, and Bacteria.
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But as new forms of life were discovered and our knowledge of life on Earth grew, new categories, called ‘Kingdoms,’ were added. When scientists first started to classify life, everything was designated as either an animal or a plant.