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In this lesson we look at how modern molecular evidence led to a fundamental rethinking of how we classify all life on Earth. The traditional five-kingdom system has been supplemented by the three-domain system, proposed by Carl Woese.
The traditional five-kingdom system was based mainly on observable physical characteristics (cell structure, nutrition, body plan). However, as scientists developed new techniques to analyse the chemical structure of organisms — particularly their RNA and DNA — they discovered that some organisms that looked similar were actually very different at the molecular level.
The biggest surprise was that prokaryotes — previously all placed in one kingdom (Prokaryotae/Monera) — actually consisted of two fundamentally different groups.
Carl Woese (1928–2012) was an American microbiologist and biophysicist. In 1977, he proposed a new classification system based on analysis of ribosomal RNA (rRNA) sequences.
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