Informational genes are supposed to be much less mutated during evolution, such as those involved in DNA processing. This group includes the lokiarchaea, which have been found to be the prokaryotes most closely related to eukaryotes when comparing informational genes. These evidences support the 2D hypothesis (Figure 1). ![]() Phylogenetic studies (comparison of DNA sequences) suggest that eukaryotes emerged from a recently discovered group of archaea named the Asgard group. ![]() Thus, by that time, archaea were already a well-established group. In the tree on the right, LECA appeared from a branch of an archaea group. In the tree on the left, eukaryote emerged from a common ancestor shared with archaea. Two theories about the emergence of eukaryotes. Recent evidences from genome phylogenetic studies place the origin of eukaryotes closer to archaea than to bacteria (Figure 1). To make things more complex, the archaea genes in the eukaryote nucleus come from different archaea taxa. However, not all genes follow these rules. For example, genes involved in DNA processing, including replication, transcription and translation are more similar to those of archaea, while those genes in charge of the metabolism, such as energy production and synthesis of amino acids, lipids and nucleotides, are more similar to the genes of bacteria. Current eukaryotes are like chimeras because genes from both types of prokaryotes coexist in the eukaryotic nucleus. There is no doubt that LECA evolved from prokaryotes, but from which one, bacteria or archaea? Choosing one of them is not easy. Comparison of genomes of today's phylogenetically distant eukaryotes suggests that LECA already had a genome as complex and rich as the genome of the current cells, and probably with similar cellular morphology and structure. So, with the evidences we have today, the eukaryote cell was "invented" only once during evolution. Eukaryotes are thought to be monophyletic, which means that all of them, including plants, animals, fungi, algae, and unicellular eukaryotes, descended from a common ancestor known as LECA (Last Eukaryotic Common Ancestor). It is not known how did eukaryotes emerge, but it should have happened as a result of the collaboration between the two prokaryotic cell types present at that time: archaea and bacteria. It means that the process of eukaryogenesis should have being started much earlier, may in the prokaryote communities that populated the Earth at that time. It is about 1500x10 6 after the prokaryotes. ![]() They were also able to integrate whole genomes (that gave rise to mitochondria and chloroplasts), developed the sexual reproduction, and led to a new type of living beings: the multicellular organisms (which independently happened several times).Į ukaryotic cells appeared 1500-2000x10 6 years ago. The eukaryote cells reached a morphological complexity level not seen before, the nucleus included, and developed a network of fibrous proteins known as cytoskeleton. It was something new, with novel features that allow it to explore new evolutionary pathways. The eukaryote cell is regarded as an evolutionary transition. Two main groups of prokaryotes are known: bacteria and archaea. Prokaryotes were thought to be the only cell type during the first 1500 - 2000x10 6 year of the early Earth. They also show a capsule lining the extracellular surface of the plasma membrane, and structures such as prokarytoe flagella and pili, for cell movement and DNA exchange, respectively, can sometimes be observed. The organization of prokaryotic cells is quite simple: a plasma membrane confines an inner space where chemical reactions take place. Prokaryote means before the nucleus, that is, they do not have DNA enclosed by an inner membrane. P rokaryote were the first cells to appear on Earth, which happened about 3500x10 6 years ago.
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