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The cell. 1. Introduction


Prokaryote were the first cells to appear on Earth, which happened about 3500x106 years ago. Prokaryote means before the nucleus, that is, they do not have DNA enclosed by an inner membrane. Prokaryotes were thought to be the only cell type during the first 1500 - 2000x106 year of the early Earth. Two main groups of prokaryotes are known: bacteria and archaea.

The eukaryote cell is regarded as an evolutionary transition. It was something new, with novel features . The eukaryote cells reached a highly morphological complexity level, integrate whole genomes, developed the sexual reproduction, and led to multicellular organisms.

1. When?

Eukaryotic cells appeared 1500-2000x106 years ago. It means that the process of eukaryogenesis should have being started much earlier.

2. From who?

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. 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). So, with the evidences we have today, the eukaryote cell was "invented" only once during evolution.

There is no doubt that LECA evolved from prokaryotes, but from which one, bacteria or archaea? Choosing one of them is not easy (Figure 1). Current eukaryotes are like chimeras because genes from both types of prokaryotes coexist in the eukaryotic nucleus. 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 are more similar to the genes of bacteria. However, not all genes follow these rules.

Recent evidences from genome phylogenetic studies place the origin of eukaryotes closer to archaea than to bacteria (Figure 1).

Origin fo eukaryotes
Figure 1. Two theories about the emergence of eukaryotes. In the tree on the left, eukaryote emerged from a common ancestor shared with archaea. This is the 3D model. In the tree on the right, LECA appeared from a branch of an archaea group. Thus, by that time, archaea were already a well-established group. This is the 2D model. LECA (Last Eukaryote Common Ancestor).

3. How? Eukaryogenesis

There is a key event during the emergence of LECA: what was the impact of the endosymbiosis of the mitochondrial ancestor? There are many theories about how the evolutionary process that gave rise to LECA happened. They can be summarized in two (Figure 2):

Origin of eukaryotes
Figure 2. Two models for the origin of the first eukaryotes (LECA). The colors of membranes indicate which cell is in charge of a particular compartment (modified from López-García y Moreira, 2015)

Eocyte or autogenous model . It suggests that an arcahea lineage progressively increased the cellular complexity to develop into the first eukaryotic common ancestor (FECA), and then into LECA. It means that a proto-eukaryote cell (FECA) was first developed, which emerged from the Asgard archaea lineage. These cells independently acquired most of the cell complexities found in the eukaryote cells, such as the endomembrane system and the cytoskeleton, but lack mitochondria. They also got the ability to do phagocytosis, that later allow them to acquire an ancestral alfa-proteobacterium.

Symbiotic model. This model proposes that the direct fusion of a archaea and a bacterium gave rise to the first eukaryotic cell. There was no proto-eukaryote intermediate stage between prokaryote and eukaryote. In the phylogenetic tree there would be two initial branches, archaea and bacteria, and then a third branch, the eukaryote. There are current examples of bacteria cells with prokaryote cell inside. The fusion of the two cell triggered the increase in cell complexity, and the bacteria became the mitochondria. In this new cells, archaea rules the DNA processing and bacterium the metabolism and membranes.

4. Endomembranes

One feature of eukaryotes, different from prokaryotes, is a rich and complex endomembrane system. The evolutionary origin of this system is still unknown. All the inner membrane-bound compartments may have been formed from archaea plasma membrane invaginations that were later detached. Thus, the interior of the endoplasmic reticulum or the Golgi apparatus is actually extracellular space. However, how the archaea lipids (isoprenoid fatty acid chains and ether bonds) were replaced by the bacteria lipids (no isoprenoids and with ester bonds) remains to be revealed.

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