It is now accepted that the first cells derived from simpler "objects", and that their descendants became more and more complicated and ordered until their evolutionary transformation into modern cells, namely, cells endowed with the same basic structures and mechanisms as those existing today. Although the appearance of the first modern cells goes back at least two billion years, many scholars believe that it occurred almost four billion years ago. In that case, the Earth's formation, which took place quite quickly about 4. 6 billion years ago, is in close proximity to the originoflife. There…mehr
It is now accepted that the first cells derived from simpler "objects", and that their descendants became more and more complicated and ordered until their evolutionary transformation into modern cells, namely, cells endowed with the same basic structures and mechanisms as those existing today. Although the appearance of the first modern cells goes back at least two billion years, many scholars believe that it occurred almost four billion years ago. In that case, the Earth's formation, which took place quite quickly about 4. 6 billion years ago, is in close proximity to the originoflife. There is disagreement not only about the moment ofappearance ofmodern cells, but also as to their early evolution. Ideas on the first steps in cellular evolution vary considerably, and our present knowledge is so limited as to be little more than vacuous. Why discuss it then? Precisely because it is an empty space that needs to be filled! Does it make sense only to discuss things which are well-known and well documented? Is this the best way to interpret that spirit of exploration of the unknown which is at the very heartofscientific endeavor? The idea that it is idle to venture into fields which are poorly documented has been long supported, even in cases similar to those dealt with here, for example, the evolutionary phases which took place before the first cell.
1 Introduction.- 2 The First Cell.- 2.1 Properties attributed to the first cell: reproduction.- 2.2 Properties other than reproduction.- 2.3 Derivation of the first cell from a precellular aggregate.- 2.4 There are many kinds of putative aggregates.- 2.5 A detailed hypothesis on the formation of the first cell.- 2.6 Properties attributed to modern cells.- 2.7 Temporal frame.- 3 Modern Cells: Prokaryotic and Eukaryotic Descents.- 3.1 The general pattern of descent lineages remains controversial.- 3.2 Descent is traditionally inferred from morphological comparison.- 3.3 Descent inferred from gene sequences is not truly universal.- 3.4 The possible origin of a protein complex.- 3.5 What was the cellular organization of the common ancestor?.- 3.6 Major evolutionary transitions.- 4 Prokaryotes: The Flagellum.- 4.1 The bacterial flagellum is a complex structure.- 4.2 Complex structures appear only once.- 4.3 From what did the flagellum derive?.- 4.4 Possible derivation series.- 5 Prokaryotes: The Outer Membrane.- 5.1 The outer membrane is also a complex structure.- 5.2 Was the outer membrane an alimentary adaptation?.- 5.3 Did the outer membrane appear in a flagellated bacterium?.- 5.4 The role of interspecific gene transfers.- 6 Prokaryotes: Trends towards Complexity.- 6.1 Cell differentiation.- 6.2 Multicellularity.- 6.3 Particles which escaped from cells.- 7 The Appearance of the Nucleus.- 7.1 Eukaryotes are defined by the nucleus.- 7.2 The nucleus requires an intracellular skeleton.- 7.3 Did the nucleus appear inside an indeterminate cell?.- 7.4 Did the nucleus appear inside a bacterium?.- 7.5 Did the nucleus derive from an intracellular bacterium?.- 7.6 The origin of nuclear linear chromosomes.- 8 Eukaryotes: Dictyosomes.- 8.1 From what did dictyosomes derive?.- 8.2 Progress in membrane traffic.- 8.3 Did some vesicles derive from intracellular symbiosis?.- 9 Eukaryotes: The Mitochondrial Symbiosis.- 9.1 Mitochondria are bacteria adapted to intracellular life.- 9.2 Primitive engulfment and origin of mitochondria.- 9.3 Alternative hypotheses on the origin of mitochondria.- 9.4 Have Eukaryotes without mitochondria always been so?.- 10 Eukaryotes: Plastidial Symbioses.- 10.1 Plastids are also bacteria adapted to intracellular life.- 10.2 Intermediate adaptations on the way to plastidial symbioses.- 10.3 Parallel symbioses.- 10.4 Serial symbioses.- 10.5 Plastidial regressions.- 11 Eukaryotes: The Cilium.- 11.1 A surprising and complicated geometry.- 11.2 Ideas to explain the geometry of the cilium.- 11.3 Did the cilium derive from a specialization of the intracellular skeleton?.- 11.4 Did the cilium derive from extracellular symbiosis with a Spirochete?.- 11.5 Before and after the appearance of the cilium.- 12 Eukaryotes: Complexity and Gigantism.- 12.1 The fixation of sex.- 12.2 Cell differentiations.- 12.3 Multicellularity.- 13 Conclusions.- Acknowledgements.- References.
1 Introduction.- 2 The First Cell.- 2.1 Properties attributed to the first cell: reproduction.- 2.2 Properties other than reproduction.- 2.3 Derivation of the first cell from a precellular aggregate.- 2.4 There are many kinds of putative aggregates.- 2.5 A detailed hypothesis on the formation of the first cell.- 2.6 Properties attributed to modern cells.- 2.7 Temporal frame.- 3 Modern Cells: Prokaryotic and Eukaryotic Descents.- 3.1 The general pattern of descent lineages remains controversial.- 3.2 Descent is traditionally inferred from morphological comparison.- 3.3 Descent inferred from gene sequences is not truly universal.- 3.4 The possible origin of a protein complex.- 3.5 What was the cellular organization of the common ancestor?.- 3.6 Major evolutionary transitions.- 4 Prokaryotes: The Flagellum.- 4.1 The bacterial flagellum is a complex structure.- 4.2 Complex structures appear only once.- 4.3 From what did the flagellum derive?.- 4.4 Possible derivation series.- 5 Prokaryotes: The Outer Membrane.- 5.1 The outer membrane is also a complex structure.- 5.2 Was the outer membrane an alimentary adaptation?.- 5.3 Did the outer membrane appear in a flagellated bacterium?.- 5.4 The role of interspecific gene transfers.- 6 Prokaryotes: Trends towards Complexity.- 6.1 Cell differentiation.- 6.2 Multicellularity.- 6.3 Particles which escaped from cells.- 7 The Appearance of the Nucleus.- 7.1 Eukaryotes are defined by the nucleus.- 7.2 The nucleus requires an intracellular skeleton.- 7.3 Did the nucleus appear inside an indeterminate cell?.- 7.4 Did the nucleus appear inside a bacterium?.- 7.5 Did the nucleus derive from an intracellular bacterium?.- 7.6 The origin of nuclear linear chromosomes.- 8 Eukaryotes: Dictyosomes.- 8.1 From what did dictyosomes derive?.- 8.2 Progress in membrane traffic.- 8.3 Did some vesicles derive from intracellular symbiosis?.- 9 Eukaryotes: The Mitochondrial Symbiosis.- 9.1 Mitochondria are bacteria adapted to intracellular life.- 9.2 Primitive engulfment and origin of mitochondria.- 9.3 Alternative hypotheses on the origin of mitochondria.- 9.4 Have Eukaryotes without mitochondria always been so?.- 10 Eukaryotes: Plastidial Symbioses.- 10.1 Plastids are also bacteria adapted to intracellular life.- 10.2 Intermediate adaptations on the way to plastidial symbioses.- 10.3 Parallel symbioses.- 10.4 Serial symbioses.- 10.5 Plastidial regressions.- 11 Eukaryotes: The Cilium.- 11.1 A surprising and complicated geometry.- 11.2 Ideas to explain the geometry of the cilium.- 11.3 Did the cilium derive from a specialization of the intracellular skeleton?.- 11.4 Did the cilium derive from extracellular symbiosis with a Spirochete?.- 11.5 Before and after the appearance of the cilium.- 12 Eukaryotes: Complexity and Gigantism.- 12.1 The fixation of sex.- 12.2 Cell differentiations.- 12.3 Multicellularity.- 13 Conclusions.- Acknowledgements.- References.
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