Evolution 33

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Geological clock

Kinds of cells

The age of the Earth is about 4.6 billion years. In the image above those 4.6 billion years is depicted as a 12 hours clock. The first signs of life are fossil cyanobacteria (single-celled organisms without a nucleus). Their age is between 3.8 and 3.5 billion years (that means at about 2 o'clock). During the entire Precambrian, which lasted till 543 million years ago, the cyanobacteria were the most common life form. About 2 billion years ago (at 5.35) cells with a nucleus came into being, the Eukaryotes, and only by the end of the Precambrian (at 10.18) the first multicellular organisms occurred. At the beginning of the Paleozoic (at 10.20) an enormous outburst of life forms must have happened for from that moment on the sediments are full of fossils. The Paleozoic ends (at 11.35) with a disastrous extinction at the end of the Permian, 250 million years ago. Then the Mesozoic starts, the period of the dinosaurs. It ends with a great extinction at the end of the Cretaceous, 65 million years ago (at 11.50). One of the causes was probably the impact of a huge meteorite. Next begins the Cenozoic, in which period we still live now. Modern man came into existence about one second and a half before twelve. And we are now the cause of a large extinction ...

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Cyanobacteria

The cyanobacteria were prokaryote, that means that they were single-celled and without a nucleus. Their hereditary material was floating loose in the cell. Cyanobacteria could produce food from carbon dioxide and water and they brought oxygen in the atmosphere. Until about 1.8 billion years ago this oxygen was absorbed by free iron in the seas and fixed in thick layers of iron compound. Only thereafter gradually free oxygen came into the atmosphere.
Around 2 billion years ago the eukaryotes came into being: single-celled organisms with a nucleus and different kinds of organelles (very small organs). The chromosomes were situated in the nucleus. Probably dents from the surface have formed a protection around the DNA molecules and this structure has evolved into a nucleus. Several kinds of organelles seem to have been prokaryotes living inside another cell. This was a kind of symbiosis, in this case called endosymbiosis. An important indication for this is the fact that the mitochondria (the energy factories in the cell) have their own DNA. Some of the eukaryotes have a flagellum as well. It is not impossible that this flagellum is also the result of a symbiosis with a prokaryote. Eukaryotes are much more complicated than prokaryotes. They came into existence by a kind of evolution which is totally different from the type discussed before with mutations and variations. But surely natural selection will have played an important role.

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From Prokaryotes to Eukaryotes

Evolution 33

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