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I know that the probability of life existing on a habitable planet is pretty slim. But, how likely is it that it hapenned millions of times in the same planet? Or was it only a few species that evolved into millions of other different species over time?

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Hello! I am not sure I understand your question. Do you mean: "How many times did life originated de novo (from non-living things) on earth?". If this is your question, you might want to change your title which is misleading. We have the feeling that you ask about how many species exist today. –  Remi.b Dec 18 '13 at 18:07
    
I mean if life is very rare to happen spontaneously how do we have millions of different species? –  MAyman Dec 18 '13 at 18:10
    
Because of speciation (evolutionary process by which one species becomes two!). Would something like this answer your question? –  Remi.b Dec 18 '13 at 18:12
    
Yes. Thank you. –  MAyman Dec 18 '13 at 18:14

2 Answers 2

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We can be reasonably certain that ALL species that exist today share a common ancestor, a common origin. Every single species we have even encountered, for example, uses a long chain of nucleotides (DNA or RNA1) to store its genetic information. If life had arisen through multiple independent sources, we would expect to find different solutions to the problem of passing genes to offspring.

To put it another way, all forms of life we have found can be placed in one of three main classes (superkingdoms): eukaryotes, prokaryotes and archea. All other types of life come under one of those three categories (viruses are special and not really considered alive2):

enter image description here

Now, any organism we have ever seen uses the same basic systems of storing its genetic information. In fact, there are even certain genes that are conserved (similar in sequence) across all domains of life (see my answer here for some examples). That is why this version of the tree has a common root. All lines start from a single original line (commonly known as the Last Universal Common Ancestor, LUCA).

In summary, all the evidence we have points to a single origin of life. There are too many shared features to assume multiple origins. Despite their enormous diversity, all organisms share certain very basic systems.


NOTES

  1. As far as I know, only certain viruses use RNA instead of DNA, all truly living creatures use DNA. Even if I'm wrong about this, the differences between RNA and DNA at this level are small enough that the common origin still holds.

  2. See here for more in why viruses are not alive.

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Another proof of the common origin of life is the use of (almost) the same codons for coding proteins. Or that the sequence DNA -> RNA -> Protein is also always the same. –  Chris Dec 18 '13 at 18:34
    
@Chris well, there are actually at least 20 different genetic codes (granted they don't change that much) so I did not want to use that one. Good point on DNA->RNA->Protein though. –  terdon Dec 18 '13 at 18:37
    
I know that there are some differences. But given that the three kingdoms seperated quite a long time ago, its pretty amazing, that we can still translate human proteins in E.Coli and they are functional. And the difference is quite small if you think about the sheer number of species. –  Chris Dec 18 '13 at 18:41
    
@Chris absolutely, I agree. I just didn't want to use it as an argument (though I would if debating another biologist) since there are actually some differences. –  terdon Dec 18 '13 at 18:47
    
Thats a good point. :-) –  Chris Dec 18 '13 at 19:16

The fantastic number of species that one can observe today is not a fixed value. This number changes over time. These changes are not caused by De Novo origin of life but by two processes which are:

  • exctinction
    • Process by which a species disappear
  • speciation
    • Process by which one species (or one lineage) becomes two.

Therefore the absolute change in number of species equals the difference between speciation and extinction.

$$\delta_{species} = speciation.rate - extinction.rate$$

There is a ton to say about extinction and speciation and it would take several books to tell you about this science. If you want to study the subject a bit further, I would suggest you to first grab a book of general biology or an introductory book in evolutionary biology before jumping into a book that talk about speciation and extinction.

One can map how speciations events occurred frmo a unique common ancestor in order to create the diversity of life you can observe today. Such mapping are called Tree of Life. Here are the roots of the tree of life of living things on earth. You can spot Animalia, Fungi and Plants on the far right of the picture. The Common ancestor of all living things on earth is called LUCA (Last Universal Common Ancestor).

enter image description here

As a sidenote: The concept of species itself accept several definitions.

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