Why life appeared only once on Earth? Did it?

[LifeIsStrange]

Interesting, I have another question:

We don't really know the precise conditions that allowed life to appear, we just know that life appeared extremely early in Earth formation.
Life being so quick to appear make it sounds like it is extremely probable to appear once the basic condition appears.
But to my understanding it is believed that every single known alive and extinct species descend from a same, unique common ancestor. In other words, life would have only appeared once on earth.

So we face a paradox:
Life is extremely probable to appear because it appeared right after some basic conditions were met on earth. The same conditions did hold and still holds nowadays but somehow life appeared only once.

To resolve this paradox here are some tentative hypothesises:

1. reject the premise, life would have appeared many times on earth. There exists a few life forms that share very few in common of their DNA with others and the existing common DNA could be attributed to either horizontal gene transfer or to the idea that there is necessary common minimal design for life.
   E.g some components of the cell structure might be necessary invariants for life. But in the same way as in natural language many sentences can represent the exact same meaning, I believe many different DNA sequences, outputting many different proteins, might generate the same invariants.
   Therefore if life appeared many times but reduced to the same cell invariants, they could still have 0% common DNA. But no living being exist with such a property. To explain this we need the following sub-hypothesies:
   1.1) active DNA has to be extremely efficients and therefore such optimization selection reduce the number of different implementations.
   1.2) horizontal gene transfer has affected every known living being to significant extents
   Another way to defend the 1) hypothesis would be to believe that life has appeared many times but all alternatives forms of life have been quickly made extinct by competition with our first life form.
   But if we think that, that doesn't explain why we couldn't detect new life appearing out of nowhere, nowadays and being quickly killed by current beings.

2. life has only appeared once because the necessary conditions did not last. Which condition did not last then?
   We could find some condition that no longer holds nowadays such as much higher CO2 level, acidity, etc
   But condition that quickly disappeared? No idea.

3. once life appeared, it spread so quickly, and on every single place where new life could have appeared that it prevented new life forms to appear. Because existing life would take the resources necessary for life to appear e.g eating a pre-life form.
   That being said I personally don't believe that life spreaded as much and as quickly.

4. despite everything making us believe that life is extremely likely to appear, maybe that life is actually extremely, extremely unlikely to appear.

What do you think? Do you have any additional hypothesises?
On a related topic, single life cell life forms appeared only one time even if it specialized third times (eukaryote, bacteria and archae).
Contrary to single cell lifeforms, it is known that multicellular life forms appeared many times, ~15 times if I remember well. Could new multicellar life forms still appear today?

[LifeIsStrange]

Why Cambrian explosion?
The natural nuclear reactor?

2 points by The_rationalist 3 hours ago [–]I wonder how much such natural nuclear reactor played a role in biological evolution. If I remember correctly the oldest ediacarian fossils were found at the same place, such nuclear reaction and thus accelerated mutations could have been a starter for multicellar lifereply	2 points by The_rationalist 3 hours ago [–]I wonder how much such natural nuclear reactor played a role in biological evolution. If I remember correctly the oldest ediacarian fossils were found at the same place, such nuclear reaction and thus accelerated mutations could have been a starter for multicellar lifereply
2 points by The_rationalist 3 hours ago [–]I wonder how much such natural nuclear reactor played a role in biological evolution. If I remember correctly the oldest ediacarian fossils were found at the same place, such nuclear reaction and thus accelerated mutations could have been a starter for multicellar lifereply
Ultimatt 2 hours ago | unvote [–]A fun thought, but absolutely zero to do with biological evolution. Multicellular life isn't especially hard to form once you have cells with controlled division/replication either. With the simplest multicelled life essentially being cells that don't completely divide as the start of it. https://www.sciencemag.org/news/2018/06/momentous-transition...reply

https://news.ycombinator.com/item?id=24739302