MadSci Network: Evolution

Re: Is the panspermia hypothesis credible?

Date: Wed Jun 14 13:26:20 2000
Posted By: Sarah Earley, Grad student, CU Boulder
Area of science: Evolution
ID: 960238025.Ev

Hi Hakan,

Since we do not know how life began on Earth, the panspermia hypothesis 
cannot be ruled out.  This hypothesis is credible in the sense that it has 
not been disproven or eliminated as a possibility for the origin of life.  
I can tell you that origin of life studies are very difficult, since we can 
never know for certain whether any of the hypotheses hit the jackpot or 

Though I haven't read much of Francis Crick's writing on this topic, I have 
been to several conferences where the origin of life is a main subject.  
Some origin of life hypotheses include the primordial soup hypothesis, a 
more geochemical origin of life, and panspermia.  

1. The primordial soup hypothesis involves an origin of life in the midst 
of an ocean, where organic molecules interact to form larger organic 
molecules such as amino acids, which theoretically might form proteins.  
(According to the RNA world notion, formation of nucleotides might be more 
convincing, since scientists have known for about two decades that RNA can 
have catalytic functions.  For more information about this, there is a book 
out titled The RNA World (Tom Cech, editor) that has lots of fascinating 
information about RNA and implications for the origin of life.)  

2. The second hypothesis involves interactions between aqueous substances 
and minerals.  The minerals provide a scaffold and possibly contributions 
that would lead to the formation of complex molecules that might eventually 
become biological.  Certainly, this method for the origin of life in more 
probable in the sense that molecular interaction at a surface is more 
likely to occur than interaction in an ocean.  The molecules at a surface 
are localized in a more 2-dimensional environment, rather than floating 
around in a body of water.  

3. Panspermia is just what you said it is: the seeding of life on Earth by 
extraterrestrial matter such a the Martian meteorites.  When I first got 
into exobiology research, I read that regardless of whether there were 
organics on the Earth's surface early on, large impact events would 
effectively volatilize the organics.  That means that Earth would be 
sterilized with regard to organics, and an extraterrestrial contribution 
such as comet debris would be required to get any sort of organic 
synthesis.  There is a paper by Carl Sagan and Chris Chyba about this 
topic. In that sense, life on Earth was clearly seeded from 
extraterrestrial matter.  If you've heard anything about the putative worm 
in the martian meteorite ALH84001, you might know that people are thinking 
of more obvious panspermia.  Actual organisms rather than organic 
molecules.  This is a hotly debated topic at the moment.  It is possible 
for organics to exist within an object traveling from one location to Earth 
without being degraded. 

There is another option that involves an extraterrestrial geochemical 
origin of life.  If life originated at the rock-water interface, the 
abundance of biologically relevant elements in cells should be similar to 
those in the rocks.  This is not a necessary relationship, but it makes 
sense.  If this is in fact the case, the statistics for how much phosphorus 
is in martian rocks agrees roughly with how much phosphorus is in a cell.  
This is not the case with rocks that are from the Earth. I do not know if 
this is the case for any other biologically relevant elements or not, 

This is obviously very complicated.  Panspermia is just one of the many 
theories about the origin of life.  Though you specifically asked about 
that hypothesis, I figured that explaining some of the other prominent 
ideas would be beneficial so you could get a feel for what ideas are out 
there and decide for yourself which makes the most sense.  If you want more 
definitive literature citations, please write back and I'll look them up 
for you.  

Sarah Earley
CU Boulder  

Admin Note: While panspermia gives a possible hypothesis for the origins of life 
on Earth, it still doesn't solve the pre-biotic origins of life, but simply moves 
it to another place, e.g. if life on Earth was seeded by meteorites from Mars, 
then how did life evolve on Mars?

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