Fermi's paradox suggests that there are little or no other intelligent 
civilizations within the Milky Way galaxy.  On the other hand, intelligent 
life should exist on a substantial fraction of planets with life because 
natural selection broadly increases intelligence with time.  Here on the 
Earth, for example, numerous mammals have a high degree of intelligence and 
I suspect many of them could reach human intelligence with a few more 
million years of evolution.


This contradiction can be resolved if the origin of life is far harder than 
commonly believed.  That is, in the Drake equation, f_L should be far 
smaller than most people think it is.  Even on planets that are life 
friendly the formation of life should be extremely rare for the below 
reasons.


For life to start, a molecule must arise that can make approximate copies of 
itself.  Once that happens then natural selection can work its magic.  But a 
molecule that can make approximate copies of itself must be a fairly 
sophisticated nano-machine being comprised of dozens, if not hundreds, of 
molecules and it must arise via inorganic and non-evolutionary processes.


From the study of DNA and genes, it is known that all life on the Earth has 
a common origin (undoubtedly from a molecule of the aforementioned kind). 
Since Earth is a life friendly planet, why hasn't another molecule (of the 
aforementioned kind) arisen?  If it had, then life on the Earth would have 
organisms with two different molecules for genetic codes: DNA and something 
else.


Since all Earthly life is based on DNA, this suggests that, over the four 
billion years of life on Earth, this has never happened again.  That is, 
over the last four billion years, no other molecule has arisen by inorganic 
and non-evolutionary processes that can make approximate copies of itself 
and evolve other organisms.  And Earth is a life-friendly planet so chances 
are optimal that such a molecule should arise.  This fact is pointed out 
clearly in the New Scientist magazine article "Second Genesis" by Bob 
Holmes: "Many scientists argue that there is no reason why a second genesis 
might not have taken place, and no reason why its descendants should not 
still be living among us".


There is no evidence that other instances of the origin of life, with a 
different genetic basis, would be consumed by any other life prior to 
establishing its own survival.  A genetic code based on a different set of 
atoms and molecules would not necessarily be palatable to any other life. 
In fact, it could be toxic.  Two different sets of biochemistry could have 
their progeny ignore each other like many species on Earth only have a very 
small set of predator and prey.  Obviously there would be co-evolution 
because of mutual interactions and symbiotic relationships would exist.


If multiple instances of the origin of life happened on the same planet then 
there is no reason to think that they would not all have long-term progeny. 
People must drop the old western mentality that says "This planet is not big 
enough for the both of us".


This suggests that the formation of such a molecule is a very rare event. 
In other words, the reaction rate of inorganic chemistry per square meter 
times the surface area of the Earth, times the average depth such reactions 
take place, times four billion years is <<, much less, than the number of 
such reactions needed before an approximately self reproducing molecule 
arises by chance.


If that first molecule had not arisen here on the Earth then the Earth would 
probably have been lifeless ever since.  This same reasoning applies if life 
first started somewhere else in the solar system and then migrated to Earth. 
During the late heavy bombardment, any life in the solar system could have 
been moved to any other place inside the solar system.  If life rose 
independently on Mars once, over the past four billion years, then that 
suggests that the reaction rate of inorganic chemistry per square meter, 
times the surface area of a Mars sized world, times the average depth such 
reactions take place, times four billion years is about the number needed so 
that an approximately self reproducing molecule arises by chance once, ~ 1.


It seems too much of a coincidence that the laws of chemistry work out in 
such a way that life arises, on average, once per terrestrial world per 
several billion years.  Rather, for such cases, it seems much more likely 
that life arises multiple times or almost never.  The latter possibility 
makes sense from a combinatorial perspective.  A self reproducing molecule 
will be composed of dozens to hundreds of other molecules.  But the total 
number of permutations for such a molecule's components will far exceed the 
total number of inorganic chemical interactions that take place per 
terrestrial world per several billion years.


A simple combinatorial thought experiment explains why.  The number of ways 
of stacking a deck of playing cards is so huge that if 67.8 billion solar 
masses were converted entirely into protons then each proton could represent 
a different way of stacking the deck of playing cards.  But there are 92 
naturally occurring chemical elements and a self reproducing molecule will 
probably be composed of hundreds of atoms from the set of 92 different 
kinds, whereas there only 52 cards in a playing deck.  The number of 
permutations for any `genesis' molecule could dwarf the number of chemical 
reactions occurring in the observable universe over the past 13.7 billion 
years.


So, in the Drake equation, f_L could be something really small like 10^-90. 
In this case the fact that life exists on the Earth simply shows that the 
universe is super huge and its true size far exceeds the visible universe. 
During both cosmic inflation and dark energy inflation the universe falls 
down its own gravity well converting huge quantities of its gravitational 
potential energy into vacuum energy and expansion energy.  This probably 
explains why the universe is so much larger than just the observable 
universe.


So the universe could contain 10^150 planets, for example.  If f_L is 10^-90 
then the total number of planets in the universe that have life is around 
10^60.  So there are a lot of planets with life out there but none of them 
are close by.  So this is one possible explanation for why there is only one 
kind of life in the solar system.  And this explanation is consistent with 
Fermi's paradox.  It also suggests that any other life in our solar system 
got there via migration.


In light of all this, it cannot be concluded that water, oxygen, and 
methane, for example, are indicators of extraterrestrial life.  The presence 
of these simple gases in the atmospheres of other planets can easily be 
explained by inorganic processes.  Since little is known about the geology 
and chemistry of planets in other solar systems, there could be many ways 
that an oxygen rich atmosphere arises by non-biological means.  Check out 
the below link for just such an example.  To claim that oxygen in a planet's 
atmosphere is a litmus test for life is unfairly stacking the deck against 
more prosaic possibilities.  It is unlikely that alien life would use the 
exact same photosynthesis that biological processes employ on Earth, or even 
have O2 as a waste product.


  http://www.physlink.com/News/020304ExopanetOC.cfm


If Earth is the only planet in 10^150 with life then that suggests that the 
universe is fine tuned for Earthly life.  If a substantial fraction of the 
10^150 planets have life then that suggests the whole universe is finely 
tuned for life.  If the universe if not fine-tuned for life then that 
suggests the number of planets with life should be around the logarithmic 
middle of 10^150 or around 10^75.  If the universe is not fine tuned for 
intelligence then the number of planets with intelligent life should be 
around the logarithmic middle of 10^75 or about 10^38.  It seems there are 
lots of planets out there with life and intelligence but none of them will 
ever communicate with humans.


The Fermi paradox, and the vast combinatorial possibilities for atoms and 
molecules, plausibly suggests that both extraterrestrial life and 
extraterrestrial intelligence are relatively rare.


The evolution of life and intelligence may occur in the following way.  The 
evolutionary tree of life may be like a shrub and the height of each shrub 
leaf, say, is proportional to the intelligence of the species represented by 
that leaf.  As the shrub grows, it has branches growing in all directions, 
from zero degrees to ninety degrees relative to the shrub's base.


A leaf at the end of a branch at zero degrees is almost at ground level and 
that leaf corresponds to a species whose intelligence has not changed much 
over billions of years, for example primitive bacteria like life.  Leafs at 
the top of the shrub, around ninety degrees, correspond to species with the 
most amount of intelligence (for the biosphere represented by that shrub).


Here on the Earth, for example, the hominoid family, and probably a few 
others species like Dolphins, are represented by leafs that are around 
ninety degrees on Earth's `shrub of life'.  As a shrub grows, it has 
branches that grow in all directions, from zero degrees to ninety degrees. 
In this sense evolution is not selecting for intelligence since the branches 
are randomly growing in all directions.


But there is a broad increase in intelligence since the average height of 
the shrub increases while it grows.  On some biospheres, as its `shrub of 
life' grows there will probably come a time when a leaf or two reaches a 
sufficient height that its corresponding species is capable of radio 
astronomy.  Once this happens then that species reworks that planet's biota 
which prevents any other species from evolving into high intelligence.  It 
is certainly possible that most planets with intelligent life follow this 
pattern.


There is no evidence that (1) DNA is the only basis for life, (2) multiple 
instances of the origin of life have occurred on the Earth, (3) on any 
planet one origin of life make other such origins implausible, and (4) 
primitive self-replicating molecules are forming all the time on Earth.  In 
fact, there may never have been an origin of life in the solar system.  Life 
may have migrated to the solar system on debris from an earlier solar system 
and this could explain Earthly life so soon after the Earth's formation.


With just today's technology, astronomers are able to map about a million 
galaxies in the Sloan digital sky survey.  So it is fair to assume that a 
civilization in our galaxy, that is 200,000 years ahead of ours, would have 
mapped all, or most, of the stars and planets within the Milky Way galaxy. 
To see why note that, in the past century, the technology was developed to 
automate the production of hundreds of millions of cars.  A civilization 
200,000 years ahead of ours would easily have automated the production of 
millions of large space based telescopes capable of discovering most of the 
planets within the Milky Way.


Such a civilization would already know about the Earth and would be capable 
of sending space probes to Earth.  Furthermore, a civilization like that 
could easily automate the long term continuous broadcasting of 
multi-frequency signals toward millions of favorable planets, especially 
since its space based automatic broadcasting equipment would have automated 
self maintenance systems and therefore require little or no effort to 
maintain.  Fermi's paradox applies not only to extraterrestrial life 
visiting the Earth but also to extraterrestrial life broadcasting to the 
Earth.


In conclusion, it is quite possible that f_L is a very small number and both 
life and intelligence is quite rare.



K