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