K_h wrote:

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.

Does it? News to me. What evidence do you have that this is the case?

Here on the Earth, for example, numerous mammals have a high degree of intelligence and many of them could reach human intelligence with a few more million years of evolution.

Yes, if there were indeed strong selection pushing them toward greater intelligence. What makes you think there is?

This contradiction can be resolved if the origin of life is far harder than commonly believed.

Easier to resolve it by doubting your central claim, that there is a general trend toward increasing intelligence. The great majority of the world's living species will not disagree with you, because they're bacteria without so much as a nerve among them.

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.

Not necessarily. What if DNA is the only reasonable basis? It would be invented independently more than once. But of course all life we know of is related. Either life arose here once because it's just that unlikely, or it arose several times and only one survives (look up coalescence, if you will), or the first origin to happen changed conditions to make it unlikely for a second origin to happen. You're going to have to rule out the other two alternatives if you want to pick the first one. Another problem is that life arose comparatively soon after the world became hostpitable to attempts; it doesn't sound that unlikely.

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 Earth is a life-friendly planet so chances are optimal that such a molecule should arise.

Not true. Earth is now a very life-unfriendly planet. Organic molecules are eaten before they have much chance to evolve, and that nasty poisonous oxygen degrades organic compounds.

[snips]

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.

Who says water is an indicator of life? It's only claimed to be necessary for life. Methane, as far as I know, is never mentioned. Oxygen is the indicator of life, and if you want to suggest an inorganic process that can make a lot of free oxygen in an atmosphere, feel free.

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.

That's what we might call number salad. Can you present a real argument why any of these numbers would mean what you claim?

In conclusion, it seems there are lots of planets with life out there but none of them will ever communicate with humans.

In conclusion? You have just denied the entire rest of your post. First you claim that life is rare but intelligence is inevitable given life. And to conclude you claim that life is common but intelligence is rare. What exactly are you smoking?