I would argue it's a very, very good bet that we (intelligent man) would not be here now if one totally random event in the history of earth had not occurred. I refer to being hit by that dinosaur killing, mammal preserving, (little) 6 mile wide meteorite 65 million years ago that created the Cretaceous and-Tertiary boundary!

        "The question of questions for mankind, the problem which underlies all others, and is more deeply interesting than any other, is the ascertainment of the place man occupies in nature, and of his relation to the universe of things. Whence our race has come.
                                                                                            Thomas Huxley, 1863

Scientist views
        In popular astronomy writing and TV of the 60's and 70's the most commonly heard viewpoint on extraterrestrial life was that it considered to be very likely. One famous astronomer of earlier years who often talked about the high likelihood of life in the universe was Carl Sagan. Early in his career he coauthored a book titled: Intelligent Life in the Universe", and has been quoted as saying,  "I personally have been captured by the notion of extraterrestrial life, and especially extraterrestrial intelligence from childhood".

         The 'life is everywhere' view was based on two simple assumptions: high likelihood of huge number of habitable planets x  high likelihood of life just starting up (given enough time). (But I never really bought it.). In the 21st century the scientist belief/hope that 'life is everywhere' seems to have faded badly. The first quote that we may be all alone that I've seen is below.

        Richard Dawkins, famous evolutionary biologist, in his new book expresses a view much like my own:

        "Most scientists hunch is that there are independently evolved life forms dotted around planetary islands throughout the universe -- though sadly too thinly scattered to encounter one another. ... But is it also possible that we really are alone in the universe, in which case Earth, with its greatest show on earth (meaning life, and not coincidentally the title of Dawkins new book!), is the most remarkable planet in the universe." (WSJ 9/09)

       Old, early generation, dim stars, like those in globular clusters, are likely to have been formed from clouds that were poor in 'metals. In astronomy metallicity of an object is the proportion of its matter that is not hydrogen or helium. In this sense earth and life are mostly made of 'metals'. Metallicity of stars goes up when stars are formed later in the life of the universe and in regions of high star concentration. The metallicity of the Sun, formed in the last 1/3 of the life of the universe, is approximately 1.6 per cent by mass.

        The black hole cores of young galaxies tend to emit huge amounts of radiation (we see them as quasars). Planets in young galaxies or anywhere near the center of any galaxy when its large black hole is 'feeding' are likely to be bathed in nasty, high intensity, life killing radiation.

        Most stars are in binary (or triplet) systems, and any planets they have are very unlikely to have stable, constant temperature orbits.

        Large stars run hot and burn out in much less than the 4 billion years it takes, or seems to take, intelligent life to develop. This argument, of course, may be parochial because we have only one data point, the time required for intelligent life to appear on earth.

        Small, dim stars, while much longer lived, have only narrow, relatively circular, orbital regions where solar flux stays in an earth like range. This argument is somewhat weakened by the fact that earth seems to have accommodated a 30% change in solar flux by (apparently) varying the concentration of the greenhouse gas CO2 in the atmosphere (Gaia hypothesis?).

        A planet in a highly elliptical orbit around a dim start would tend to have wicked temperature variations on land (if there is any). But such a planet might be able to harbor life its oceans because the ocean temperature, due to its large thermal inertia, would be controlled by the average temperature of the planet over its whole orbit. Large temperature swings on land would seems to be a negative for the development of intelligent life.

         A lot of stars are tightly packed in the center of galaxies or in globular clusters. When stars are close, near collisions are common which would disrupt planetary orbits. Also when stars are close a nearby supernova can easily sterilize the surface.

Solar system
        The fact that the solar system contains a very large mass planet (Jupiter) has protected earth from (too many) catastrophic impacts in last few billion years. Jupiter with 318 times the mass of earth (about 0.1% the mass of the sun) with help from Saturn (30% the mass of Jupiter) has over time cleaned out the inner solar system of most of the chaotic or widely circulating smaller bodies. If they come near Jupiter, they either get sucked in or accelerated hugely and thrown out of the (inner) solar system.

        This cleaning out of potential comets and meteorites has (over time) hugely reduced the number and size large bodies hitting the earth. Computer simulation shows that with Jupiter and Saturn reduced to the size of Uranus and Neptune (about 15 times the mass of earth) the earth is hit with a mass extinction size comet or meteroid 1,000 times more often, roughly every 100,00 years instead of every 100 million years. This would greatly increase the chances that earth (or the earth's surface) would be sterilized, or pushed back to the single cell evolution stage, by a large impact.

Astronomical history of earth
        Earth has a single, large moon, which based on evidence from moon rocks is thought to have formed from the debris of a huge collision earth had with a Mars sized object  In other words the creation of our large moon, which is huge relative to the size of the planet, looks to be pretty unlikely.  It is thought that without our moon the earth's axis tilt would be unstable and chaotic. How significant this is would probably depend on the time scale, which I have never seen anything on. The effect of a tilt change to life (I would think) would be somewhat on the order of continents moving around, which happens on a time scale of a 10-100 million years.

        Earth's orbit around the sun, though technically an elipse, is nearly circular. Hence orbit induced variations in solar radiation on earth are small. But even the earth's very small solar radiation variation due to its orbit is thought to be a (fundamental) driving forces behind cyclical ice ages in the last few million years.

Geologic history of earth
       Earth needed enough mass (and radioactive elements) to keep its core molten for four+ billion years. The molten core provides a magnetic field that protects the atmosphere from erosion by the solar wind. Current thinking is that Mars, which has only about 1/9th the mass of the earth, lost most of its atmosphere because it lost its magnetic field a few billion years ago it cooled enough so that its iron core 'froze out'.

        The hot interior of the earth is also necessary for  plate tectonics. Many people think that  plate tectonics, which results in (so called) drift of continents and volcanos is a fundamental driving force for evolution and also strongly affects the make up of the atmosphere and temperature of the earth. Venus, with nearly the same mass as earth but no plate tectonics, has a blazing hot surface and is held up as an example of how important  plate tectonics is for life.

          Snowball earth --- a recent theory is that glaciers covered all land on earth (down to the tropics) several times between 580 and 750 million years ago. Though controversial, this theory has gained a lot of support. In a 1999 paper titled, The Snowball Earth, by Hoffman and Schrag of Harvard, the authors write

        "A series of global "freeze-fry" events would cause population "bottlenecks and flushes", observed to accelerate evolutionary rates in some species. The crash in population size accompanying a global glaciation would be followed by millions of years of comparative genetic isolation in high-stress environments. This is a favorable scenario for genomic reorganization and the evolution of new body plans. Finally, repopulations following each glaciation would occur in transient selective environments quite different from those preceding the glaciation, favoring the emergence of new life forms."

        http://www.eps.harvard.edu/people/faculty/hoffman/snowball_paper.html
        http://en.wikipedia.org/wiki/Snowball_Earth

Gaia history of earth
        How come the earth's temp has remained so stable and in the right zone for life as the sun has warmed 30% (predicted by solar models) in last few billion years?  Is this another stroke of dumb luck, or is this a (Gaia like) negative feedback effect with life changing the CO2 in the atmosphere and/or the albedo of the earth to hold the temp in the right zone for life?

        For about the first third of the earth's history there was little to no oxygen in the earth's atmosphere. Only when photosynthesizing cyanobacteria, which put out oxygen as a waste product, evolved 3 million years ago did the atmosphere begin to develop the high oxygen levels that nearly all land and marine animals need to live. --- This is a standard argument, but it bothers me. I would think the oxygen build-up in the atmosphere would cause high oxygen levels in the ocean too and kill off the anaerobic cyanobacteria making the oxygen. So why didn't the oxygen content of the atmosphere level off at much lower levels?

      Newer findings indicates that the Earth's oceans were largely anoxic and highly sulfidic up until about 600 million years ago, which is the time of the Cambrian explosion.

Mass extinctions on earth
        The classic mass extinction is the last major extinction that occurred 65 million years ago (Cretaceous-Tertiary extinction) that killed off the dinosaurs, clearing the way for mammals. The leading theory (Alvarez hypothesis), which is pretty well confirmed at this point, is that this extinction was caused by a 6 mile dia (really pretty small!) meteorite hitting the Yucatan peninsular in Mexico (Chicxulub Crater). A study reported in the Geological Society of America Bulletin in 2006 says new samples provide compelling evidence that one and only one (meteorite) impact caused the KT mass extinction  Overview article here:

             http://en.wikipedia.org/wiki/Cretaceous-Tertiary_extinction_event

        250 million years ago the biggest mass extinction occurred with 95% of marine species killed and 70% of land species. This is the Permo-Triassic extinction. This extinction cleared the way for eventual dominance by dinosaurs. It's unclear what the cause of this extinction was. It could have been an asteroid, but more likely was a long series of enormous volcanic eruptions (resulting in flood basalts) in Siberia..

Implications of mass extinctions on earth
           The earth seems to have had about the 'right number' (a slippery term if ever there was one) of catastrophic mass extinctions (about 5 big ones).  Not too many and not too big, and not too few and not too small. Mass extinctions by killing off competition opens up resources, thus jumps starts evolution.

        Clearly there was risk that an extinction causing meteorite hitting the earth might have been just a little 'too big'. A meteorite with x10 dia likely has x1,000 times the mass and (potentiall) x1,000 times the energy. 'Too big' could kill off not 95%, but 100% of life, sterilizing the earth, or at least the surface. Or maybe just killing all 'higher' (multi-cellular) life, setting back the evolutionary clock by a couple of billion years to the single cell stage.

       There is also the risk that earth would have had too few (or too small) mass extinctions. If the Cretaceous-Tertiary extinction had not happened, would mammals have been able to out compete dinosaurs?  Who knows? But more than a hundred million years (a long, long time) of dinosaur dominance argues that mammals would never have become dominant. It's a very good bet that mammal evolution would have been radically different if mammals had had to compete with dinosaurs (for a long time).

  Think about the implications of this one truly random event
       A  metorite, just big enough to kill all the dinosaurs without sterilzing the earth, hits earth when mammals (with their superior temperature regulation and internal fetus development) already exist, and some mammals survive the extinction. To me this speaks volumes about the incredible dumb luck that we are here.

Time scale of single cell germs, animals and plants
        For about 80% of the history of the earth life has existed on earth at the maximum complexity level was one cell.  This is a long, long time, several billion years. It is apparently (but not surprisingly) a huge (and highly unlikely?) evolutionary jump to increase complexity level to more than one cell.

        For about the first 50% of earth's history the complexity level of the cell was at the level of bacteria and archaea., which are 'simpler' cells with no nucleus. Archaea are a group of early microorganisms, which today thrive primarily in extremely harsh environments, are a separate life form from bacteria and cells with nucleus (eukaryotes).

        About 1.5 to 2 billion years ago, the last 44% (2 bil/4.5 bil = .44) of earth's history, the complexity level of single cells increased with the development of eukaryote cells. It was a big evolutionary step to go from bacteria and archaea, with simple internal cell structures, to the much more complex eukaryote cells of animals, plants, fungi and certain unicellular organisms, whose cells have a visible nucleus.

        Not only do eukaryote cells they have their DNA separated into the nucleus but they are sort of a hybrid cell with enclosed, specialized regions like chloroplasts (in photosynthetic eukaryotes like green algae) and mitochondria (in aerobic eukaryotes chemical energy is trapped in the microchondria).  The prevailing theory is that these specialized regions were originally independent bacteria/archea cells that have become incorporated into eukaryote cells in sort of a symbiotic relationship. Microchondria and chloroplasts even have their own separate DNA!

        Only about 600 million years ago, the last 13% ( 0.6 bil /4.5 bil = 0.12) of earth's history do the first multicellular simple animals (worms, etc) begin to develop.This is the so-called Cambrian Explosion. These little, soft body, animals were rarely preserved. Most of what we know abou them comes from a bed of fine grain oil shale in Canada about 520 million years old, known as the Burgess shale.

        Ttimeline of life's evolution is here (Wikipedia)

        http://en.wikipedia.org/wiki/Timeline_of_evolution

Time scale of human evolution
        It's took 4-6 million years for a series of more and more 'man-like' hominids to develop with the latest (homo sapiens) appearing 100k years ago. Note, since the earth is (about) 4.5 billion years old , 4.5 million years is only 0.1% the age of the earth. From another point of view nothing even reembling man (in any way) walked on earth for 99.9% of earth's history. 4-6 million years for human evolution means quasi-men were around for maybe 400 to 600 times the 10k years there has been civilization (Holocene era). That's a long, long time for men to be around and no civilization.

        Even the last humanoid variation, homo sapiens (otherwise known as us),  apparently 'goofed around' for 90k years (during an ice age) before starting to develop civilization. This is 3,600 generations and no civilization. How come? No one really knows, but based on cultural artifacts, like cave paintings and carved figurine, that first appear about 40,000 years ago, the guessing is that language developed about this time. In other words it took homo sapiens about 60,000 years to learn to speak, and even after he learned to speak he apparently 'goofed around' for another 30,000 years (1,200 generations) before starting to develop civilization. Hey, what gives guys?

        All this long, tortuous 'human' history does not give me a 'warm and fuzzy' that intelligent life (in the sense of civilization) was likely to arise, even with all these men and quasi-men running around on this planet.

        Here is a link with a summary of the 14 quasi-men preceding our species (homo sapiens) over the last 6 million years.

             http://www.pbs.org/wgbh/evolution/humans/humankind/o.html
 

Civilization --- fraction of the age of the universe
        All civilization, including development of farming and domestication of animals, has occurred in the last 10 thousand years, fitting nicely into the last 10k interglacial period of 100k ice age cycles. This is 10^4/13.7 x 10^9 =7.3 x 10^-7 or 7.3 x 10^-5 % or 0.00007% the age of the universe. Boy, god sure took his time developing civilization.

Ape extinctions
        A huge number of apes went extinct in the Miocene (5 to 24 million years ago). This was probably because continental changes and developing grasslands caused tropical forests, where apes lived, to dramatically shrink. It's a good thing a few apes linages survived to produce hominids.

Bootleneck in human evolution shown by genes
        "There are many reasons to believe that there may have been a number of severe population size bottlenecks on the lineage leading to living humans."  This paper estimates the most recent bottleneck at 1.5 million years ago "at the beginning of our lineage, the end of the Pliocene some 2 million years ago." with a (worldwide) human population of 10k to 100k (Molecular Biology and Evolution 17:2-22 (2000) paper)

References
        Extraterrestrial Life and our World View at the Turn of the Millennium Lecture by Steven J. Dick
Dibner Library Lecture, Smithsonian Institution Libraries, May 2, 2000. (not very interesting)

       " Many evolutionary biologists, such as George Gaylord Simpson and Ernst Mayr, seeing the great complexity of life and its evolution on the way to human intelligence, are very skeptical about the existence of intelligence in the universe."

        http://www.sil.si.edu/silpublications/dibner-library-lectures/extraterrestrial-life/ETcopy-KR.htm