Life's Origins at Four Billion Years Ago; Implications for Our Future

A group from the University of Tokyo (Tashiro et al, 2017) argues in a Nature paper that carbon isotope ratios in rocks in northern Labrador, Canada means that those rocks harbored life almost four billion years ago. This pushes back the early bound on origin of life almost two hundred million years, almost to the Hadean eon. To be sure this finding has not been universally accepted, but it's worth thinking about what it would mean. In particular, and perhaps not coincidentally, this is also right about when the Earth's surface transitioned from molten to solid. [Added later: it turns out the Moon may have been briefly "habitable", i.e. had an atmosphere and liquid water. Look for the same signature in rocks there?]

A recent paper reconstructing the last universal common ancestor (LUCA's) genome from a massive tree of millions of genes showed that it was pretty clearly a sulfur-vent organism. This is good news if you're looking for life on Europa or Enceladus, because that means that life on Earth didn't need the sun (and neither would any life that could evolve along vents under Europa's icy crust.) If you assume that the chance of life evolving by 3.5 billion years ago on Earth was 50%, and that the chance of life evolving is based on surface area, and all other things are equal (admittedly speculative when we don't even have all the information for our N=1) then there is a one in three chance of life on Europa. (If that probability correlates instead with the volume of water, then it was overwhelmingly more likely for life to evolve on Europa!)

[Added several days later: someone has finally run the numbers. A model of RNA polymer formation by Pearce et al suggests that the first RNA world molecules were most likely to have formed in small surface pools rather than sulfur vents - but even earlier, 4.17 billion years ago. If a wet-dry cycle is needed, this suggests ocean worlds like Europa are less likely than once-wet places with exposed land like Mars. The lesson of this paper is that you need puddles, not bone-dry deserts or world-spanning oceans. In this model, a world with puddles and organics seems all but certain to develop into an RNA world. A paper by Cardenas et al from the Geological Society of America Bulletin strongly suggests that 3.5 billion years ago, Mars was exactly the kind of place to have puddles. The logical argument is that life, or at least an RNA world, also developed very quickly there, and we should look for similar deposits to the ones found by Tashiro et al. If Pearce's argument does not produce findings like Tashiro's on Mars, we at least can start looking for differences in the early environments of the two.]

Two things to keep in mind about the LUCA paper: 1) LUCA is the last universal common ancestor. There could be a long lineage before it; and 2) the smaller and simpler a system, the more profound the changes possible in that system. If at one point Earth was an RNA World, molecular clock techniques developed based on modern DNA metabolism would probably be pretty bad at retrodicting LUCA. That two hundred million year gap map be exactly that. All that carbon might be free-floating ribosomes, or peri-biotic viroids.

Even more importantly, this has implications for the likelihood of the evolution of life. This discovery should worry you if you consider the Great Filter. The idea is that it seems very likely that life would evolve anywhere there's liquid water. Yet the universe is not obviously filled with intelligent life. Something is therefore stopping the progression from the evolution of life, to that life spreading from its home planet. (This is typically assumed to be some natural event and need not be some science fiction plot of an alien menace stamping out intelligence wherever it appears.) And every time that the origin of life is pushed back a bit further - that gives greater cause to worry, because where probabilistic events are concerned, the faster something happened, the more likely it was. If this paper is correct, then life on Earth appeared essentially as soon as the surface cooled from magma to solid. [Added several days later:

The real question is whether the Great Filter is behind us (we're freaks that got more complicated than algae) or in front of us (every intelligence is powerful but short-sighted and wrecks its own ecology before it can escape its home planet.) Therefore, a very reassuring discovery would be simple life - the local flavor of blue-green algae - under the ice Europa of Enceladus,* and in the ancient mud of dried Martian riverbeds, and baked into Venusian bedrock. That would mean that somehow, we got past the gate - still no guarantees, but we already passed the filter. This would mean that if we do manage to get out of the solar system, we'll find a lot of alien bacterial mats, but no alien minds. Boring? That idea is actually quite reassuring.

On the other hand, a bad discovery would be mass fossil beds of complex multicellular things (like the radioactive squid in Europa Report), especially ones with extrasomatic adaptations (tools.) We have had a number of landers on Mars and Venus, and none of them captured any obvious macroscale life. But a positive finding by SETI would be even more harrowing, especially because it's unlikely that there would be only one other intelligence that happens to be even within a million years of our technology - even if they're within 1% as old as we are, that's a gap of 40 million years in either direction! In such a situation we would have to include they must be legion. In such a situation, we would have to reason: we can hear them, but for some reason they never get away from their home planet - and we are unlikely to be any different.

*If indeed we believe that Enceladus only formed in the Cretaceous, then there is much less likely to be life there than Europa, and we should focus on Europa.

Previous post about alien evolution, Vast Cool and Unsympathetic: Other Worlds Detecting Earth


REFERENCES
Benjamin T. Cardenas, David Mohrig, Timothy A. Goudge. Fluvial stratigraphy of valley fills at Aeolis Dorsa, Mars: Evidence for base-level fluctuations controlled by a downstream water body. GSA Bulletin, 2017; DOI: 10.1130/B31567.1

Pearce BKD, Pudritz RE, Semenov DA, Henning TK. Origin of the RNA world: The fate of nucleobases in warm little ponds. 10.1073/pnas.1710339114 PNAS October 2, 2017

Tashiro T, Ishida A, Masako Hori M, Motoko Igisu M, Mizuho Koike M, Pauline Méjean P, Naoto Takahata N, Yuji Sano Y, Komiya T. Early trace of life from 3.95 Ga sedimentary rocks in Labrador, Canada. Nature 549, 516–518 (28 September 2017) doi:10.1038/nature24019

Vast, Cool and Unsympathetic: Other Worlds Detecting Earth

If aliens visited Earth in any numbers, the result would likely be a disaster for our species. This has been the consensus of the not-inconsiderable number of scientists who have given this topic serious thought, Stephen Hawking not least among them.

And yet, there are people who deliberately try to signal our presence to aliens. If you take the possibility of intelligent aliens seriously, you should support a ban on this activity. These people either don't seriously believe they'll be heard, or they're willing to risk the end of life on Earth at some future time, all for their little project. It's as if the Mayans had built signal fires on the beach to show the locations of their cities to any helpful people navigating the coast in giant canoes.

If you think life on Earth is the result of evolution, and for some reason you're hesitant to extend Darwinian principles to the rest of the universe, think about it this way:

• a) the Mayans encountered individuals from the same species, a mere four thousand years more technologically advanced than themselves, and the results were catastrophic to Mayan civilization and the New World's ecosystem, and


• b) any idea that humans are somehow nastier than any advanced organisms that might visit from the stars is based on nothing, except wishful thinking and a desire for moral signaling.

If you like Earth's ecosystem, and you despair of the way that invasive species from the Old World (for the most part) have rolled over those in the New World and on island ecosystems like New Zealand, imagine the damage to Earth's biomes from invasive alien microorganisms. (Again, if we take the possibility of aliens seriously, then this should be considered as low probability, very high consequence threat, i.e. an existential threat, along the lines of an asteroid impact or gamma ray burst.)

Therefore, it's worth worrying about how easy we are to detect. This paper proposes a way to cloak the Earth with lasers. Another way to think about it is to establish a detectability index, and a useful one might be: how far away could a parallel Earth (with the same EM emissions) be, for us to detect it? Or, for them to detect us? I call this the C-index, and XKCD's What If addressed the same question. Now, astronomers have asked what other solar systems are ideally positioned to witness a transit of Earth against the sun, even without hearing EM emissions.

Astronomers have debated what types of planets are most likely to develop life, and a good summary might be that we should like for habitable-zone super-Earths that are closer to the center of the galaxy and have had frequent proper motion close passes by other stars. We should see if any of these stars meet those conditions, and study them exhaustively.

Previous post on alien evolution, Influence of Interstellar Proximity on Interstellar Exploration and Evidence of Extraterrestrial Visitation

The Battle of Mohi

Today, this place is in Hungary. In 1241, it was also in Hungary - and it's where the Mongols eviscerated the only army that had a serious chance of stopping their advance to the Atlantic...and then soon after, vanished back into Central Asia. What if...

The Paradigm Shift Problem in Science Fiction Movies: How Will Blade Runner 2049 Solve It?

A paradigm shift is a plot twist that changes the setting and therefore the whole meaning of what the characters are doing. Science fiction suffers uniquely from this because it's unique in having paradigm shifts (the rare action movie notwithstanding.) In Alien, we find that the crew is not dealing just with a face-crab, but with a predator that bursts from people's stomachs. In V, it's that the friendly human-like aliens are literal cold-blooded reptiles. In the Matrix, it's that we're all in a simulation. In Star Wars it's that Darth Vader is Luke's father - a bit more like a traditional plot twist, but still profound.

When a movie depends on the paradigm shifts for its money shot, what do you do to keep the fires burning? You could either make an entirely different reveal in the next movie - and that's hard. You can just keep having things bursting out of people's stomachs (boring!) or just have, I don't know, the personification of the Matrix reveal you know, a little bit more (so lame!) Or, you could add a whole NEW paradigm shift. That's difficult and dangerous. Not only are good paradigm shifts are hard to come by, but introduce a new one, and suddenly you've changed the whole tone, and it no longer seems like a movie from that franchise. This is hardly ever done, although Highlander 2 does come to mind, and it was so bad that Highlander 3 ignored it as essentially un-canonical.

Of course audiences are getting smarter, and harder to please, as we see the man behind the curtain, because we're saturated in media and we've become a bit too savvy and aware of how plot structures work and we roll our eyes at call-backs (Sarah Connor telling Kyle Reese "come with me if you want to live", WOW! I can't believe that it's reversed now!) The innovation here is the J.J. Abrams trick of making the characters aware of their past and having them consciously do the call-backs - Kylo Ren looking at Vader's helmet - and possibly even having the reversed Vader-Luke interaction, and having the bad son succeed in killing the good father.

I'm nervous about Blade Runner 2049 running into the same problem, but there's a difference, and here I'm afraid the title of the post is a little misleading. Blade Runner kind of doesn't have to solve the paradigm shift problem. Blade Runner doesn't rely on "Is Deckard a replicant?!?" for its punch to the same degree that the franchises above do. Its success is in the overall tone set by the first movie, so Villeneuve isn't as trapped into focusing on the paradigm shift as the core of the story. The Fifth Element was probably the movie that was at its debut most compared to Blade Runner, and the film-making is such that, if it has a sequel, it also won't be forced to make such difficult no-win writing decisions. The new short that takes place in 2048 is obviousy a Blade Runner film, by the sound just as much as the famous vision. My main complaint is that it seems that nothing much in that world seems to have changed in 30 years. But we will see!

Anticipation: Blade Runner 2049 - and Kubrick?

At least one person has the theory - and enough confidence in it to post a Youtube video about it - that Blade Runner is deliberately full of references to Kubrick's 2001. Of course, the ending credits are actual Kubrick footage from the Shining, but there are a couple other things. And of course, these are my own two favorite movies in the world, so I would love for this to be true. Consequently I'm immediately skeptical.

In Blade Runner there are clear references to other movies - for instance, the lighting in the Tyrell Corp. office where Deckard interviews Rachel is unambiguously straight out of an early scene from Orwell's Citizen Kane (another great one.) But I have in fact noticed two odd connections recently, not between these two movies, but between Kubrick and the (now we must use the word) franchise. To wit: Tyrell is played by Joe Turkel - who also played the bartender in The Shining. Perhaps more significantly, Denis Villeneuve, the director of Blade Runner 2049, is a certified Kubrick freak.

Review: Alien Covenant

tl;dr Not terrible, and definitely not as bad as Prometheus, with good acting and better-developed and organized themes but still with some ridiculous plot holes.

I had said I was not going to worsen my experience of the Blade Runner sequel by over-anticipating it, and I am failing spectacularly. I'm beginning an effort to see the recent Ridley Scott movies (he executive produced Blade Runner 2049) and will also be seeing all Villeneuve's movies before I see the new Blade Runner. This is my excuse for having seen Alien: Covenant.

I reviewed Prometheus quite negatively several years back and while Covenant is not as bad, nothing will ever live up to the original, through no fault of Scott's. Science fiction relies on paradigm shifts more than other genres. Paradigm shifts are a subspecies of plot twists. Plot twists are an event unexpected to plot and characters (the Red Wedding in Game of Thrones), or the revelation of information about characters or events in the past that changes the motivaitons of characters and the meaning to audience of events. A paradigm shift is a revelation about the very nature of the world that the characters are living in; e.g., we actually live inside a simulation! (the Matrix); the V aliens are not really humanoid but actually reptiles!

And therein is the problem. Soap operas and action movies can and do have multiple plot twists. But since the paradigm shift is about the whole world on the screen, it necessarily becomes the central aspect of the story. And you're trapped: either you just rely on the same shift (and wear it out) or you shift to a new one. Rely on it, and you bore your audience (the second and third Matrix movies; the utter lack of surprise that the V aliens are reptiles; the eye-rolling boredom of "Wow, aliens are bursting out of people's bodies? Who knew!") Shift to a new one and you're making a different movie. Studios and directors really don't want to change the underlying formula of a successful franchise, so more xenomorphs bursting out of chests it is! J.J. Abrams developed a neat trick for coming into mature franchises and simultaneously appealing to fans and casual viewers, but even this trick has worn out a bit. Without further ado then, the good bad and ugly of Covenant.


Positive:

- Great location and cinematography. For that matter I even liked that in Prometheus. Then again, you could film anything in South Island, New Zealand and I would probably like it.

- Scott did a better job of developing the theme that we live in a pitiless and mechanistic Darwinian universe, and that maybe the machines really are seeing accurately (in the original Alien, "Survival - unclouded by conscience, remorse, or delusions of morality.") The cruelest can, and probably usually do, win, despite our strivings and the trappings of our comforting values. I also like the animalistic nature of the aliens throughout the franchise, again on display. Nature doesn't produce culture, it produces survival, and culture is a by-product that helped survival (for a while.) David's creations have a predatory cleverness but there won't be any exchanging art or technology with them any time soon. I really liked the very biological touch of the dead facehugger curling up like a dying spider. There's also the theme of Weyland creates android; android creates xenomorph. Weyland wasn't very nice to begin with. They also explained why the later version of the android didn't just run over the earlier version (Walter explains that David disturbed people and so there were limitations placed on later models.)

- Fassbender is an outstanding actor and I might even have enjoyed the movie more if it was just David and Walter talking and fighting. The flute-teaching scene was very suspenseful, erotic, and in general made me uncomfortable, which I very much appreciated; by far the scene in the movie that bothered me most (yes, even including creatures bursting out of organs.) There was a criticism of Prometheus that the only character that didn't act like a robot (Fassbender's David) was, in fact, playing a robot, especially against Charlize Theron's woodenness. (Did you see her in Mad Max? She stole the show. If somehow you made Charlize Theron boring, you really screwed up.) I wonder if Scott was creating this contrast intentionally, but it's hard to defend this as intentional direction when the rest of that movie seemed like a disorganized pastiche of half-developed ideas. Here, the contrast is clearly a central feature of the themes of a much better developed story.

- In some of the lines and themes, there was more a sense of the film-maker's call back to previous movies (and his own outside the franchise)

• While Daniels is resisting David's attack, David says, "That's the spirit." I'd argue that's an intentional Roy Baty reference.
• David refers to the xenomorph as the "perfect organism", which Ash calls it in Alien. This one is particularly interesting because you wonder if somehow David influenced Ash, or if androids have a tendency to be psychopaths that arrive at the same admiration for the Darwinian killing machine.
• At first I thought Scott was plagiarizing himself in the flush-the-alien-out-the-airlock scene, but he's actually plagiarizing Cameron, who had taken over the franchise.
• It has nothing to do with Scott, but when David and Walter fight, you can't help but think of scenes from a certain other Cameron series with two terminators going mano a mano. Their relationship very much invites Data and Lore comparisons as well.

- David is a character study of a psychopath. His narcissism, manipulativeness and lack of empathy are clearly on display (an android admiring himself in a mirror?) Walter points out his flaw in misattributing a poem. David's crying is clearly sometimes for show, but he does seem to have been genuinely moved, at least abstractly, by the slaughter of the Engineers. On the other hand, Walter's innocent Kantian adherence to "duty" in contrast makes him seem safe, boring and even child-like.

- More interesting to think about in terms of where Scott could take the franchise - if David's point in creating the xenomorphs was to wipe out organic life, or at least both humans and engineers, then it makes sense that in Aliens 4, Ripley and her new android friend finally make it back to an obviously ruined Earth. If Scott continues to make movies, he's either going to have to declare other directors' work on the franchise not cannon, or do a lot of maneuvering. (Until this decade Scott had made only one of the four Alien movies. Each one had a different director; and of course this is even more complicated since xenomorphs have also been kidnapped into another franchise entirely, i.e. Predator.)

Interesting, so I guess positive: James Franco's role in the movie was near zero, other than two lines of dialogue on a video the protagonist was watching, as well as being a Pittsburgh-rare corpse. There was really a lot that was released online that was not in the movie.

Neutral, but I expected it to be bad, so I guess that means good: I was expecting to hate Danny McBride in this (boy, what a casting decision) and I had no feelings one way or the other.

Puzzling: Mr. Foreign-Accent McBurny-Face had a facehugger on him for all of 30 seconds and still managed to get a xenomorph implanted. Why do the things hang on to faces (in later-in-the-timeline-movies) for hours to days? The chest bursters popped out in a matter of minutes too. It could be that the later ones had deteriorated somewhat from David's original design. Or, it could be that Ridley Scott cares more about rapid action than continuity, because audiences tend to ignore plot problems when there's higher action density.


The Negative

- So stupid I can't even believe it: trained interstellar professionals go gallivanting off the ship into the brush on an alien planet, one they weren't even planning on landing on, and no concern for pathogens? (You could get rid of some of this with a single line in the plot without changing the visual story much at all, e.g. "remember when we got our universal vaccine shots", etc.) And very little comment about "Wow, we're setting foot in an alien ecosystem!" And very little comment about "Wait, there's wheat here, humans must have been here before us!" Fine, maybe Scott thought audiences are a little tired of every movie showing the wonder that the first settlers felt...but worst of all, when the crew goes out with THE BIOLOGIST to collect samples, a crewmember smokes and just casually tosses the cigarette butt. Seriously!?!? And then when the crew is returning with THE BIOLOGIST, two of them are obviously, suddenly, severely sick with a local pathogen, and THE BIOLOGIST is in full contact with them and doesn't think to put them in quarantine outside the ship. I mean come on. This is worse even than a bad horror movie where they say, "Hey, inhuman howling in the basement of this abandoned house, we should go down and see what it is!"

- Also really stupid: Yes Scott went out of the way to show us that Billy Crudup was a bad captain. But still, he doesn't appear to be a complete moron or an emotionally vacant nitwit. "Hey, a creature just killed one of my crewmembers and not only were you communing with it, but you were mad at me when I killed it!...okay, I'll follow you into your horror chamber and lean my face into this weird pod when you tell me to."

If Scott had made the characters behave a little more circumspectly - maybe even have viruses that could get through air filters on space suits, have David do his explication to Walter and have a facehugger drop on bad-captain from above as he was about to shoot David - this would have been a much, much better movie.

- Confession, I knew how it ended, but come on - how was it possible not to realize that it was David and not Walter when he was proudly watching the xenomorph run up and down the deck? Waiting for the reveal at the very end was quite annoying for that reason.

- Also, once they're in the dead colonial city - I guess there's barely a word uttered about the massive carved monoliths around them because they've already heard of the Engineers from the earlier mission? (Although again, a little more awe might still be on order when you walk into an alien city.)

- A neutrino burst? Come on, you can do better than that. There are only enough neutrinos to do serious damage if you're right next to a supernova, and then you have other things to worry about. If David can trigger neutrinos and supernovas, why doesn't he just go back to Earth and do that? The plot would've lost nothing if a stray shower of comet fragments damaged the ship and woke them up.

- A colony ship with a specific destination goes off-route to land on a planet emitting a strange (but ultimately human) signal. Alright; that's fair enough. But how did they miss a planet that was habitable? Yes it's a trap that David set, but the planet was habitable prior to any interference by David. Note that the ship has hyperdrive and it's the early twenty-second century, and it's interesting that we feel we can say that won't happen. (This is a reality criticism, not film criticism.)

- The bad android has an English accent and the good one has an American accent. I want to see an evil robot from Alabama or Queensland. (Gary Oldman as the villain in Fifth Element did have a Southern American accent, to that movie's credit.)

General AI: Computation versus Survival, Superintelligent is Not Omniscient

It is usually assumed that a superintelligent AI would maniacally focus on improving computation. Just to highlight the centrality of computation, a recent paper in the British Interplanetary Society Journal argued that the reason we don't see aliens is they're sleeping, waiting for a time when the universe is cool enough that their computations are more efficient. The alien singularities are waiting until they don't need to be cooled.

The most common concern associated with this line of thinking is that the technological singularity would be bad because the AIs would use all available resources - starting with all matter on Earth, including us - as computational resources. While I think a technological singularity would be catastrophic, I think the reason is eve more mundane.

Of course, this assumes that the AIs in all their power are maximizing computation. I don't think this is questioned nearly enough, and a good bit of the inertia around it stems from the cultural assumptions of the programmers and engineers making the argument. The singularity is thought of as a logical outcome of Moore's law, which concerns exponential growth in computation. It's not clear that this is what an AI would necessarily be maximizing. For our part, humans and other animals maximize a host of confused and often contradictory goals. Of course we remain in this mess because we are not recursively self-modifying. Assuming that AIs with such an ability aren't automatically condemned to wirehead, it's not unreasonable to ask whether there are things to maximize that increasing computations just wouldn't fix.

Replicators whose descendants are present into the future are the result of selection for one thing - making copies - and to the extent that extra computation can improve that, then the AIs present in the future will be selecting for computation that helps them reproduce and sustain themselves. But even a superintelligent AI is not an omniscient AI, and cannot see infinitely into the future and understand ahead of time the impact of all its actions in maximizing its survival OR computation. My strong suspicion is that a hard takeoff will likely be an apocalyptic gray goo explosion, much more thorough and faster than the mass extinctions so far in the much more comparatively mildly ecocidal anthropocene, and that furthermore this is a strong candidate for the Great Filter and the Fermi paradox. That is to say, we're more likely to find the simple but fecund survivors of such an event as something that looks like post-singularity AI-algae (or free-roaming AI "cancer") than alien AIs that are interested in philosophy.

Influence of Interstellar Proximity on Interstellar Exploration and Evidence of Extraterrestrial Visitation

It's easy to despair at the gulf between stars and the millennia of time it would take to get a ship there. The fastest spacecraft humans have yet produced was Helios 2, which after a slingshot maneuver in 1989 was moving at about 103 kilometers per second. There are two things to note about that statement. The first is that it was a slingshot maneuver, not an acceleration achieved under its own power (which is always the case in space exploration.) The second is that breakneck speed would deliver Helios 2 to our nearest neighbor Alpha Centauri right around 13,000 years from now. (This also means there are stars in our neighborhood that even if we aimed our fastest-yet probe at them, we could never reach, because they're moving away from us faster than our fastest spacecraft.)

Those frustratingly quarantine-like time spans suddenly seem much shorter when we consider the geologically brief time spans between close passes of the sun and nearby stars, resulting from proper motion. These changes in interstellar geography actually occur much faster than plate tectonics. In the space of a few tens of thousands of years we might go from having our nearest neighbor five light years away, to close enough to impinge on the Oort cloud and send comets falling toward the inner system. It's amazing to think but since our ancestors were first using fire, multiple close-passes between other stars have occurred. Merely 70,000 years ago we had a star 0.82 light years away (Scholz's Star), and in another 1.3 million, we'll have another (Gliese 710).


There are a number of clear inferences to be drawn from the frequency of such close passes.

1) It bears repeating, in geologic time, 70,000 years is really not long. Humans may have already started leaving Africa when this occurred. We should assume the sun is not unique in this, and our close-pass rate is about once per million years. That means, since the solar system formed, this has occurred 4,500 times. The Wild-2 comet, from which we retrieved material that we've analayzed on Earth, has a nitrogen isotope ratio that strongly suggests it's a comet formed around another star. It also has the amino acid glycine.

2) Impact ejecta from large bodies like Earth can make it into orbit. We have Martian rocks here on Earth from such events having happened on Mars. Space is not hospitable, but even metazoans have survived fairly harsh exposures; for example, C. elegans worms from space shuttle Columbia experiments survived uncontrolled re-entry and were found alive on the ground weeks after the crash. They weren't even protected inside large space rocks. Some exobiologists expect that for this reason, if we do find life elsewhere in the solar system, it will be related to life on Earth (ejected and diffused during the Archaean?) essentially a long-lost branch of archaebacteria. While such a process would be less likely, i.e. take longer in the much greater volume of the outer solar system, if it is not less than 1 in 4,500, it has probably already occurred.

3) It is very likely that the amazingly short 66 years from first manned powered flight to first human on the Moon occurred in part because of the Moon's relative proximity. More than twice that duration has now elapsed and we are still only in the talking stages about a landing on Mars. A species that has the good fortune to "come of age" in terms of space faring technology, when the next closest star is a mere 0.82 light years from their own, has an easier task of proving the possibility of interstellar space flight than we do, coming of age when we're about equidistant from everything. We might therefore narrow our search for intelligent life to super-Earths around sun-like stars with close neighbors. (Of course, there is also a not-unreasonable argument to be made that close passes, or any distant large bodies disturbing the local Oort cloud, increase the chance of major impact events and decrease the chance of the kind of complexity developing that would allow long-distance space travel.) It's worth noting that in view of the high frequently of close passes, a problem for the Oort shower hypothesis of mass extinctions is that it does not happen more frequently, i.e., every million years.)

4) Recalling that our fastest spacecraft have all used gravitational slingshot maneuvers - while we might speculate wildly about the amazing propulsion technology visiting aliens would have, we can be 100% certain that they will have gravity maneuvers at their disposal, because we use it. It's easy and cheap (free, really.) Therefore, there may be interstellar "backwaters" that will not necessarily be empty spots in the galaxy, but places that are difficult to approach from nearby stars and then slingshot away from to another nearby star. If you're a species in such a backwater, you're not going to get visited very much, and you'll ask "Where is everybody?" As the stars shift, your interstellar geography status may change quickly, within a few thousand years. One of the problems with detecting aliens, particularly well-advanced ones, is we don't know what we're looking for. We may be looking right at evidence of their existence and miss it because they don't use our provincial communication methods, or because we're used to it and we explain it in terms of the background operation of "dumb matter". Or, the periodic mass extinctions that are sometimes claimed to be associated with close passes could in fact be associated with close passes - but because of an ecosystem-collapsing alien visitation as Stephen Hawking envisions, rather than because of Oort cloud impactors.

Previous post on alien evolution, There's (at least) a 1-in-3 Chance of Life on Europa