A couple of years ago, the so called 'Rare Earth hypothesis' was proposed by paleontologist Peter Ward and astronomer Donald Brownlee.
As the name quite clearly implies, this hypothesis states that planets like Earth may be far rarer than previously believed.
Among the things it states is that simple, microbial life may be very common in all of the universe, but that complex, multicellular life, such as for instance plants and animals, on the other hand is extremely rare - this because it took so long for life to evolve from single- to multicelled here on Earth.
It claims further that many of Earths special attributes, such as plate tectonics and a large, stabilizing moon that almost makes the Earth-Moon system into a double planet, may have had a determining effect on the rise and evolution of life on the planet.
These and other attributes of our planet, are - according to Ward and Brownlee - extremely rare other places in the universe, indeed possibly unique to our planet Earth.

Of course this provoking hypothesis has gotten its responses, and the responses are based largely on concrete discoveries made over the latest years, and not so much on speculation as the Rare Earth-hypothesis. Especially the discovery of so called extremophiles in very inhospitable environments here on Earth, have provided science with new knowledge about life's toughness and adaptability. It has, for instance, been discovered that many primitive, formerly unknown organisms thrive in volcanic environtments, both on dry land as well as on the bottom of large oceans.
These and other discoveries have provided many astrobiologists with new faith in the possibility that life can be far more adaptable and rich in diversity, than what hypotheses such as Rare Earth claims.

In this post I will focus on what we actually know about life here on Earth. As you all know, our own planet is currently the only frame of reference at our disposal, but we still have obtained some hard facts during the latest years, such as that the number of stars in the observable universe is approximately 70 000 000 000 000 000 000 000, and that the vast majority of these stars are surrounded by orbiting planets.
I therefore greatly doubt the possibility that Earth is the only place in the universe where life has emerged at all.
I'll now try and explain why I also think there must be other species out there, similar to our own, and not just primitive germ analogues or collections of organic slime and foam:

What is it that defines us humans? What makes us unique in the animal kingdom on Earth? One could point out several things, such as the combination of an obviously high intelligence, great social skills, and the ability to use language, grammar and physical tools.
We modify our natural surroundings and in this way we manage to keep nature's threats at a safe distance. If we still somehow should be confronted with such threats, we have the ability to use weapons and other tools to defend ourselves.
This has made us into a superior species among the mammals on the planet, and little by little we have settled on every continent and have developed a very large population, totally out of proportion compared to the populations of other large, mammalian species.
The way I see it there is however another factor that is crucial to our success, and that is man's instinctual drive for exploration and discoveries. We have a built in hang to go places, to find and conquer new land. Because of this, we have all the way from the beginning of our emergence, wandered from continent to continent, and through trial and error we have crossed vast expanses of oceans and mountain ranges, not knowing what we're looking for, simply to explore new areas and break frontiers.
We are driven by a fundamental curiousity towards all that is unknown, and where does this curiousity come from? It comes from the genes, of course. We are genetically programmed to explore, and this exploration- and curiousity-gene has made us into an extremely successful species on this planet.
One might speculate if a global, technological civilization has any chance to survive for long, in a geological or astronomical time perspective, but the human species as a whole will doubtfully die out even if world society breaks down and a couple of billion human lives are lost in nuclear wars. Human beings, as a species, will persist through the millennia, and new civilizations will see the light of day only to possibly go under at a later time.

Imagine then, a scenario on another planet, a planet of the same size and composition as Earth, that orbits a G2-star like the sun, in the same distance to its star as the one we have to ours. All is set for life as we know it to arise, and therefore that's exactly what it does. When life first arises it is very tough. That's a fact we've learned from the studies of extremophiles here on our own planet. If life first emerges, it's unlikely to go extinct in a forseeable future, even if its conditions change dramatically. It will simply adapt to new conditions. The ones best fit for survival, survives. The ones least fit, goes extinct.
Assume further that the planet has almost exactly the same conditions as ours, it's not unlikely that Earth has such a twin planet somewhere in our own galaxy, considering the fact that there are approximately 400 billion stars here.
Evolution goes at a slow pace in the beginning, life dwells for a long time at the microbial form until the plant kingdom finally emerges and starts to evolve. The plant kingdom then lay the ground for animals, life and evolution experiences an accelarating degree of complexity.
Which of the life forms on such a planet is best fit for success, which genes would such an organism be equipped with? It would of course be equipped with genes that make the species spread over the largest possible area. And how should the species spread over the largest possible area? It would of course have a built in hang to continuous exploration, a genetically programmed curiousity.
In this fashion these genes would eventually spread all across the globe, and if we assume that the well renowned biologist Richard Dawkins is correct, humans, as all other life, are primarily controlled by their genes, and not the other way around.
We are steered by our selfish genes, that all fight a mortal struggle for survival.
What gene is it then, that in the long term will have the greatest ability of survival?
It would of course have to be the gene that conquers the entire planet. And when the planet is conquered, it neccessarily has to look towards new horizons, and where does it look then? It looks to the stars.
Such inheritable traits are not unique to Earth. I do not discount the possibility that man could be the_first_spacefaring species in the universe, but I'm convinced that other species, analoguous with ours, are at least evolving in a large number of planetary systems, through out the entire universe. It is highly unlikely that we are a unique species, in the sense that we're spacefaring, as seen from a cosmic perspective.

The short version of my hypothesis goes like this:
If life emerges on a planet, and it will as long as the conditions for it are met, the genetic material that in the long, evolutionary perspective proves to be the most successful, is the one that manages to spread over the largest possible area.
When the home planet no longer has room for more individuals carrying this genetic material, the individuals are neccessarily forced to find new places to settle, in other words: They're forced to become spacefaring.

The Rare Earth hypothesis does not take into account that evolution always will favour those individuals that are best fit for survival. When simple life first emerges on a planet, something that the Rare Earth hypothesis don't regard as unusual, it is inevitable that this life will increase in complexity as long as it gets enough time, something that the Rare Earth hypothesis on the other hand sees as highly unlikely and improbable. There are enough sun-like stars in the universe, with life spans of 10 billion years, so that there're a large number planets where conditions are met for life, is not to be doubted with our present knowledge.
The genes that have the biggest potential for leng term survival in a biosphere of complex lifeforms, are - as previously stated - the ones that spread over the largest area, and these genes will therefore have to create a high level of intelligence and an instinctual curiousity in the individuals that carry them, like us humans ourselves are a good example of.
I therefore see no reason to believe that man is the only spacefaring species in the universe.

Did you actually read "Rare Earth"? If you did, I think you misunderstood it. While I agree that Ward and Brownlee's definition of what constitutes a "habitable planet" is far too narrow, it is actually the less important constraint on the development of civilization.

The more important constraint is occasional catastrophic events, such as asteroid impacts and nearby supernovae. I think you are absolutely right that life adapts to the local conditions - if a planet has 100 atm surface pressure, water will be comfortably liquid at 200 C, and life will evolve accordingly. In other words, life fits the box that it came in. But Ward and Brownlee's main point - and I think in that they are right, - is that ANY box gets violently shaken every once in a while. And the more complex life forms are, the less of a shake it takes to destroy them. Permian Extinction 250 million years ago came awfully close to wiping out every multicellular creature on Earth. Recent evidence supports the asteroid impact hypothesis; if that asteroid massed twice as much, Earth would be back to bacteria - and would still be there now.

Good analogy are card castles. On any given day thousands of people throughout the world build card castles. Necessary "environment" is easy to come by - a flat surface with no wind, and several card decks. Yet very few 5' tall card castles exist anywhere in the world. 10' tall ones probably do not exist at all - even though there are endlessly trying fanatics. The probability of something knocking them down before they grow that complex is too high - and grows higher as the castle grows. A vibration from a passing truck won't harm a 10-card castle, but will bring down a 100-card one.

10' tall house of cards? I'm lucky if I can get three cards to stand up.

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What is a "hang"? I am not aware of any definition that fits the context.Your run on sentences also leave me confused.

Anyways, I think life has the ability to survive on a fairly large percentage of planets. However, that does not mean they evolved there. And no matter how they got there, it's even less likely they would become copmplex animal life. And even more rare that they would become intelligent life that could leave. I mean come on, if you didn't have a small planet orbiting a big planet, would you be tempted to see what exists between the dirt and the stars? People can talk about the tides or axial stabilization, but I think the biggest role of the moon is to be our first step into space. It's a convenient target. It is the proof that Tantalus is not doomed to his fate eternally.

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J Pax

I would assume by the context that "hang" means that we have a built-in genotype that is expressed phenotypically ;-) as a need to go and explore... or in other words, a predisposition...

I'm not sure why you might assert that life would not evolve into more complex organisms; given earth's own fossil record. (Keeping in mind that life on Earth has not been the result of some accidental collision of atoms but rather a pretty inevitable thing as sure as Na & Cl do not just accidently form NaCl when in contact with each other. This last part in brackets because I may be misundertanding where your statement is coming from.)


There is nothing I'd like better than to know we are not alone in the universe. To me, if such a discovery were ever to occur... that would be the most wonderful time to be alive.

But I'm a bit "agnostic" on ET intelligence. How inevitable is it that "high level" intelligence eventually forms wherever and whenever life exists? I don't know.

But let's say, for instance, that humans never existed. Let's say that all the human precursors were also wiped out by that big meteor. Would something else, some other species, here on Earth eventually have developed the intelligence necessary to travel off to another planetary body? Like a really smart dexterous rat? Or a really smart octopus-like humanoid thingy? Wasn't there a TV series called Red Dwarf that had a humanoid cat that had evolved a few million years?

Is it always pre-ordained that planetary life must evolve human-like intelligence? Then out of all the millions of species on Earth, after all the multi-millions of years of evolution, why are we the only rocketeers?

Perhaps all that is required (of evolution) is for a species to be incredibly fit for one's niche environment. Meaning if you (or a particular species) are simply surviving to breed, then the physics of evolution is "being satisfied." There are no other biological pressures to do anything else. So evolution slows down or maybe even stops when the organism becomes perfectly suited to its environment.

Thus a shark or cockroach may absolutely be the highest form of life on this (or some other) planet, under that definition. I suppose life found near undersea volcanic vents could stay exactly as it is... "forever"... if it turned out there is not one single other more efficient reproducing organism it could turn into. So much for yearning to disperse and go explore other planets.

So what I am stating here is that there is nothing: no rule; no physical law no evolutionary requirement for human-like intelligence to develop on any given life-bearing planet. Organisms simply evolve as they do in response to their surroundings if it aids their replication.

And that's as far as it goes. Great intelligence isn't necessarily necessary.

Our own intelligence is a mere anomaly (as are all evolutionary changes, I suppose) brought on as the result of a precise and inevitable evolutionary response to a specific set of rare environmental pressures. In our case, turns out we breed better (more successfully) if we're smarter. (Up until now, anyway.)

Well, that's one possible take on the situation. And I hope it's a wrong one.

RBG

One other thing I should add:

So often in biology there is a "wink-wink" short-hand used where it is said that a species "needs" or "wants" to evolve into such & such.

There are better examples of this but I'm just too tired to think of them.

It's quite easy to slip into that mode of thinking while writing and I tried to excise that kind of verbiage above - maybe not always successfully.

But, to be clear, there is no conscious forethought or master plan or desire or anything of that nature with respect to natural selection / evolution.

RBG

Well, actually I haven't read it yet. But I've ordered it, and at this moment I'm eagerly awaiting its arrival along with other astrobiology books.
The start of my post was based on what I knew(or what I thought I knew) about the Rare Earth hypothesis.
I do agree that life probably would have looked totally different today, if it weren't for all the incidents and mass wipeouts occuring in the distant past. Humans, or for that matter creatures with human-like intelligence would probably not have existed today if the dinosaurs didn't go extinct 65 million years ago.

Sorry. My post was originally written in my native language, norwegian, and then translated into english in a hurry. English is a secondary language to me, so sometimes I'm having trouble finding the correct words and building sentences properly. Especially so last night, since I didn't have a dictionary at my disposal when translating the text.
RGB put it right though, with 'predisposition'.

your 'Hang' is probably related to the word hanker in english;
as in 'to hanker after' something.

after all there were plenty of Norse speakers in the viking city of Yorvik a thousand years ago, which has become my own home town of York.

Whether all humans have a natural 'hankering' to go to the stars I don't know.
However I know that I would go if it were at all possible.

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Would even microbes be common? They are AWFULLY complex and unlikely to just "come together". The simplest bacterium is closer to a human than it is to a clay particle!

Start with a simple organic or inorganic "oily" membrane surrounding some liquid water. (I believe you can produce this mechanically by shaking up oil & water, as an example.

Or

Water spheres held together by surface tension, absorbing outside materials via osmotic physics that begin to accrete or stick together.


What you can get are particles that look suspiciously like rudimentary cells. Maybe incorporate naturally occurring organic molecules at this time. (Such a concoction would probably be capable of incorporating just about anything.) Add in some mechanism for simple division: ie: when it gets too big and unstable, it cleaves off. (Others more qualified could comment here.)

And the real important part: give it a half billion years for billions upon billions of these things to slowly add in structure changes spanning every physical possibility. These spheres would be great little test tubes for natural laboratory experiments. Shear numbers and time would ensure that some are going to be more and more interesting from the point of view of efficient replication and ever-increasing interior complexity. There's no "come together" involved.

With so much activity going on, it's a sure bet not all these cells are going to develop identically. And at some point there would be a synergistic, symbiotic, reproductive advantage for some of these differing cells being stuck together.

I just realized that this is similar to the way I approach digital photography. Take zillions of shots and eventually I get a good one.

I'm no expert here, but the mechanism would be something like that.

No one can know for sure what exactly happened 4 billion years ago, though there are some hints in a nebulous fossil record. I believe the best evidence is that a lot of this can be reproduced in lab environments suggesting this is what could have happened.

RBG

I wasn't trying to be overly critical, I really did have a hard time understanding. Was "hang" a mistranslation or a transliteration from norwegian?

I was thinking that too.

I know a lot of people like to talk about how a species' genes "wants" to evolve, but that is an especially slippery form of animism. I say it is "especially slippery" because we are talking about living things, which are therefore animated, but we attribute some extra personification to these things of which they are not capable. Unless we posit some sort of mental ability for even the simplest association of the molecules of life, DNA can't want to do anything.

I think we should avoid turning this into a biological evolution debate, which the BA seems to find distasteful. Suffice it to say, that if you think biological evolution can happen here, then you should assume it can happen elsewhere pursuant to local conditions. And of course, if you think biological evolution did not happen on earth, then you will probably need some other instigating event in order to think it could happen elsewhere.

My earlier post was comparing local evolution to panspermia. No matter how life started in the universe, it may be able to spread without high intelligence. However, for that same rugged single-celled organism to evolve into a highly intelligent life-form may be limited by local factors. And even if that life-form is evolved, it may never be able to develop spacefaring capabilities for reasons of resource, ingenuity, or drive. In other words, if humanity evolved on earth, but earth had no surface radioactives for energy, could they leave the surface? If there was no moon to use as a first step, could they get off the surface an on their way to the stars without any intermediate goals? If there was no moon, would they even realize that space was a three dimensional construct and not just a dome of stars in some crystal sphere beyond their reach?

__________________
"Oh no no no I'm a rocket man Rocket man burning out his fuse up here alone." -- Sir Elton John

J Pax

No harm done. It was a mistranslation, not quite sure where I got it from actually, I guess I just thought I'd seen it used in that context somewhere. Well, nevermind about that now.


Interesting point. But the belief that space is a dome has been widely accepted in numerous cultures, even with the moon in place. There's no easy and intuitive way to realize space's three dimensionality or the distance to stars or other celestial objects. If human's, or human like creatures had evolved at all - with no moon present - they probably would have come to the same conclusions as we have, it might take them some more time though. But then there's the point that's raised by the Rare Earth hypothesis among others, that the presence of our moon has been crucial for development of complex life in the first place.
If that's true then humans probably wouldn't exist anyway unless there was a moon like ours in place, and therefore they wouldn't be there to witness the lack of a moon.

Yes,but wich humans?