How rare is life in the universe?

[Dinosaur]

Mr. Anomymous: If you recall correctly, I consider Drake way off base.

If I recollect correctly, Drake pegs the number of advanced civilisations existing out there in the Galaxy alone at being something in the region of 10,000 or so. Actually quite conservative when you consider the sorts of numbers involved.

He made up a plausible equation, but guessed at parameters to be plugged in without making any sort of analysis.

The first time I saw his equation, I wondered what he based his estimated numbers on. I do not remember his discussing such concepts as habitable zones in a solar system and the same for a galaxy. He seemed to assume that an intelligent creature is an inevitable result of evolution, which is counter indicated by the history of the Earth.

I stand by my guess that intelligent life is etremely rare in the universe, and that we might be the only intelligent species in our galaxy. I would be amazed to discover that there are more than 2-3 intelligent species in our galaxy. Our galactic habitable zone is not wide and the habitable zone of a solar system is not very wide. I would expect many galaxies to have no intelligent life.

BTW: In various gross characteristics, I would expect evolution elsewhere to produce results similar to what happened here on Earth.

• Similar organic chemistry based on carbon compounds.
  
  
• Land animals larger than insects with 4 limbs, rather than having 3, 5, or 6 limbs. Id est: No Centaurs.
  
  
• Animals with two eyes for binocular vision rather than cyclops or 3-4 eyes.
  
  
• Intelligent creatures (if any) with a hand similar to ours. A thumb (possibly 2) opposing 3 to 5 fingers.

The above are a few of the characteristics which seem to be designs that work and are likely to be more efficient than various alternatives.

 

[Mr Anonymous]

Hello old man.

Oh, as far as the actual numbers Drake comes out with, the formulae is all Greek to me I'm afraid. I recollect reading his book on the subject only and the figure quoted would be that as I recall Drake himself as arriving at. A Galaxy 200,000 light years across, a billion, billion stars or some such...

But no, indeed. I can't clearly recollect at all Drake taking into account increased stellar density around the galactic core virtually righting off any likelihood of life even starting.

But still, with what y'have left - it's a fair swath of sky. Largest dispersal area, lot of stars good and far apart...

But in terms of:

BTW: In various gross characteristics, I would expect evolution elsewhere to produce results similar to what happened here on Earth.

as you put it:

• Similar organic chemistry based on carbon compounds.
  
  
  Land animals larger than insects with 4 limbs, rather than having 3, 5, or 6 limbs. Id est: No Centaurs.
  
  
  Animals with two eyes for binocular vision rather than cyclops or 3-4 eyes.
  
  
  Intelligent creatures (if any) with a hand similar to ours. A thumb (possibly 2) opposing 3 to 5 fingers.

These remain in no way unreasonable assumptions, but as I endeavour to point out - nothing in any model produced takes into account cataclysm. Everyone happily envisions the creatures which may possibly inhabit the rock-pool, so as to speak, but not what happens to them immediately after the rock gets thrown into it. What survives, how it adapts and then how much variation has taken place to allow some fraction of that new population to survive and endure the next cataclysm.

And so on, and so forth. World without either end or exception.

Nature, periodically, virtually wipes the clock almost clean to the bone. Evolutionary trends, established and enduring get taken out completely. Life in general terms, no one has a problem with.

But the evolution of intelligent life....

Oooo. It's more tricksy than a thieving Hobbit to nail down quite what it takes to do that using just math - I don't care how unimpeachable the source.

We are looking at odds and the bucking of reasonable expectation for Intelligent Life to emerge elsewhere in the Galaxy. We happen to be on a planet awash with brains, the ocean alone is literally swimming with the things - but there is intelligence and there is us - and we remain so unexpectedly unlikely with regards to the previous course of biological history we buck the likelihood of even our own existence even though we remain present enough to noodle the math.

At so many points this world of ours could have died, still could the presence of Intelligent Life such as our own not withstanding. That it didn't bucks the odds, bucks reasonable assumption. Of course, there remain reasons. But only because we can see evidence for them.

The history of some other world with the potential of life upon it - of this we can know nothing. We proceed based upon gut, instinct and belief. We don't have a single solitary factual reason to base anything on anything really...

Except, of course, what we believe...

 

[spuriousmonkey]

Similar organic chemistry based on carbon compounds.

could be


Land animals larger than insects with 4 limbs, rather than having 3, 5, or 6 limbs. Id est: No Centaurs.

land animals larger than insects refer on earth to vertebrates. They have 4 limbs because the ancestor had 4 limbs. There is no reason to assume why an ancestor couldn't have had 6 limbs. Or 8.


Animals with two eyes for binocular vision rather than cyclops or 3-4 eyes.

There are animals with 3 eyes on earth! Many reptiles have a third eye:
For instance:
http://www.panda.org/news_facts/edu...ies_home/remarkable_animals/tuatara/index.cfm



Intelligent creatures (if any) with a hand similar to ours. A thumb (possibly 2) opposing 3 to 5 fingers.

That is antropomorphic thinking. The octopus has excellent 'dexterity'.

 

[PsychoticEpisode]

My guess is that the universe is quite full of simple living organisms but finding intelligence may be rare.

 

[Novacane]

My guess is that the universe is quite full of simple living organisms but finding intelligence may be rare.

Yea.......I agree. It's certainly rare on planet Earth, isn't it?

 

[Dinosaur]

Novacane: Your post reminds me of a wit who once said something like the following.

Sure, I expect intelligent life to exist elsewhare in the universe. It almost happened here on Earth.

SpuriousMonkey: For vertebrates, having more than 4 legs does not seem to result in being able to run, walk, or jump more efficiently. An extra pair of limbs require additional brain circuitry as well as a more extensive network of motor & sensory nerves. The extra set of limbs would also require additional energy resources (Id est: More food eaten per day) for maintenance & repair. Thus the extra pair of limbs do not seem to have any evolutionary advantage, but have significant costs. Hence, I would not expect to find vertebrates elsewhere in the universe with more than 4 limbs.

Oddly enough, a centaur is likely to be a very worthwhile design. However, it seems difficult to imagine how it evolves from a simpler design.

The following does not seem to me to qualify as an eye in the ordinary sense of the word.

There is a cluster of light-sensitive cells situated on top of its brain, just below a small hole in the skull. This third "eye" probably serves to regulate the tuatara's exposure to the sun and so regulate its body temperature.

Did you actually read the description of what is being called a third eye? Note that the author of the article put quotes around eye. Two eyes like those common to all vertebrates provide depth perception, not possible with one eye, providing a definite advantage. As for an extra set of limbs, more eyes do not seem to provide an advantage worth the extra brain circuitry and maintenance requirements. I would not expect to find creatures anywhere iu the universe with more than two true eyes.

Considering its environment, the appendages of an octopus are very useful

That is anthropomorphic thinking. The octopus has excellent 'dexterity'.

Tentacles do not seem very useful for tool making purposes. It seems to me that the evolution of intelligence involves a feedback cycle: Tool use, brain development, tool making, more tool use, more brain development, more sophisticated tool making.

It starts with picking up a rock or a stick and using it as is. This initiates brain development in the direction of developing techniques for using the stick or rock more effectively and/or in additonal contexts. Next comes some minor modification of the rock or stick followed by a period of using the enhanced device. More use, more dexterity, more modifications, eventually resulting in actual tool making. It all starts with a hand including at least one opposable thumb and two or more fingers, providing the potential for tool making. I would expect the evolutionary process taking millions of years to be a slow motion version of what an exceptionally intelligent human could do if raised by animals as was the fictional Tarzan.

In the absence of some good arguments or evidence (unlikely to be available) to the contrary, I will stand by the view of a previous post.

• Carbon based organic chemistry, 4-limbed vertebrates, two eyes, and intelligent creatures (if any) having hands with one or two thumbs opposing 3 to 5 fingers.

 

[Novacane]

Novacane: Your post reminds me of a wit who once said something like the following.

SpuriousMonkey: For vertebrates, having more than 4 legs does not seem to result in being able to run, walk, or jump more efficiently. An extra pair of limbs require additional brain circuitry as well as a more extensive network of motor & sensory nerves. The extra set of limbs would also require additional energy resources (Id est: More food eaten per day) for maintenance & repair. Thus the extra pair of limbs do not seem to have any evolutionary advantage, but have significant costs. Hence, I would not expect to find vertebrates elsewhere in the universe with more than 4 limbs.

Oddly enough, a centaur is likely to be a very worthwhile design. However, it seems difficult to imagine how it evolves from a simpler design.

The following does not seem to me to qualify as an eye in the ordinary sense of the word.Did you actually read the description of what is being called a third eye? Note that the author of the article put quotes around eye. Two eyes like those common to all vertebrates provide depth perception, not possible with one eye, providing a definite advantage. As for an extra set of limbs, more eyes do not seem to provide an advantage worth the extra brain circuitry and maintenance requirements. I would not expect to find creatures anywhere iu the universe with more than two true eyes.

Considering its environment, the appendages of an octopus are very usefulTentacles do not seem very useful for tool making purposes. It seems to me that the evolution of intelligence involves a feedback cycle: Tool use, brain development, tool making, more tool use, more brain development, more sophisticated tool making.

It starts with picking up a rock or a stick and using it as is. This initiates brain development in the direction of developing techniques for using the stick or rock more effectively and/or in additonal contexts. Next comes some minor modification of the rock or stick followed by a period of using the enhanced device. More use, more dexterity, more modifications, eventually resulting in actual tool making. It all starts with a hand including at least one opposable thumb and two or more fingers, providing the potential for tool making. I would expect the evolutionary process taking millions of years to be a slow motion version of what an exceptionally intelligent human could do if raised by animals as was the fictional Tarzan.

In the absence of some good arguments or evidence (unlikely to be available) to the contrary, I will stand by the view of a previous post.

• Carbon based organic chemistry, 4-limbed vertebrates, two eyes, and intelligent creatures (if any) having hands with one or two thumbs opposing 3 to 5 fingers.

Take notice of the insect world. 'Where there is no intelligence, there is no stupidity' I believe the human race is the exception to this rule (My opinion of course).

 

[Pete]

Hi Dinosaur,
Why land animals?

 

[Dinosaur]

Pete: The only evidence we have relating to life forms & intelligent creatures is the history and current state of our own planet.

Why land animals?

It seems reasonable to believe that the laws & mechanisms driving evolution have basic principles which are universal just as we expect the laws of physics to be the same elsewhere in our universe.

Until there is some compelling reason to believe otherwise, I will try to base my beliefs relating to life elsewhere on what I know about life here on Earth.

In spite of various claims about intelligence in sea going creatures,, I see no evidence for it. Modern humans, chimps, and other primates seem to display more intelligent behavior than any other creatures. Hence, I surmise that their basic design is best suited for the evolution of intelligence. Most species of mammals seem more intelligent than most species of fish and other aquatic creatures. Beavers build dams; Rabbits & moles create their own living quarters by digging; Squirrels gather nuts as a food supply for the winter months. Aquatic animals do not display similar behaviors which seem to me to be precursors to the evolution of intelligence.

Another interesting thought is that mammals (and birds) care for their young longer than reptiles, fish, crustaceans, et cetera. This might be a necessary precursor to the evolution of intelligence. It leads to the passing on of learned experiences to the next generation. The next generation need not start from scratch and reinvent good ideas.

I do not expect life elsewhere to be close copies of life here. I do expect the basic designs to be very similar, with the differences in the details like the number of fingers. Bilateral symmetry for land animals seems to be a basic design, while both bilateral & radial symmetry seem to work for aquatic creatures. I therefore expect land creatures elsewhere to have bilateral symmetry only, while sea creatures elsewhere are expected to have both bilateral & radial symmetry.

I think the above indicates the basic thoughts behind my beliefs about life elsewhere.

BTW: A concept not closely related to evolutionary thoughts. I am surprised that many intelligent & well educated people do not realize that creature size dictates all sorts of constraints on basic design and function. The huge SciFi insects are a physical impossiblity. An elephant scaled down to the size of a deer would be totally disfunctional and could not survive. A deer scaled up to the size of an elephant would break his legs merely walking. An elephant would not survie a fall from 10 to 20 feet, while a small creature might not be injured at all. A wet mouse in cold weather could die quickly due to loss of body heat, while a large animal would easily survive being drenched.

 

[spuriousmonkey]

SpuriousMonkey: For vertebrates, having more than 4 legs does not seem to result in being able to run, walk, or jump more efficiently. An extra pair of limbs require additional brain circuitry as well as a more extensive network of motor & sensory nerves. The extra set of limbs would also require additional energy resources (Id est: More food eaten per day) for maintenance & repair. Thus the extra pair of limbs do not seem to have any evolutionary advantage, but have significant costs. Hence, I would not expect to find vertebrates elsewhere in the universe with more than 4 limbs.

Speculation. Why can insect get by with 6 legs? what about a milipede? What about a snake? No legs.

The following does not seem to me to qualify as an eye in the ordinary sense of the word.Did you actually read the description of what is being called a third eye? Note that the author of the article put quotes around eye. Two eyes like those common to all vertebrates provide depth perception, not possible with one eye, providing a definite advantage. As for an extra set of limbs, more eyes do not seem to provide an advantage worth the extra brain circuitry and maintenance requirements. I would not expect to find creatures anywhere iu the universe with more than two true eyes.

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

5 eyed creature.

http://en.wikipedia.org/wiki/Spider#Sense_organs

6-8 eyes in spiders.

Considering its environment, the appendages of an octopus are very usefulTentacles do not seem very useful for tool making purposes. It seems to me that the evolution of intelligence involves a feedback cycle: Tool use, brain development, tool making, more tool use, more brain development, more sophisticated tool making.

It starts with picking up a rock or a stick and using it as is. This initiates brain development in the direction of developing techniques for using the stick or rock more effectively and/or in additonal contexts. Next comes some minor modification of the rock or stick followed by a period of using the enhanced device. More use, more dexterity, more modifications, eventually resulting in actual tool making. It all starts with a hand including at least one opposable thumb and two or more fingers, providing the potential for tool making. I would expect the evolutionary process taking millions of years to be a slow motion version of what an exceptionally intelligent human could do if raised by animals as was the fictional Tarzan.

Neither does the hand of a monkey seem very good at tool making purposes. You think too much about humans and their perspective. An octopus can unscrew a cap. Many primates can't manage that. The point is that there are other engineering solutions that can solve the problem of toolmaking. Give the octopus a larger brain and it might pick up a rock and clip a flint of it against another rock.

 

[francois]

There's nothing wrong with the Drake Equation. The equation is extremely simple and it makes intuitive sense. The problem is in how most people use it.

You gotta substitute the variables with good numbers if you're going to get a good/accurate answer.

where:

N is the number of civilizations in our galaxy with which we might expect to be able to communicate at any given time

and

R* is the rate of star formation in our galaxy
fp is the fraction of those stars that have planets
ne is average number of planets that can potentially support life per star that has planets
fl is the fraction of the above that actually go on to develop life
fi is the fraction of the above that actually go on to develop intelligent life
fc is the fraction of the above that are willing and able to communicate
L is the expected lifetime of such a civilization


R* = 10/year, fp = 0.5, ne = 2, fl = 1, fi = fc = 0.01, and L = 50,000 years
Here, the variables are assigned optimistic values. These are the values that Drake et al. used a lot.
N = 10 × 0.5 × 2 × 1 × 0.01 × 0.01 × 50,000 = 50

As you see, N = 50. Put in another way, according to the equation, there are 50 civilations inhabiting our galaxy which we should be able to communicate with at any given time. (Assuming they're not ignoring us.)



R* = 6/year, fp = 0.5, ne = 2, fl = 0.33, fi = 1×10-7, fc = 0.01, and L = 420 years

Now change a few of the variables and you get an entirely different answer.
N = 6 × 0.5 × 2 × 0.33 × 1×10-7 × 0.01 × 420 = 8.316×10-7 = 0.0000008
Number of civilizations we should be able to communicate with in this galaxy = .0000008. Not a very high number.

It's all about the variables you feed the equation. It's a good equation, however, in of itself.

 

[swivel]

Hi Dinosaur,
Why land animals?

For technological races? Metallurgy, for one. Electronics would be the other. Think about what fire meant for our species.


And francios, the Drake equation is only as good as the variables, and the variables are bunk.

 

[Novacane]

There's nothing wrong with the Drake Equation. The equation is extremely simple and it makes intuitive sense. The problem is in how most people use it.

You gotta substitute the variables with good numbers if you're going to get a good/accurate answer.

where:

N is the number of civilizations in our galaxy with which we might expect to be able to communicate at any given time

and

R* is the rate of star formation in our galaxy
fp is the fraction of those stars that have planets
ne is average number of planets that can potentially support life per star that has planets
fl is the fraction of the above that actually go on to develop life
fi is the fraction of the above that actually go on to develop intelligent life
fc is the fraction of the above that are willing and able to communicate
L is the expected lifetime of such a civilization


R* = 10/year, fp = 0.5, ne = 2, fl = 1, fi = fc = 0.01, and L = 50,000 years
Here, the variables are assigned optimistic values. These are the values that Drake et al. used a lot.
N = 10 × 0.5 × 2 × 1 × 0.01 × 0.01 × 50,000 = 50

As you see, N = 50. Put in another way, according to the equation, there are 50 civilations inhabiting our galaxy which we should be able to communicate with at any given time. (Assuming they're not ignoring us.)



R* = 6/year, fp = 0.5, ne = 2, fl = 0.33, fi = 1×10-7, fc = 0.01, and L = 420 years

Now change a few of the variables and you get an entirely different answer.
N = 6 × 0.5 × 2 × 0.33 × 1×10-7 × 0.01 × 420 = 8.316×10-7 = 0.0000008
Number of civilizations we should be able to communicate with in this galaxy = .0000008. Not a very high number.

It's all about the variables you feed the equation. It's a good equation, however, in of itself.

So far, based on all of the 'confirmed' intergalactic alien radio signals encountered todate, .0000008 seems to be the number...........and just maybe it's even lower than that

 

[Dinosaur]

SpuriousMonkey: You do not seem to understand that various creatures design are fundamentally different due to being functional in different evolutionary niches.

Speculation. Why can insect get by with 6 legs? what about a milipede? What about a snake? No legs.

Above posted as arguing against my discussion of 4-legged vertebrates.

• Primates, ungulates, rodents, et cetera have considerable brain and nervous system circuitry allocated to control of their 4 limbs, which can be controlled independenttly of each other. There is also an energy cost (food intake) per limb for usage, maintenance, and repair (Each pair of vertebrate limbs is a significant percentage of the overall body mass). An additional pair of limbs is costly for a vertebrate, which might be one reason that snakes get by with no limbs. Do you think that a deer, squirrel, feline, canine, et cetera would run faster or more efficiently with an extra set of limbs? Would it be more agile or able to jump higher? An extra set of limbs for vertebrates (surely for larger ones) has a significant cost with no corresponding evolutionary advantage.
  
  
• Insects like centipedes do not have the brain or nervous system circuitry to control their limbs independently, and a pair of limbs is not a significant percentage of the insect body mass. The cost of an extra set of limbs is not high for most insects, while I suppose there is some advantage to having extra sets.

Similarly your remarks about the sea creature with 5 eyes overlooks the fact that it seems to have been an evolutionary dead end. From the artists guesses at what the creature looked like, I am not sure that the eyes were as functional as those used today by birds, reptiles, and mammals. I imagine that an extra eye facing backwards might confer some advantage, especially for prey creatures, and wonder why there are no animals with this feature. Note, however, that a second eye provides a very important advantage: The ability to judge distance accurately. A third eye or an extra pair would not provide any advantage as important as depth perception. As for extra limbs, extra eyes have a cost for vertebrates without much evoutionary advantage, which is why I would not expect such a design to evolve.

Evolution seems to result in reasonably cost effective designs, which is what encourages creationists to speculate about divine intervention. I strongly suspect that what has happened here on Earth is indicative of what to expect of life forms elsewhere.

Your claim that an octopus can unscrew a cap might be valid. I do not know much about what they can do. However, tentacles do not seem capable of tying knots or doing performing other functions easily accomplished by a hand with a thumb opposing several fingers. Perhaps (as you claim) an octopus with a better brain could make much better use of his tentacles. The point is that the primate hand was capable of making use of rocks or sticks when there was very little brain power. This led to the evolutionary development of more brain power.

If a space ship lands on Earth tomorrow, I will bet on it having primate like creatures as pilots and navigators. If it disgorges intelligent centaurs or intelligent creatures with tentacles instead of hands, I will have lost my bet. Would you make a large bet with me before seeing the crew of a star ship?

 

[Pete]

If a space ship lands on Earth tomorrow, I will bet on it having primate like creatures as pilots and navigators. If it disgorges intelligent centaurs or intelligent creatures with tentacles instead of hands, I will have lost my bet. Would you make a large bet with me before seeing the crew of a star ship?

I'll take that action!
It's a bet that would be worth losing.
The loser must extend their hospitality to the winner. If you win, I pay for you (and your family) to come and stay with me and my family for a couple of weeks, and vice versa.

What do you think?

I'll allow a fairly broad definition for "Primate like creatures": creatures with two limbs primarily for locomotion, two limbs primarily for manipulation with jointed sub-limbs (fingers) at the extremeties, and a head containing sensing organs. Tails are optional.

Personally, I strongly expect that any extra-terrestrial ambassador would be non-biological, or at least very different from their naturally evolved ancestors... So, there are some other cases to consider.

If the ambassadors are clearly not primate-like, and all known ancestors are not primate-like, then I win.

If any ambassadors are artificially primate-like, but it is demonstrable that their naturally evolved ancestors are/were clearly not primate-like, then I win.

If any ambassadors are artificially primate-like, and the nature of their naturally evolved ancestors is not known, then you win.

If the ambassadors are artificially and significantly different from naturally evolved primate-like ancestors, then we both win!

Disputes to be settled by sciforums poll.

 

[Facial]

I have read several articles on the Drake equation, and the most recent studies surprisingly seem to confirm its validity to finding life in the universe.

Another study shows that Earth-like planets may be fairly common.

As long as the impetus exists for evolution, most probably by geological or meteorological events, then it will probably take several billion years for intelligence to develop - or thereabouts, since evolution is almost essentially random (given that the environmental variables occur at random, ie asteroid impacts). For earth, it took between one to two billion years before the evolution of humans. It's quite possible that it can take less than a billion years. For earth the appearance of intelligence in humans was on the third 'try' : first try was the Paleiozoic, with perhaps the highest degree of species diversity, second try being the Mesozoic, with domination of reptiles (but no intelligence), and the third winning try being the Cenozoic, with the domination of mammals (humans being amongst them).

To me it is rather unusual that I am a mammal - it would've seemed much more likely for intelligence to evolve during the Paleozoic. As Spurious pointed out, octopods are quite dextrous, and I am stymied by the fact that amongst the millions and millions of invertebrate species during the Paleozoic, none of them happened to evolve intelligence to the degree that humans have.

My guess is that there may be as many as 10,000 planets with life on them in the Milky Way, out of hundreds of billions of stars. Perhaps 20-30 of them may have the potential for intelligence, and 10 may already have developed at least to the extent that humans have. Just a rough guess. Where is a real analysis?

 

[spuriousmonkey]

SpuriousMonkey: You do not seem to understand that various creatures design are fundamentally different due to being functional in different evolutionary niches.Above posted as arguing against my discussion of 4-legged vertebrates.

• Primates, ungulates, rodents, et cetera have considerable brain and nervous system circuitry allocated to control of their 4 limbs, which can be controlled independenttly of each other. There is also an energy cost (food intake) per limb for usage, maintenance, and repair (Each pair of vertebrate limbs is a significant percentage of the overall body mass). An additional pair of limbs is costly for a vertebrate, which might be one reason that snakes get by with no limbs. Do you think that a deer, squirrel, feline, canine, et cetera would run faster or more efficiently with an extra set of limbs? Would it be more agile or able to jump higher? An extra set of limbs for vertebrates (surely for larger ones) has a significant cost with no corresponding evolutionary advantage.
  
  
• Insects like centipedes do not have the brain or nervous system circuitry to control their limbs independently, and a pair of limbs is not a significant percentage of the insect body mass. The cost of an extra set of limbs is not high for most insects, while I suppose there is some advantage to having extra sets.

Similarly your remarks about the sea creature with 5 eyes overlooks the fact that it seems to have been an evolutionary dead end. From the artists guesses at what the creature looked like, I am not sure that the eyes were as functional as those used today by birds, reptiles, and mammals. I imagine that an extra eye facing backwards might confer some advantage, especially for prey creatures, and wonder why there are no animals with this feature. Note, however, that a second eye provides a very important advantage: The ability to judge distance accurately. A third eye or an extra pair would not provide any advantage as important as depth perception. As for extra limbs, extra eyes have a cost for vertebrates without much evoutionary advantage, which is why I would not expect such a design to evolve.

Evolution seems to result in reasonably cost effective designs, which is what encourages creationists to speculate about divine intervention. I strongly suspect that what has happened here on Earth is indicative of what to expect of life forms elsewhere.

Your claim that an octopus can unscrew a cap might be valid. I do not know much about what they can do. However, tentacles do not seem capable of tying knots or doing performing other functions easily accomplished by a hand with a thumb opposing several fingers. Perhaps (as you claim) an octopus with a better brain could make much better use of his tentacles. The point is that the primate hand was capable of making use of rocks or sticks when there was very little brain power. This led to the evolutionary development of more brain power.

If a space ship lands on Earth tomorrow, I will bet on it having primate like creatures as pilots and navigators. If it disgorges intelligent centaurs or intelligent creatures with tentacles instead of hands, I will have lost my bet. Would you make a large bet with me before seeing the crew of a star ship?

vertebrates have 4 legs because they are vertebrates. i doubt aliens will be vertebrates.

 

[swivel]

vertebrates have 4 legs because they are vertebrates. i doubt aliens will be vertebrates.

:bugeye:

Man. You say the craziest things. Nothing about being a vertebrate implies 4 legs. There is no connection between those two things.

And I sure hope you don't think that aliens will be giant bugs, because exoskeletons will not work on large creatures. Even with weaker gravity, you have a problem with the surface of a sphere increasing much slower than the volume. Basically, the guts will overwhelm the structural stability of the carapace. That's why bugs never got very big here, and the same physical principles will apply elsewhere.

Most of biology is chemistry. And chemistry does not change according to the whims of science-fiction authors. Chemistry is as unyeilding as physics. No matter what life-form you have, it is going to build itself up from the most abundant elements on the periodic table. And the distribution of these elements will only vary slightly on other planets. They have to do with rates of nuclear fusion in stars... which is why you will not find planets loaded up with gold anywhere. When you look at the elements available, calcium phosphite is so advantageous, that it only has to occur once to take over. Current evolutionary theory suggests that this has evolved in parallel many times over, like vision and flight.

The point that should be taken away from the pre-Cambian explosion shouldn't be how unusual life can be... the point should be that very basic plans and stability are what SURVIVED. Planet Earth tried most of the crazy things that sci-fi writers dreamt about, and they didn't work for very long. They were novelties.

Meanwhile, the shrew, the fish, the cockroach, and the pidgeon carry right along. Looking much the same as before - and as each other.

 

[spuriousmonkey]

:bugeye:

Man. You say the craziest things. Nothing about being a vertebrate implies 4 legs. There is no connection between those two things.

There is. The ancestor of terrestrial vertebrates had 4 limbs. That's why they call them tetrapods.

To assume that all intelligent life must have 4 limbs because tetrapods have 4 limbs is unsustainable.

 

[RHaden]

A vertebrate is defined by having a backbone, not four limbs. The number of limbs on terrestrial vertebrates is a derived characteristic.

Are we done trolling now?