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PostSubject: Book: Rare Earth   Tue Jun 19, 2018 9:25 pm

Yesterday, while waiting for lunch with a friend in the U. District, I picked up a used copy of a book that’s been on my want list since 2000 when it was published.  It’s by two U.of W. professors, and it’s subtitled “Why Complex Life is Uncommon in the Universe.”

Reviews from the cover:
Geoff Marcy, University of California, Berkeley wrote:
This book offers a fresh and accurate perspective on the most profound question in science — is intelligent life in the Galaxy a dime-a-dozen occurrence or is it a cosmic fluke here on Earth?
James Kasting, Pennsylvania State University wrote:
Microbial life is common in the universe, but multicellular animal life is rare.  A controversial thesis, but one that is well-researched and well-defended.
David Levy, discoverer of Comet Shoemaker-Levy wrote:
If we’re to believe what we see in the movies, extraterrestrials thrive on every world.  But this unique book, written by two of the top scientists in the field, tells a different story.  Complex life might be very rare, and very precious.
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PostSubject: Re: Book: Rare Earth   Thu Jun 21, 2018 1:01 pm

I found a pdf file of the book online.
Much of the information deals with matters I have read in articles over the years. Really no big surprises in the book.
It appears to me to support what I have always believed: microbial life is very common, while advanced life forms (mammals, etc.) are rare.
Much of the book is nothing more than supposition and conjecture.
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PostSubject: Re: Book: Rare Earth   Thu Jun 21, 2018 2:27 pm

You think??? Shocked
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PostSubject: Re: Book: Rare Earth   Wed Jul 04, 2018 6:25 pm

Summary (to be updated as I progress through the book):

  • 13.799 (± 0.021) billion years ago (bya) - Big Bang.  Universe starts (Thursday, 4:00 pm sharp).
  • 13.8 to 4.6 bya - Early universe is dominated by chaos.  Stars come and go, entirely hydrogen and helium fusion.  Only late in the game are enough heavy elements created for planets to start forming.  Many collisions and annihilations keep things popping.
  • 4.54 (± 0.05) bya - Our solar system coalesces out of a giant molecular cloud.  Our system is unusual in its prevalence of heavy elements and the size of our sun (95% of all stars are smaller).  Our system is between arms of a spiral galaxy, out near the tips, in an area of low density so we experience fewer collisions and annihilations than normal.  
  • The rocky inner planets of our solar system grow by accretion, sweeping up dust and debris in their path.  The outer gas planets grow much slower from lighter elements farther from the sun, and end up far larger because more gravity is required to hold gasses together.  The gas giants together account for 99% of all the mass circling the sun -- but the sun itself contains 99.8% of all mass in the solar system.
  • Earth's distance from the sun is unique in our solar system (the so-called "habitable zone") -- far enough from the sun to allow nitrogen and carbon and water to occur (the basic elements of life), but not so far that these elements are gaseous (inside Earth's orbit) or permanently bound up in the composition of the planet (beyond Earth's orbit, before the gas giants).  By some estimates, this delicate balance has only a 0.0001% variance possible.  Venus and Mars show what happens outside the HZ.
  • 4.6 to 3.9 bya - Earth is constantly bombarded by debris and planetoids as it sweeps the orbit.  The surface remains molten and is reshaped many times.  One of these Mars-sized impacts ejects the Moon (4.51 bya) and gives us our spin and polar tilt.
  • Our Moon is uniquely large (in our solar system) in comparison to the planet it orbits, 1.2% of Earth’s mass.  This causes tidal effects unique to Earth, which are important to geothermal activity.  The cratered surface of the Moon shows what the period 4.51 to 3.9 bya was like.
  • 3.9 bya - the path that Earth travels has basically been cleared (other planets too, so they stop hurling stuff in our direction).  The surface begins to cool, and Earth separates into a molten core, a mantle, and a rocky crust.
  • Oldest known rocks on Earth, 4.031 (± 0.003) bya
  • The Earth's super-heated atmosphere at this point is mostly carbon dioxide, with some water vapor, ammonia and methane.  As the Earth cools, the water vapor gradually condenses and falls from the sky.  The Earth becomes totally covered in a sea that is 4,000 meters deep.
  • Due to the unusual composition of the Earth, including metals like iron and nickel and even heavier elements like uranium, thorium, platinum, iridium, and osmium, the core remains hot from radioactive decay.  This allows volcanism and plate tectonics which continue to this day.  No other planet in the solar system has plate tectonics.
  • Plate tectonics causes the crust to become uneven, leading to deep open trenches and ever-moving continents which rise above the surface of the water.  Dry land appears.  Rifts appear undersea where the plates are pulling apart, causing undersea magma and superheated gas vents.
  • The earliest forms of life (prox. 3.8 bya) are PROBABLY extremophiles (archaeans that use hydrogen sulfide, methane and carbon dioxide and the extreme heat of undersea geothermal vents in the ocean trenches to grow bacteria-like non-cellular structures).  These are "alive" in the sense that they accrete and reproduce, but they are not mobile, do not evolve (still exist unchanged today), and have no functions other than to exist.  They're more a chemical process than a living being.  If life exists elsewhere in the universe -- given all the unlikely requirements listed above -- this is the most likely extent of it.
  • Being in the deep ocean protects these bacteria from the destructive forces on the Earth’s surface like impacts and ultraviolet radiation
  • ”Below the photic zone—the sunlit, upper reaches of the ocean—many microbes have evolved chemosynthetic (instead of photosynthetic) processes that create organic matter by using oxygen in seawater to oxidize hydrogen sulfide, methane, and other chemicals present in vent and seep fluids.”
  • ”Major types of bacteria that live near these vents are mesophilic sulfur bacteria. These bacteria are able to achieve high biomass densities due to their unique physiological adaptations. For example, Beggiatoa spp. is able to carry an internal store of nitrate as an electron acceptor that helps with the harvesting of free sulfide in the upper sediment region of the vents.”
  • In this era there was no oxygen in the atmosphere (or sea), so ultraviolet radiation from the sun would’ve prevented any emergence of life on land.
  • 3.8 to 3.5 bya -- through a process of evolution which is described in the book but too complicated for me to understand -- in a relatively short span of time (300 million years) these self-reproducing extremophile bacteria evolved into stromatolites (such as have been found in the Warrawoona Series in Australia).  These microbial mats are the first fossilized evidence of life, sandwiched between lime deposits. “They have been found on every continent in rocks half a billion years old and older.  Today, they are found only in one type of environment on Earth, in quiet, briny tropical waters.”  Everywhere else they would be eaten today.
  • Cyanobacteria, the so-called "blue-green algae mats" (although in today's usage "algae" refers to eurkaryotes, not prokaryotes like the first cyanobacteria), evolved next.  These photosynthesize, so they must reside on the surface of the seas (the details here are contradictory in different sources online).  They metabolize water, hydrogen sulfide and carbon dioxide and produce oxygen as waste.  Between 2.7 bya when they took over the seas and today (they still exist) they fundamentally changed the chemistry of Earth by adding oxygen, a poisonous gas which causes all sorts of chemical compounds to break down (oxidize).
  • Between 3.2 and 1.4 bya another microfossil, called acritarches, are also found.  Even after reading about then I'm confused what they were.
  • A few times in its history Earth froze over completely: once 2.5 bya and again between 800-600 mya.  The first of these may have been triggered by the cyanobacteria outgassing oxygen, and subsequent loss of carbon dioxide in the atmosphere.
  • With the oceans covered over with up to 1500 meters of ice (for as long as 30 million years, 2.5 to 2.47 bya), the oceans and the atmosphere became decoupled for the first time in Earth's history.  
  • With the oceans and atmosphere no longer exchanging gasses and minerals, the atmospheric carbon dioxide begins to climb again from volcanic activity.  Meanwhile dissolved metals and oxygen in the oceans also climbed under the ice lid from undersea outgassing.  
  • Eventually (~2.47 bya) the atmospheric CO2 triggers a greenhouse effect, raising Earth's temperature , which melts the ice covering the planet, which releases all the built-up oxygen and minerals from the sea.
  • At 2.45 bya, with the sudden rise in Earth's temperature, the release of iron and manganese and oxygen from the oceans, there was a phytoplankton bloom.  Cyanobacteria releases more oxygen into the atmosphere, which kills prokaryotes not used to it but creates an ecological niche for new life forms (like eurkaryotes) who can use oxygen to metabolize.  The old order -- which has endured virtually unchanged on Earth from 3.8 bya to 2.5 bya -- is swept away and a new diversity of oxygen-utilizing organisms evolve (2.5 bya to ~800 mya).  Little direct fossil evidence exists however, because these soft-bodied organisms left no trace.
  • This sudden increase in oxygen (called the Great Oxygenation Event, 2.45 bya), first in the oceans (causing most of the dissolved iron to oxidize and drop out of solution as rust), and then, as the oceans reach oxygen saturation, in the atmosphere, where it provides the mechanism for the metabolism of terrestrial minerals (as oxides) leading to ways for plants and animals to consume their environment.
  • This newly-oxygenated atmosphere for the first time protects the surface of the Earth from solar ultraviolet radiation as well as cosmic rays.  Without this protection anything assembled on land would be quickly split apart again.
  • Under this new protective canopy, and with the sudden abundance of metals and oxygen, and sudden return to warmth, the era between 2.45 bya and 1.2 bya sees an explosion of life.  Simple self-replicating bacterial mats and virus-like proto-living arrangements of amino acids and proteins, evolve into prokaryotes (single cells with no nucleus and a single strand of DNA) and then to eurkaryotes (single cells with a nucleus containing multiple strands of DNA, and mitochondria to supply energy, and chloroblasts to photosynthesize, and other organelles).  This 'creation of life' is the least understood stage, because nobody has yet figured out how to get from amino acids to RNA.  But once the ball starts rolling the diversification expands exponentially.
  • It is assumed eurkaryotes evolved from prokaryotes "eating" other prokaryotes and finding that having cells of differing specialties living inside you can be advantageous.
  • The date of the first appearance of eurkaryotes and subsequent multi-celled life is still contentious (no fossils exist of course).  The oldest estimates are 1.2 bya, youngest 550 mya, median somewhere around 750 to 800 mya.  
  • Sexual reproduction -- reproduction by combining two cells rather than simply dividing -- is dated to around 1 bya.  This sped up the pace of evolution by some order of magnitude.
  • Also during 2.5 to 1.2 bya, loads of comets and meteors may have been impacting the Earth, bringing water and amino acids from outer space.  Something in this barrage MAY have triggered RNA, although nobody outside of science fiction can say what.
  • Between 2.7 and 1.25 bya stromatolites dominate the fossil record, but beginning about 1.25 bya -- which is both the end of the bombardment of Earth's surface, and the beginning of eurokaryotic and multi-celled life -- they begin to disappear.  They apparently start getting eaten.  Today they are rare.
  • Between 800-600 mya several more ice ages occur, as the atmosphere bounces back and forth between too much CO2 and too much oxygen.  Each time life is winnowed, then the survivors are boosted with a fresh supply of raw materials, and eventually the number of oxygen-breathers reaches a stasis point, where they begin to control the planet and prevent these "Snowball Earth" cycles.
  • When the last ice age recedes about 600 mya, the result is the so-called Cambrian Explosion usually dated to 541 mya.  All kinds of body styles evolve (according to Stephen Jay Gould's "Wonderful Life", many more than eventually survive.  As many as 100 phylum were created.  Today 28-35 survive).  This explosion in diversity is due to the evolution of expression genes (according to Sean Carroll's "Endless Forms Most Beautiful"), where small changes in the timing of development can affect the number and size and placement of limbs, the overall size of organisms, and a myriad other variations.  Life explodes in diversity -- and consequently habitats.
  • 580 to 550 mya - The first to appear are the ediacarans, leaf-like primitive multicellular organisms which could be either colonies of individual cells (like sponges) or simple animals (like jellyfish) or even plants (or something in between, or something totally unknown today).  They left impressions in the ocean floor, probably because they fed on the algae mats at the bottom of the sea and got buried by sand-falls.  Their numbers after 550 mya dropped precipitously; probably something evolved to feed on them.
  • 550 to 500 mya - "trace" fossils and SSFs dominate.  Trace fossils left no body parts, but left long trackways or feeding patterns in ocean sediments.  Unknown what they were exactly, but they were mobile (unlike ediacarans) and they fed on other organisms.  SSFs are small shelly fossils, under 1/2", and so pulverized as to be unreconstructable.  They might be the trace fossils source.
  • 530 to 500 mya - The age of trilobites, brachiopods, mollusks and echinoderms.  The first bony parts to be fossilized.  Trilobites range from microscopic to over three feet long.  During this 30 million years the variety and quantity of life on earth is unprecedented, even today.
  • Not a single new phylum has emerged since the Cambrian Explosion.  The last 550 MY has been a period of whittling down, of slow extinction.
  • In addition to the evolution of sexual reproduction, another driver for the Cambrian Explosion was undoubtedly the evolution of predation.  Rather than having to make their own living, larger and more sophisticated organisms could take advantage of another creature's success by devouring them.  This led to an arms race / size race / teeth / claws / shells.
  • Also, the Cambrian Explosion marks the first appearance of shells and skeletons.  Available free calcium in the environment was one factor.  The sudden appearance in the fossil record of bones and shells probably makes this period look like more of an anomaly than it was.
  • Another factor in the Cambrian Explosion was plate tectonics.  The continents migrated from the poles to the equator.
  • "On Earth there have been about 15 [mass extinctions] during the last 500 million years, 5 of which eliminated more than half of all species then inhabiting our planet."  Ordovician–Silurian extinction events (450-440 mya), Late Devonian extinction (375-360 mya), Permian–Triassic extinction event (252 mya), Triassic–Jurassic extinction event (201 mya), Cretaceous–Paleogene extinction event (65 mya), Holocene extinction (10,000 ya to present).


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PostSubject: Re: Book: Rare Earth   Thu Jul 05, 2018 8:14 am

My iPad app that shows the Earth’s history only goes back 541 million years.
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PostSubject: Re: Book: Rare Earth   Thu Jul 05, 2018 2:48 pm

There are too damned many assumptions and just plain made-up stuff in that book for me to give any credence at all to their conclusions.

"Earth's distance from the sun is unique..."
"Our Moon is uniquely large..."

They should never use the word unique in guessing  about the universe.

"No other planet has been shown to have plate tectonics."
That may be true, but how many planets have they examined closely enough to determine whether they have plate tectonics? Three or four, maybe?

"If life exists elsewhere in the universe -- given all the unlikely requirements listed above -- this is the most likely extent of it."
To draw that conclusion, you must accept that the suppositions the author makes are  universally true, and that Earth-like life is the only kind that could possibly exist.

Worthless book.
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PostSubject: Re: Book: Rare Earth   Fri Jul 06, 2018 2:50 pm

I have added "of our solar system" to several of these statements. I'm sorry if I wasn't being clear.
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PostSubject: Re: Book: Rare Earth   Fri Jul 06, 2018 4:50 pm

Yes, it does sometimes make a big difference,

But consider that of the eight planets we have observed (which happen to all be in our solar system) one of them is teeming with life.
So 12.5% of the planets we have observed have life.
Although I do think that's unusual, I have never seen any scientific data that indicates it is one of a kind, or even exceptionally rare.

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PostSubject: Re: Book: Rare Earth   Fri Jul 06, 2018 5:45 pm

This is a philosophy discussion, not a scientific one -- can't be proven, one way or the other -- so you're free to formulate whatever opinion you wish.

I disagree with you, and this book is providing a lot of good scientific evidence for how unlikely life was on Earth... but I'm not out to change your mind.
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PostSubject: Re: Book: Rare Earth   Sat Jul 07, 2018 11:24 am

The scientific data presented in the book is nothing new; note the two dozen or so pages of references.

What is lacking - what makes me use the phrase "scientific data" rather than "scientific evidence" - is that the data provide no evidence for what has happened, is happening, or will happen on other planets (excepting those in our solar system to some extent). We just do not have the ability to examine these other planets. Suppositions are inadequate.

Yes, in the distant past there were some extreme situations that prevented life on Earth from developing or evolving, but ... here we are. The author presents no sound evidence that such dire conditions on other planets could not be overcome by life, giving sufficient time and planetary changes as we have experienced.

I am not arguing that there are Star Wars creatures flying around in space. In fact I would argue strongly that they are not and never will be. But I will not accept an argument that denies the existence of advanced life (and I am not talking about technological life; cats and dogs would suffice for that definition) without evidence. And the book provides none.
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PostSubject: Re: Book: Rare Earth   Mon Jul 09, 2018 5:08 am

The point is, and I'm sure you see this, is that conditions on early Earth were challenging and changing. It is not necessary, of course, for another planet to undergo exactly the same devastating apocalypses that Earth did in order for life to evolve into higher organisms, but without such motivation life was perfectly happy to remain at a bacterial level on Earth for 2.9 of its 3.9 billion year span.

Advanced life is not guaranteed. It's not inevitable. It's not a given that life "progresses" toward anything. It's not even advantageous, except under certain extreme circumstances.

Life getting started is unlikely enough.

Having it evolve into something mobile and sentient is a whole other level of unlikely.
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PostSubject: Re: Book: Rare Earth   Mon Jul 09, 2018 9:18 pm

The Kola Superdeep Borehole in Murmansk is the deepest hole ever made by mankind.  It extends 7.5 miles into the crust.

The boring had to be stopped in 1994 because the bit was encountering temperatures of 325 degrees Fahrenheit, and the rock was turning soft and plasticy.

Even though the crust is up to 100 miles thick by most estimates.

The biggest surprise?  Samples brought to the surface showed microfossils, even at that depth.
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PostSubject: Re: Book: Rare Earth   Wed Jul 11, 2018 4:25 pm

NoCoPilot wrote:
Advanced life is not guaranteed.  It's not inevitable.  It's not a given that life "progresses" toward anything.
True.

NoCoPilot wrote:
It's not even advantageous, except under certain extreme circumstances.
Please explain that. I really don't understand what you mean.


NoCoPilot wrote:
Life getting started is unlikely enough.  
Having it evolve into something mobile and sentient is a whole other level of unlikely.
Not you, not I, not a person on the planet has sufficient data to validate those claims.
They may be true, but there is no concrete reason to believe - or disbelieve - them.

You  have to keep in mind the huge fucking numbers we are dealing with. If only one out of a trillion planets had life, that would leave one to three trillion planets that do - or did - have life, using the current estimate of one to three septillion planets in the universe. You can't use a sample size of one and make sound conclusions.
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PostSubject: Re: Book: Rare Earth   Thu Jul 12, 2018 6:52 am

_Howard wrote:
NoCoPilot wrote:
It's not even advantageous, except under certain extreme circumstances.
Please explain that. I really don't understand what you mean.
Bacteria can survive in extreme cold.  Bacteria can survive in extremely hot undersea thermal vents.  Bacteria survives inside rocks thousands of feet underground.  The biomass of all bacteria on Earth far outweigh the biomass of all other life.  Bacteria are hardy.

Multicellular life not so much.  Vulnerable to cosmic rays, ultraviolet radiation, mass extinctions and thrives only in a very narrow temperature range.

However, bacterial mats outgas oxygen, which is poisonous to most life.  Once the atmosphere of Earth stabilized at 21% oxygen (after several bouts with too hot or too cold due to imbalance), the prevalence of this poisonous gas -- which causes organic and inorganic material to break down, to oxidize -- life on Earth had to adjust to living in poisonous gas, and actually used the reductive qualities of oxygen to break down and metabolize the environment for life's own benefit.

Life which is not oxygen-based probably would never grow beyond microscopic, because you need a way to break down and distribute nutrients.

_Howard wrote:
NoCoPilot wrote:
Life getting started is unlikely enough.  
Having it evolve into something mobile and sentient is a whole other level of unlikely.
Not you, not I, not a person on the planet has sufficient data to validate those claims.  They may be true, but there is no concrete reason to believe - or disbelieve - them.
Scientists have been trying to create life in the laboratory since 1953.  Urey and Miller created amino acids and primitive proteins, the building blocks of life, in a test tube using electricity but the leap from there to RNA composed of amino acids and proteins is still a leap of unknown origin.  If life were easy to create, we would have created it by now.

And once you have self-replicating life, the book I just read explains some of the unlikely events that led it to mutate into something beyond algae mats (although in Washington DC, I have my doubts).

_Howard wrote:
You have to keep in mind the huge fucking numbers we are dealing with. If only one out of a trillion planets had life, that would leave one to three trillion planets that do - or did - have life, using the current estimate of one to three septillion planets in the universe. You can't use a sample size of one and make sound conclusions.
Nor can you use a sample size of one to extrapolate any "one-out-of-a-trillion" odds.  It could just as easily be one-out-of-three-septillion.

All life on Earth descended from a single source.  There is no second source that uses a different mechanism. So even under circumstances that have turned out to be "ideal" life arose only once.
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PostSubject: Re: Book: Rare Earth   Thu Jul 12, 2018 9:43 am

In the afterword, the authors attempt to update the Drake Equation with all the specific weirdnesses found on Earth. The result -- highly speculative of course -- yields a result between 0 and 1.
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PostSubject: Re: Book: Rare Earth   Thu Jul 12, 2018 1:26 pm

The Drake Equation is nothing more than a mental exercise.
Quote :
The Drake Equation is an attempt to encapsulate all the variables that would be relevant to establishing the number of intelligent civilizations that existed in the Milky Way galaxy and which were broadcasting radio signals at this particular point in time.

Until proven values are discovered to assign to the variables, it is meaningless.

NoCoPilot wrote:
All life on Earth descended from a single source.  There is no second source that uses a different mechanism.  So even under circumstances that have turned out to be "ideal" life arose only once.
And how do you know that? Is it not possible that life arose in various places around the planet over a period of many millions of years? It could have died off in some places and continued in others.

That's my issue: we don't know many of the assertions being made.
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PostSubject: Re: Book: Rare Earth   Thu Jul 12, 2018 2:15 pm

I think scientists feel the amount of shared DNA among all god’s critters — as well as the fact that all critters have DNA — points to a single origin.

Could there have been more than one strain of life, now gone extinct? That left no trace in the fossil record and left no survivors? Well to argue that, you’d really need to buy a strainer for your credulity.
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PostSubject: Re: Book: Rare Earth   Thu Jul 12, 2018 2:17 pm

By the way, there’s a Wikipedia page on the Rare Earth Hypothesis, and it details a comprehensive list of counter arguments.
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PostSubject: Re: Book: Rare Earth   Thu Jul 12, 2018 4:55 pm

NoCoPilot wrote:
I think scientists feel the amount of shared DNA among all god’s critters — as well as the fact that all critters have DNA — points to a single origin.
I don't give a rat's ass what they feel. I want to know what they have concrete proof of. I would suspect that we share very little DNA - if any - with the very early single cell creatures, and such things as the tardigrade.

NoCoPilot wrote:
Could there have been more than one strain of life, now gone extinct?  That left no trace in the fossil record and left no survivors?  Well to argue that, you’d really need to buy a strainer for your credulity.
I wasn't necessarily suggesting different forms of life. With the huge oceans, the same or similar environment, and the extreme time spans, I would not dispute the possibility that the same, or very similar, life forms popped up in various places at different times. I would suspect that any remnants of billion-year-old bacteria are very difficult to tell apart.

Again, I am talking about irrefutable data, not assumptions - or feelings.
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PostSubject: Re: Book: Rare Earth   Sun Jul 15, 2018 7:26 am

Science Alert, in 2015 wrote:
...new research has shown that approximately 6,000 of the tardigrade’s genes come from foreign species, which equates to around 17.5 percent.  Horizontal gene transfer occurs in humans and other animals occasionally, usually as a result of gene swapping with viruses, but to put it into perspective, most animals have less than 1 percent of their genome made up of foreign DNA. Before this, the rotifer – another microscopic water creature – was believed to have the most foreign genes of any animal, with 8 or 9 percent.

So where is the tardigrade getting all its genes from? The foreign DNA comes primarily from bacteria, but also from plants, fungi, and Archaea. And it’s this incredible variety of genes that researchers suggest has allowed the water bear to survive in such extreme conditions.
Phys-Org, in 2017 wrote:
Tardigrades became more famous recently when it was suggested that their DNA was a mix of animal and bacterial segments, making them "Frankenstein" hybrids. The new research has now laid the Frankenstein idea to rest by arguing that tardigrade DNA looks "normal," with no evidence that these special animals use extraordinary means to survive. Previous ideas that they might have taken up large numbers of foreign genes from bacteria are shown to be due simply to contamination.
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PostSubject: Re: Book: Rare Earth   Sun Jul 15, 2018 7:30 am

_Howard wrote:
And how do you know that? Is it not possible that life arose in various places around the planet over a period of many millions of years? It could have died off in some places and continued in others.
_Howard wrote:
With the huge oceans, the same or similar environment, and the extreme time spans, I would not dispute the possibility that the same, or very similar, life forms popped up in various places at different times.
Yes, one could assume the unlikely, with zero evidence.

But why would you?
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PostSubject: Re: Book: Rare Earth   Sun Jul 15, 2018 9:53 am

NoCoPilot wrote:
Yes, one could assume the unlikely, with zero evidence.

But why would you?

I was not presenting any assumptions, just possibilities. And you have used the word "unlikely" numerous times to present as facts things which have little or no evidence to support them.

NoCoPilot wrote:
I think scientists feel the amount of shared DNA among all god’s critters — as well as the fact that all critters have DNA — points to a single origin.
Wouldn't you describe this sentence as merley making assumptions?
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PostSubject: Re: Book: Rare Earth   Sun Jul 15, 2018 10:01 am

I was trying to write gently.

What I could have, and probably should have written is:
Quote :
Scientists point to the amount of shared DNA among all critters as proof of a single common ancestor.

More than half of our genetic code is the same as a banana's.  Well, YOURS anyway.
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PostSubject: Re: Book: Rare Earth   Sun Jul 15, 2018 10:13 am

_Howard wrote:
And you have used the word "unlikely" numerous times to present as facts things which have little or no evidence to support them.
Nocopilot wrote:
All life on Earth descended from a single source.  There is no second source that uses a different mechanism. So even under circumstances that have turned out to be "ideal" life arose only once.

The fact that all life shares A) DNA and B) a lot of it, is pretty strong support for the idea of a single source.  There is not a variety of life mechanisms on Earth, which has been the ideal environment for the emergence of life.  All life is traceable to a common ancestor.*

Not dozens of different ancestors.

Not a hundred different ways for inorganic matter to organize into self-replicating patterns.

One.




* - On this planet.  By extension, any life on other planets would have had to come from Earth.  Which is impossible. Except possibly with something like the Martian meterorites. But we know Mars is dead. And outside our solar system that wouldn't work.
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PostSubject: Re: Book: Rare Earth   Sun Jul 15, 2018 10:36 am

When life was getting started, most of the DNA transfer was horizontal. Once some things had stabilized a bit and evolution kicked in, then the transmission went seriously vertical. I think the implication is life got started at one location, but there were a variety of things floating around in that tidal pool (or clay substrate, or wherever it was). Once evolution got going, the evolving cell would outperform it's competition, and they would indeed disappear, likely without trace. If the insistence on a single ancestor is taken to be "only one original cell", well, no. That's not what happened.
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