An interesting phenomenon I rarely see brought up in support of panspermia is the apparent universality of the genetic code. The fact that all known life shares a common genetic structure implies a common ancestor. The lack of competing genetic codes could be interpreted to imply that life only started on earth once, or that it only made it to earth once. Natural selection is a constant force, presumably new genetic codes could offer an advantage, e.g., robustness against damaging mutation, resulting in multiple competing genetic codes.
The notion of abiogenesis occurring only once on earth seems improbable considering complex organs like eyes have evolved independently several different times. Or the fact that convergent evolution "reinvents" similar species in response to similar selective pressures. Selective pressures constantly "push" life in the same directions, so it would seem that whatever forces gave rise to life would have happened so repeatedly, but there doesn't seem to be any evidence for that.
A clean explanation for this would be that life didn't start on earth, and earth may have never had the right conditions to originate life, hence no competing genetic codes. But it was still hospitable enough to maintain life, so one universal genetic code flourishes with no emergent competition.
On the contrary. If life started multiple times on earth competitive exclusion probably took care of all other primitive forms rapidly. Even small differences, energy efficiency and reproduction lead to situation where only one form of primitive life survives.
Once the life really started going, it changed the chemistry of the earth rapidly. Similar chemical evolution starting from scratch was not possible once the life started. Eventually when photosynthesis started, biologically induced molecular oxygen caused so called Oxygen Catastrophe where Earth's chemistry turned from reducing into oxidizing. Oxygen also changed the chemistry in the seas and rocks. Molecular oxygen is great disinfectant.
Doesn't that just beg the question? If life originated on some other planet and made it to Earth, why did only the DNA-based lifeform(s) make it here?
If we assume that different kinds of life based on different nucleic acids can co-exist, then it seems unlikely that a single kind would have happened to be carried to Earth by a natural process.
If we assume DNA has some inherent fitness advantage (so that only DNA-based life survived the journey), than we don't need the additional assumption of panspermia: perhaps only the DNA-based life survived some early period in Earth's history.
Also, for abiogenesis, given that we still can't reliably reproduce something like that, and that we have never seen new kinds of life spontaneously appearing on Earth, we do have some circumstantial evidence for the possibility that it is an incredibly unlikely event, such that it might have only happened once in Earth's history.
Edit: There is also another simple possibility: perhaps given the resources available on Earth, either forever or in the period where abiogenesis was possible, perhaps DNA(+RNA) is the only molecule with the right properties to act as genetic material. Perhaps there is no common ancestor to all life on Earth, but a "forest" of inter-breeding, spotnaneously arisen original organisms, but they were all DNA/RNA-based simply because of the available resources.
> If life originated on some other planet and made it to Earth, why did only the DNA-based lifeform(s) make it here?
The problem is that we can't see through survivor bias. Maybe life has lots of possible configurations and only DNA-based life was highly successful on Earth. It's also possible that non-DNA-based life exists, we just haven't noticed it because it is too small, too slow, or too something-else that we missed it.
> perhaps only the DNA-based life survived some early period in Earth's history.
My thoughts exactly.
Also, the emergence of an intelligent, technologically capable species (i.e. humans) is a relatively very recent occurrence in Earth's history. It's as if it just suddenly happened, even though there was a long history of dynamic evolution for many hundreds of millions of years. But the sudden emergence of humans doesn't mean there had to have been an external cause.
What is surprising to me is not just that it's always DNA, but that it's always translated in the same way (codons to aminoacids, and the mapping function is fixed). Is there any simple reason for that?
Changing it makes the organism non-viable, so a change there never enters a population. In contract DNA/RNA-mutations can be viable so happen all the time.
That's what we currently know based on what we discovered so far. You can't really rule out the existence of a relatively advanced civilization - although it might advance in a completely different way than ours - whose remains lie buried somewhere under the Mariana Trench for example.
It'll be easier to send a DNA without a life-form. A life-form is very expensive (need to eat and breath) and not necessarily adaptable to earth conditions.
A very well encapsulated and protected DNA will land on earth and create the most adaptable life-form there through mutation and competition. All of these life-forms have the trait of wanting to pass their genes, multiply as much as possible and then spread around.
So the engineers probably had that in mind:
1- Extremely small: Probably so that they can propel it through space at very high speed + propel lots of it.
2- No maintenance required.
3- Will create a life-form if possible that is adaptable to the target country.
4- The life-form has the property of multiplication, and spreading.
5- The new superior life-form will propel more of these DNA.
>A life-form is very expensive (need to eat and breath)
Not necessarily, many organisms can undergo cryptobiosis, where all metabolic activity ceases, and then restarts when conditions are appropriate (sometimes decades later).
Fungi & bacteria can turn into spores & travel through space. There was also recent article about fungus growing on the outside (!) of the space station.
There are also things like tardigrades [1] which are pretty much the epoch of survival evolution. And this is all just stuff on our little planet. It's hard to say anything about life on a large scale, since we are basing everything we know on a sample of one. But it's probably safe to say that we have not even scratched the surface of what's possibility through just plain old evolution alone.
Another possibility often overlooked is that there's life out there on completely different timescales than we can recognize. We're only likely to notice those within a few orders of magnitude to our own.
Maybe DNA is simply geared well compared to other mechanisms for the timescales we're interested in.
This is an interesting thought. Plants are actually very animated at their own timescale, adapting to their environment through phenotypic plasticity. However this happens far too slow for humans to notice without a time lapse.
>why did only the DNA-based lifeform(s) make it here?
Possibly genetic drift. The odds of life making a successful journey to earth seem very poor, so maybe it was a small population that only made it here once, and then began to speciate.
OK, I'm not any kind of expert in this area, but is RNA not considered an early form of genetic coding? And DNA evolved from it? Also, don't viruses replicate by "borrowing" the mechanism of their hosts?
My understanding is that life visible to the naked eye is a minority of life forms on the planet. Maybe other forms of genetic coding than DNA are more prevalent than we anthropocentrists think.
Or there might have been a catastrophe that killed off almost everything. In which case the organisms that survived might not have been very widespread before the catastrophe, or typical of what existed before the catastrophe.
If a large asteroid hit the Earth today it might be only a few subterranean extremophile microorganisms that survive. Asteroids were more frequent during the early history of the solar system.
Or that the self replicating systems of molecules merged into one (organic life on earth is base 4, containing two sets of two complimentary pairs - it potentially could've done it with just one of those sets),
Or that the same self replicating systems were the most (or only) stable systems that could be forged from the locally available inorganic molecules.
This is true, but if that did happen presumably we could find some evidence for it, since it would be a relatively common occurrence during the period where earth was capable of abiogenesis. Because if it occurred at least twice, it probably happened many more times.
The counterpoint is that the fossil record is very incomplete when going that far back, seeing as tectonic subduction literally wipes the slate clean.
Why are there no other human species (anymore)? Because there was no room for them anymore and they could not compete. In the same way, it it very well possible that life utilizing other genetic code got stamped out (early on).
Interesting but your thought experiment only "proves" that the likelihood for something that can replicate itself to emerge is far smaller than the likelihood that that something eventually evolves into something extremely complex. Nothing more imho.
Probably, although a not-widely-accepted but interesting theory is that there might be microorganisms that we can't detect because we're not looking for the right things (because the things we look for are based on macroscopic life and things that interact biologically with it).
I think it's unlikely because it seems hard to imagine no interaction between the kinds of life ever after billions of years of coevolution. On the other hand, there are tons of microorganisms that do share our biochemistry that we haven't noticed or identified yet, so maybe it's not impossible to imagine some that don't that we also haven't noticed or identified (since it would be more difficult for us to recognize them).
You can't quantify the complexity of something like an eye. The eye could actually be very very simple to form, due to circumstances and variables outside of our knowledge.
Scientists have discovered that the animal with the most genes--about 31,000--is the near-microscopic freshwater crustacean Daphnia pulex, or water flea. By comparison, humans have about 23,000 genes.
One of the most astonishing features of the D. pulex genome is its compactness: despite being around 200 Mb in size (around 16-fold smaller than the human genome which is 3,200 Mb in size); its 12 chromosomes contain a minimum set of 30,907 predicted protein-coding genes, more than the 20,000–25,000 contained in the human counterpart.
The unexpected complexity of the water flea genome compared with a human genome illustrates unknown dimensions of complexity that we cannot fully comprehend.
What this means is this. You do not know how complicated the original template of life is... the complexity for the first genome to produce instructions that allow for self replication to kick off evolution may be so complex that it could only happen once on our planet, once in this galaxy or once in the universe.
We simply do not have the knowledge to know the probability space of life on earth or life as we know it.
Methane is not nearly as good a signal to look for life as larger organic molecules are. Methane can easily be created by abiotic processes and remain stable when not directly exposed to radiation or oxidizers indefinitely: this happens on earth, and we do have oxidizers everywhere. Larger organic molecules are less likely to have abiotic origins by any known process and are almost always much less stable.
On earth you can test whether methane has organic or abiotic origins by testing its isotopes. We can do this because we understand decently well how the geologic past of earth unfolded. We could probably do a similar thing for mars
>The Earth is 4.5 billion years old. And the universe, at least based on estimates from the Big Bang, is something like fourteen billion years. So, if life evolved somewhere else, that buys you about ten billion years of time.
Obviously I'm no expert, but you have to keep in mind, it took a decent amount of time for non-violent stars to become the norm / stellar neighborhoods to calm down and for overall metallicity to become significant enough to allow for the formation of terrestrial planets.
Carbon dating depends on the details of the biosphere. C14 is produced in the upper atmosphere by cosmic rays, and then (once it's fixed into a tree) it decays with a half-life of 5700 years.
So if a chair is made of wood with a C14 fraction that's 1/4 of the atmospheric fraction, you can say it's 11400 years old (as a first-order approximation -- the actual science calibrates out a lot of things.)
But it's not clear that the presence of life would affect the C14 ratio in the upper atmosphere. The fraction should be where the production rate (proportional to the cosmic ray flux and percentage of nitrogen) matches the decay rate (1/5700 / year)
But Mars also has an atmosphere (although thin) of CO2 exposed to cosmic rays, no? So we could determine if there is some sort of CO2->methane cycle by seeing if there is any appreciable amount of C14 in the methane we find on Mars?
Apologies if that's a stupid question, I'm out of my element here
C14 is actually produced from cosmic rays hitting nitrogen, not carbon. The impact turns a proton into a neutron. Both CO2 and CH4, once the chemical bond is made, only decrease in C14 fraction as it spontaneously decays.
Astronomers are pretty good at deducing things from the few, imprecise measurements they can make from millions of miles away, so you should never rule out them being able to figure out something. But it'll depend on a lot of assumptions.
Of course, not. Universe is infinite in size, time, and scale.
The farthest object we can see in _visible_ light is at least 32bly away, so it is at least 32by old, or just 13.4by old by mainstream theory called «Big Bang».
>> and for overall metallicity to become significant enough to allow for the formation of terrestrial planets.
You don't need much metallicity before terrestrial planets start forming. They would certainly be rarer in the early universe, but stars filter material very quickly (pushing light stuff further away, stripping new planets to their dense cores). So even with 10% of today's metallicity, there would still be plenty of terrestrial planets.
(lol. My spellcheck corrects "metallicity" to "Metallica".)
There's a great Imax film called Dark Universe were Neil deGrasse Tyson explains how it's likely that if an observer in the vicinity of Earth were to travel to the furthest point in space that we can observe, they would see the exact same thing in all directions that we see from Earth. They would see beyond our Earthly visible region of the universe and see more universe beyond it. If the observer travelled further again, they would again see the same thing in all directions. Basically we can only see so far, but it's likely no matter how far you travel in the universe, you'd just see more expansive universe in all directions. None of this is proven of course but it's an interesting and plausible idea.
The question of where life originated will have no good answer until we figure out how to create life from basic components. We still don't know how to do that yet.
Yes, Urey and Miller showed that you could get simple organic molecules by passing an electric charge through gasses thought to be present around the early Earth.
But going from low molecular weight inputs to life is a vastly different problem. You could think of it as the most complicated bootstrapping problem in the universe.
Nobody has managed to do it in the lab, either. What can't be built from scratch can't be understood very well, and therein lies the problem.
If we had a stepwise procedure for building a self-replicating, self-feeding organism of any level of complexity from base components, we would know exactly what to look for.
Until then, the idea of sending a DNA amplifier to Mars isn't a bad fallback position.
If we succeed, it would be the most important breakthrough in the history of humanity, and would have very deep existential implications. It wouldn't definitely rule out a supreme being as the creator of the universe we live in, but it would make the gap between us and that being much smaller than millions of people on earth believe now.
> It wouldn't definitely rule out a supreme being as the creator of the universe we live in
Yes, that's exactly what the believers will tell you even then.
Think about it: the science knows today that every atom in our bodies is either produced 13.8 billion years ago (is it is H2) or in the explosion of some star. It's true for every element that you learn in the chemistry class. We know that the Earth is 4.7 billion years old. And we already know that humans share 99% percent of the DNA with chimps. And that 0.006 billion years ago on the Earth neither humans nor chimps existed, but their common ancestor.
Do people who believe in a "miracle" divine intervention that supposedly happened only some 0.000002 billion years ago (or even only 0.0000014 billion years ago) when only at that point their deity became involved with the believers, do these people in anyway feel affected by all that? No.
Compared to what we know today, their religious "messages from the deity" obviously prove that they were written by plain humans of older times, because these messages don't contain any knowledge we have today, but instead the "eternal" truths like "at the end of the day the Sun sets in the muddy pond" or that the Earth's sky is a solid dome: https://en.wikipedia.org/wiki/Firmament#Biblical_use
Even though we have only one datapoint, I wonder how much can be inferred from this planet's life characteristics.
First, life came about rather shortly after the planet was able to sustain it. So this timing is either very unlikely, or biogenesis has a high probability of happening, or the initial life got seeded from off planet (from a very rare initial phenomenon).
Second, all the biosignatures from all forms of life are similar. For example, corality (left handed versus right handed molecules) is the same in all life examples we had. So that means that biogenesis happened only once. Otherwise we would have multiple unrelated examples (yes, there is the possibility that one form "ate everything else", but we have all kinds of variety instead of only one organism becoming dominate). So if biogenesis happened only once (for all of our bio examples), then it is either a rare event that just happened to occur very shortly after the planet cooled, or it happened sometime before Earth was hospitable for life and elsewhere in the universe.
Does any of this make sense? Or am I reading too much into the tea leaves?
I think this is why those of us interested in this are very much hanging out for results from our explorations of Mars. It's pretty much our only opportunity to see if life emerged twice in the same planetary system under similar conditions.
What are the chances?
On the positive side Mars had pretty much all the ingredients for life (chemically) and had a long enough period for at least something intersting to develop. On the negative side, the environment was still different to ours (gravity, climate, whether, etc).
We still aren't clear on what conditions need to met for like to get going. This is why both positive or negative results on Mars are exciting.
My favourite theory is that life actually takes ages to get started so much so that it started on Mars and finished on Earth (from Mars being impacted and debris making it to Earth). Kind of like a passing of the torch.
Mars: "Crap. I'm dying. Hey Earth, can you take this thing I've been working on?"
Earth: "Yep. No worries. I got this. Will try to send some back in a few billions years. Take care."
>It's pretty much our only opportunity to see if life emerged twice in the same planetary system under similar conditions.
Well there's also Titan, Enceladus, and Europa right? Titan is by far the most exotic but it could serve as an example of what happens when life evolves under a radically different environment. It has some complex organic molecules there and a very interesting/varied geology and topology. Enceladus and Europa could exhibit very similar conditions to those of the deep sea on Earth as well, which could very well be where life originated. Also at one point even Earth could have became a "snowball" planet and still able to sustain life: https://en.wikipedia.org/wiki/Snowball_Earth
Unfortunately, we cannot be certain life on other planets in our solar system did not originate on our own. It has been determined that meteor that killed the dinosaurs possibly seeded planets and mas far out as Saturn with microbes.
> So that means that biogenesis happened only once.
No, it just means that only one lineage did well enough in competition that we can find examples to study its biomarkers.
Or that there are factors we don't yet understand that favor similar biomarkers in independent lineages.
Or...luck.
> if biogenesis happened only once (for all of our bio examples), then it is either a rare event that just happened to occur very shortly after the planet cooled, or it happened sometime before Earth was hospitable for life and elsewhere in the universe.
Or biogenesis itself changed the environment in a way which made it no longer conducive to biogenesis.
> So this timing is either very unlikely, or biogenesis has a high probability of happening, or the initial life got seeded from off planet
Or life can only appear on young planets (maybe while it's hot), or some astronomical event triggered it (Moon?), or there are enough hard steps on the path to intelligence that it couldn't appear if life took too long to form. It's a good question anyway.
> So that means that biogenesis happened only once.
Life has repeatedly shown a tendency of consuming all available raw matter and populating every niche. Biogenesis happening only once is completely non-surprising.
But I would be cautious calling a billion years or so "shortly" even in the context of a four-something billion year old planet. A billion is a very big number, especially when you multiply it by how many trillions of little potential multi-dimensional petri dish situations were around for each second of each year of those billion years.
And it could have happened only once because once established, it tends, on a timescale orders of magnitude shorter than a billion years, to squelch the appearance of other forms, at a minimum by guiding them along with its template, or alternatively by smothering them.
But this all says nothing about whether it happened on Earth or elsewhere. Either one could still be true.
I thought it was more like 400 million years between water and the first life -- and that is the oldest life we have evidence of. It could conceivably take a couple hundred million years for life to go from the initial spark, to having evolved enough (and have a sufficient population) to leave detectable fossils.
I think we can wrangle a few more datapoints if we are creative with perspective, for example, we have extremely good evidence to suggest that the planet Mars is currently inhabited by alien robots.
The new life may have spread very rapidly and ended the conditions that abiogenesis required, in a sense it had a massive "leg up" on the "evolution" of non-living compounds, and we already know that species that get out competed are quickly driven to extinction.
Consider for example that abiogenesis is probably impossible in an atmosphere containing oxygen, because everything gets oxidized too fast, although free oxygen in the air happened quite a long time after life arose.
To abiogenicly form long chain carbons you need a free oxygenless reducing environment, oxygen from the new microbes could have easily poisoned all the viable sites for abiogenesis.
> The reason this news registered among scientists is that methane is often a sign of life; although the gas can be produced by various chemical reactions, most of it comes from animate beings.
This doesn't sound right at all. There are literally lakes of methane on Titan. The atmospheres of Jupiter, Saturn, Uranus, and Neptune all contain significant methane--it's what makes Uranus and Neptune blue. Pluto has methane ice.
IIRC from the original reporting, some recent research suggested that there's more methane than contemporary geological models of Mars predict. Which likely says more about the [non-biological] deficiencies in those models than it does the likelihood of life.
I'm not an astro-anything, so feel free to correct me. But it's telling that the interviewee never comes close to confirming the reporter's claims; they're simply quoted as saying, "I think probably many people would like the idea of methanogens on Mars", and "the idea that they might be related to methanogens on Earth is not crazy." Isaac Chotiner is a name I'll try to remember so I can avoid his articles.
There is ancient methane in those places. But something on Mars is producing methane.
Methane breaks down pretty fast in Mars' atmosphere, and yet it reappears there occasionally. That can certainly have non life explanations. But they're fairly exotic.
IIRC, methane should not persist on Mars, and theres not been a global phenomenon detected that can explain its occasional detection. Therefore, transient detections are considered indicators of possible life, since nothing else seems to explain it.
It's not that it's rare everywhere, it's that it's rare on Mars.
The author (Chotiner) has indeed made a mis-statement about the state of knowledge on planetary CH4, but that doesn't pertain directly to most of the article's content, which is about DNA/RNA.
As you point out, the state of Mars CH4 is quite complex and extremely controversial, with 3 different measurements (Curiosity + 2 orbiters), low concentrations observed (~10 ppb) and multiple physical processes going on. Here's a pretty good recent summary:
Minor fun fact: Fred Hoyle, possibly the most public proponent of Panspermia in the last few decades, also coined the phrase "Big Bang theory" to dismiss an alternative to his preferred Steady State theory of the universe.
Sometime champion of arch positions, he was once quoted as saying "it is better to be interesting and wrong than boring and right".
It is a pretty out there theory, but at one point the universe would have cooled enough from the very hot big bang origins to allow liquid water to exist everywhere, but not cooled sufficiently to cause everything to freeze.
If any rocky planets existed at the time, maybe that was the origin of life which was much more able to spread widely due to the small size of the early universe.
I suspect that a formal, anonymous survey of astrobiologists would find that the idea is far more prevalent among experts than Ruvkin's estimate that "one per cent would buy into the idea of life spreading the way I’m sort of promoting it".
It's just risky to express, for cultural reasons, until there's tangible proof. Many scientists are temperamentally reluctant to speculate without conclusive evidence, especially given the penchant of the press to sensationalize any such theories. It'd raise difficult questions about our place in the universe, and possibly popular fears.
Thinking science had already settled on a consensus that simple life is everywhere might even paradoxically reduce funding for new missions to test the idea. After all, there have been tantalizing hints of life-processes on Mars going back to the Viking lander experiments of the 1970s – and yet we still can't seem to send a lab package that'd definitively answer the question! But the hope of figuring it out motivates new missions.
There was this interesting article posted here some time ago that plotted the complexity of life found on earth vs time, and by using the right methodology and logarithmic scales, it arrived at a nice linear line over many data points and several billions of years. And the line converged to a 0 point, but that point was a few billion years before earth was created 4.5 billion years ago, adding more credibility to a panspermia theory. Can anybody remember that article and its url?
What if we began a 'seeding' experiment, suppose there could be more life but isn't because life doesn't spread well in the vacuum of space. What if we began sending bacteria and other micro-organisms to potentially habitable planets via tiny spacecrafts. If we were to seed as many planets as we could possibly find, then eventually (in a million years or so), this entire galaxy could be teaming with life and the fermi paradox will be refuted as there will be no way not to see the life that is out there. We may still not be able to traverse the stars by that time at least not without generation ships but our descendents could at least wave from afar and maybe communicate across vast distances with intelligent life that we today helped seed/create.
I'm not really on board with the idea of just blindly infecting all habitable planets with Earth life. We should look but don't touch until we have some clue what the heck we are doing and why.
This right here. Provided a body that can retain and emit heat from a star, it intuitively makes sense that given correct conditions the warm body would trend towards more complex head dissipation mechanisms.
Life is just a more complex — and sustainable — exothermic reaction than standard oxidative mechanisms (fire).
By the way: this isn't a new concept. Jeremy England at MIT has been espousing the concept for a while [0].
Seems like a bad bet given that from the evidence we have so far, life is an unbelievable statistical anomaly. There's no reason why life had to have evolve on Earth or survive long-term; it is sheer luck that we haven't been hit by gamma ray bursts or a supernova or any one of the other nasty, super-deadly things the cosmos could throw at us.
I wish he had posited a theory for how life from Mars would have even gotten here. Some comments have said an asteroid hit Mars and then redirected to Earth with early life on it? Is that possible/likely?
Maybe we could test this someday with petri dishes in orbit. Outfit satellites with catch basins that funnel into various potentially habitable pods and see what grows. Park them in a lagrange point for better isolation.
But if panspermia is true such critters in the cosmic wind would be cousins of life on earth, and it would be difficult (impossible?) to distinguish an alien microbe from domestic contamination. If it's false we see nothing, and wonder if it's because space is sterile or we're just not yet offering it the right primeval soup.
Panspermia doesn’t only work (if it works) by microbes floating in space. Rather the idea is that microbes are embedded and somewhat protected in asteroids which then land somewhere habitable. The occasional close brushes between stars and their asteroid clouds provide the medium where microbes could be transferred between stars quickly enough to launch and land before the radiation destroys any hope of survival (like 20 thousand years.)
What I find more interesting is the potential frequency with which new life from origins other than earth has been mixed together with what is here. A lot of stuff falls into our atmosphere and I doubt we know where all of it comes from.
I commented today on a bit of dermatological discussion where they claim our skin has a biome of bacteria. This is in addition to the gut biome that is starting to look like it contributes to a wide array of human health issues. Part of me wonders how much of those biomes (e.g. how many bacteria within that biome) are from sources outside our planet.
Well, if they did, then the fact that the basic DNA machinery is the same implies that we and the alien bacteria had a common ancestor somewhere back there.
> implies that we and the alien bacteria had a common ancestor
My intuition suggests that wouldn't be necessary. For example, if I saw a snowflake like structure on another planet I wouldn't be surprised since that kind of structure is a result of the physical properties of the elements involved. It could be the case DNA is very similar in that respect, a mathematical necessity given the properties of hydro-carbons. I suspect the science on that issue isn't settled.
Not a biologist, but my impression is that the concepts involved in DNA/RNA, and especially the machinery around it, are so complicated and have so many degrees of freedom, that snowflakes, which are rather simple crystallizations of only water, may not be an apt comparison.
Maybe it's closer to, say, someone having just learned the basics of analogue electronic circuits (but nothing beyond that), going into a basement, and coming out with a fully formed IBM-compatible PC with USB ports.
Again, not my field, and I'm happy to be taught otherwise on that one.
If you're saying that DNA is too complex to have evolved on earth, where else would it evolve? Panspermia just puts the origin of life in another location in the universe. It doesn't answer any questions about how that complexity would evolve.
Thats not what is being said, hes saying that if life evolved twice, it would be almost certainly completely different and "incompatable" at a molecular level.
Consider the design of processor architectures and operating systems, similar design goals but typically one cant run code from the other.
I'm thinking someone may have proved by construction that DNA isn't the only possible code. Didn't someone make "artificial" DNA by substituting something chemically for some of it?
You appear to be saying: "It is possible something DNA-like could be constructed from different materials in different codes and it would still be viable".
I was saying: "It is possible hydrogen, carbon, oxygen, etc. will form into DNA as long as suitable conditions exist and this is common enough occurrence that multiple sources of DNA independently developed across the galaxy/universe". I only say this to assert that the appearance of DNA from a source other than Earth should not necessarily imply a common ancestor.
Given the complexity of DNA I think it is near obvious life did not originate on earth. If you assume it did not originate on earth that gives you at least another 10 billion years to work with.
We haven't yet collected evidence that says terrestrial life originating on Earth is impossible or even unlikely, nor has anyone produced an alternative explanation that doesn't involve introducing additional external factors. It may indeed turn out to be incorrect (although I wouldn't be on it), but it's in no way comparable to geocentrism which simply couldn't be reconciled with experimental observation.
If you lived on Long Island, and didn't know where humans came from, but you knew about how big the earth was, would it be rational or in accordance with Occam's razor to assume homo sapiens originated in your vicinity? Most people would in fact be wrong if they even assumed humans originated on their continent.
I feel like people get sidetracked into talking about evidence, when the issue is that life originating on Earth should not be the default assumption. I feel like someone more lucid than me could make a good Bayesian sort of argument.
It's not exactly the default assumption though. Historically the default is that life was magicked into existence or came here from a previous world or some other creation story. The current abiogenisis story of molecules->proteins->cells->Life is relatively recent.
The other thing is that because we have a fairly... ?linear record of evolution: if life did originate somewhere else it got to Earth in something like prokaryotic single cell form 3.5 billion years ago. Which means we can't use the idea of panspermia to tell us anything. It's neither predictive or explanatory. It tells us nothing of how or where life did originate or how life got started on Earth or what or where we should look for or expect in the rest of the Universe.
The current theory suggests life could pop up extremely quickly, which itself already implies life is likely to exist in the rest of universe on some level so panspermia doesn't even mean anything with regards to how likely life is anywhere else.
The difference is that you've never seen evidence of humans living anywhere outside of Long Island.
Clearly if you see humans living all over the globe they almost certainly originated elsewhere. But if humans only live on LI, then it's reasonable to conclude they originated on LI.
Given the gaps in our knowledge of early life, a definite statement in either direction is unlikely to be in the offing. However, terrestrial life is decidedly unlikely. Life is decidedly unlikely, and no one has so far suggested an even remotely plausible mechanism for abiogenesis on Earth. This means if you are betting, an extraterrestrial origin is the safe bet; if the odds of abiogenesis are so slim, while life once formed is tenacious and hardy enough to travel through space, it is more likely life traveled here.
It's not clear to me how much panspermia really helps with the abiogenesis problem.
Panspermia might increase the window of time and set of planets where the abiogenesis of Earth's life could have occurred. But are there any grounded estimates of the factor by which it expands those windows? For example, is it safe to assume that if abiogenesis occurred on any planet in the Milky Way before the Earth formed, then samples of that life would have reached Earth within say 200M years after the Earth formed? That would require estimating the rates at which samples leave the planetary gravity well, how likely those samples are to reach Earth's gravity well, how likely the samples are to survive the trip, etc. I have not yet read any detailed estimates of this kind (but I'd like to!).
Another problem ... if the answer to the previous question is "yes", then that might expand the abiogenesis window by, say, 10^13 (say 200B planets and a 50-fold increase in the available time window, ignoring other galaxies). Is that enough to flip abiogenesis from "not likely" to "likely"? 10^13 is a big number but if abiogenesis is extremely unlikely then we're already dealing with very low probabilities. In other words, panspermia only helps if the probability of abiogenesis occurring on a planet in a given time window falls in a certain range, and in context that range might be quite narrow.
We have a seemingly pretty firm age for the universe of 13.7 billion years; earth is 4.5 billion years old, and life on earth is at least 3.5 billion years old.
So I think the step of going from nothing to the first single celled life is implausibly large to happen in a few million to a billion years, and to me, Occam's razor would imply that something necessary was developing during the previous 9 billion years, such as the development of chemotrophs underground in deep space.
Hence why I draw exactly the opposite conclusion. It's my thinking that if life existed throughout our 13 billion year old galaxy it would have evolved to saoience and consumed all available resources long before the rise of humanity. The fact our planet hasn't been strip mined or colonized is, absent hard data, good reason to believe we're alone.
Why would aliens need to stripmine earth? What do we have that they need so much that isn't abundant elsewhere? There's an asteroid out there alone between Mars/Jupiter with possibly more Gold than all the gold on earth combined. Likely there are billions and trillions of these things. Advanced races probably can create just about anything from base components and minerals easy enough so they'd just need to mine whatever is nearby.
Now, they could possibly need stars and galaxies (i.e. all stars in a galaxy) to capture as energy for large enterprises, but pretty sure there's not kardashev 2-3 races in our vicinity --we'd surely know it if there were. Energy is probably the only resource that aliens might go looking for, everything else is presumably abundant.
I think we should really begin colonizing and expanding our biome to other planets asap so we can possibly beat other civilizations to it. I'd trust a civ that grew from our basic microbes over one from silicon -because we know how we turned out at least, we can't be so sure about them.
I'd be interested to see (if I could live long enough) what would happen if we sent microbe probes to every semi-habitable planet we find, then wait 1 million years to see what life progresses from each planet.
There's a big difference between microbial life and intelligent life.
Bostrom and others have argued that, based on how long evolutionary milestones took on Earth, the "difficult step" is actually complex multicellular life.
I agree with your conclusion that Fermi's paradox simply means there is no other intelligent life, though I think the jury's still out as to where the filter is.
Indeed. There's quite a lot of debate surrounding it and a couple of mathematicians arguing we haven't hit the filter yet, but I maintain hope we're past it. It wounds my sci-fi loving heart to say this but humanity is probably better off alone.
"It could be that I’ve never had a religious thought in my life."
The very idea that life has to originate from somewhere/somewhat and cannot just happen sounds like a secular reformulation of a typically religious statement.
The notion of abiogenesis occurring only once on earth seems improbable considering complex organs like eyes have evolved independently several different times. Or the fact that convergent evolution "reinvents" similar species in response to similar selective pressures. Selective pressures constantly "push" life in the same directions, so it would seem that whatever forces gave rise to life would have happened so repeatedly, but there doesn't seem to be any evidence for that.
A clean explanation for this would be that life didn't start on earth, and earth may have never had the right conditions to originate life, hence no competing genetic codes. But it was still hospitable enough to maintain life, so one universal genetic code flourishes with no emergent competition.
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