Why woud a pop science author make free advertisement for 23andMe? genome sequencing and genome ancestry are older than some random company which does not even do full genome sequencing 23andMe only does genotyping...
the only avoidable reason it's more familiar to the public is because of all the previous pop sci authors preferring to say 23andMe instead of "genome sequencing" and "genome analysis" etc
if enough pop sci articles stop dumbing down the public would better understand what this is all about...
Genetics is both popular-culture topic and part of high school curriculum nowadays, so it doesn't need too hard explaining, and the kind of person that wouldn't understand the explanation probably also doesn't know what 23andMe is (which reinforces this being a sneaky ad).
also like calling making yourself fries making yourself "french" fries :P
But the developed descendants of the first life outcompeted the later ones immediately so we only ever discover life that is related to us.
But if we search where life related to us couldn't go - maybe we'll find completely independent trees of life?
Of course, sterile experiments in a laboratory might give us a lower bound on just how trivial the requirements for abiogenesis are. So far nobody has noticed it happening in a lab, despite more than a few trying.
Consider endosymbiotic hypothesis. The evidence is that it seems to have occurred at least multiple times (mitochondria, chloroplast, nucleus) in different geological eras. I would expect if life had a high enough probability to happen multiple times, it could occur in different geological eras.
You can’t extract a (meaningful) probability from a single sample no matter how intuitive it seems.
It's possible to get insights from a single sample. Science journals, especially in fields like medicine, do it to talk about atypical case studies. It's a good learning experience, too, when reflecting on the structure and context of those edge cases.
For instance, people frequently talk about the probability a given voter will cast the deciding vote in an election.
Indeed, posit n repeated trials over a given time line, it seems equally implausible that an observation of a positive outcome earlier in the timeline contains no information on implied probabilities relative to a positive observation occurring later in the time lines.
How would you prove that though?
A theory that's not disprovable isn't valuable. See Russel's tea pot.
Edit: phone auto typo.
More likely, in all seriousness, that life originated in Mars and came here via panspermia during the early bombardment period. Mars was warm and wet while Earth was still suffering cataclysmic impacts and only relatively mature examples of Martian life (e.g. bacteria) would have survived the trip.
If life originated multiple times on Earth there would be evidence for that. Heck if life originated on Earth there should be evidence for that, which there bizarrely isn’t.
Viruses are much simpler, for one. Or perhaps you don't consider them to be life forms. Either way, if you understand how bacteria operate at even the most crude level, you'll realize that they (all bacteria, collectively; individually, they're specialized, of course) eat basically anything and everything in terms of organic molecules.
What are the ramifications for this? It means any sort of abiogenesis that takes place today needs to somehow avoid being immediately eaten by bacteria and then continue to survive for potentially millions of years in order to evolve into an advanced life form. Needless to say, this seems to be vanishingly unlikely these days.
Many eukaryotes carry transposable elements, which are DNA sequences that exploit DNA and RNA machinery to create numerous copies of themselves in the genome. That is, they're like viruses, except that for the most part they don't get transmitted, except through cell replication.
But I believe that there are examples where transposable elements do get transmitted like viruses. Or sometimes as parasites on actual viruses. So that's separation. And there are also examples, where viruses become integrated into the genome. Sometimes reversibly, as with HSV, which is why you can never really cure an HSV infection.
Anyway, it seems most likely that simple stuff evolved first, and then combined into more complicated stuff. But the simple stuff has survived, to some extent, as transposable elements and viruses.
That is what makes all the difference. It's an enormous leap to go from a couple of amino acids to something able to survive and evolve and compete against other complex organisms.
It is possible, I suppose, that remnants of other "starts" could have survived somewhere that's totally inhospitable to ours. Or perhaps, so different that they're not even food.
Discovering them would be very difficult, though. I mean, we have a hard enough time with all those "unculturable" bacteria. We only know them now through DNA technology.
Anyway, I think we're still talking an order or two of magnitude difference between DNA/RNA (and supporting infrastructure) and the point at which proto life 2.0 transitions from "background chemistry" to "bacteria food".
I've so far read in detail about only one hypothesis on abiogenesis (in aforementioned book), and there the necessary environmental conditions were since changed by very existence of life.
Something like "proto life A" and "proto life B" get incorporated into "proto life C" to make "proto life D". And after enough steps like that, you get life.
In a sense, that's a trivial statement. Because that's just what chemistry is about. Or deeper, just what physics is about. But regarding GP's point, I'm saying that it's too simplistic to argue about whether life originated once or multiple times. Maybe there were multiple beginnings, which combined and interacted in multiple ways. As we know now for evolution from primate-like ancestors to modern humans.
The machinery of DNA and RNA is one example. It's a chunk that worked. Or maybe multiple chunks that got combined. And the same with ATP synthase and the flagella motor.
Think in terms of lines, not individuals. Mutations are acquired on individual level, so out of a whole family of bacteria, one individual may reproduce into something slightly different, while the rest of the family continue unchanged.
(Modulo fast mutation rate in bacteria, and modulo the wide use of horizontal gene transfer. The more I learn about bacteria and archaea, the more I feel there's little point in trying to find the tree of life.)
I am always surprised at the number of people willing to make assertions like this. Panspermia itself is a hypothesis. Panspermia from Mars specifically is a huge stretch. There is absolutely no basis for claiming that this is likely.
> only relatively mature examples of Martian life (e.g. bacteria) would have survived the trip.
Why do you make this assumption? Life complexity is not generally correlated with space survivability. At least not positively correlated.
I would point you towards the research that went into ALH-84001. Even if you don’t buy that it was an example of fossilized Martian life, analysis of the thermal stresses it suffered from ejection from Mars, time in flight, to impact on Earth showed conclusively that it was survivable. Had it been the other way around (Earth to Mars) it would have seeded Mars with life.
We’ve shown that rock which could be carrying microbes can and has been exchanged between earth and Mars. Lots of such specimens land each and every year. You can quibble that we haven’t found actual life yet, but
(1) lots of people think that’s exactly what ALH-84001 was.
(2) we don’t have that many samples of Martian meteorites. We know lots of it falls every year, but most is either not found/collected/identified or contaminated on impact.
(3) Mars meteorites falling now, ejected from a dry and sterile surface, would not be representative of what was falling in the early earth that was thrown from a wet, habitable Mars.
To be clear, from an astrobiology perspective a negative result from the search for life on (or from) Mars is harder to explain than finding it. Both panspermia and second abiogenesis are simpler explanations.
I think "from Mars specifically" is something that requires evidence to treat as likely.
However, I don't think "Earth is the unique place where life originated" should be the default assumption according to Occam's razor. Nothing nearby is anything like Earth on the surface, but it's not speculation that there were billions of years and unimaginably large amounts of volume for life to develop underground in the rest of the universe, where it's warm and sheltered from radiation. The idea that nothing happened until Earth became "Earth-like" isn't the logical default. Naming the contrary of this far-fetched claim "panspermia" doesn't make it a "thing" that needs a "basis". By analogy, I would say that a creationist claim of humanity originating with Adam and Eve is what needs evidence, not the contrary assumption, that all humans had parents.
The Moon appears to have formed without the resources to sustain life as we know it. There are great candidates for life in the outer solar system (Europa, Enceladus, Titan), but ejecta from these mostly get eaten up by the giant planets.
The only likely candidates for solar system origin panspermia is Mars, and maybe Ceres. Only Mars (and Earth) had the long wet period with open oceans, geothermal heat sources, plate tectonics, and sizable atmosphere. Mars isn't the only option, but the odds are way, way higher.
At least for life as we know it. But the point of panspermia is that it would be life as we know it ;)
Perhaps not 'likely', but there is reason to believe it's plausible. Extremophile organisms plausibly capable of surviving in space for the requisite timespans are certainly known to exist, and there is also known to be small amounts of material transfer between earth and mars.
The part I wonder about is whether any extremophile organism could survive the doubtlessly violent circumstances that are involved in ejecting rocks from either the earth or mars gravity well.
Exactly because they were outcompeted?
If I understand correctly the early life would be living much slower and reproducing much less efficiently. So it would just get eaten by more efficient life that had millions years more to develop.
How do you know life isn't arising every few million years somewhere on Earth and immediately being eaten by something before it could reproduce 10 times? What evidence would you expect to find that it happens?
We can see all this in the rocks around us. TFA is about finding such things in mines deep underground. Remarkably we can even find such evidence in fossils more than 3 billion years old.
Yet.. the oldest things we find are microbial sedentary structures indicative of remarkably complex life: with genes, ribosomes, proteins, etc. We know this because those same life forms still exist today and leave pretty much exactly the same tell tale signs. We call them simple life, but that is just by comparison. They are insanely complex compared to the random geochemistry going on alongside them.
Why don’t we find ANYTHING older than this which differs from expected geological features? Where are the fossils from the origin of life? Also, the oldest fossils we have are of an organism that still exists: simple life remarkably doesn’t get competed to extinction. It turns out that simpler life forms have some advantages from their simplicity allowing them to be more adaptive to micro niches. So why don’t we have extant examples of even simpler life forms, surviving where complex life doesn’t fare as well?
If it happened fast enough (maybe up to a couple million years, I dunno), there would be an infinitesimal amount of evidence that could easily be missed forever. If it only happened once, we would need to look at exactly the right rock, across the whole planet, with a microscope. Or it could already be recycled into the mantle. Ocean crust doesn't have a long lifespan, IIRC.
Dunno about the rest, but I do think that rapid advancement from proto-life to something pretty sophisticated is likely, wherever it starts.
How do we know if the tell-tale signs produced by modern simple life couldn't be also produced by proto-life? Maybe they used some of the same processes without all the other stuff modern life does?
Maybe life was bound to one location before it invented cells, and we just never checked that location? Maybe that location is now thousands meters below some ocean?
Considering the age of life on this planet, panspermia from Mars is unlikely; in the early life of the solar system, the sun was much dimmer and Mars was significantly colder than today. Any liquid water on the surface, on which life as we know it could have developed, would only be present in fairly recent history (no longer than a billion years really).
These organisms are close to the root of the tree. Photosynthesis came much later.
His paper was about microbial life feeding on petroleum and so he postulated that it would have to reach several KM in depth at least.
The oxygen only appeared 2.4 billion years ago, as a side effect of photosynthesis and the event is sometimes called the "Oxygen Holocaust", since it caused almost all life on Earth to go extinct.
As is often the case:
What we don't know > what we think we know
tldr: we don’t know, and may never know