To the above comment I just wanted to add that a samples from the asteroid contain uracil one of the four building blocks of RNA. In September NASA’s OSIRIS spacecraft will be bringing back even larger samples from an asteroid called Bennu, so it will be interesting to see what can be found.
Thank you Paul a wonderful article again with such clear explanations. I do wonder though…. The common language of life is complex and just as we have evidence for the emergence of modern English, is there evidence for all the many complex pre-stages of for the emergence of DNA? If not could this lack of evidence be itself evidence for an off planet (Earth) origin of our common language (supported I understand by the existence of some non-earth originating “organic molecules” ) … , OR can we construct a reliable story of Earth origin even without evidence of the prototypes.
It's certainly an interesting question. You're right that the discussion in the article only implies that all life on Earth shares a common ancestor; it doesn't necessarily mean that this common ancestor lived on Earth, let alone that the first lifeforms were on Earth. My understanding is that do we have very little or no empirical evidence for how the earliest emergence and evolution of life unfolded.
That said, I'm not sure that we could expect to have much evidence for this even assuming life did arise on Earth. It would be an extraordinary stroke of luck if any fossils survive from this time, since the earliest lifeforms would likely have been very fragile, and certainly not included the material like bone that fossilises most readily, and over billions of years most of the rocks in which fossils might have formed would have been sucked back into the mantle. This means that we are dependent on evidence such as molecular biology, which becomes difficult to trace once we get back to a point where the molecular biology itself was still in flux. So to me at least, it doesn't seem that the lack of evidence for the processes prior to the emergence of DNA and LUCA is evidence that these processes did not occur on Earth.
It is certainly possible that the material for life could have come from outside the Earth. But I think that believing that this is what happened depends on arguing that the conditions on the early Earth were less amenable to the emergence of such material and primitive life than conditions elsewhere in the solar system, and I'm unsure whether there is any compelling evidence or argument for this.
Thanks Paul, and just to play a bit more on this theme.. I agree we are in an evidence free realm of speculation here, but one possible argument for Panspermia is the more monkeys typing argument, i.e. you increase the probability of a particular complex (self-replicating) pre-LUCA patterns accidentally emerging the more monkeys you have and the longer they are typing... So while almost all of our astroids were formed with the formation of our solar system (as I understand it) some few come from other star systems in our galaxy. This being so, we at least had/have several thousand more solar systems typing away and many of those for quite a bit longer than our solar system has been around (about 4.5B years ago I understand).
It's certainly an interesting idea, and I think you're right that with an incredibly rare event, increasing the number of places in which it could take place from just this solar system to the wider galaxy could be a way to make it more probable. But as you say, it is difficult to follow this idea too far because it is piling speculation on speculation. As far as I can see, the core uncertainty is really over how unlikely the emergence of life from abiotic matter is, given a planet with the right conditions and a timescale of hundreds of millions or billions of years. Certainly, we do not understand the process by which this emergence could happen, but even if the event occurs as commonly as once per million years it would be very difficult to study as it would not spontaneously occur in feasible experiments and would involve processes that might not seem plausible on human timescales. However, if the emergence of life in the environment of the early Earth is a once in a million year event, then the life would have spontaneously emerged extremely quickly on a geological timescale, without a need to appeal to a larger number of other solar systems in which it could have arisen. Of course, it could alternatively be that the emergence of life on early Earth was a once in a trillion year event, in which case its occurrence would require explanation and increasing the number of planets from which the life on Earth could have originated would then be helpful, giving credence to panspermia, as you suggest.
Without a way to discriminate between these possibilities, my inclination is to treat the hypothesis that the emergence of Earth was a sufficiently probable event that we do not need the additional explanation of panspermia as the simpler of the options and so to prefer the hypothesis that life did originate on Earth. However, this of course does not rule out panspermia, and I think that your argument points to the fact that if future evidence emerges that the emergence of life from abiotic matter is even less probable than we think, then this could encourage more credence for panspermia.
Thanks again Paul and at the risk of stretching the friendship and piling on more speculation, it occurs to me that if " the hypothesis that the emergence of Earth was a sufficiently probable event" is true, then it is also more likely that life has "formed" on other planets in the galaxy and so we then bump up against the Fermi Paradox.
That's fair, although I think that the same issue arises if we accept that interstellar panspermia is plausible enough to have originated life on Earth, since then we would also expect that life would have been seeded on many planets rather than just the Earth by this same process.
Quokka bonus points!
To the above comment I just wanted to add that a samples from the asteroid contain uracil one of the four building blocks of RNA. In September NASA’s OSIRIS spacecraft will be bringing back even larger samples from an asteroid called Bennu, so it will be interesting to see what can be found.
Thank you Paul a wonderful article again with such clear explanations. I do wonder though…. The common language of life is complex and just as we have evidence for the emergence of modern English, is there evidence for all the many complex pre-stages of for the emergence of DNA? If not could this lack of evidence be itself evidence for an off planet (Earth) origin of our common language (supported I understand by the existence of some non-earth originating “organic molecules” ) … , OR can we construct a reliable story of Earth origin even without evidence of the prototypes.
It's certainly an interesting question. You're right that the discussion in the article only implies that all life on Earth shares a common ancestor; it doesn't necessarily mean that this common ancestor lived on Earth, let alone that the first lifeforms were on Earth. My understanding is that do we have very little or no empirical evidence for how the earliest emergence and evolution of life unfolded.
That said, I'm not sure that we could expect to have much evidence for this even assuming life did arise on Earth. It would be an extraordinary stroke of luck if any fossils survive from this time, since the earliest lifeforms would likely have been very fragile, and certainly not included the material like bone that fossilises most readily, and over billions of years most of the rocks in which fossils might have formed would have been sucked back into the mantle. This means that we are dependent on evidence such as molecular biology, which becomes difficult to trace once we get back to a point where the molecular biology itself was still in flux. So to me at least, it doesn't seem that the lack of evidence for the processes prior to the emergence of DNA and LUCA is evidence that these processes did not occur on Earth.
It is certainly possible that the material for life could have come from outside the Earth. But I think that believing that this is what happened depends on arguing that the conditions on the early Earth were less amenable to the emergence of such material and primitive life than conditions elsewhere in the solar system, and I'm unsure whether there is any compelling evidence or argument for this.
Thanks Paul, and just to play a bit more on this theme.. I agree we are in an evidence free realm of speculation here, but one possible argument for Panspermia is the more monkeys typing argument, i.e. you increase the probability of a particular complex (self-replicating) pre-LUCA patterns accidentally emerging the more monkeys you have and the longer they are typing... So while almost all of our astroids were formed with the formation of our solar system (as I understand it) some few come from other star systems in our galaxy. This being so, we at least had/have several thousand more solar systems typing away and many of those for quite a bit longer than our solar system has been around (about 4.5B years ago I understand).
It's certainly an interesting idea, and I think you're right that with an incredibly rare event, increasing the number of places in which it could take place from just this solar system to the wider galaxy could be a way to make it more probable. But as you say, it is difficult to follow this idea too far because it is piling speculation on speculation. As far as I can see, the core uncertainty is really over how unlikely the emergence of life from abiotic matter is, given a planet with the right conditions and a timescale of hundreds of millions or billions of years. Certainly, we do not understand the process by which this emergence could happen, but even if the event occurs as commonly as once per million years it would be very difficult to study as it would not spontaneously occur in feasible experiments and would involve processes that might not seem plausible on human timescales. However, if the emergence of life in the environment of the early Earth is a once in a million year event, then the life would have spontaneously emerged extremely quickly on a geological timescale, without a need to appeal to a larger number of other solar systems in which it could have arisen. Of course, it could alternatively be that the emergence of life on early Earth was a once in a trillion year event, in which case its occurrence would require explanation and increasing the number of planets from which the life on Earth could have originated would then be helpful, giving credence to panspermia, as you suggest.
Without a way to discriminate between these possibilities, my inclination is to treat the hypothesis that the emergence of Earth was a sufficiently probable event that we do not need the additional explanation of panspermia as the simpler of the options and so to prefer the hypothesis that life did originate on Earth. However, this of course does not rule out panspermia, and I think that your argument points to the fact that if future evidence emerges that the emergence of life from abiotic matter is even less probable than we think, then this could encourage more credence for panspermia.
Thanks again Paul and at the risk of stretching the friendship and piling on more speculation, it occurs to me that if " the hypothesis that the emergence of Earth was a sufficiently probable event" is true, then it is also more likely that life has "formed" on other planets in the galaxy and so we then bump up against the Fermi Paradox.
That's fair, although I think that the same issue arises if we accept that interstellar panspermia is plausible enough to have originated life on Earth, since then we would also expect that life would have been seeded on many planets rather than just the Earth by this same process.