Assume, arguendo, we choose our standard for “these two critters are the same species” as “They can interbreed and produce live offspring at the usual rate for the species.”
I wonder how far back we could take a current era e.g. horseshoe crab and interbreed it with the similar looking critters from then and get successful offspring at the usual rate?
If indeed it’s 450Mya, that says something significant about the rate of genetic drift in the absence of significant selective pressure. Conversely, if it’s only a few thousand years ago because they did some rapid significant adapting to survive the last Ice Age, that’d also be very interesting to learn.
Of course without a time machine or magic Jurassic Park tech to somehow recover ancient DNA from fossils with perfect fidelity the question is unanswerable.
Doubtful. We don’t know every metabolic detail of a 450-million-year-old horseshoe crab, but we do know modern horseshoe crabs have accumulated hundreds of millions of years of genetic change. Molecular clocks, comparative genomics, and studies of living arthropods all tell us they’ve continued evolving. The fact that the body plan stayed similar doesn’t imply the underlying physiology, immune system, development, or metabolism remained unchanged. We certainly know there was considerable variety in subtle morphology, as well as behaviour/environment differences from earlier species. We also know at least 3 of the 4 extant species diverged just tens, not hundreds, of millions of years ago.
See above about speciation timeline, and even that is an upper bound.
We can still make very good guesses based on molecular clocks…work that’s already been done.
Yeah, over hundreds of millions of years, still being close enough to be the same species is supremely unlikely. But we don’t know what the speciation age is, for any given species, and it probably does vary from one species to another.
Thank you for the correction. I apparently misinterpreted “body plan basically unchanged” as meaning “no evolutionary changes at all”.
But I think we can agree that creatures like the horseshoe crab, dragonfly, and many sharks have a direct ancestry that is very, very old, and dates back to the time of the dinosaurs.
Everything has a direct ancestry that goes back to a protoplasmal primordial atomic globule. What’s different about horseshoe crabs, sharks, and cockroaches is that their ancestors, even very far removed, were at least very similar to them.
I just finished an excellent book on this mentioned several times in the last thread but I believe not here, If the Universe Is Teeming with Aliens … WHERE IS EVERYBODY?: Seventy-Five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life, 2nd edition by Stephen Webb.
One double filter that impressed me is that the great majority of rocky planets in a habitable temperature zone almost surely do not stay in said zone for enough tens of millions of years for evolution to have enough time to get close to what we are. And yet, the planet’s climate and atmosphere cannot be too stable, or else the weaker evolutionary pressures will move too slowly.
This isn’t how Webb explains it, but say there are ten bottlenecks each of which only one in a thousand planets can make it through over a billion year period. This would give a ridiculously low chance of advanced industrial civilization arising in a galaxy of, say, fifty billion rocky planets. Of course these numbers are pure speculation, but do suggest it is hardly crazy to think we are alone.
The only thing I saw in the book to mitigate the idea of the bottlenecks being overwhelmingly strong is the idea that both radio and laser sending and receiving gear, without much improvement on what we have today, can be made so tremendously directional that a message from consciousness on another galaxy, specifically directed to this solar system, might be received here. Of course, they would have to have a reason to think this was a good target to spend a lot of energy on, and would have to deal with us and them being in relative motion, constantly undergoing slight but critical changes due to the influence of everything around them.
But the bottlenecks could easily be so strong that galaxies outside of ours, anywhere remotely in our region of space, are all empty of anyone technologically advanced.
When you’re sending messages to other galaxies, you’re not going to send them to an individual planet in that galaxy. You probably can tighten your beam more than the size of the whole galaxy, but it’d still be a pretty big chunk of it.
Where a star is on the HR diagram is key-pretty much any star more massive than the sun would reach the red giant phase (game over man phase) pretty much right about now. A K type star would have a longer lifetime, but its habitability zone would be too narrow over its lifetime, tho the h.z. may be wider than supposed (as both Venus and Mars apparently had oceans early on). [Yes I am well aware of the arguments for red dwarfs, the most common star type by far, but the tidal locking and flares may prevent intelligent life from arising.]
Even by the unavoidably low scientific level of a thread like this, I think it would be overly speculative to suggest that a star has to be exactly like ours for technological civilization to be possible. The harder to avoid issue, concerning the likely rarity of a planet staying in the habitable zone for tens or hundreds of millions of years, has more to do with having a moon just like ours, in its relative size and orbit, to stabilize the earth, and just the right kind of atmosphere and carbon cycle to keep temperature and atmospheric composition isolations from being too extreme (without being too small to create evolutionary pressure).
Another one that has to be just right is the planet’s magnetic field. This is supposed to be why Mars lost its atmosphere. Maybe an acceptable magnetic field is pretty common. But the more bottleneck fractions you multiply the big numbers (stars and years) by, the less the big numbers matter.
The Fermi Paradox is usually defined as something like this:
The striking contradiction between the high statistical likelihood that extraterrestrial civilizations exist and the complete lack of evidence for or contact with them.
Added to that supposition is this:
Even at slow interstellar travel speeds, a spacefaring civilization could colonize or explore the galaxy in a few million years.
First of all, I challenge the “high statistical likelihood”. The truth is we have no idea what the likelihood of life spontaneously arising. It’s often said that life arose as soon as conditions were right. But that ‘soon as conditions were right’ was actually hundreds of millions of years. Maybe a short time if compared to the life of the planet, but that’s a flawed comparison to my way of thinking. If conditions were right for hundreds of millions of years, and life popping into existence is inevitable, I would have thought it would have happened much sooner.
Life forming was not a cumulous thing. It wouldn’t have needed hundreds of millions of years. If inevitable, why didn’t happen within the first thousand years? Or hundreds?
The second thing is this non-sense that any spacefaring civilization could colonize the galaxy that fast. That’s math based on technology that just doesn’t exist. We don’t have self-replicating probes and probably never will. And if that herculean task could be overcome, theoretically, would any civilization want to funnel that much resources into it? I don’t think so.
I have no need to slap it down, I just do not see a paradox.
IMHO that’s the big one. The assumption seems to be that if a technological civilization doesn’t destroy itself via things like nuclear war, a bioengineered plague, or some other form of self destruction, that it is inevitable that it will colonize the galaxy because of the large time scales involved. I don’t buy that. I think there is a very high probability that the engineering hurdles of a species like ours being able to leave their solar system (or local systems in the case of binary or trinary systems like Alpha Centauri) are just too difficult to overcome, even if we take into account timelines in the hundreds of millions to billions of years.
We are self-replicating probes. As for our technology, humans with technology are able to make more technology. Even if it takes a few hundred years at each planet to rebuild our tech base, that’s hardly a delay to galactic expansion.
In fact I agree with you, and largely take back what I said.
There are two possible framings here. One is to make the positive claim that there should be evidence of lots of ETIs and, as you say, that it’s possible to travel interstellar distances.
The other is to simply ask the question of why we don’t see any ETIs given that, based only on the physics, biology etc that we know today, it’s entirely possible that there could have been millions in the night sky.
The former framing is trivially defeated by questioning the various premises.
Unfortunately, it does seem to be the definition used by wikipedia and many of the other main reference sites. Hence why I am taking back my prior point.
The other framing is fascinating because it points to the vast set of unknowns about life, intelligence, civilizations and (future) technology. And it’s a moving target because, for example, we can tick off planets being rare as a possible explanation; we couldn’t originally do that.
Unfortunately only a minority of sites, like the planetary society, use this definition.
I’ll have to switch to just talking about the Drake equation in future. I’m not a fan of the drake equation either, but at least discussion of this leads to speculation about the various unknowns rather than focusing on what basis we can make a claim.
I think it’s obvious in this context that when discussing self-replicating probes, we are speaking of machines that intelligent species build to help us colonize the galaxy.
But let’s play the concept out. Humans are the product of unguided, iterative natural selection over billions of years rather than manufactured engineering. True self-replicating machines are typically conceptualized as solitary units—a single machine making an identical copy of itself. Human reproduction is largely sexual, requiring two distinct organisms (bodies) to combine genetic material, which deviates from the strict concept of a single machine “self-replicating” autonomously.
Our bodies require raw materials (food, water, oxygen), and convert them into energy, to maintain and repair themselves, and use genetic code (DNA) as an algorithmic blueprint to build new copies of themselves. Obviously this requires a HUGE infrastructure.
So, you can’t take one human, drop them on an exoplanet and have them start reproducing and building copies of themselves (that is what is claimed that we will eventually have with a space probe). Even if you dropped a breeding pair of humans on another planet, they would not live long enough to reproduce.
Humans are self-replicating (given the earth as a infrastructure) but we are not probes in the ‘colonizing the galaxy’ sense.
I think this is a major drake equation issue. We have gotten into space in large part because there has been a billion+ years of life on this planet capturing and storing solar energy so we could dig or pump it out of the ground. Life is continuously replicating because we have a constant source of energy in the sky, and an amazingly complex ecosystem to convert that energy into bio available nutrients.
You get to the Oort Cloud and there’s no significant solar energy anymore, not for humans at least, you’ve just got thousands of years worth of bitter cold dark to survive until more energy becomes available.
Interstellar travel and self-replication technology require a number of significant advances; such as fusion power or better (I prefer particle beam propulsion myself, as described by Jordin Kare); advanced artificial intelligence technology, and versatile autonomous manufacturing technology, among others. None of these are completely impossible.
I would, however suggest that any or all of these could be used as weapons of mass destruction, long before the first interstellar probe is built. One future filter that could elimate many, most or all advanced interstellar species is this simple fact; dangerous, high-energy technologies can destroy any civilisation soon after they develop them, and before they allow that civilisation to spread.
If we wanted to, we could have already destroyed ourselves with nukes (granted, we still might). We don’t need those other technologies to do so. Are they completely impossible? No. But my guess is that some of those are highly unlikely to ever be developed, even with millions of years to work on them. The big problem is how to keep an interstellar ship functional while traveling at a decent fraction of the speed of light and also big enough to carry the energy supply to slow down to the appropriate velocity at the destination. IMHO the tech needed to do so is probably going to turn out to be too difficult to actually create.