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.