@Swamidass has done a lot of nice work on this idea, and it’s a valuable contribution to the conversation. He is also motivated to bring unity to the church wherever possible, and for that he has my respect. It’s not for everyone, but it’s quite important for some of our brothers and sisters in Christ.
This seems to be mixing issues. Partly my fault. So sorry about that.
Adam and Genome’s final theological point is against a genealogical Adam. Its sounds like you will continue to leave no mention of the fact that genealogical universal ancestors are common and recent. That leaves key information out. I’m not sure that is upfront given what we all know now.
Also genealogical science is part of population genetics. Moreover the the theological section focuses on genealogical ancestry, not genetic ancestry. Intentionally excluding established and relevant science is not going to serve readers. It’s certainly not upfront.
Thanks for the kind words. I hope you at least make mention of it. Yes, Scott McKnight (and you) do not think its important or taught in Scripture. I respect that, and being convinced this way I see why its not important to you. However, its only plausible to take that position for theological reasons, not scientific.
I was surprised when he disagreed with it on scientific grounds too. He is seems to think it is pseudoscience. No surprise, on the other hand, because there is no mention of it in Adam in the Genome. How could he know unless scientists are upfront with him?
However, it is a side issue to this thread. Sorry about raising it here.
Much more importantly, however, is how you plan to rework the sections involving the claims that @RichardBuggs have raised. Clarifying how you plan to revise those sections would be interesting. It seems worth revising both to fix some of the errors, and also for clarity. Any thoughts on that yet?
The Big Tent ... and Genealogical Adam!
From many theological starting points, a genealogical Adam has a great deal of consilience. I’m an agnostic about the details, but see a lot of beauty here.
Though this is not the only model I would put forward. I’m working on more than one.
It was background I wasn’t aware of, and improves my understanding of your position. I also want to emphasize that I have enormous appreciation for the work you’ve done on genetic modeling, on breaking down and explaining that modeling for everyone so that it was highly accessible, and on your commitment to bridge-building. Despite keeping on with my “lawyerly defense,” I do have great respect for what you’re doing here and with the genealogical Adam concept!
Getting back on topic, and setting aside genealogical science…
We know that @DennisVenema’s first claim is unsubstantiated by evidence. Whether or it is true or not, we do not know from evidence whether Homo sapiens were ever just a single couple. To think that population genetics forecloses this possibility is to proffer the ecological fallacy. Yet this is how heliocentric certainty was built; going well beyond an accurate account of science.
That, still, is a scientific error.
About the second claim (our ancestors as a whole were never just a couple after 3mya), its seems that has been walked back too. We still have to look at trans-species variation, but we likely will not get to high certainty as far back as 3 mya, and no one is even discussing 10 mya as was done in the start of this conversation. Maybe its true, but its not substantiated be evidence.
So the second claim, therefore, appears to be an error too (unless tran-species variation comes to the rescue).
Whether or not @DennisVenema decides to clearly retract these confident statements as anything more than hypotheses is beside the point. It seems that ultimately @RichardBuggs’s instincts ended up correct here.
Can you point to any other examples from biology where two specific organisms are identified as “starting” the species? I remain completely unconvinced that this is how the concept of “species” works, but I know I might be wrong.
Great question. Turns out that sexual species usually arise as populations, but not always. In biology, there are always exceptions to the rule. One well known exception to the rule is grasshoppers, which (it turns out) frequently speciate from single couples. Plants are another exception. It is very common for them to speciate from a single couple or even a single individual.
The usual pattern with mammals is speciation as a population, but it is possible to speciate from a single couple. Without getting into the details, this is an open question in the human lineage. It is certainly technically possible. Evidence one way or the other is most likely going to be equivocal, so I doubt we will ever know for sure.
Among mammals, consider the infamous Mouflon sheep of Corsica. https://en.wikipedia.org/wiki/Mouflon
Let’s imagine it’s true that they descend from an initial founding couple, and have been isolated for the last 8,000 years. At some point, if not already, they will no longer be able to produce fertile offspring with other Mouflon, and would be a distinct species.
Even if it ends up not being true in this specific case, there is no reason to think that this mechanism of speciation would not work. As long as there is enough separation for long enough period of time (by geography, genetic incompatibility, behavior, etc.) we expect new species to arise. So as long as an isolated population can arise from a single couple, this is one possible way for a species to arise.
Great question though. Always remember, there are rules in biology, but there are almost always exceptions to the rules.
But the original founding couple would have simultaneously been a member of the other species, right? And many of their offspring would have been able to reproduce with the “original” species had they not been geographically isolated correct? So it is only looking back in time that one could identify a “founding couple,” and it doesn’t have to do with uniqueness from the rest of their population, just circumstances of their environment. And in these cases where one mating pair becomes isolated from the rest of the species and “founds” a new species, can’t all the members of the new species be traced back to the “founding pair” genetically? Isn’t that exactly what these genetic studies being discussed are showing didn’t happen with Homo sapiens?
If we look in the relevant literature, how many scientists are saying homo sapiens speciated as a single couple, and how many are saying homo sapiens speciated as a population?
In the case of the Mouflon sheep, that is correct. In the case of plants and grasshoppers, it often is a new species in literally a single generation. They would not be the original species, and are not reproductively compatible.
Depends, as I just explained.
Looking back in time, however, I’m not sure we can even distinguish these two cases from genetics. The data would end up looking essentially the same.
However, from genetics alone, we would not be able to easily distinguish (it seems) between the grasshopper and Mouflon sheep scenarios.
Only within a time range. However, we are also looking at the “human lineage” (see Dennis’s claim #2), which include all our ancestors even before they are Homo sapiens.
Right now, we are looking at evidence against a single couple origin within the last 500 kya. With further analysis (e.g. using PSMC), I suppose that might increase up to about 700 or 800 kya, but we do not know for sure yet.
However, before that point, say at about 750 kya or 2 mya, I’m not sure we can rule out a single couple bottleneck in our ancestors. Those are interesting dates too. 750 kya is when Homo sapiens, Neanderthals and Denisovans (for which we only have a knuckle) share a common ancestor. This group of species (or subspecies if they are the same species) appear to be the only hominids with hyoid bones, which might be a marker for modern linguistic capability. 2 mya is when Homo erectus arises and becomes cosmopolitan, and here is where we think clothing might arise, as does language/tools beyond other animals, and more.
At either transition, ignoring theology entirely, was their a tight bottleneck? The consensus is, right now, that there was a very tight bottleneck, at least 2 mya. Perhaps not consensus, but definitely a live option when I have talked to anthropologists is a bottleneck 750 kya. How tight would those bottlenecks be? I’m not sure we can know from extant evidence.
@Christy, these are great questions. Thanks for putting them forward. Hopefully this clarifies where it seems the science stands.
I’m confused. Back that far, Homo sapiens did not exist as a species, correct? Homo sapiens is 200,000 kya and forward. So if we have “ruled out” a single pair bottleneck in the time frame when Homo sapiens emerged as a species, how can it be possible that as you say, “we do not know from evidence whether Homo sapiens were ever just a single couple”?
Typo there that I fixed.
Though, the genetics data only is about “our ancestors” not about Homo sapiens in particular.
It is possible that Homo sapiens arise as a single couple, but then there is subsequent interbreeding with other hominids.
Well, it is a misunderstanding that affects multiple posts and deleting all of them would affect the flow of the conversation. So, we are talking about a single couple in the ancestry of Homo sapiens, not a single couple within the species.
When would this have hypothetically had to to happen?
That is what I originally meant, however…
Probably sometime between 350 kya and 100 kya, depending on exactly when we think Homo sapiens arise in the fossil record. Substantial interbreeding before 100 kya would probably be undetectable (though, perhaps that date has to move back a bit).
If there is substantial interbreeding, how can you definitely identify a species boundary? Aren’t species boundaries usually identified when interbreeding stops?
Sometimes. Sometimes not.
For example, we know Neanderthals interbred substantially with Homo sapiens. However, some scientists would still classify them as a different species. Homo sapiens are defined by a cluster of anatomical and behavioral traits, a cluster that some are certain do not apply to Neanderthals.
I just want a simple question answered. Is a two person bottleneck possible without common ancestry? Could we be descended from two people that were created de novo and not descended from a previous previous population or part of a larger population, but the sole progenitors of Homo sapiens (not taking into account ad hoc miracles)?
I raised that question some weeks back.