The idea that the first two hominins on the planet were dropped off by visiting aliens (Star Trek, anyone?) is also possible and not ruled out by evidence. So are any number of other ideas. I don’t see those criteria as particularly helpful.
Strong genetic interference can do it too, especially if it is selectively advantageous.
It is possible, also, that there were jumps or leaps in abilities. That is a live possibility. We do not know exactly how a theory of mind arose, at it has only arisen once.
Though I agree, many of those scenarios are inferring something special about that bottleneck couple.
An intergalactic space stork?
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Star Trek tries to retroactively explain why so many species resemble humans through some sort of DNA mumbo-jumbo. I thought it might be fun to riff on that idea a bit. 
Edit: here you go. Star Trek’s ancient hominin panspermia.
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But that is just part and parcel of denying the fossil record and insisting there is no such thing as “cavemen”, not an actual reasoned to position based on facts or “current thinking in the field.” It’s completely irrelevant to the discussion at hand, where consensus timelines are being assumed.
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How is this even possible? “Language” is not a concrete thing one possesses or doesn’t possess like the knowledge of how to make fire. Language development surely happened on a continuum that moved from a simpler verbal/gestural semiotic system to a more complex one. It was surely communicated socially from one generation to the next. Deciding exactly when this semiotic system qualified as a full-fledged language would be an arbitrary designation and there wouldn’t be much difference between how the parents talked and how the children talked. Language has to be acquired as a baby/child from a caregiver. You’re talking about it like some couple somewhere invented human verbal communication de novo and then had no one else to talk to except their own children. This sounds utterly ridiculous to me from a linguistic perspective. You should strike this “maybe” from your list.
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Thank you for the helpful explanation! That makes sense.
It seems to me that if you had a situation with multiple alleles where the number of alleles was greater than four and all of them were observable in chimpanzees, this would be a serious challenge for the bottleneck of two. But what I’m reading between the lines is that perhaps the vast majority of genes don’t have that many alleles. This is the kind of fact that’s probably patently obvious to anyone who’s worked with genetics, but not obvious to a complete layman like me. 
Thanks again!
AMW
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You’ve got it. We have examples of complete lineage sorting (i.e. the same alleles in two species) with up to four alleles - but, four alleles could possibly pass through 2 people (two different alleles in each person). If we ever do observe more - and it could happen, because we don’t have a decent survey of chimpanzee or gorilla variation - we would have a situation where those alleles could not have passed through 2 people. The remaining fallback position here would be convergent evolution - that the excess allele(s) arose independently in the two lineages after the bottleneck.
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Even the knowledge of how to control fire and eventually produce it from scratch was probably gradually obtained. But the idea that there was a first couple with language and that isolated them from their previous population is even more of a stretch. If it was so discontinuous, how did two hominins - and only two - acquire this ability de novo?
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We do not know how quickly these things progressed. Do we? You are putting forward rationale for why it might be gradual, but there is rationale for why it might not be gradual too, at least at points. As far as a I know, from evidence, we cannot discriminate which one is correct.
I understand you are working with a mental model of fine-grain gradualism, but there is at times saltations in evolutionary processes.
I’m not asking you to change your mental model, but to just see things from a different view for just a moment. Even from a secular point of view, there are evolutionary biologists that have posited sharp breaks or saltations in the evolution of the human mind (see here). Even Wallace, the equal co-discoverer of evolution with Darwin, wondered about the need for a leap in the evolution of the human mind. None of this means non-natural processes are required, but the insistence on fine-grained gradualism is not a requirement of the data.
As to why just two, maybe it was luck or maybe providence. Varki puts forward in the link above that perhaps a full theory of mind is improbable to evolve because awareness of one’s own death is profoundly non-adaptive. Maybe it required two people evolving it at the same moment to be able to be adaptive; and that is very unlikely, but that is the most likely way to cross that barrier. Varki, to be clear, is not writing from a Christian point of view, but in an entirely scientific speculation.
There are mysteries about the details of our origins. Rather than insisting our personal instincts are correct, its better to just say “we do not know.”
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I haven’t read the book, but from the reviews that describe it, it seems that there author is working within a population, and that a theory of mind (TOM) was gradual, not a sharp break. The idea that two individuals discontinuously achieve TOM in the absence of their contemporaries doesn’t seem to be what is being advanced here. Am I reading this correctly?
As a scientist, I’m trained to be skeptical and seek plausible explanations. I’m not insisting that my instincts are correct. What I am interested in is the best possible case for a 2-person bottleneck. These issues arise out of that interest. I’m especially interested in what folks who favour a 2-person bottleneck have to say about it. Paging @RichardBuggs and @agauger once more…
That is not what scientists do, as far as I know.
Instead, we look to test hypotheses with data, and we are very careful not go beyond what the data tells us when making scientific claims. Plausibility, after all, is in the eye of beholder. Skepticism as you are using it is selective. The same skepticism could and should be directed to all positions, including yours.
I’ve already said that the evidence is not going to tell us one way or another. I do not think there is a strong evidential case for a bottleneck, or against it. Science is not good with singular events in the distant past. So this should be no surprise.
You are not reading it correctly. (no suprise because you are not reading it)
Varki is arguing that there may have been largely smooth development of the human mind, but at some point there was a very difficult to cross barrier. The indirect evidence he marshals to this point is that there are no other animals with a full theory of mind. None of our cousin species seem to have achieved it, so it must be hard to achieve. If it is so adaptive too, why is that? His resolution is that there is a difficult barrier when it first arises that usually ends it before it can become adaptive. It is a very unlikely event.
That is a reasonable theory. Very difficult to test, but its just as “plausible” as yours. And if this is the case, central to thesis is that there is a sharp difference between before and after in our minds. A sharp difference that can easily account for the absence of interbreeding. He even explains this at length, all from a secular point of view.
You’re saying that he posits this happening to just an individual? or a pair? and not to the rest of those individuals’ contemporaries?
Edit: it’s not a theory. It’s at best a hypothesis.
He argues that it happens many times in individuals, but usually ends in that individuals failure to reproduce. Then in one rare case in a single individual it persists, because s/he is so unlikely to have simultanuosly evolved the capacity for reality denial. This get’s him/her over the barrier of knowing his/her mortality, but not letting this become non-adaptive.
A critical feature of this model is that there is a sharp transition in the evolution of the human mind, which you do not seem to allow for.
I’m suggesting that one other way it’s possible that the barrier (if it exists) could be crossed is if two individuals (a mating couple) are so unlikely as to simultaneously evolve the capacity for a theory of mind at the same time. Perhaps their companionship and ability to connect gets them over. That is my innovation for the purpose of putting forward an alternate reason for why something so profoundly adaptive only appear to have evolved one time in all history.
Wither its a theory or a hypothesis, its not clear how to rule it in or out with evidence. We just do not know.
I’d be happy to consider evidence for it. These ideas, while interesting, are not evidence. They’re speculation, and I don’t see an easy way to test the idea.
Taking this speculation further, that this happened simultaneously to two individuals in close proximity and that it became reproductively isolating for them… again, this is speculation, and I see no way to even begin to test it.
@DennisVenema, I am OK with moving the TMR4A back. I have said based on fossils it could go back to the origin of Homo. I know this puts it well beyond what most would consider Adam and Eve, but then that was true for 500-700 kya anyway.
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@Christy
Have you seen this paper? These are some pretty big names, and they see no evidence for the gradual emergence of language.
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Thanks for clarifying. I guess the real question then is why you see a divide between Homo and the australopithecines, but that’s a conversation for another day.
I skimmed the paper, and I’m not seeing how you draw this conclusion. Can you elaborate?
They also take a view that language is specific to Homo sapiens - not even Neandertals have it, etc. How would that square with your preference to include Neandertals, Denisovans, and possibly H. erectus as the descendants of Adam and Eve if they lacked language (to accept their claims for the sake of argument)?
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