Size of first human generation, not mitochondrially most recent

I’m trying again to figure out approximately how many humans constituted the first generation, especially reproducers, else in total. An answer would help but also knowing what terminology would ease finding scientific answers online would help a lot.

Two being the limit is statistically almost impossible: If a predecessor species produced only one human offspring, unless that offspring reproduced with a nonhuman (possibly possible) and begot a human (even less likely) then that human lineage would have ended. If a predecessor species produced two or more human babies, they would have to have been of both sexes, one baby of each sex would have had to have lived long and healthily enough to reproduce, the babies who grew up would had to have reproductive ability during overlapping time periods, they had to have been near enough each other to meet, there couldn’t have been effectively a taboo against sibling intercourse or incest (such as if their ancestral species did not have that taboo), and the new offspring would have had to have a 100% chance of survival for enough further reproduction until the odds could drop to below 100% without threatening species survival. Genetic diversity is another matter.

For terminology, mitochondrial Eve or Adam turns out to mean most recent mitochondrial Eve or Adam, producing an age somewhere like 50-200,000 years ago as of 2013 (https://www.livescience.com/38613-genetic-adam-and-eve-uncovered.html (50,000) and https://www.nature.com/news/genetic-adam-and-eve-did-not-live-too-far-apart-in-time-1.13478 (200,000)) (the durations may have changed with newer research), whereas humans have existed for 6-7 million years (https://humanorigins.si.edu/evidence/human-fossils/species/sahelanthropus-tchadensis) and counting, or if all of that species’ descendant species long since died out then 1.4-2.4 million years ago (Homo habilis (https://humanorigins.si.edu/evidence/human-fossils/species/homo-habilis)).

I’ve been told that today’s genetic diversity gives a clue to the number needed for the first mitochondrial generation (Were there multiple lineages from primate to human?), so I suppose the same principle applies to the first generation ever.

I had thought that evolution was so gradual that if we could travel in a time machine with all of our lab equipment and identify human #1 and the last prehuman who was the human’s direct ancestor and there was at least one generation in between then we with the time machine likely would not know in what species the generation/s in between belonged. But scientists who got published in Human Evolution in 2018 (https://phys.org/news/2018-05-gene-survey-reveals-facets-evolution.html) say maybe not, in that the boundary is relatively distinct (although maybe not knife-sharp).

I originally posted at Were there multiple lineages from primate to human? roughly four years ago. Apparently, the science has been added to since.

(I skipped https://biologos.org/articles/series/genetics-and-the-historical-adam-responses-to-popular-arguments/adam-eve-and-human-population-genetics except for the January note, due to revisions pending, and the related three in a series, because I don’t know how to separate the biology from the theology in them and because I found the statement “our lineage became human as a population – one that has not numbered below about 10,000 individuals over the last 18 million years or more” unclear given the presently-known shorter age of humanity. The language evolution article link is broken or dead. The URLs are as accessed Jun. 20, 2020.)

(One tag available was for “pear-reviewed”, either a typo or a mystery. I’m interested in peer-reviewed science or the closest we have to it.)

Since biologically our lineage ultimately goes back to before multicellular organism and before sexual reproduction the “first” generation could consist of a single pre-biotic cell or collection of chemical cycles which reproduced in some manner. But talking about a first generation of the species homo sapiens is drawing a fairly arbitrary line.

However, some people like myself believe that we are more than just biological organisms, with an inheritance of mind transmitted via human communication. Thus a Christian like myself can believe that Adam and Eve were the first human generation even though they were not the first homo sapiens. The claim then is that we are all descended from them memetically even if we are not genetically related to them (though that is not excluded by the scientific evidence either). It is just this idea of a single couple being the sole genetic propagators of humanity which will not fly in the face of the evidence.

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I dunno. Sounds “ap- peeling” to me in a fruity sort of way.

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I don’t know what you mean by ‘humans’ here. There were probably thousands of individuals (although we don’t really know how many) in a population that generation by generation became more like modern humans. It’s not a question of a non-human giving birth to a human – it was a 49.99% human giving birth to a 50.00% human, who looked like nothing special among the others in the population.

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Yes, we could trace back to the beginning of life, but I’m interested in when humans began, arbitrary though that be.

The opening post referenced two species, either of which could serve as the stsrting point: Sahelanthropus tchadensis and, if S. tchadensis completely died out, Homo habilis.

I understand that there is now an open question about whether transition from an ancestral species to the next descendant species is quite as gradual as we had thought (as discussed in the opening post); the percentages make sense, and maybe there’s a convention that arbitrates the dividing line.

The subject that is “probably thousands” is what I’m trying to quantify. That, and I’m tryiing to find standard terminology.

This is like saying that because I exist, my grandmother had a 100% chance of survival up until the point she ensured my mother’s growth to reproductive age. It doesn’t work that way. My grandmother could have died, or had her life interrupted in many other ways, and I would either not exist or have been descended from a different ancestor. You can’t prove anything retroactively by saying that because a thing happened, it had to have happened.

I only now looked at the paper you referenced. You’re better off pretending you never read it – it’s a bad paper, published in an obscure journal, that tells us nothing about how species originate. You can find a summary of the problems with the paper here.

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From that article:

Think about that for a moment. They are looking at one mitochondrial gene, and you think this gene can tell us about big changes in morphology and physiology?

If my understanding of population genetics is correct, all that data can really tell us is that it takes about 200,000 years for a COI variant to become dominant in a population.

Here is another perplexing quote from the article:

They are only looking at living organisms. The reason for the lack of in between variation is that they’re dead. For the same reason, we don’t see any modern in between languages for the various Romance languages, and that’s because no one speaks them anymore. Using the same reasoning, each leaf on a tree is a separate thing unto itself.

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@Lynn_Munter: If there were only two humans, one of each sex, and they gave birth to only two human babies, one of each sex, then, among other requirements, the babies had to have a 100% chance of survival to reproductive age or humanity would have soon ceased to exist. As the numbers enlarge laterally, then the survival percentage could drop without termination of humanity. Your analogy reverses the direction of time, so, yes, that analogy is faulty. You would not have descended from another ancestor (assuming no surrogacy), although someone else might have (and perhaps your not being born could have made it easier for someone else to be born). I agree that some events don’t have to have happened even though they did.

@glipsnort and @T_aquaticus: Gradualness of evolution from species to descendant species makes more sense to me, but I’m not expert enough to stick with that (but thank you for the link). I see that Human Evolution, although reportedly open access, is not listed at predatoryjournals.com and not being listed there is good news for the journal’s credibility. I don’t know enough to evaluate most of the specific claims in the article. I thought Rockefeller U. is pretty well respected, so I assume logical sloppiness would be less likely, but maybe the critiques are on point.

@T_aquaticus: I’m not sure about the language analogy. If proto-Indo-European speakers split so that two groups stopped speaking with each other long enough so that random chance alone would lead to linguistic evolution, one group eventually speaking proto-Germanic and the other eventually speaking proto-Russian, languages in between proto-Germanic and proto-Russian might never have died out because they might never have begun.

It’s not predatory, but it has a very low impact factor, i.e. it is seldom cited. That doesn’t by itself mean a paper is wrong, of course, but there’s often a reason that a paper is published in an obscure journal.

Then why don’t those languages still exist? It would seem that all the people who spoke those in between languages are dead.

The in-between DNA sequences would be found in the dead ancestors of the living populations.

And? So? I don’t believe that any particular individuals were mandated to have existed in order for the human race to have evolved. If it didn’t happen one way, something similar enough would have happened another way, like water finding channels to flow down elevations.

This is also a small detail compared to the larger issue, which is that in any given generation between ape and human, individuals could reproduce with a wide variety of other individuals. Just because a fleck of blue paint would stand out against a purple background, it does not follow that if you mix a long series of shades between blue and purple that you will be able to tell them all apart.

@T_aquaticus: I guess your linguistic analogy holds up. If a speaker invents a word and then drops it as useless, that’s like a gene mutating and the mutation being killed before it can advance reproduction. The only trace we’d have of either event would be an outdated dictionary with the nonce word or a fossilized bone with preserved DNA with the mutation, each followed by a new dictionary or DNA sample without the new word or mutation. We don’t have dictionaries or DNA samples to document all in-between states but that doesn’t mean those states didn’t exist.

@glipsnort: Whether low-impact journals should be avoided (you didn’t say that but it could be inferred) is interesting; it may take a while before we can check textbooks to see if the low-impact point has been excluded from scientific consensus. In this case, I’ll leave the critique to others. For now, I lean toward gradualness of evolution.

@Lynn_Munter: We agree on the larger point.

I’m trying to figure out how many individuals had to have existed at the beginning of humanity, the minimum number. I don’t think it was just two. It could have been just two but the statistical odds are against that. I think you agree it could have been more than two. I think it almost certainly had to be more than two.

You and I agree on the possibility of a human reproducing with a nonhuman to produce a fertile human; but I haven’t seen much from anyone else or anywhere else supporting that point.

We agree on gradualness of evolution and so do other people, including in this thread.

Ancient DNA is a very exciting field of study, and it does hold a lot of potential information related to recent evolutionary history. However, the study under question didn’t use any ancient DNA and instead focused on living populations. All you would see in this type of study is the ends of the lineages, not the in-between states that lead from a common ancestor to modern organisms.

Good. The bit about the journal was an aside – the results in the paper in question simply did not tell us anything about how or when species originated, regardless of where it was published.

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