Original poster here.
Any analogy can be problematic. If it’s disputed, try abstraction.
I leave theology to others.
@DennisVenema: I take your angle as correcting some of what I’ve said above, although I think we were closer than you thought. We agree, for example, that change was not sudden. Since everyone in the first 10,000 or more beings was human and change is incremental, then there’s no clear demarcation between them and the preceding number, perhaps 10,000, and with 10,000 as the minimum then some earlier beings may also have been human. I already acknowledged as much. I steered clear of what happened 200,000 years ago and the MRCA and asked instead about the beginning of all of humanity, 4.3 million years ago (i.e., the oldest or least recent common ancestry), and in that context I wrote of an in-between state for a time when nonhumans slowly evolved into humans; that roughly corresponds to what you wrote of hominins. It’s not likely that anyone in any lineage who first met the definition of “human” could have been identified as such at the time or distinguished from their nonhuman contemporaries and nonhuman parents (as opposed to great-. . .-grandparents, who are more easily distinguishable), because of the gradualness of evolution. There’s no nonarbitrary method of identifying the first humans (in any quantity) from their immediate ancestors, given as nonhumans. But that means that whoever we with hindsight were to deem the earliest humans would have been able to reproduce with the nonhumans who were less than a generation older (maybe a bit more), because the genetic similarity was so nearly complete that reproduction between what we might with hindsight only arbitrarily call separate species would have been possible. Pressures on a population could affect the offspring of a human mating with a nonhuman so as to create another nonhuman-to-human lineage within the same community; that is a correction from what I posited above and that is based on or extended from what I understand of what you wrote. I hope I haven’t misunderstood your summary.
However, the same seems likely to be true of, say, the evolutionary invention of eyes (not to mention whales, chameleons, et al.). Perhaps skin became sensitive to visible electromagnetic frequencies and transmitted what it sensed to the brain which applied it to further its owner’s survival; perhaps some areas of the skin became more light-sensitive than other areas of the same owner’s skin; perhaps a more-sensitive area developed means to protect itself leading eventually to eyelids; and so on. That, too, would have been gradual and the pressure leading to it would have been on a population, probably not 10,000 but probably more or less and probably not exactly one or exactly 40. Yet, as I understand this, scientists have identified 40 places on the evolutionary tree where eyes begin. I think what that means is that at each of the 40 a population of one species evolved eyes but reproduction (here assuming reproduction was sexual) between a light-sensitive individual and a light-insensitive individual could result in a light-insensitive individual who in turn might reproduce with a light-sensitive individual producing a light-sensitive descendant, resulting in two light-sensitive lineages for that species at one spot on the evolutionary tree, because the tree is shorthand for what we know and does not map every individual.
Assuming the same principles apply to all complex evolutionary steps, they’d also apply to humans, so if one early human male were to be identified as “Adam”, that would be arbitrary within a particular generation and community (bounded by available transportation, not necessarily on foot), as there could be other “Adam”-like humans, i.e., male humans born of nonhuman parents, but not enough evidence has been preserved to allow identifying a non-Adam human lineage. Absent that evidence, the shorthand is still useful, even as scholarly shorthand. Thus, as shorthand, there’s only one locus of nonhuman-to-human evolution: one locus defined as within an age range allowing members to reproduce with each other and with sufficient proximity to be considered for our purpose to contain one community.
It still appears that the first male humans did not have to come into existence at the same time or place as the first female humans, although humans of the two sexes would have had to meet in order to continue humanity by the time humans had evolved enough to prevent any more reproducing with nonhumans, and perhaps had to earlier.
Another argument favoring community reproduction rather than being by only two individuals starting humanity comes to mind, but it’s speculative: There’s a taboo now against relationships that result in inbreeding. I think some animals exhibit this, too. My speculation is that perhaps that taboo also existed 4-5 million years ago. If so, if conditions supported pro-human evolution then multiple evolution would have been likelier and many nonhumans might have given birth to humans, albeit only where and when those pro-human-evolution conditions prevailed and not elsewhere or at other times.
I gather that all of the other taxonomic categories of humans, such as Neandertal, evolved later and not directly from nonhumans, so they’re irrelevant to my question opening this topic.
That evidence does not support multiple evolution is something I got from, “while evolution can lead multiple times to similar solutions, this has not happened (yet?) for our own unique capacities”, according to this topic’s moderator, Casper_Hesp, and perhaps other passages, supra. But I acknowledge your point here.
It’s possible, I speculate, that our species has evolved in its few million years of existence. If we could clone and try to reproduce with a million-year-old human, we might be genetically different enough not to be able to reproduce. But we don’t notice that, although million-year-old DNA might exist and even be analyzable, if we haven’t seen evidence of branching of species from the modern human lineage. Branching is a strong reason to label separately.
@Argon: Family names are not as tightly bound to families as are genotypes. Family lineages sometimes change names and one family may have multiple sons. If you’re right about the future of the Amish, they may decide that differentiable names are important for family identity and social relationships and some families will either change family names or adopt another convention.
@AMWolfe: Linguistics is a different kind of field. Languages evolve, but some words are more stable than others. Place names, the name of an entire people who speaks a given language, and the words with the most frequency and distribution of use tend to be evolve more slowly or hardly at all. Written and spoken forms may evolve at different rates. Phrases and words may evolve at different rates.
@GJDS: Different disciplines may have different givens and different standards for sufficiency of evidence. Within a discipline, different groups of specialists can have different standards. At one time, two mathematicians disagreed on a point about infinity and one found himself unable to get his work published in the nation where they both lived, but eventually some of the work was published in another nation. While standards tend to converge, they don’t always, and good work may get done and recognized even without convergence.