Here, context helps. Context is important because it is not practical, nor normally expected, that every sentence include every condition needed so the sentence can stand by itself. In this case, the sentence you are quoting is the second sentence of a paragraph. The first and second sentences of that paragraph say this: “Since lay people, by definition, lack specialist knowledge and skill, that virtually all of them would see A. afarensis as not human (I agree with you on that) is not determinative of specialist knowledge. On many points specialists and laity agree, but on many they don’t.” Here, what I’m saying is that on many points lay people do not agree with specialists and specialists do not agree with lay people. And that’s true. For example, in many fields, specialists often learn something before anyone else does and then tell the general public (composed mostly of lay people). The rate of acceptance by lay people may be fast or slow. If Harvard and the FDA announce a new 10-cent pill that will wipe out cancer starting tomorrow, acceptance will likely be fast. But by the time I learned that gravity is measured on the basis of two masses, Einstein had already pronounced that wrong, so my source or sources had been late. The Brontosaurus is also a case in point. According to Scientific American, scientists first thought it existed as a genus, then decided that what had been found was actually of another genus so that there was no Brontosaurus, and then, as of 2015, believe it should be a new group, all at the same time that the public believed in the Brontosaurus as a distinct creature. And, I think if you ask passersby on a street to name all the planets, virtually everyone will name nine. But scientists would say either eight or eleven, because they’d either exclude Pluto as no longer a planet or count Pluto, Eris, and Makemake as dwarf planets. Scientists re-evaluated Pluto in 2003 but I doubt most of the public has caught up even though it’s now 13 years later. Learning that “my very educated mother just served us nine peanuts [or another p-word]” as a mnemonic makes change harder. Applied to this thread, it’s likely that most lay people would look at a picture of “Lucy” and say “that’s no human being” but some scientists say yes, it is. That scientists disagree with each other on that point I acknowledged above. But while almost no lay person shown a picture of “Lucy” would say “human”, some specialists disagree with laity and are entitled to. That entitlement is a consequence of the specialists having specialized, having spent time and energy studying the question and developing reasons for their conclusions. I accept the scientists’ differing conclusions (differing with each other) about “Lucy”'s humanness and reject granting the lay view dominance or equality with scientific views.
As a side point on whether I acknowledged that scholars disagree with each other, I also wrote above, “[i]t seems to me that, in every major field, scholars disagree on a minor proportion of the field’s intellectual content, which, I think, enhances the field’s scholarly validity.”
On the inference: In the opening post, I wrote, “It’s likelier that ‘Adam’ and ‘Eve’ each had reproductive intercourse with a primate who was very similar to themself until evolution occurred again.” In post #23, I wrote, “That ‘one’, however, does not deny that there could have been an original 10,000 or so, if they were 10,000 or so beings who could reproduce (to yield fertile offspring) with both ancestors and contemporaries who were exclusively primate and contemporaries and descendants who were exclusively human.” In that same post, I wrote, “Either (a) there’s an in-between state whereby a being (from a potentially small number) can reproduce with either an exclusively primate or an exclusively human or (b) both men and women humans had to come into existence at the same time, i.e., be born within a few years and in sufficiently close proximity so they could reproduce with each other if they couldn’t reproduce with predecessor primates. The former (‘(a)’) seems much more likely. . . .” That post goes on to say much more on point. I could rewrite #23 to be clearer but then it would be substantially longer, not shorter, and I don’t think you want that, and post #53 (to find it search for “I take your angle as correcting some of what I’ve said above”) adds substantially. Posts #28 (search for “I don’t think today’s human could reproduce even with the primate of 4.3 million years ago”), #34 (search for “I think I did leave plenty of room for uncertainty”) (that post seems to have gotten mangled), #56 (search for “I’m not surprised you thought”), and #59 (search for “Considering linear evolution”) say a little of what #23 and #53 say but probably don’t add much to them. That my inference holds up was not because everyone agreed with it but rather because no one successfully refuted it. You’re welcome to have a go at it, but I wouldn’t suggest rushing through the text.
I’ll try this: Two possibilities are available. One is that, despite the differences between the 10,000 or more first humans and the immediately-ancestral primates being as extremely slight as could have been, they were not able to reproduce with each other, in which case the first human females and the first human males had to come into existence within roughly 15 years of each other, and maybe sooner, and in physical proximity, and that does not seem as likely as the other possibility. The other possibility, which I think is likelier, is that, because the differences between the 10,000 or more first humans and the immediately-ancestral primates were so extremely slight that they could not tell themselves apart as separate species (even if they had all of our modern knowledge) and reproduction could occur when a human of either sex mated with an immediately-ancestral primate of the other sex, in which case the first human males (in any quantity) and the first human females (in any quantity) did not have to have met or have to have been of the same generation or physical neighborhood. In either case, humans of both sexes eventually had to mate with each other and exclusively of nonhuman primates because all would have evolved too much to support any other reproductive possibility.
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