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> And there are circular arguments: DNA is identified as, say, Neanderthal because the fossil was identified as such, and then it is used to identify other foss

by jjk166 5y ago

> And there are circular arguments: DNA is identified as, say, Neanderthal because the fossil was identified as such, and then it is used to identify other fossils as Neanderthal, even if they don’t look Neanderthal-like. This is the opposite of a circular argument, this is the foundation of inductive reasoning. A = B and B = C thus A = C. If something has unambiguous neanderthal DNA but doesn't look neanderthal-like, then your idea of what is neanderthal-like is discredited.

AnthonyMouse 5y ago
The issue is that humans share ~99% of their DNA with chimpanzees. Pick any of that, say "this is chimpanzee DNA," and you're now identifying billions of humans as chimpanzees (or vice versa).
- jjk166 5y ago
  And 60% of our DNA is shared with a banana, but that's not the dna used to classify species. The overwhelming majority of both our and chimps DNA comes from our common ancestor, and all of that is neither human nor chimp DNA. But there are specific genes which Chimps evolved after our lineages diverged, and any animal containing those genes must be in the chimp lineage. In the case of hominid interbreeding, we can see genes that are common in neanderthals appearing in non-african modern humans. Had these genes existed in our common ancestor, they would be present in african populations as well. The opposite being the case, the only way for those genes to get there is if only some people descend from neanderthals, which in turn requires successful interbreeding.
  - AnthonyMouse 5y ago
    You still have the question of directionality. Suppose humans migrate to Europe from Africa, then have some local divergence from their ancestors. Then those humans interbreed with neanderthals, so that those divergent genes spread within the neanderthal population as well. Putting aside the question of whether the existence of interbreeding makes neanderthals not a separate species, it's going to be difficult to tell where those genes originated. Suppose the first person to have those genes was a human who bred with both humans and neanderthals, but it so happens we have fossils of some of the earliest neanderthals with those genes and not the earliest humans with them. Then you could have the result that some "neanderthals" descended from humans but not vice versa, and be unable to distinguish that from the other way around.
    - jjk166 5y ago
      This would be an issue if you were naively looking at a single mutation in a single gene in a single individual without context, but the problem disappears when looking at large numbers of whole genes in many individuals with fossil evidence for context. You can see more basal forms of genes, combinations of genes which get passed on together, biological clocks indicating when a mutation occurred, and geographic information indicating where it originated. If a gene variant exists for hundreds of thousands of years in one population, and then a nearly identical variant suddenly starts showing up in another population, it's safe to assume that some hanky panky took place. We do indeed find neanderthals who descend from humans, interbreeding went both ways. For example we can see that around 100,000 years ago, the neanderthals' y chromosomes were almost completely replaced with a modern human y chromosome whereas even as late as 38,000 years ago there were still neanderthals with purely neanderthal mitochondrial DNA.
ImaCake 5y ago
This is also the point where the author lost me. But it was because of the dismissal of the molecular evidence. That molecular evidence is very compelling and has vastly improved our knowledge of early homo. Disagreeing with what the molecular evidence means would be valid, but dismissal is a sign of ignorance. It is even more surprising that people dismiss genetics in a age when national borders are closed based on such analyses of COVID genomes. The nextstrain resource is a great way to get an intuitive grasp of what genetics gives us. https://nextstrain.org/ncov/gisaid/global https://nextstrain.org/ncov/gisaid/global

by jjk166 5y ago

AnthonyMouse 5y ago

The issue is that humans share ~99% of their DNA with chimpanzees. Pick any of that, say "this is chimpanzee DNA," and you're now identifying billions of humans as chimpanzees (or vice versa).

jjk166 5y ago
And 60% of our DNA is shared with a banana, but that's not the dna used to classify species. The overwhelming majority of both our and chimps DNA comes from our common ancestor, and all of that is neither human nor chimp DNA. But there are specific genes which Chimps evolved after our lineages diverged, and any animal containing those genes must be in the chimp lineage. In the case of hominid interbreeding, we can see genes that are common in neanderthals appearing in non-african modern humans. Had these genes existed in our common ancestor, they would be present in african populations as well. The opposite being the case, the only way for those genes to get there is if only some people descend from neanderthals, which in turn requires successful interbreeding.
- AnthonyMouse 5y ago
  You still have the question of directionality. Suppose humans migrate to Europe from Africa, then have some local divergence from their ancestors. Then those humans interbreed with neanderthals, so that those divergent genes spread within the neanderthal population as well. Putting aside the question of whether the existence of interbreeding makes neanderthals not a separate species, it's going to be difficult to tell where those genes originated. Suppose the first person to have those genes was a human who bred with both humans and neanderthals, but it so happens we have fossils of some of the earliest neanderthals with those genes and not the earliest humans with them. Then you could have the result that some "neanderthals" descended from humans but not vice versa, and be unable to distinguish that from the other way around.
  - jjk166 5y ago
    This would be an issue if you were naively looking at a single mutation in a single gene in a single individual without context, but the problem disappears when looking at large numbers of whole genes in many individuals with fossil evidence for context. You can see more basal forms of genes, combinations of genes which get passed on together, biological clocks indicating when a mutation occurred, and geographic information indicating where it originated. If a gene variant exists for hundreds of thousands of years in one population, and then a nearly identical variant suddenly starts showing up in another population, it's safe to assume that some hanky panky took place. We do indeed find neanderthals who descend from humans, interbreeding went both ways. For example we can see that around 100,000 years ago, the neanderthals' y chromosomes were almost completely replaced with a modern human y chromosome whereas even as late as 38,000 years ago there were still neanderthals with purely neanderthal mitochondrial DNA.

ImaCake 5y ago

This is also the point where the author lost me. But it was because of the dismissal of the molecular evidence. That molecular evidence is very compelling and has vastly improved our knowledge of early homo. Disagreeing with what the molecular evidence means would be valid, but dismissal is a sign of ignorance. It is even more surprising that people dismiss genetics in a age when national borders are closed based on such analyses of COVID genomes. The nextstrain resource is a great way to get an intuitive grasp of what genetics gives us. https://nextstrain.org/ncov/gisaid/global https://nextstrain.org/ncov/gisaid/global