In school textbooks, “Neanderthals” (Homo neaderthalensis) have been described as “ancestors of modern humans (Homo sapiens)”, or as “a separate line living geographically in close proximity to Homo sapiens, but not interbreeding”. In recent years, however, things have become increasingly complicated. Each of us has two genomes –– genomic DNA (gDNA) in the nucleus of every cell (except mature red blood cells, RBCs), and mitochondrial DNA (mtDNA) located in the several hundred mitochondria situated in the cytoplasm of each cell (except mature RBCs). It is well known that mtDNA is derived completed from the ovum (mother’s gamete).
The [attached] editorial details a recent studiy (described in a Nature Commun article) of mtDNA from a Neanderthal who lived ~100,000 years ago in southwest Germany. Researchers found this mtDNA resembled that of early Homo sapiens instead of Neaderthals. It is speculated that a female member of the lineage that gave rise to Homo sapiens in Africa mated with a Neanderthal male >220,000 years ago –– much earlier than other known encounters between the two groups –– and her children spread her mtDNA through the Neanderthal lineage. And, over time, her African mtDNA completely replaced the ancestral Neanderthal mtDNA.
Although other researchers in this field are enthusiastic about the hypothesis, they caution that “this conclusion will require more than one genome to be certain.” Most Neaderthal DNA samples have been from those who lived close to the time of extinction of the species on Earth (~40,000 to 50,000 years ago). Researchers also have studied gDNA and mtDNA genomes of another archaic group from southern Siberia, called Homo denisovans. The gDNA had suggested that Neanderthals and Denisovans were each other’s closest kin, and that their lineage split from Homo sapiens >600,000 years ago.
However, the Neanderthal mtDNA from these samples had posed a mystery: It was not like Denisovans’ and was closely related to Homo sapiens –– a pattern inconsistent with the ancient 600,000-year-divergence date. In this latest (2o17 Nat Commun) study, paleo-geneticists of the Max Planck Institute for the Science of Human History (Jena, Germany) examined ancient mtDNA from a Neanderthal thigh bone [originally found in 1937 in the Hohlenstein-Stadel cave (HST) in Germany]. Researchers compared the coding region of the HST Neanderthal’s mtDNA with that of 17 other Neanderthals, three Denisovans, and 54 modern humans. The HST Neanderthal’s mtDNA was significantly different –– even from that of proto-Neanderthals that date to 430,000 years ago at Sima de los Huesos in
Spain –– suggesting that their mtDNA had been completely replaced. But the HST sample was also surprisingly distinct from that of other Neanderthals, allowing researchers to build a phylogenetic tree and study how Neanderthal mtDNA evolved over time.
Using modern humans’ mtDNA mutation rate to calculate the timing, the researchers conclude that the HST mtDNA split from that of all other Neanderthals at least 220,000 years ago. The ancient H. sapiens’ mtDNA therefore must have entered the Neanderthal lineage before this time, but after 470,000 years ago, the earliest date for when modern human and Neandertal mtDNA diverged. That is sufficiently enough time for the new form of mtDNA to have spread among Neanderthals and replaced all their mtDNA. Stay tuned! We will continue to learn more about modern human ancestry with each passing year of paleo-anthropology and DNA-sequencing research.
Science 7 July 2o17; 356: 14