As these GEITP pages have often described, there is no simple diagram to illustrate migrations of modern humans (Homo sapiens) out of southeast Africa during the past 1-2 million years. Both Neanderthals (Homo neanderthalensis) and modern humans are likely to have evolved from Homo erectus, an ancestor that left Africa ~1.8 million years ago (MYA) –– most likely reaching (what today is) Georgia, on eastern edge of the Black Sea, by way of the Levant, as well as China ~1.7 MYA and Iberia (now Spain) ~1.4 MYA. Numerous “molecular clock” genetic studies place the divergence time of the Neanderthal and modern human lineages between 800K and 400K years ago. Other scholars believe Neanderthals descended via Homo heidelbergensis (another distinct Homo erectus migration out of Africa that also had occurred during this time-frame). Neanderthal traits are also seen in Homo heidelbergensis specimens beginning between 600K and 350K years ago. An additional subspecies (not to be covered further today) is the Denisova (Homo denisova) which is of South Asian descent and lived around the same time-frame as the Neanderthal. By convention, European hominins younger than ~250K years are called Neanderthals. The genome of modern humans contains bits and pieces of both the Neanderthal and Denisovan genomes.
When Neanderthals mated with modern humans ~250K to 30K years ago, they “gave back” thousands of ancient African-gene variants that Eurasians had lost when their ancestors migrated out of Africa in small tribes, perhaps 80K to 60K years ago. This diversity might been a “genetic gift” to Eurasian ancestors as they spread around the world. Today, however, some of these ancient single-nucleotide variants (SNVs) are a burden: they appear to boost the risk of becoming addicted to nicotine, as well as differences in pigmentation, and having wider waistlines. (??) At the recent Annual Mtg of the ASHG [see brief summary in attached 1-page article], researchers reported that some “Neanderthal” SNVs inherited by modern humans outside of Africa are “not peculiarly Neanderthal genes,” but represent the ancestral human condition; this finding highlights just how much diversity can be lost when people pass through a genetic bottleneck as they move out of Africa.
When researchers examined closely the genomes of >20,000 people in the 1000_Genomes_Project and Vanderbilt’s BioVU_data_bank of electronic health records –– they noticed that distinct stretches of chromosomes inherited from Neanderthals also carried ancient alleles, or SNVs, found in all Africans studied (including the Yoruba, Esan, and Mende peoples). Researchers found 47,261 of such SNVs across the genomes of Europeans and 56,497 in Asians. Most intriguingly, in Eurasians these alleles are only found next to Neanderthal genes, suggesting “this entire chunk of DNA was acquired at the same time,” when ancestors of today’s Eurasians mated with Neanderthals ~50K years ago. The most stringent explanation is that these alleles represent the ancestral human condition –– inherited by both Neanderthals and Homo sapiens in Africa from their common ancestor.
Geneticists at the meeting also focused on archaic DNA “deserts,” where modern humans have inherited no DNA from Neanderthals or other archaic sublines. One of these regions includes the FOXP2 “language” gene. Absence of archaic DNA in these deserts suggests that, in our ancestors, natural selection flushed out the Neanderthal version of this gene. Neanderthal versions of FOXP2 would have produced much less of its protein than the amount expressed in modern human brains. In fact, a rare mutation –– that causes members of a family to produce half the usual amount of FOXP2 protein –– is known to trigger severe speech defects. It has been suggested by several researchers that enhancing FOXP2 expression may have been key to modern human language. 🙂
Science 27 Oct 2o17; 358: 431