Post by Admin on May 5, 2022 18:03:57 GMT
An early modern human from Romania with a recent Neanderthal ancestor
Qiaomei Fu,1,2,3,* Mateja Hajdinjak,3,* Oana Teodora Moldovan,4 Silviu Constantin,5 Swapan Mallick,2,6,7 Pontus Skoglund,2 Nick Patterson,6 Nadin Rohland,2 Iosif Lazaridis,2 Birgit Nickel,3 Bence Viola,3 Kay Prüfer,3 Matthias Meyer,3 Janet Kelso,3 David Reich,2,6,7 and Svante Pääbo3
Abstract
Neanderthals are thought to have disappeared in Europe ~39,000–41,000 years ago but they have contributed one to three percent of the DNA of present-day people in Eurasia1. Here, we analyze DNA from a 37,000–42,000-year-old2 modern human from Peştera cu Oase, Romania. Although the specimen contains small amounts of human DNA, we use an enrichment strategy to isolate sites that are informative about its relationship to Neanderthals and present-day humans. We find that on the order of six to nine percent of the genome of the Oase individual is derived from Neanderthals, more than any other modern human sequenced to date. Three chromosomal segments of Neanderthal ancestry are over 50 centimorgans in size, indicating that this individual had a Neanderthal ancestor as recently as four to six generations back. However, the Oase individual does not share more alleles with later Europeans than with East Asians, suggesting that the Oase population did not contribute substantially to later humans in Europe.
Between 45,000 and 35,000 years ago, anatomically modern humans spread across Europe, while the Neanderthals, present since before 300,000 years ago, disappeared. How this process occurred has long been debated1,3,4,5. Comparisons between the Neanderthal genome and the genomes of present-day humans have shown that Neanderthals contributed approximately one to three percent of the genomes of all people living today outside sub-Saharan Africa6,7 suggesting that human populations ancestral to all non-Africans mixed with Neanderthals. The size of segments of Neanderthal ancestry in present-day humans suggests that this occurred between 37,000 and 86,000 years ago8. However, where and how often this occurred is not understood. For example, Neanderthals share more alleles with East Asians and Native Americans than with Europeans, which may reflect additional interbreeding in the ancestors of eastern non-African9–12. Surprisingly, analyses of present-day genomes have not yielded any evidence that Neanderthals mixed with modern humans in Europe, despite the fact that Neanderthals were numerous there and cultural interactions between the two groups have been proposed13,14.
More direct insight into the interactions between modern and archaic humans can be obtained by studying genomes from modern humans who lived at a time when they could have met Neanderthals. Recent analyses of genomes from a ~43,000–47,000-year-old modern human from western Siberia15 and a ~36,000–39,000-year-old modern human from eastern Europe16 showed that Neanderthal gene flow into modern humans occurred before these individuals lived. The Siberian individual’s genome contained some segments of Neanderthal ancestry as large as 6 million base pairs, suggesting that some Neanderthal gene flow could have occurred a few thousand years prior to his death15.
We report genome-wide data from a modern human mandible, Oase 1, found in 2002 in the Peştera cu Oase, Romania. The age of this specimen has been estimated to ~37,000–42,000 years by direct radiocarbon dating2,17,18. Oase 1 is therefore one of the earliest modern humans in Europe. Its morphology is generally modern but some aspects are consistent with Neanderthal ancestry19–21. Subsequent excavations uncovered a cranium from another, probably contemporaneous individual, Oase 2, which also carries morphological traits that could reflect admixture with Neanderthals17.
We prepared two DNA extracts from 25 and 10 mg of bone powder removed from the inferior right ramus of Oase 1. We treated an aliquot of each of these extracts with E. coli uracil-DNA-glycosylase (UDG), an enzyme that removes uracils from the interior parts of DNA molecules, but leaves a proportion of uracils at the ends of the molecules unaffected. Uracil residues occur in DNA molecules as a result of deamination of cytosine residues, and are particularly prevalent at the ends of ancient DNA molecules9,22. Among the DNA fragments sequenced from these two extracts, 0.18% and 0.06%, respectively, could be mapped to the human reference genome. We prepared three additional DNA libraries from the extract containing 0.18% human-like molecules, but omitted the UDG treatment to increase the number of molecules where terminal C to T substitutions could be seen and used to identify putatively ancient fragments. Because the fraction of endogenous DNA is so small, we used hybridization to DNA probes to isolate human DNA fragments from the libraries23. Applying this strategy to the mitochondrial (mt) genome allowed the mtDNA from the five libraries to be sequenced to an average coverage of 803-fold (Supplementary Information section 1). At the 3′-ends of the DNA fragments, cytosine residues (C) appeared as thymine residues (T) relative to the human mtDNA reference in 21% of fragments, reflecting appreciable levels of cytosine deamination. This suggests that at least some of the human mtDNA is of ancient origin. We determined mtDNA consensus sequences in two ways: using all mtDNA fragments, and using only deaminated fragments that carry C to T substitutions at either end relative to the consensus mtDNA sequence based on these sequences, an approach known to enrich for endogenous DNA9,24–26. The mtDNA sequence based on all fragments clusters with present-day Europeans (Extended Data Figure 1) (Supplementary Information section 1). In contrast, the mtDNA sequence based on deaminated fragments is related to a large group of present-day Eurasian mtDNAs (haplogroup N) but diverges from these before they diverged from each other. This Oase 1 mtDNA carries a few private mutations based on which its age can be estimated to 36,330 years before present (14,520–56,450; 95% confidence interval). Using six positions where the mtDNA sequence differs from at least 99% of 311 present-day humans, we estimate the contamination rate among all mtDNA fragments to 67% (95% confidence interval (CI) 65%–69%). When we restrict to mtDNA fragments that carry terminal C to T substitutions, the contamination estimate is 4% (95% CI of 2%–9%) (Supplementary Information section 1).
To isolate nuclear DNA from Oase 1, we used three sets of oligonucleotide probes that cover about two million sites that are single nucleotide polymorphisms (SNPs) in present-day humans and captured DNA molecules from the five libraries. Of the SNPs targeted, 51% (n=1,038,619) were covered by at least one DNA fragment, and 13% (n=271,326) were covered by at least one fragment with a terminal C to T substitution. To estimate nuclear DNA contamination, we tested whether Oase 1 DNA fragments with or without evidence of deamination share more alleles with present-day Europeans or with East Asians. We find that significantly more Oase 1 fragments without deamination match Europeans, implying European contamination of 17% to 30% (Supplementary Information section 1). Based on these findings and those from mitochondrial DNA, we restricted all subsequent analyses to DNA fragments that carry terminal C to T substitutions. After doing this, the fractions of SNPs from the X and Y chromosomes are similar, indicating that Oase 1 carried both an X and a Y chromosome and thus that he was male. The Y chromosome alleles belong to the F haplogroup, which is carried by most males in Eurasia today (Supplementary Information section 2).
Qiaomei Fu,1,2,3,* Mateja Hajdinjak,3,* Oana Teodora Moldovan,4 Silviu Constantin,5 Swapan Mallick,2,6,7 Pontus Skoglund,2 Nick Patterson,6 Nadin Rohland,2 Iosif Lazaridis,2 Birgit Nickel,3 Bence Viola,3 Kay Prüfer,3 Matthias Meyer,3 Janet Kelso,3 David Reich,2,6,7 and Svante Pääbo3
Abstract
Neanderthals are thought to have disappeared in Europe ~39,000–41,000 years ago but they have contributed one to three percent of the DNA of present-day people in Eurasia1. Here, we analyze DNA from a 37,000–42,000-year-old2 modern human from Peştera cu Oase, Romania. Although the specimen contains small amounts of human DNA, we use an enrichment strategy to isolate sites that are informative about its relationship to Neanderthals and present-day humans. We find that on the order of six to nine percent of the genome of the Oase individual is derived from Neanderthals, more than any other modern human sequenced to date. Three chromosomal segments of Neanderthal ancestry are over 50 centimorgans in size, indicating that this individual had a Neanderthal ancestor as recently as four to six generations back. However, the Oase individual does not share more alleles with later Europeans than with East Asians, suggesting that the Oase population did not contribute substantially to later humans in Europe.
Between 45,000 and 35,000 years ago, anatomically modern humans spread across Europe, while the Neanderthals, present since before 300,000 years ago, disappeared. How this process occurred has long been debated1,3,4,5. Comparisons between the Neanderthal genome and the genomes of present-day humans have shown that Neanderthals contributed approximately one to three percent of the genomes of all people living today outside sub-Saharan Africa6,7 suggesting that human populations ancestral to all non-Africans mixed with Neanderthals. The size of segments of Neanderthal ancestry in present-day humans suggests that this occurred between 37,000 and 86,000 years ago8. However, where and how often this occurred is not understood. For example, Neanderthals share more alleles with East Asians and Native Americans than with Europeans, which may reflect additional interbreeding in the ancestors of eastern non-African9–12. Surprisingly, analyses of present-day genomes have not yielded any evidence that Neanderthals mixed with modern humans in Europe, despite the fact that Neanderthals were numerous there and cultural interactions between the two groups have been proposed13,14.
More direct insight into the interactions between modern and archaic humans can be obtained by studying genomes from modern humans who lived at a time when they could have met Neanderthals. Recent analyses of genomes from a ~43,000–47,000-year-old modern human from western Siberia15 and a ~36,000–39,000-year-old modern human from eastern Europe16 showed that Neanderthal gene flow into modern humans occurred before these individuals lived. The Siberian individual’s genome contained some segments of Neanderthal ancestry as large as 6 million base pairs, suggesting that some Neanderthal gene flow could have occurred a few thousand years prior to his death15.
We report genome-wide data from a modern human mandible, Oase 1, found in 2002 in the Peştera cu Oase, Romania. The age of this specimen has been estimated to ~37,000–42,000 years by direct radiocarbon dating2,17,18. Oase 1 is therefore one of the earliest modern humans in Europe. Its morphology is generally modern but some aspects are consistent with Neanderthal ancestry19–21. Subsequent excavations uncovered a cranium from another, probably contemporaneous individual, Oase 2, which also carries morphological traits that could reflect admixture with Neanderthals17.
We prepared two DNA extracts from 25 and 10 mg of bone powder removed from the inferior right ramus of Oase 1. We treated an aliquot of each of these extracts with E. coli uracil-DNA-glycosylase (UDG), an enzyme that removes uracils from the interior parts of DNA molecules, but leaves a proportion of uracils at the ends of the molecules unaffected. Uracil residues occur in DNA molecules as a result of deamination of cytosine residues, and are particularly prevalent at the ends of ancient DNA molecules9,22. Among the DNA fragments sequenced from these two extracts, 0.18% and 0.06%, respectively, could be mapped to the human reference genome. We prepared three additional DNA libraries from the extract containing 0.18% human-like molecules, but omitted the UDG treatment to increase the number of molecules where terminal C to T substitutions could be seen and used to identify putatively ancient fragments. Because the fraction of endogenous DNA is so small, we used hybridization to DNA probes to isolate human DNA fragments from the libraries23. Applying this strategy to the mitochondrial (mt) genome allowed the mtDNA from the five libraries to be sequenced to an average coverage of 803-fold (Supplementary Information section 1). At the 3′-ends of the DNA fragments, cytosine residues (C) appeared as thymine residues (T) relative to the human mtDNA reference in 21% of fragments, reflecting appreciable levels of cytosine deamination. This suggests that at least some of the human mtDNA is of ancient origin. We determined mtDNA consensus sequences in two ways: using all mtDNA fragments, and using only deaminated fragments that carry C to T substitutions at either end relative to the consensus mtDNA sequence based on these sequences, an approach known to enrich for endogenous DNA9,24–26. The mtDNA sequence based on all fragments clusters with present-day Europeans (Extended Data Figure 1) (Supplementary Information section 1). In contrast, the mtDNA sequence based on deaminated fragments is related to a large group of present-day Eurasian mtDNAs (haplogroup N) but diverges from these before they diverged from each other. This Oase 1 mtDNA carries a few private mutations based on which its age can be estimated to 36,330 years before present (14,520–56,450; 95% confidence interval). Using six positions where the mtDNA sequence differs from at least 99% of 311 present-day humans, we estimate the contamination rate among all mtDNA fragments to 67% (95% confidence interval (CI) 65%–69%). When we restrict to mtDNA fragments that carry terminal C to T substitutions, the contamination estimate is 4% (95% CI of 2%–9%) (Supplementary Information section 1).
To isolate nuclear DNA from Oase 1, we used three sets of oligonucleotide probes that cover about two million sites that are single nucleotide polymorphisms (SNPs) in present-day humans and captured DNA molecules from the five libraries. Of the SNPs targeted, 51% (n=1,038,619) were covered by at least one DNA fragment, and 13% (n=271,326) were covered by at least one fragment with a terminal C to T substitution. To estimate nuclear DNA contamination, we tested whether Oase 1 DNA fragments with or without evidence of deamination share more alleles with present-day Europeans or with East Asians. We find that significantly more Oase 1 fragments without deamination match Europeans, implying European contamination of 17% to 30% (Supplementary Information section 1). Based on these findings and those from mitochondrial DNA, we restricted all subsequent analyses to DNA fragments that carry terminal C to T substitutions. After doing this, the fractions of SNPs from the X and Y chromosomes are similar, indicating that Oase 1 carried both an X and a Y chromosome and thus that he was male. The Y chromosome alleles belong to the F haplogroup, which is carried by most males in Eurasia today (Supplementary Information section 2).