Neanderthal DNA in Modern Humans: Immune Genes, COVID Risk, and 1–4% Ancestry

Forty Thousand Years Later, Their DNA Still Shapes Your Immune System

When Svante Pääbo's team published the first draft Neanderthal genome in Science in May 2010, they reported a finding that changed human evolutionary biology overnight: people of non-African descent carry approximately 1–4% Neanderthal DNA in their genomes. The interbreeding had occurred around 50,000–60,000 years ago, as anatomically modern humans expanded out of Africa and encountered Neanderthal populations already established across Europe and western Asia. The two groups met. They mixed. And traces of that meeting persist in billions of living humans today.

We are not entirely Homo sapiens.

The Geographic Distribution of Admixture

Neanderthal ancestry is absent or negligible in sub-Saharan African populations, whose ancestors did not encounter Neanderthals during the out-of-Africa expansion. In all other populations, the proportion varies by region:

The higher Neanderthal ancestry in East Asians compared to Europeans may reflect a second admixture pulse in East Asia after the initial out-of-Africa bottleneck, or differential selection against Neanderthal alleles in European populations over time.

Immune System Inheritance: HLA and Toll-Like Receptors

Not all inherited Neanderthal genes survived by chance. Many persist because they were advantageous — particularly genes involved in immune function. Three categories stand out:

• Toll-like receptors (TLR1, TLR6, TLR10): A 2016 study in American Journal of Human Genetics (Dannemann et al.) found that a haplotype at this cluster in chromosome 4, derived from Neanderthals, is carried by up to 50% of Europeans and East Asians. The Neanderthal-derived version enhances inflammatory responses to bacterial infections.
• OAS1, OAS2, OAS3 antiviral genes: These genes encode enzymes that degrade viral RNA. Neanderthal-derived variants of OAS1 have been associated with reduced severity of several viral infections.
• HLA class I and II genes: Human leukocyte antigen (HLA) genes govern immune recognition of pathogens. Several HLA alleles common in modern non-African populations — including HLA-A*02 and HLA-C*07 — may have been acquired or reinforced through Neanderthal admixture, expanding the immune repertoire of early modern humans entering pathogen-rich environments.

Neanderthal Variants and COVID-19 Risk

In September 2020, Svante Pääbo and Hugo Zeberg published a paper in Nature identifying a genomic risk factor for severe COVID-19 — a cluster of variants on chromosome 3 — as Neanderthal in origin. Key findings:

• The chromosome 3 haplotype is carried by approximately 16% of Europeans and 50% of South Asians
• Carriers face a significantly elevated risk of respiratory failure from SARS-CoV-2 infection — odds ratios ranging from 1.6 to 2.1 depending on the study
• The haplotype is rare in East Asia (1.8%) and absent in Africa, consistent with Neanderthal admixture origin
• The functional mechanism involves genes regulating immune cell receptor expression in the airways

Conversely, a Neanderthal-derived variant at the OAS1 locus appears to be protective against COVID-19, demonstrating that the Neanderthal genomic legacy cuts in both directions — conferring risks and benefits depending on which ancient variants a person carries.

Neanderthal Cognitive Capability: The Ongoing Debate

Neanderthals possessed brains equal in volume to modern humans (averaging approximately 1,600 cc versus 1,350 cc for modern humans). Archaeological evidence of Neanderthal cognitive complexity has accumulated significantly:

• Eagle talon jewelry from Krapina, Croatia (~130,000 years ago) predates H. sapiens arrival in Europe by 80,000 years
• Pigment use and possible body decoration at multiple European sites
• Burial of the dead — debated but supported at Shanidar (Iraq) and La Ferrassie (France)
• Production of the Châtelperronian tool industry, more refined than earlier Neanderthal industries

A persistent counter-argument holds that some "Neanderthal" sophisticated behaviors date to the period of contact with H. sapiens and may reflect imitation rather than independent invention. The gene FOXP2 — associated with human speech and language — appears in Neanderthal genomes in a form identical to the modern human version, though whether Neanderthals had language comparable to modern humans remains unresolved.

The Neanderthal "Desert": Purifying Selection

While Neanderthal DNA persists across the modern human genome, it is conspicuously absent from certain regions. The X chromosome and gene-dense regions involved in brain development and spermatogenesis are nearly devoid of Neanderthal ancestry — a pattern consistent with hybrid incompatibility. Male hybrids may have had reduced fertility, limiting transmission of certain chromosomal combinations to subsequent generations. This "Neanderthal desert" on the X chromosome was described by Sankararaman et al. in Nature in 2014 and has been replicated in multiple subsequent analyses.