Effects of introgressed Neanderthal alleles on present-day brain morphology

By analyzing MRI data from nearly 40,000 UK Biobank participants, this study reveals that while Neanderthal alleles causing major neuroanatomical divergence were largely purged, a subset of tolerated introgressed variants continues to subtly shape present-day human brain morphology and influence susceptibility to neuropsychiatric disorders.

The Gist

The Big Picture: A Genetic "Heirloom" in Our Brains

Imagine that modern humans and Neanderthals were two different families living in neighboring villages thousands of years ago. About 40,000 years ago, these families started intermarrying. As a result, most of us non-Africans today carry a tiny bit of "Neanderthal DNA" in our genetic makeup—about 2% of our total genome.

Think of this DNA like an old, dusty heirloom passed down through generations. For a long time, scientists knew this DNA existed, but they didn't know what it actually did to us. Did it make us taller? Did it change our skin? Did it affect our brains?

This study is like opening that dusty heirloom box to see what's inside, specifically focusing on how Neanderthal DNA shapes the shape and structure of the modern human brain.

The Mystery: Why Did the Brain Change?

If you look at fossilized Neanderthal skulls, you can see their brains looked different from ours.

• Neanderthals: Had long, oval-shaped brains (like a football).
• Modern Humans: Have round, globular brains (like a beach ball).

Scientists have always wondered: What genetic instructions caused our brains to become rounder? And more importantly: Did the bits of Neanderthal DNA we still carry today still have any power over our brain shape or our mental health?

The Investigation: A Massive Brain Scan

The researchers went to the UK Biobank, a massive database containing MRI brain scans of nearly 40,000 people. They didn't just look at the whole brain; they broke it down into 370 different measurements, like:

• How thick the outer layer (cortex) is in different spots.
• How deep the folds (sulci) are.
• The size of the inner "wiring" (white matter tracts).
• The volume of specific areas like the frontal lobe or the cerebellum.

They then cross-referenced these brain measurements with the people's DNA to see if the "Neanderthal bits" were linked to any specific brain shapes.

The Findings: The "Bad" Was Removed, The "Good" Remained

Here is the most interesting part of the story, explained with an analogy:

1. The "Purging" (The Low-Frequency Variants)
Imagine the Neanderthal DNA as a shipment of furniture delivered to a modern house. Most of the furniture was old, broken, or didn't fit the modern style.

• The Result: The study found that the rare pieces of Neanderthal DNA (the ones only a few people have) were mostly "broken." They had been weeded out by natural selection because they were harmful or didn't work well with modern human biology. They didn't seem to affect brain shape much because they were too rare to matter.

2. The "Survivors" (The Common Variants)
However, some pieces of furniture were sturdy and actually useful.

• The Result: The common Neanderthal DNA (the ones many people have) did have an effect. These are the "survivors" that natural selection kept because they weren't harmful, or maybe they were even helpful.

The "Star" Discovery: The DAAM1 Locus

The researchers found one specific spot in the genome (near a gene called DAAM1) that was a superstar.

• What it does: This Neanderthal DNA snippet influences the shape of the cuneus and precuneus (areas in the back of the brain involved in vision and self-awareness).
• The Twist: Here is the surprise. You might expect Neanderthal DNA to make our brains look more like Neanderthal brains (longer, flatter). But this specific snippet actually does the opposite. It pushes the brain structure in a direction that makes it less like a Neanderthal brain.
• The Analogy: It's like inheriting a recipe from a great-grandparent that you thought would make a heavy, old-fashioned cake, but instead, it turns out to be a secret ingredient that makes the cake rise higher and fluffier than modern recipes do.

The Mental Health Connection

The study didn't just stop at brain shape; it looked at mental health, too.

• Schizophrenia: The Neanderthal DNA at the DAAM1 spot seems to offer a protective shield against schizophrenia.
• Depression: Conversely, other Neanderthal DNA snippets seemed to slightly increase the risk for major depression.

This suggests that the same genetic changes that shaped our brains also shaped our mental health. The "heirloom" DNA isn't just about how our brains look; it's about how they work.

The "Desert" Analogy

The paper also talks about "Neanderthal Deserts."

• The Concept: Imagine the human genome is a map. There are huge areas on this map where you will never find Neanderthal DNA. These are the "Deserts."
• Why? These are the most critical areas for human survival (like genes that build the brain). If a Neanderthal gene landed there, it would have been so harmful that the person carrying it wouldn't have survived to pass it on. So, nature "deserted" those areas of Neanderthal DNA.
• The Finding: The study confirmed that these deserts are real and are packed with genes that make our brains unique.

The Bottom Line

This paper tells us that while our Neanderthal ancestors left us a genetic legacy, nature was very picky about what it kept.

1. Most Neanderthal DNA was junk for our modern brains and was thrown away.
2. The DNA that stayed is still actively shaping our brains today, influencing everything from the thickness of our cortex to our risk of mental illness.
3. It's a mix: Some of it protects us (like against schizophrenia), while some might make us more vulnerable (like to depression).

In short, we are walking around with a few Neanderthal "ghosts" in our brains, and they are still whispering instructions on how our brains are built and how they function.

Technical Summary

1. Problem Statement

While the morphological differences between Neanderthals and anatomically modern humans (AMH) are well-documented via fossil endocasts (e.g., Neanderthals had elongated brains, while AMH evolved a more globular shape), the specific genetic mechanisms driving these differences remain largely inaccessible. Previous studies on Neanderthal introgression have identified impacts on neuropsychiatric traits (e.g., depression, schizophrenia) and some brain globularity loci, but these were limited by small sample sizes (e.g., <300 individuals) or a lack of high-resolution neuroimaging data.

The central question addressed by this study is: How do Neanderthal-derived genetic variants (introgressed alleles) influence present-day human brain morphology, and what is the selective regime acting on these variants? Specifically, the authors aim to determine if alleles affecting divergent neuroanatomy were purged by selection or if a subset of tolerated alleles continues to shape human brain structure and mental health.

2. Methodology

The study leverages the UK Biobank (UKBB) dataset, analyzing a cohort of 38,406 unrelated individuals of European ancestry with available brain MRI scans and imputed genotypes.

• Data Integration & Variant Identification:

  • Neanderthal-Specific Variants (NSVs): The authors utilized a previously inferred set of Neanderthal introgressed segments (from Morez Jacobs et al., 2025) identified using the SSTAR algorithm. They defined NSVs as variants present in Neanderthal genomes (Altai, Vindija, Chagyrskaya) but with a derived allele frequency <1% in Yoruba (YRI) populations.
  • LD Expansion: To capture variants in near-perfect linkage disequilibrium (LD) with NSVs ($R^2 > 0.99$, distance < 200kb), the set was expanded to include 422,195 NSVs.
  • Categorization: Variants were classified into four categories: NSVs, variants overlapping introgressed segments but not NSV (OVPint), variants in "Neanderthal deserts" (regions devoid of introgression), and "other" standing variation.

• Phenotypic Analysis:

  • 370 Brain Imaging-Derived Phenotypes (BIDPs): The study analyzed a comprehensive suite of MRI traits, including cortical regional areas/volumes/thicknesses, subcortical volumes, cortical folding metrics (sulcal depth/length/width), and diffusion tensor imaging (DTI) tract diffusivity.
  • GWAS & Aggregation: Genome-wide association studies (GWAS) were performed for each BIDP using REGENIE. To handle the polygenic nature and correlations between traits, Aggregated Cauchy Association Test (ACAT) was used to combine p-values across all 370 traits, identifying loci with significant effects on any brain trait.

• Statistical & Functional Analyses:

  • Frequency/Depletion Analysis: The authors compared the fraction of significant variants across different Minor Allele Frequency (MAF) and LD score bins to test for purifying selection.
  • Fine-Mapping: SuSiE (Sum of Single Effects) was used to generate credible sets of causal variants. Loci where NSVs constituted the majority of the credible set were flagged as Neanderthal-driven.
  • Functional Annotation: Variants were checked against single-cell ATAC-seq data (open chromatin) and gkmSVM models to predict effects on chromatin accessibility in specific brain cell types (e.g., microglia, neurons).
  • Directional Effects: Two methods were employed to assess genome-wide directional alignment:
    1. SLDP (Signed LD Profile) Regression: Correlating GWAS effect sizes with the frequency difference between Neanderthals and YRI.
    2. Individual Burden Analysis: Calculating a "Neanderthal burden score" for each individual and regressing it against BIDPs, both genome-wide and restricted to trait-associated loci.

3. Key Contributions

• Scale: This is the largest study to date linking Neanderthal introgression to high-resolution brain morphology, utilizing nearly 40,000 individuals and 370 distinct neuroimaging traits.
• Methodological Refinement: The study introduces a rigorous framework for distinguishing between low-frequency and high-frequency introgressed variants, revealing a dual pattern of selection that was previously obscured.
• Causal Inference: By combining fine-mapping with functional genomics (ATAC-seq/gkmSVM), the authors move beyond association to propose specific regulatory mechanisms (e.g., chromatin accessibility changes) for Neanderthal alleles.
• Link to Mental Health: The study explicitly connects morphological changes driven by Neanderthal alleles to neuropsychiatric outcomes (schizophrenia, major depression).

4. Key Results

A. Selection Patterns: Frequency-Dependent Depletion

• Low-Frequency Depletion: Low-frequency NSVs are significantly depleted for associations with brain phenotypes compared to other variant categories. This confirms strong post-admixture purifying selection against deleterious low-frequency Neanderthal alleles.
• High-Frequency Retention: Conversely, common NSVs (MAF > 0.25) show no depletion and are actually enriched for significant associations with brain morphology. This suggests that a subset of Neanderthal alleles affecting brain structure escaped purifying selection or were positively selected, likely because their effects were compatible with modern human development or offered adaptive advantages.
• Neanderthal Deserts: Regions devoid of Neanderthal DNA ("deserts") are consistently enriched for functional effects, reinforcing the idea that these regions were under strong negative selection.

B. Specific Loci and Fine-Mapping

• 28 Significant Loci: Out of 521 genome-wide significant loci associated with BIDPs, 28 contained at least one significant NSV.
• 8 Causal Loci: In 8 loci, NSVs comprised the majority of the fine-mapped credible set, strongly suggesting they are the causal variants.
  • DAAM1 (Chromosome 14): The most prominent locus. A ~48kb Neanderthal haplotype containing four NSVs is associated with 39 traits, including occipital and parietal volumes. Notably, the effect direction on the precuneus is opposite to the expected Neanderthal morphology (which typically has a smaller precuneus), suggesting complex regulatory effects. These variants are eQTLs for DAAM1 and overlap open chromatin in microglia and neurons.
  • PRDM5 (Chromosome 4): Associated with frontoparietal cortical thinning and linked to Major Depressive Disorder (MDD) and schizophrenia.
  • GMNC (Chromosome 3): Linked to ventricular volume and diffusion tracts, consistent with its role in ciliated cell differentiation.
  • Other Loci: JUN, SLC13A3, SETD9, EYA4, and TSTD3 were also identified with specific impacts on white matter tracts and subcortical volumes.

C. Directional Effects and Morphology

• No Global Globularization Signal: Genome-wide directional analysis (SLDP) did not reveal a strong signal mirroring the known "globularization" of the modern human brain (i.e., Neanderthal alleles did not uniformly push traits toward the archaic shape).
• Regional Divergence: When focusing on suggestive loci, frontal and parietal areas were the most consistently affected. Neanderthal burden was associated with decreased area but increased thickness in frontal regions, suggesting a divergent arrangement of grey matter.
• Internal Structures: Significant associations were found with the caudate nucleus, an internal structure not visible in fossil endocasts, highlighting the unique power of genetic approaches to reveal "invisible" evolutionary changes.

D. Neuropsychiatric Implications

• Schizophrenia: The DAAM1 locus showed a protective effect against schizophrenia, consistent with previous findings of an inverse correlation between Neanderthal alleles and schizophrenia risk.
• Major Depressive Disorder (MDD): A positive association was found between Neanderthal burden (restricted to BIDP-relevant loci) and MDD, suggesting a pathway where introgressed variants influence brain morphology which, in turn, impacts mental health.

5. Significance

This study fundamentally shifts the understanding of Neanderthal introgression in the human brain:

1. Purifying Selection vs. Adaptation: It demonstrates that while most Neanderthal alleles affecting brain development were deleterious and purged (especially at low frequencies), a specific subset of common regulatory variants was retained. These variants continue to shape human cortical organization and connectivity.
2. Beyond Endocasts: The findings reveal that genetic introgression affects brain structures (like the caudate nucleus and specific cortical folding patterns) that are not resolvable from fossil endocasts, providing a "genetic window" into archaic brain morphology.
3. Evolutionary Trade-offs: The retention of alleles that influence both brain morphology and neuropsychiatric risk (e.g., schizophrenia protection vs. MDD risk) suggests that the evolutionary forces shaping the modern human brain involved complex trade-offs between structural optimization and mental health susceptibility.
4. Regulatory Mechanism: The study highlights that the primary mechanism of Neanderthal influence is likely regulatory (altering chromatin accessibility and gene expression in specific cell types) rather than protein-coding changes.

In conclusion, the paper establishes that Neanderthal introgression is not merely a relic of the past but an active, albeit filtered, component of the genetic architecture governing modern human brain structure and mental health.