The Paper’s Claim vs. The Critical Question
The study identifies a human-specific BRCA2 mutation (M2662T) that reduces DNA double-strand break repair efficiency by ~20% compared to the chimpanzee variant. The authors propose this reduction was positively selected during human evolution, speculating it might represent an evolutionary trade-off favoring other biological functions[1].
But does breaking a pre-existing DNA repair system demonstrate the creative power of mutation/selection? Or does it highlight the limits of evolutionary mechanisms to build fundamentally new biological systems?
Critical Analysis: A Closer Look at the Evidence
Finding 1: Functional Reduction as “Adaptation”
The paper documents a loss of interaction between BRCA2 and RAD51/DSS1 proteins due to the M2662T mutation, decreasing homologous recombination repair capacity[1]. While framed as an evolutionary innovation, this represents:
- Degradation of existing machinery: The mutation replaces hydrophobic methionine with polar threonine in a conserved domain, weakening protein-protein interactions critical for DNA repair[1]. This mirrors documented loss-of-function mutations in cancer biology rather than novel functionality.
- No new information created: The change modifies an existing molecular interface but doesn’t create new protein domains, regulatory networks, or repair mechanisms. It’s analogous to sanding down a gear in a watch – the mechanism still works, but less efficiently.
Finding 2: Evolutionary Storytelling vs. Mechanism
The authors propose four evolutionary models to explain BRCA2M2662T fixation, ultimately favoring a “mutational trade-off” where reduced DNA repair capacity might confer unspecified advantages[1]. However:
- No experimental evidence links the 20% repair reduction to any selectable benefit in human ancestors
- Teleological reasoning: The assumption that reduced repair “must have been beneficial” replaces mechanistic explanation with just-so storytelling
- Contrast with engineering: No human engineer would intentionally degrade a critical repair system without clear compensatory innovation – yet the paper identifies none
The Bigger Picture: Why This Isn’t Evidence for Microbes-to-Man Evolution
This study inadvertently highlights three key problems for grand evolutionary narratives:
- The Paradox of “Constructive Degradation”: If breaking existing systems counts as evolutionary progress, it reverses the logic of biological innovation. New organs, body plans, and molecular machines require adding functional complexity, not subtracting it.
- The Limits of Tinkering: Even if this mutation provided survival advantages (unproven), it operates entirely within the existing framework of eukaryotic DNA repair systems. It offers no model for how such systems originated from prokaryotic precursors.
- The Information Gap: The mutation involves a single amino acid substitution in a 3,418-residue protein[1]. This scale of change is orders of magnitude removed from the coordinated mutations needed to build new protein folds, let alone new cell types.
Broader Context: When Loss Masquerades as Gain
This pattern of framing functional losses as evolutionary innovations recurs throughout evolutionary biology. Geneticist Michael Behe notes in Darwin Devolves that “the first rule of adaptive evolution is: Break or blunt any functional gene whose loss would increase the number of offspring.” While such changes may enable microevolutionary adaptation, they operate fundamentally differently from the constructive mutations required for large-scale biological innovation.
The Bottom Line: Evolution’s Unsolved Paradox
If mutation/selection could truly build new biological systems from scratch, we’d expect studies to document:
- Novel protein domains arising through random mutation
- Stepwise construction of irreducibly complex systems
- Clear trajectories from prokaryotic to eukaryotic molecular machinery
Instead, this paper – like much evolutionary research – documents variation within existing frameworks. A 20% reduction in DNA repair capacity might help explain cancer susceptibility differences between humans and chimpanzees. But it does nothing to explain how the infinitely more complex DNA repair system itself came to exist. Until evolutionary studies can bridge this chasm between modification and creation, the grand narrative remains an extrapolation beyond the evidence.
Paper Analyzed:
Selection for Decreased BRCA2 Functional Activity in Homo sapiens After Divergence from the Chimpanzee-Human Last Common Ancestor
Jinlong Huang, Yi Zhong, Alvin P. Makohon-Moore, Travis White, Maria Jasin, Mark A. Norell, Ward C. Wheeler, Christine A. Iacobuzio-Donahue
Abstract Excerpt:
“Humans have an increased incidence of epithelial neoplasia compared to non-human primates… We conclude that a 20% reduction in BRCA2 DNA repair ability was positively selected for in the course of human evolution.”