The evidence ladder begins with a reporter assay: 561,410 human–ape substitutions were tested and 15,077 loci showed significantly different regulatory activity. Hybrid human–ape cells then revealed 4,463 human-specific differences in nearby, or cis, expression. Pathway analysis found that 17 of 21 differently expressed glycosaminoglycan genes were downregulated, and human joint tissue contained approximately one-quarter to one-third as much glycosaminoglycan as joint tissue from non-human great apes. Together these observations support regulatory remodeling of cartilage biology, but they do not prove that any one substitution changed skeletal shape, caused bipedalism or produced arthritis.
Experimental layerQuestion and testObservation and limit
Can a human DNA letter change regulatory activity relative to its ancestral great-ape version?
561,410 substitutions tested; 15,077 loci showed significantly different activity.Does not establish: a change in bone shape, walking or disease.
2Shared cellsHybrid-cell comparison
Does the difference persist when human and ape genomes operate in the same cellular environment?
4,463 human-specific differences in cis-regulated expression were reported.Does not establish: how a complete, moving joint develops or functions.
3PathwayConverging signals
Do many regulatory and expression differences point toward the same biological system?
Glycosaminoglycan biology emerged repeatedly; 17 of 21 differently expressed GAG genes were downregulated.Does not establish: which skeletal trait selection favored.
Does actual cartilage show a corresponding biochemical difference?
Human joints contained approximately one-quarter to one-third as much GAG as joints from non-human great apes.Does not establish: that one variant caused the difference or that reduced GAG caused arthritis.
Read upward carefully: Convergence makes the broader hypothesis more plausible; it does not turn association into single-variant causation.