Some people have demonstrably better mitochondrial DNA, others worse. Here, a study shows correlations between some variants and old-age frailty: "Mitochondria contribute to the dynamics of cellular metabolism, the production of reactive oxygen species, and apoptotic pathways. Consequently, mitochondrial function has been hypothesized to influence functional decline and vulnerability to disease in later life. ... mitochondrial DNA (mtDNA) variation was compared in frail and non-frail older adults. Associations of selected SNPs with a muscle strength phenotype were also explored. Participants were selected from the Cardiovascular Health Study (CHS), a population-based observational study ... Three mtDNA SNPs were statistically significantly associated with frailty across all pilot participants or in sex-stratified comparisons." Given the degree to which mitochondrial composition correlates with species life span differences, we should not be surprised to find some variations significant in human life span.