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There are substantial subtelomeric interstitial telomeric sequence (ITS) in the human genome, however the origin of these sequences is not well understood. We investigate the possibility that these ITS have arisen via a process of chromosome end extension to the telomere sequence. By analysing the relationship between subtelomeric duplication and ITS, we identify multiple ITS which were the ancestral chromosome telomeric capping sequence. Comparison of chromosome terminal sequence between 15 species reveals an ongoing evolutionary process of chromosome extension, with an average extension rate of 0.0020 bp per year per chromosome. Analysis of SNP data from 1000 genomes demonstrates reduced SNP diversity in subtelomeric regions, indicating that many terminal regions are younger than the remaining autosomal sequence.