The human genome is far from fixed. Bits of DNA called SNPs (single nucleotide polymorphisms) and RIPs (retrotransposon insertion polymorphisms) have been moving around our genomes since well before we were human. More interestingly, they are still moving around today.
SNPs are simply single base changes in our DNA. For example, a person might have a A-T base pair in the middle of a gene, rather than a C-G base pair. This might make no difference or it could be the cause of a devastating disease. RIPs are much longer stretches of DNA that get plopped into the wrong place. You’d think, given the cell’s DNA correction machinery, such structural changes would be rare. Well, it depends on what you mean by ‘rare’. Haig Kazazian and Adam Ewing of Johns Hopkins University School of Medicine found over a thousand new RIPs not previously catalogued in human genome databases.
According to Kazazian:
We're not only discovering where they are and who has which ones, but also finding out that they insert with a remarkable frequency: On the order of one in every 50 individuals has a brand-new insertion that wasn't in their parents.
The researchers hope that these new insertions can be correlated with specific diseases or phenotypic traits. If so, that might help doctors develop treatments or cures. In any case, the prevalence of these mobile elements has implications for human evolution, both in the past and in the future.