Alu elements are most abundant human SINEs with 500,000-1 million copies. Alus derive from 7SL RNA as 300 bp dimers. Alus contain pol III promoter, polyA tract, CpG islands, direct repeats. Alus insert via retroposition into AT-rich genome regions. Alus impact recombination, transcription, translation. Twelve Alu subfamilies defined by diagnostic substitutions. Subfamilies classified as young, intermediate, old by retroposition age. Youngest Alu insertions remain polymorphic across humans. Alus act as molecular fossils with defined ancestral state. Alus lack homoplasy due to unlikely identical insertions. Polymorphic Alus mark human genetic diversity and pedigrees. Human-specific polymorphic Alu insertions originated in Africa. Alu polymorphisms show larger ancestral population size in Africa. Polymorphic Alu insertions indicate Asian origins of Native Americans. Alu sequences reveal recent human autosomal genetic replacement.