Alu mobile elements are sequences of 300 bases containing the sequence AGCT that randomly insert themselves into primate genomes.

They are hereditary and always occur in the same quantity and at the same location of the genome.

The quantity and location differ by species. If Neanderthals and Cro-Magnons interbred, these variables would be the same. If the alu elements are different in number and/or location, Neanderthals should indeed be attributed to their own species.

A problem with using alu elements for evolutionary purposes is that it is in the nuclear genome and spread across 23 chromosomes (not independent).

Figure 16: Alu mobile element entering the intron region and eventually undergoing exonization to become exons.