However, hybridization with or without introgression may, nevertheless, threaten a rare species' existence. Hybridization and introgression are side-effects of introduction and invasion. However, admixture and introgression between lineages probably took place. Finding components of Denisovan introgression with differing relatedness to the sequenced Denisovan, Browning et al. Several of the "Homo" lineages appear to have surviving progeny through introgression into other lines. The modern human papillomavirus variant 16A may descend from Neanderthal introgression. Notable gene introgression into European wildcat populations exists also in Italy, Hungary, Spain and Portugal. This could indicate two separate introgression events involving two different Denisovan populations. Thus, Neanderthal and Denisovan DNA has been incorporated into human DNA by introgression. Populations of "S. caprea" often show hybrid introgression. New Guineans have introgression from these two latter populations. An introgression of other species has not occurred. All these oak species show evidence of introgression with one another. There is evidence for introgression of "H. Heidelbergensis" into "H. sapiens". Importantly, the introgression is not random.