Genetic studies have found that the dogs from Southeast Asia and South China show greater genetic diversity than those dogs from other regions, suggesting that this was the place of their origin. A similar study found greater genetic diversity in African village dogs than in breed dogs. An East Asian origin has been questioned because dog fossils have been found in Europe dating around 15,000 YBP but only 12,000 YBP in far eastern Russia. The reply is that archaeological studies in East Asia lag behind those in Europe, and that the environmental conditions in southern East Asia do not favor the preservation of fossils. Although primitive forms of the dog may have existed in Europe in the past, the genetic evidence indicates that these were later replaced by dogs that have migrated from southern East Asia. In 2017, a literature review found that this East Asian study sampled only east Asian indigenous dogs and compared their patterns of genetic diversity to those of breed dogs from other geographic regions. As it is known that the genetic bottlenecks associated with formation of breeds strongly reduce genetic diversity, this was not an appropriate comparison.