Evolution of the basal ganglia in tetrapods: a new perspective based on recent studies in amphibians.

It has been postulated frequently that the fundamental organization of the basal ganglia (BG) in vertebrates arose with the appearance of amniotes during evolution. An alternative hypothesis, however, is that such a condition was already present in early anamniotic tetrapods and, therefore, characterizes the acquisition of the tetrapod phenotype rather than the anamniotic-amniotic transition. Re-examination of the BG organization in tetrapods in the light of recent findings in amphibians strongly supports the notion that elementary BG structures were present in the brain of ancestral tetrapods and that they were organized according to a general plan shared today by all extant tetrapods.