X chromosome inactivation (XCI) is the transcriptional silencing of one X in 
female mammals, balancing expression of X genes between females (XX) and males 
(XY). In placental mammals non-coding XIST RNA triggers silencing of one X (Xi) 
and recruits a characteristic suite of epigenetic modifications, including the 
histone mark H3K27me3. In marsupials, where XIST is missing, H3K27me3 
association seems to have different degrees of stability, depending on 
cell-types and species. However, the complete suite of histone marks associated 
with the Xi and their stability throughout cell cycle remain a mystery, as does 
the evolution of an ancient mammal XCI system. Our extensive immunofluorescence 
analysis (using antibodies against specific histone modifications) in nuclei of 
mammals distantly related to human and mouse, revealed a general absence from 
the mammalian Xi territory of transcription machinery and histone modifications 
associated with active chromatin. Specific repressive modifications associated 
with XCI in human and mouse were also observed in elephant (a distantly related 
placental mammal), as was accumulation of XIST RNA. However, in two marsupial 
species the Xi either lacked these modifications (H4K20me1), or they were 
restricted to specific windows of the cell cycle (H3K27me3, H3K9me2). 
Surprisingly, the marsupial Xi was stably enriched for modifications associated 
with constitutive heterochromatin in all eukaryotes (H4K20me3, H3K9me3). We 
propose that marsupial XCI is comparable to a system that evolved in the common 
therian (marsupial and placental) ancestor. Silent chromatin of the early 
inactive X was exapted from neighbouring constitutive heterochromatin and, in 
early placental evolution, was augmented by the rise of XIST and the stable 
recruitment of specific histone modifications now classically associated with 
XCI.