Another less important rice, Oryza glaberrima (African rice), is grown in east Africa, but is being replaced by Oryza sativa.
The cultivated rices and their ancestors are considered to be diploid (2n = 24), although their high chromosome number indicates that they could be ancient, diploidized polyploids.
Ten genomes have been identified among the various sections of Oryza, based on chromosome pairing relationships, molecular markers and sequencing.
The cultivated species and their closest relatives carry the A genome and form what is referred to as the sativa complex. 
The A genome is further divided with superscripts to denote small pairing aberrations and partial sterility among the various diploid species.
The origins of Asian rice cultivation are also clouded. Many authorities consider India to be its cradle, but strong cases have been made for much earlier origin in central China and South East Asia.
From its beginning somewhere in Central Asia around 10,000 BC, rice cultivation probably moved into Korean and Japan by 3000 BC. The cultivation of African rice probably began in the Niger delta about 3500 years ago and spread gradually across tropical East Africa.
Asian rices arrived in Africa about 2000 BC. Rice found its way to the New World in 1647, when its cultivation was begin in the Carolinas.
Rice and cultivation
