Yams are tuber-bearing vines of the genus Dioscorea that are exploited as a source of food by people living in the tropics and subtropics. They are monocots of the Order Dioscoreales, closely related to Pandanales (which include the Pandan or Screw Pine). It is believed that the two orders split up between 130-120 million years ago in the Early Cretaceous. Different species of yam are found in the Americas, Africa and Asia. Yam tubers are rich in starch. The major species of yam are Dioscorea cayennensis and Dioscorea dumetorum (native to Africa), Dioscorea alata and Dioscorea esculenta (Southeast Asia), Dioscorea polystachya (East Asia), Dioscorea bulbifera (across the Old World), and Dioscorea trifida (South America).
Two Indian scientists – RC Mehrotra and Anumeha Shukla from the Birbal Sahni Institute of Palaeosciences (in Lucknow, the capital of the state of Uttar Pradesh), undertook excavations in Rajasthan’s Gurha lignite mine (part of the Palana Formation, near the city of Bikaner in western Rajasthan). They found fossilized leaves of an extinct species of yam – Dioscorea eocenicus, dating back to the Eocene (56-38 million years ago). This is the first instance of yam fossils being recovered in Asia. Earlier, fossils were found in Mexico, Chile, France, Ethiopia, and Kenya. These findings hint at a Gondwanan origin of the Dioscoreacea Family (to which the yam genus Dioscorea belongs). Gondwana or Gondwanaland was the southern supercontinent formed by the coming together of South America, Africa, Madagascar, India, Australia and Antarctica. Gondwanaland broke up and India drifted north, colliding with Asia and giving rise to the Himalayas. Interestingly, the scientists also found evidence of the great change that this northward movement of India had on its climate and vegetation.
The region of Bikaner in western Rajasthan is part of a vast, sandy desert. During the Eocene, when Dioscorea eocenicus used to grow in this region, it was covered by lush, tropical forests, only a few degrees north of the equator. As India (then an island) began to move away from Africa, Rajasthan’s climate turned drier and the vegetation changed, till it assumed its current state – an arid, sun-baked land devoid of all but the hardiest xerophytes. The lush, tropical forests that once covered all of the subcontinent are now to be found far to the south, in the Western Ghats of peninsular India. The scientists’ findings were published in the Review of Palaeobotany and Palynology.
Image Attribution: The image above, sourced from Wikimedia Commons, shows vines of Purple Yam (Dioscorea alata) in Maui Nui Botanical Garden, Maui (Hawaii). The photograph was uploaded by Forest and Kim Starr.
- ‘First record of Dioscorea from the early Eocene of northwestern India: Its evolutionary and palaeoecological importance’, Research Gate (dated November, 2018)