Where bananas come from

Anne Vézina Wednesday, 07 October 2015

CIAT, the International Center for Tropical Agriculture, recently did a blog brilliantly titled Mi cassava es tu cassava to remind us that many, if not most, of our locally grown crops originated somewhere else. Depending on where you live, chances are that the potato, cassava, wheat, sweet corn, tomato or chili pepper grown in your country were domesticated in another region and introduced sometime in the last 10,000 years. When it comes to food crops, no country is an island.

Source CIAT
Source CIAT

I highly recommend the blog and its accompanying policy brief, which make the case for putting in the public domain the genetic diversity of all crops, not just the 64 covered by the Plant Treaty[1]. My admittedly nit-picking quibble is that banana is not listed among the crops domesticated in the Pacific region. In the map on the right, which shows the primary centres of diversity of the world’s major food crops, the banana is tagged to South and Southeast Asia only. It's not surprising. It's what many textbooks say. I blame Norman Simmonds for this.

Simmonds was not only a respected plant breeder (besides banana, he also worked on potato and sugar cane), he was also a prolific writer who helped popularize the idea that bananas originated in Southeast Asia. In Where our bananas come from, published in 1962 in the New Scientist magazine, he wrote that to answer the question on where cultivated varieties of banana come from, “we must go back to Malaysia several thousand years ago, for it was there that men took the first decisive steps in converting the inedible, wild, seedy bananas of the jungles into the lush, parthenocarpic and sterile fruit that we know today”.

Photo by A. D’Hont
Photo by A. D’Hont

Fair enough. Nobody denies that Southeast Asia is a centre of domestication of banana. It’s just that Simmonds — the author of the reference book on bananas — should have known better, if only because a few years earlier he had been in a collecting mission to Southeast Asia and the Pacific. During that trip, he visited what is now Papua New Guinea, where he observed a diversity of bananas he had not seen anywhere else. The unusual thing about these edible bananas is that like their wild ancestors they have two sets of chromosomes (diploids)[2], whereas most of the bananas cultivated nowadays are triploids, in that they three sets of chromosomes (see From diploids to triploids in The Pacific connection). Because in Asia edible diploids had been largely displaced by the more productive triploids they had given rise to, Simmonds assumed that the diploids he saw in PNG had been introduced from Southeast Asia early in the domestication process and had owed their survival to the late arrival of triploid bananas. Except for the unusual Fei bananas – which Simmonds recognized as having been domesticated in the Pacific because their wild ancestor(s) are not found elsewhere – he could not imagine that bananas tracing their origin to Musa acuminata alone, or hybridized with Musa balbisiana, had been domesticated outside Asia.

Subsequent genetic analyses and the discovery of 7,000-year-old banana phytoliths[3] at Kuk Swamp in the highlands of PNG, now a UNESCO World Heritage Site, proved Simmonds wrong. New Guinea and nearby islands, including the Solomon Islands, are definitely centres of domestication of banana. But the story doesn’t stop here. Actually it dovetails quite nicely with the message in CIAT’s blog.

Analysing the genetic make-up of hundreds of banana cultivars revealed that more than one subspecies of Musa acuminata have been implicated in the domestication of bananas. Since these subspecies occupy distinct geographic regions (have a look at the maps in an open access paper on banana domestication), different subspecies at various stages of domestication must have been moved around in order to come into contact and hybridize. In technocratic jargon, it means that the meeting of Malaysian and New Guinean bananas, among others, was facilitated by people sharing genetic resources.

The rest, as they say, is history. Maybe as early as 2,000 to 4,000 of years ago, the ancestors of East African highland bananas and Plantains went west to Africa, where they continued to diversify, making Africa a secondary centre of diversity. In the other direction, many of the Western Pacific bananas were taken east by seafarers, eventually landing in Hawaii. And maybe South America, but that’s another story.

The more recent introductions to the Middle East, Latin America and the Caribbean are better documented, but we should not forget that as these movements were happening, bananas were also moved between regions where they are native. The world is a richer place as a result, although sometimes I wish people had been more disciplined in naming their bananas. But then again, we would be missing out on the fun of doing a checklist of banana cultivar names.

1. For those who are wondering, banana is on the list of 64 crops. This means that the accessions conserved at the International Transit Center, which celebrates its 30th anniversary this year, are available for sharing with scientists and other bona fide users.
2. Humans also have two sets of chromosomes, one inherited from the mother and the other from the father.
3. Phytoliths are microscopic stones made of silica that form in plant cells and are used as archaeological markers for plants that, like domesticated bananas, do not produce seeds and pollen. To see what they look like, click here.