Purslane, Portulaca oleracea, is a worldwide crop and weed (see last week's post link). It has been cultivated for thousands of years in both the Old World and the New. That doesn't make much sense: how did it cross the oceans before Columbus?
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| purslane, Portulaca oleracea |
It is basic to biology that every species became a species in some limited area. Maybe it got cut off from other populations, maybe it evolved an adaptation that set it apart from its ancestors, whatever the detail, somewhere there was a beginning, and in some valley or on a particular mountain, not across a whole continent.
So we have purslane. It is currently found on every continent except Antarctica, from the equator to quite high elevations and latitudes.
But that's today. What of the past?
Seeds were found on Slovenian coast, dated to the early Neolithic (6000 to 4000 BCE). Archaeologists suggest the Phillistines brought purslane into Israel, in the 12th century BCE. It is reported from classical Egypt. Dioscorides, a Greek in the Roman army, wrote of purslane in 64 CE.
On the other side of the Atlantic, purslane seeds and pollen were found in southern Illinois dated to between 350 BCE and 250 CE, and archaeologists think its use goes back thousands of years. It was also documented in southern Louisiana about 500 CE. Purslane was found in Crawford Lake, Ontario in sediments from between 1340-1540 CE, ascribed to humans using the plant there. Columbus reported it in Cuba in 1492.
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| purslane, Portulaca oleracea |
Origin of Purslane
When botanists try to determine where a plant originated, they look for areas where it is numerous, has lots of relatives, or the species is particularly diverse. For Portulaca oleracea, its current worldwide distribution obscures the search for its origins. Purslane's relatives, about 100 species of Portulaca, are found all over the world (look on Wikipedia link). So these don't give much information.
However, studies of purslane chromosomes point to the Americas. In particular, purslane is a polyploid complex. There are plants with 2n = 18 (diploid), 2n= 36 (tetraploid), and 2n = 54 (hexaploid) chromosomes. Diploids are known from Africa, Florida, and Nicaragua. Tetraploids from India and North and South America. Hexaploids are known from Africa, Europe, North and Central America, and Hawaii. Other chromosome numbers such as 45 have been reported (Japan).
The lowest number is believed basic, a group of 9 chromosomes, which then doubled and tripled to make tetraploids and hexaploids. From diploid to tetraploid is not difficult: two sets of chromosomes fail to separate at meiosis but the seed lives. To get hexaploids, however, the most common way is for a gamete from a diploid (9 chromosomes) to pair with a gamete from a tetraploid (18 chromosomes), and make a living but sterile hybrid. In the hybrid, its 27 chromosomes segretate at meiosis in all sorts of uneven numbers, which form infertile gametes. But if a embryo in the hybrid doubles it chromosomes, you get a fertile hexaploid because each gamete gets 27 chromosomes (in fact, three complete packages of the original 9 chromosomes).
Thus, having both diploids and tetraploids facilitates hexaploids. Only in Florida and Nicaragua are diploids, tetraploids, and hexaploids all reported from the same population. In Africa, Asia, and Europe, not all the levels are present. It is easier to hypothesize that the hexaploids arrived in Europe from somewhere else, rather than diploids and tetraploids have gone extinct there. Thus, the evidence to date suggests a New World origin to Portulaca oleracea.
[I cannot find a world-wide search for the origin of purslane. Lots of places have never had a purslane's chromosomes counted. DNA data likewise has not yet been assembled to look for a point of origin. So this is the best hypothesis to date, there may be more to it.]
Distribution of Purslane
How did it get around the world?
It clearly did, well before Columbus, or the Vikings, or even the probable contacts between Polynesians and South America.
Looking at plant distributions, we should always consider that the people migrating from Asia to the Americas over the Bering Strait to become Native Americans, 10,000 years ago or earlier, brought or were accompanied by Old World plants. However, purslane migrated the other direction.
Thus, human transport of this plant seems unlikely. What then?
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| purslane flowers and seed pods |
Actually, purslane is very well designed to be carried by animals. The seeds are tiny and would stick in muddy feet or be caught in feathers. They survive being eaten and germinate well in mammal and bird feces. But also, the plant is succulent. It can stay alive, without water, much longer than plants typically do. It doesn't normally root at the nodes, but if broken off, the pieces readily root. Thus, not just a seed but a piece of purslane, carried by wind or water or caught on a bird, could root on the other side of the ocean.
Presumably it was carried by animals. Several authors, Ridley in particular, have made this argument in the past, but it seems to have been ignored in the last couple decades. Horses and other animals went from North America to Asia across the land bridge. Birds fly from the Americas to Europe and Asia regularly. So it is not impossible for purslane to have been eaten or grown by humans in both the New and Old Worlds long before humans crossed the oceans. We simply have to remember that plants do not require humans for their distributions and that animals have been moving around the globe for millions of years. Somewhere in the Americas Portulaca oleracea evolved from a Portulaca ancestor not terribly long ago (because, despite being quite variable, detailed investigations continue to call it a single, very complicated species) and hitched a ride to the Old World. On both continents, humans tasted it and found it delicious.
This is not a typical story. Most worldwide species were recently moved by humans. Or they have become several different species in different parts of the world. But purslane reminds us that the common patterns aren't the only ones. Purslane is an abundant, edible, weedy plant found all around the world.
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| Purslane, Portulaca oleracea |
Comment: Looking into this, I found many sites that repeat, despite the evidence, that purslane is native to the Old World. I wonder if that represents a bias: things always go from Eurasia to the Americas. But they don't and we shouldn't assume they do. Furthermore there are a number of articles on the web which read as very authoritative but in fact are old and repeat discredited ideas, although it can be difficult to figure that out. They might tell you purslane came to the Americas after 1492, while even a little research since 1975 will tell you otherwise. People uploading older material should be careful to include the author and date of publication. People reading the internet should note when the information was written and check for confirmation in things published in the last decade.
Comments and corrections welcome.
References
Byrne, R., and McAndrews, J. H. (1975). Pre-Columbian purslane (Portulaca oleracea L.) in the new world. Nature 253, 726–727.
Danin, A., I. Baker and H.G. Baker. 1978. Cytogeography and taxonomy of the Portulaca oleracea L. polyploid complex. Israel Journal of Botany. 27: 177-211.
Frumin, S. and E. Weiss. 2018, Plant use in the Bronze and Iron Ages at Tell eṣ-Ṣâfi/Gath. Near Eastern Archaeology. 81 (1): 77-80.
Green, A. J. 2016. The importance of waterbirds as an overlooked pathway of invasion for alien species. Diversity and Distribution. 22 (1/2): 239-147.
Kumar, A., S. Sreedharan, P. Singh, E. G. Achigan-Dako, and N. Ramchiary. 2021. Improvement of a Traditional Orphan Food Crop, Portulaca oleracea L. (Purslane) Using Genomics for Sustainable Food Security and Climate-Resilient Agriculture. Frontiers in Sustainable Food Systems. link (Accessed 8/9/22)
Levin, D. A. and S. V. Scarpino. 2017. On the young age of intraspecific herbaceous taxa. New Phytologist. 213: 1513-1520.
Matthews, J. F., D. W. Ketron and S. F. Zane. 1992, The biology and taxonomy of the Portulaca oleracea L. (Portulacaceae) complex in North America. Rhodora. 95: 166-183.
Parker, K. E. and B. M. Butler. 2017. Botanical Remains from the Baumer Component at Kincaid Mounds. Midcontinental Journal of Archaeology. 42 (2); 113-136.
Reed, K. 2015. From the field to the hearth: plant remains from Neolithic Croatia (ca. 6000-4000 cal BC). Vegetation History and Archaeology. 24 (5): 610-619.
Ridley, H. N. 1902 .Dispersal of plants throughout the world. L. Reeve, Ashford, Kent. 1930 edition online. link p. 260, 472, 481, 541 and others.
Zizumbo-Villarreal, D., A. Flores-Silva and P. Colunga-GarcíaMarín. 2014. The food system during the Formative Period in West Mesoamerica. Economic Botany 68 (1): 67-84.
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