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It may come as a surprise to cactus hobbyists but probably more has been written about Opuntia ficus-indica than any other cactus species. To illustrate this point, a quick Google scholar search will reveal over 12,400 results for Opuntia ficus-indica. You can learn about its medicinal properties for treating ulcers, scurvy and diabetes, lowering cholesterol, accelerating wound healing and even as a hangover cure. It is a source of dietary fibre, high in Vitamin C and essential amino acids. It is eaten by humans, grown as cattle fodder and as a host for the cochineal insect, the source of a natural red dye. Some people even admit to growing it in their cactus collections. It is thus closely linked with mankind for most of human occupation of the New World. In addition, ever since the first arrival of the Spanish in Mexico, it has been spread around much of the planet by the hand of man. Yet, despite this, its origins remain shrouded in mystery. Modern research suggests it is a native of southern Mexico. But, to better understand the potential origins of Opuntia ficus-indica (Fig. 16), we need to examine not just the plant but the landscape of Southern Mexico and the early history of human civilization in the New World.

One of the papers in the scientific literature examines molecular evidence to try and determine the closest relatives of Opuntia ficus-indica1. In this study, Patrick Griffith proposes that there is a close relationship of the tree-like, fleshy-fruited, large padded Opuntias of South and Central Mexico. Different clones of Opuntia ficus-indica appear at different places within the cladogram suggesting several independent origins of the species. This strongly supports the classical hypothesis that selected clones of Opuntia ficus-indica became a domesticated crop plant and became so important to man that it quickly spread across much of Mesoamerica. Another potential inference from the cladogram is that it is of hybrid origin. The inference is that the hybridization to form what is today known as Opuntia ficus-indica happened more than once. Opuntia streptacantha and O. tomentosa seem to be two of the most likely parents. The case for O. streptacantha as a potential parent to O. ficus-indica is strengthened as it too has significant anti-diabetic and cholesterol lowering properties. But, with the notoriety of Opuntias to hybridise, it does not tell us if this is a natural or man-made hybrid. Whichever is true we next need to look at why it became so important to our early ancestors.

The earliest generally accepted evidence of human occupation of the New World is towards the end of the last Ice Age about 14,000 to 15,000 years ago through a corridor in the melting ice cap at the Northern end of the Americas.2 On entering what is now modern day Mexico, man was met by a wealth of large mammalian species that were common across the whole of the Americas at that time. These provided for all his needs. About 9,000 to 10,000 years ago the climate began to change and become more arid. Coincident with this, large prey animals became extinct and the vegetation changed to become similar to that found today. With his main food supply gone, man had to look elsewhere for sustenance. Small game and vegetable matter quickly took their place at the dinner table. With the loss of large prey animals man became dependent on vegetable matter to supplement the diet to survive. This therefore lead to a more settled lifestyle to enable the cultivation of crops and to other innovations including hybridisation of crop plants for increased yields and the development of corn, beans, squash, chillies and avocado as we know them today. Along with these crops, Opuntia pads and fruits were also collected and eaten and probably planted near to crop fields much as they are today. By 4,000 years ago, living in villages, growing crops and making pottery was an established way of life.3

But, where exactly did this happen? The landscape of Southern Mexico is formed as a result of plate tectonic activity. The easterly movement of the Cocos ocean plate causes the formation of the Sierra Madre del Sur mountain chain. The more northerly Sierra Madre Oriental is heavily influenced by hard, immovable rock from volcanic activity. As a result of continued eastern movement of the Cocos Pacific plate, the land between the two chains is forced upwards as an inland plateau at about 1500m above sea level. The average temperature on the high altitude valley floors is about 15-20oC with about 400mm of rain per year.4 The vegetation of the valley floor is mainly pastureland or desert scrub vegetation at about 1500-2200m becoming oak forest then pine forest further up the mountains. Pastureland and desert scrub areas are the habitat for most of the wild large-padded Opuntias including O. streptacantha and O. tomentosa, the potential parents of O. ficus-indica.5,6

Thus, we can now examine the archaeological evidence of man’s early occupation of Southern Mexico and relate this to the physical geography and vegetation zones of modern day Mexico. The modern day city of Oaxaca sits on a plateau about 1500m above sea level. It sits strategically at the head of three highland valleys which contain a lot of Southern Mexico’s most important archaeological sites. Some of these are marked on the map (Fig17). Today, we have no direct evidence to show that early farming communities from 9,000 years ago remained in the same localities and eventually developed enough to produce pottery and leave behind relics some 4,000 years later. However, it would be fair to make this assumption as it is unlikely that the optimum places to farm changed much during this time-scale. These sites are mostly within the three valleys that radiate away from Oaxaca and on the valley floor or lower foothills. By 3,000 years ago farming techniques had developed so significantly that crop yields were high enough for people to be freed from working the land to take up alternate occupations such as potters or builders. This lead to the first major city-state of Mesoamerica at Monte Alban near the modern day city of Oaxaca.

In conclusion it seems likely that early man reached South and Central Mexico and with climate change at about 9,000 years ago became farmers. Archaeological evidence suggests the highland valleys around Oaxaca proved highly suitable as farming land as the population grew significantly to enable the emergence of Mesoamerica’s first city state of Oaxaca at Monte Alban. Ancient farmer’s ability to hybridise and improve foods crops such as corn played an equally important role in providing enough food for population growth. Molecular evidence from Opuntia ficus-indica suggests its polyphyletic nature means it may also have been subject to human intervention with the tree-like O. streptacantha and O. tomentosa as potential parents. Whether its origins were as a result of natural or man-made hybridization remains uncertain. The presence of multiple sites of ancient human activity within the type of vegetation favoured by the tree-like Opuntias strengthens the case that this happened in the highland valleys around Oaxaca between 4,000 and 9,000 years ago. Its multiple uses for man and ability to thrive in an arid environment accounted for its success. This is seen in its spread across Mesoamerica and later, following the Spanish conquest, across the globe.

References.
1. The origins of an important cactus crop, Opuntia ficus-indica (Cactaceae): New molecular evidence. Griffiths MP, Am J Bot 2004. 91(11): 1915-1921.
2. Evolution, the human story. Dr Alice Roberts. Dorling Kindersley 2011.
3. Ancient Oaxaca. RE Blanton et all, Cambridge University Press 2005.
4. Mixtecs, Zapotecs, and Chatinos. AA Joyce. Wiley-Blackwell 2010.
5. El Genero Opuntia en Jalisco. A Gonzales Duran. Universidad de Guadalajara, 2001.
6. Flora del Valle de Tehuacan-Cuicatlan. SA Montes. Universidad Nacional Autonoma de Mexico. 1997.

I. Crook, Manchester.