The dawn of what we now consider civilization (in the various places where it arose independently) hinged on an event that was probably little noticed at the time, namely the transition from hunting and gathering of wild foods to the active cultivation of certain desirable plants and animals. How did people first begin to grow the weeds and wildlife that came to be crops and livestock? One part of the answer, presumably, is that as people collected and ate favorite types of plants, they began sprouting up in backyards and trash dumps from the discarded seeds, and before long people were sowing them intentionally. Call it the “trash dump” hypothesis of agricultural origins.
As humans gathered (and discarded) the various plants that proved useful to them, such volunteer kitchen gardens must have accumulated a motley assortment of species that don’t normally occur together. New evidence from Mexico suggests that these prehistoric human gardeners created an evolutionary crucible in which, usually unwittingly, they conjured up new types of organisms that would not otherwise exist. And thus crops were born.
This hypothesis is supported by a combination of genetic, archaeological, ethnobotanical, and biogeographical data on small trees in the genus Leucaena, of which 22 species and a swarm of hybrids occur in the Americas. The seeds of these legume trees have been used as food crops in Mexico for 6000 years, and several are still intensively cultivated today. Interestingly, many plants now cultivated in Mexico are “polyploid”, meaning that they have multiple sets of chromosomes (usually 4 instead of the typical 2 sets). In “allopolyploids”, the combined set of chromosomes originated from two different parent species, apparently as a result of interbreeding between formerly separate species. Often such new types of plants have quite different characteristics than either parent does. Hughes and colleagues found that several of the most widely used Leucaena varieties in Mexico are hybrids of this sort, and that the key to their origin involves spontaneous “backyard hybridization”, as separate species normally found in different regions or habitats are brought into close contact human habitations and kitchen dumps. Along with genetic evidence of hybrid origins, a key clue to this process is that several favored varieties that are widely cultivated today are unknown in the archaeological record prior to about 3000 years ago.
The case of the Leucaena trees is evidently not an isolated one. The new results corroborate previous findings, notably for the Agave and Opuntia (“prickly pear”) plants also extensively cultivated in Mexico:
“Taken together Leucaena, Agave, and Opuntia comprise three of the dominant perennial plants cultivated in [south-central] Mexico today. In all three genera, domestication has apparently been facilitated by spontaneous hybridization after extensive predomestication cultivation. In each case there is evidence that the prominent species in cultivation . . . have hybrid origins most likely after cultivation. There is also evidence to suggest that hybridization has been important in many other Mesoamerican crops . . . It seems that . . . the simple step of bringing species together, consciously or casually, in dump heaps and informal backyard orchards has played a central role in Mesoamerican crop domestication.”
I see two broader implications of these results. First is the evidence provided by this study for the relatively rapid (probably over decades to centuries) evolutionary changes that can occur in species when human activities change their distributions. We can expect to see much more of this as modern technological humanity completely rerwites the book on biogeography by altering habitats and redistributing species on a planetary scale.
The second implication is a cautionary tale about preservation of wild biodiversity. One of the most compelling utilitarian arguments for benefits of biodiversity involve the “library” of genetic resources that wild organisms provide for developing new crop and livestock varieties, which will be especially important in the facing of changing climate, emerging new diseases, and so on (and recent developments support this idea). The experience of early Central American farmers in developing new crop varieties through both active cultivation and serendipitous backyard experiments emphasizes the benefits of having a variety of wild plants and animals out there that provide raw materials for such experiments. Such wild variety provides a diversified portfolio for agriculture. In the end, this will serve us better than putting all our eggs in one lucrative but highly risky basket such as corn or rice, as Enron’s stockholders discovered all too painfully.
Original source: Hughes, C.E., R. Govindarajulu, A. Robertson, D.L. Filer, S.A. Harris, and C.D. Bailey. 2007. Serendipitous backyard hybridization and the origin of crops. Proceedings of the National Academy of Sciences USA 104:14389-14394.