Between approximately 11,000 and 5,000 years ago, human societies in many different regions of the world brought a wide range of different species of plants and animals under domestication, marking the initial emergence of food production economies and the beginning of one of the major transitions in human history. This transition, often described as the ‘‘Neolithic Revolution’’ or the ‘‘Origins of Agriculture’’ has been an enduring area of inquiry in both archaeology and biology for more than a century.
Genetic and archaeological evidence provides strong support for the independent domestication of marshelder (Iva annua), chenopod (Chenopodium berlandieri), squash (Cucurbita pepo), and sunflower (Helianthus annuus) in the eastern United States and reconfirms the region as one of the world’s independent centers of domestication.
Marshelder (Iva annua)
This crop plant in fact plays an important role in establishing whether the eastern woodlands was an independent center of plant domestication. No longer cultivated as a domesticated crop, marshelder grows today as a wild plant across much of the eastern and central United States, and its range extends south into Tamaulipas, Mexico. No remains of marshelder, however, have ever been reported from archaeological contexts in Mexico. In contrast, small achenes* and seeds representing wild I. annua plants have been recovered in abundance from numerous archaeological contexts across the eastern United States dating back as early as ca. 8,000 B.P., indicating that wild populations of marshelder were widely harvested by human groups for several thousand years leading up to its initial domestication.
*(Marshelder and sunflower fruits (achenes) consist of a single seed enclosed in a dry indehiscent pericarp)
The earliest evidence for domesticated marshelder in the eastern United States comes from a shallow Titterington Phase pit (Feature 20) at the Napoleon Hollow site in west-central Illinois, which yielded an assemblage of 79 carbonized marshelder achenes and seeds, including 44 that were complete enough to be measured. When achene measurements were adjusted to correct for shrinkage due to carbonization, this Napoleon Hollow assemblage of 44 I. annua specimens exhibited a 31% increase in mean length when compared with a reference class of 11 modern wild I. annua populations, providing clear evidence that they represent domesticated plants. An indirect date obtained on dispersed charcoal within Feature 20 indicated an age of 3,920 ± 90 B.P. in radiocarbon years B.P., consistent with the age of the Titterington cultural phase as documented elsewhere in Illinois.
Each of the other three plant species identified as indigenous eastern domesticates (chenopod, squash, and sunflower) have in fact recently been proposed as representing early introductions from Mexico.
Chenopod (Chenopodium berlandieri)
A number of different wild species belonging to the genus Chenopodium grow today in eastern North America. One of these species of chenopod, Ch. berlandieri, is of interest here because it is both currently grown as a domesticated crop plant in Mexico and was present as a pre-Columbian domesticate in eastern North America.
It was suggested in 1990 that Ch. berlandieri was initially domesticated in Mexico and subsequently introduced as a domesticate into eastern North America. The suggestion that domesticated chenopod may have been initially introduced into the eastern United States from Mexico was recently reiterated, even though archaeological research in the last 15 years has provided compelling evidence to the contrary.
In eastern North America, archaeological evidence indicates that Ch. berlandieri was harvested as a wild plant as early as 8,500 B.P. and was an important domesticated crop plant in the region for >3,500 years (from ca. 1850 B.C. to A.D. 1750), with a number of regionally distinct cultivar varieties now recognized. AMS radiocarbon dating of uncarbonized thin-testa Ch. berlandieri fruits from Cloudsplitter and Newt Kash rockshelters in eastern Kentucky indicated conventional radiocarbon ages of 3,450 ± 150 and 3,400 ± 150 B.P., respectively, providing the earliest available evidence for this species being grown as a domesticated crop plant in the East.
The archaeological record of Mexico also offers clear evidence of human harvesting of wild stands of Ch. berlandieri, along with a variety of other plants producing similarly sized small seeds, for thousands of years before the arrival of the Spanish.* Despite careful and comprehensive analysis of extant collections, however, thin-testa fruits representing domesticated chenopod have yet to be found in any archaeological contexts in Mexico, leading McClung de Tapia and Rios-Fuentes* to recently conclude that Ch. berlandieri was not present as a domesticated plant in Mexico before the 16th century.
*(McClung de Tapia, E. & Rios-Fuentes, J., 71st Annual Meeting of the Society for American Archaeology, April 26, 2006, San Juan, Puerto Rico, Session 158)
A recent genetic study employing random amplified polymorphic DNA fragment analysis to identify genetic relationships among six Chenopodium taxa, including the Mexican domesticate Ch. berlandieri ssp. nuttalliae and a collection of wild Ch. berlandieri from Madison, Wisconsin, suggests a considerable amount of genetic separation between the modern Mexican domesticate and eastern North American wild samples.
Squash (Cucurbita pepo)
Of the five species of squash (Cucurbita) that were domesticated from different progenitor species of wild Cucurbita gourds throughout the Americas, only C. pepo is known at the present time to have been independently domesticated more than once, resulting in two present-day domesticate lineages, now classed as distinct subspecies. The first of these, C. pepo ssp. pepo, which includes cultivated pumpkins and marrows, was developed from an as yet undocumented wild progenitor in Mexico 10,000 years ago. The second subspecies (C. pepo ssp. ovifera, which includes cultivated crooknecks, acorn, and scallop squashes) has long been identified as a potential eastern North American domesticate.
Over the past two decades, a number of genetic studies have focused on identifying the wild ancestor (and location of domestication) of this proposed ‘‘eastern’’ lineage of pepo squash.
Molecular evidence supports the Ozark wild gourd (C. pepo ssp. ovifera var. ozarkana) as the best candidate to be the progenitor of the domesticated squashes of the C. pepo ssp. ovifera lineage. In addition, the archaeological sequences of both northeastern Mexico and eastern North America also provide substantial parallel evidence that ssp. ovifera was domesticated in eastern North America from an indigenous wild Cucurbita gourd.
The earliest evidence of domesticated C. pepo squash in the East was recovered from water-saturated habitation layers of the Phillips Spring site in south-central Missouri, within the present day geographical range of the wild gourd C. pepo ssp. ovifera var. ozarkana.
By the time domesticated C. pepo first appears in the archaeological record of eastern North America 5,000 years B.P., a domesticated pepo squash had already been undergoing human selection in Mexico for five millennia. A domesticated variety of C. pepo. ssp. pepo was being grown in the south-central highlands of Mexico at 10,000 B.P., and by 8,500 B.P. its seeds and peduncles were already substantially larger than those recovered from 5,000 B.P. deposits at the Phillips Spring site. A domesticated C. pepo squash with large fruits, large peduncles, and large seeds was also being grown as far north as Tamaulipas, in northeast Mexico, by 6,000 B.P., in close proximity to the modern-day range of the wild Cucurbita gourd C. fraterna. The presence of this large-seeded, large-fruited domesticated C. pepo squash in Tamaulipas at 6,000 B.P. provides further evidence against the domestication of the ‘‘eastern’’ lineage (C. pepo ssp. ovifera) from C. pepo ssp. fraterna in northeast Mexico.
Both molecular and archaeological evidence now provides compelling support for the independent domestication of C. pepo ssp. ovifera in eastern North America from an indigenous eastern North American Cucurbita gourd.
Sunflower (Helianthus annuus)
As is the case with Ch. berlandieri, wild forms of sunflower (H. annuus) have a broad present-day distribution in North America that encompasses much of the central and western continental United States and southern Canada, along with northern Mexico. Its pre-Columbian distribution, however, is less well understood.
The earliest evidence for domesticated sunflower in the eastern woodlands of the United States consists of six complete carbonized seeds recovered from Level 14 of the Hayes site in Tennessee. Direct AMS dating of one of these seeds yielded a conventional radiocarbon age of 4,265 ± 60 B.P.
At the same time that currently available archaeological evidence offers strong support for the initial domestication of sunflower in the eastern United States, recent comprehensive genetic studies of modern populations of wild and domesticated sunflower provides conclusive confirmation.
Harter et al. [Origin of extant domesticated sunflowers in eastern North America, (2004) Nature] showed that all extant domesticated sunflowers resulted from a single domestication event. This single domestication event, which involved a substantial genetic bottleneck, occurred in eastern North America.
Domestication of crop plants and the transition to food production occurs much more recently in Eastern North America than in most other parts of the world, and the region certainly does not compare with a number of the other centers of domestication in terms of having produced any of today’s most important crop plants. But the eastern woodlands of North America does represent, along with the Near East, the best-documented regional record of this major transition in human history.
(Source: “Eastern North America as an independent center of plant domestication”, by
Bruce D. Smith, 2006)
Many genetic changes essential for crop evolution have been identified, and these alleles provide useful tools for inferring how, where, and when early societies transformed wild plants into agricultural staples. Examining sequence diversity of these alleles in cultivated lineages, wild progenitors, and archaeological specimens can reveal critical information about the rate, timing, geography, and order of the domestication process. For instance, sequences of domestication loci obtained from archaeological maize cobs indicated that although domestication alleles for two traits became fixed early, fixation of domestication alleles for a third trait occurred several thousand years later. Patterns of sequence diversity around domestication alleles have yielded estimates of the strength of selection during domestication for several loci in rice and maize. In addition, the geographic distributions of domestication alleles in extant cultivated and wild germplasm have been used to assess how crops and crop alleles have spread from domestication centers and reveal whether convergent traits in independent lineages evolved from the same or unique suites of mutations.
Molecular, archaeological, and linguistic evidence has long supported the presence of a domestication center in eastern NorthAmerica (ENA). However, pre-Columbian archaeological finds at several sites in southern Mexico have suggested the possibility of a second independent domestication center and challenged the view that domesticated sunflower reached Mexico only after introduction by Spanish traders. The archaeological evidence suggests two possible alternative explanations: an independent origin of domesticated sunflower in Mexico or dispersal to Mexico through trade routes established before Spanish colonization. The identification and significance of the archaeological remains, the historic record, and modern linguistic evidence have been debated heavily. Previous molecular studies using neutral nuclear and chloroplast DNA markers have indicated that extant Mexican landraces are descended from the same lineage as ENA landraces, suggesting that if an independentMexican lineage evolved, it did not contribute to modern cultivated sunflower germplasm. However, the provenance and genetic purity of the twoMexican landrace samples included in those studies have been questioned. In addition, few Mexican wild H. annuus populations were included in these analyses.
We have made new, extensive collections of Mexican wild sunflowers and domesticated cultivars grown by indigenous Mexican populations. Sequencing genes demonstrated to have experienced selective sweeps during domestication in ENA provided an opportunity to test whether extant Mexican landraces descended from the same or a unique lineage. In particular, if domesticated sunflower reached Mexico through trade routes from the North, we predicted that ENA and Mexican landraces would share the same haplotypes at these genes. If two independent domestication events occurred in isolation, it is highly unlikely these parallel events would have selected for the same causative mutations arising on the same haplotype backgrounds at multiple loci. In addition, the presence of the domesticated haplotype in ENA wild populations and its absence from wild Mexican populations would exclude the possibility of a parallel selective sweep from equivalent standing variation.
The findings from three candidate domestication loci and additional neutral markers indicate that domesticated sunflowers grown in Mexico today are descended from the same cultivated genetic lineage as ENA domesticated sunflower.
Since c4973 domesticated alleles segregate in both ENA and Mexican wild populations, the possibility of a parallel selective sweep on the same allele cannot be excluded. Nevertheless, the higher frequency of domesticated alleles in ENA wild populations suggest that the eastern United States is the more likely geographic source.
Although the data presented herein are more consistent with a single origin and dispersal before the arrival of Spanish explorers, our findings do not invalidate the possibility that a separate center of sunflower domestication in Mexico did indeed occur during the pre-Columbian period. An ancient lineage may not be represented in modern germplasm because no descendants of the hypothesized independent Mexican lineage have survived.
(Source: “Sunflower domestication alleles support single domestication center in eastern North America”, by Benjamin K. Blackman et al., 2011)
Research-Selection for NovoScriptorium: Maximus E. Niles