An Asian origin for a 10,000-year-old domesticated plant in the Americas
David L. Erickson, Bruce D. Smith, Andrew C. Clarke, Daniel H. Sandweiss, and Noreen Tuross. 2005. An Asian origin for a 10,000-year-old domesticated plant in the Americas. Proceedings of the National Academy of Sciences, Vol. 102, No. 51.
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Abstract
New genetic and archaeological approaches have substantially improved our understanding of the transition to agriculture, a major turning point in human history that began 10,000–5,000 years ago with the independent domestication of plants and animals in eight world regions. In the Americas, however, understanding the initial domestication of New World species has long been complicated by the early presence of an African enigma, the bottle gourd (Lagenaria siceraria). Indigenous to Africa, it reached East Asia by 9,000–8,000 before present (B.P.) and had a broad New World distribution by 8,000 B.P. Here we integrate genetic and archaeological approaches to address a set of long-standing core questions regarding the introduction of the bottle gourd into the Americas. Did it reach the New World directly from Africa or through Asia? Was it transported by humans or ocean currents? Was it wild or domesticated upon arrival? Fruit rind thickness values and accelerator mass spectrometer radiocarbon dating of archaeological specimens indicate that the bottle gourd was present in the Americas as a domesticated plant by 10,000 B.P., placing it among the earliest domesticates in the New World. Ancient DNA sequence analysis of archaeological bottle gourd specimens and comparison with modern Asian and African landraces identify Asia as the source of its introduction. We suggest that the bottle gourd and the dog, two “utility” species, were domesticated long before any food crops or livestock species, and that both were brought to the Americas by Paleoindian populations as they colonized the New World.
Introduction
Innovative approaches in genetics and archaeology continue to provide substantial new information regarding the origins of agriculture and the independent domestication of different species of plants and animals between 10,000 and 5,000 years ago in at least eight separate regions of the world (1–4). Not surprisingly, as they come into clearer focus, the developmental histories of each of these independent centers of domestication are turning out to be far more complex and nuanced than previously thought. Here we consider one of these imbedded complexities, the consistent occurrence of the bottle gourd (Lagenaria siceraria), an Old World plant, in close association with the earliest indigenous New World domesticates. In the process of answering basic questions regarding the early presence of this African plant in the Americas, we reach a number of unexpected conclusions regarding the cultural, environmental, and temporal contexts of initial human domestication of plants and animals.
The bottle gourd has been grown worldwide for thousands of years, usually not as a food source, but for the value of its strong, hard-shelled, and buoyant fruits, which have long been prized as containers, musical instruments, and fishing floats (5). This lightweight “container crop” would have been of particular importance to human societies before the advent of pottery and settled village life. Along with the five wild perennial species that also belong to the genus Lagenaria, the bottle gourd has long been recognized as being indigenous to Africa (5, 6). …
Charles Pickering’s discovery of bottle gourds in Peru in the 1840s (13, 14) extended these questions of the timing, routes, and mechanisms of the plant’s global diffusion to include the Americas, and a complete spectrum alternative explanations, Asia vs. Africa, wild vs. domesticated, and ocean current vs. human-mediated have subsequently been proposed (15): i.e., the bottle gourd was carried from Africa to the New World by ocean currents (6, 14–16), by a boatload of African fishermen (17), by ocean currents from Asia (16), or perhaps by boat (14, 16, 18). Because of observed morphological similarities between present-day bottle gourds grown in Africa and the Americas, a substantial majority of researchers writing on the topic over the past 150 years have formed a consensus that L. siceraria reached the Americas by ocean currents directly from Africa.
Because of its Old World origins and equivocal status as a domesticate, L. siceraria has long been a difficult and unresolved aspect of initial plant domestication and the origins of agriculture in the Americas. Here we address the complex and longstanding set of questions that surround the initial arrival and early history of this plant in the New World via an interdisciplinary approach involving direct accelerator mass spectrometer (AMS) radiocarbon dating, morphological analysis, and ancient DNA (aDNA) sequencing of bottle gourd rind fragments recovered from early archaeological contexts in South America, Mexico, and the eastern United States. …
Identifying Chloroplast DNA Markers
To identify DNA sequence polymorphisms that would enable us to accurately and consistently distinguish between African and Asian landraces of bottle gourd, it was important to address the problem presented by the increasing availability of bottle gourds and seed stock worldwide, and the associated possibility of African genotype gourds currently being grown in Asia, and Asian ones in Africa. We addressed this problem in the construction of our modern reference collections by acquiring a majority of our plant material either directly from rural farming contexts or from well provenienced ethnographic specimens…
Screening of modern reference classes focused on markers that exhibited fixed differences between the landraces. In doing so, we sought to identify polymorphisms that represent the broadest geographic patterns of migration, as opposed to local, random differences among populations. Overall levels of genetic variation were extraordinarily low, and very few fixed differences between the landraces were identified. The entire internal transcribed spacer regions 1 and 2 from ribosomal DNA as well as >10 kb of mitochondrial sequence revealed no sequence differences between the landraces. We did identify one nuclear microsatellite marker that exhibits fixed differences between the modern African and Asian landraces; however, it could not be amplified from the ancient bottle gourd material.
Sequencing within the chloroplast genome was more successful, and we observed three diagnostic polymorphisms…
Ancient DNA Extraction
DNA extractions were conducted on the ancient bottle gourd fruit rind (exocarp) fragments (Table 1), by following procedures outlined for specimens considered as representing a “medium risk” for contamination (domesticated animals and plants)…
A minimum of eight clones were sequenced for each PCR product. Point mutations in nondiagnostic sites were observed among the samples and different clones from the same sample. These mutations may be due to amplification of random mutations that are attributable to accumulated physical and chemical damage over time. For all of the reported loci, we were able to replicate the diagnostic nucleotide sequences, either through independent extractions and amplifications within the LAB, or through independent verification in a different laboratory.
Results of aDNA Analysis
The three markers we used proved to be robust in diagnosing genotypes in the ancient samples, in that all ancient samples that could be PCR-amplified contained alleles that were consistent with either the African or Asian genotype. Thus, whereas there were point mutations at other locations along the molecule for some of the ancient samples, at the diagnostic bases the ancient samples contained alleles that were represented in either the modern African or Asian landrace samples. No novel alleles were observed in the ancient samples, and the markers proved to be informative for samples as old as 9,000 calendar years B.P. …
We observed that all of the archaeological rind fragments predating the arrival of Europeans from which DNA could be amplified were identical to the modern Asian reference group (Tables 3 and 4, which are published as supporting information on the PNAS web site). The single archaeological specimen included in the study that postdates European contact, however, corresponds to the modern African reference genotype for the three markers used. This postcontact rind fragment, recovered from Coxcatlan Cave and dated to ca. anno Domini (A.D.) 1660, more than a century after the establishment of Spanish settlements in the Tehuacan Valley (33), likely represents only one of many early European introductions of domesticated African bottle gourd into the Americas. After multiple early postcontact arrivals, African landraces apparently spread rapidly, and on the basis of DNA analysis of modern New World cultivars, today have almost completely replaced the Asian subspecies in the western hemisphere (34). …
Discussion
In addition to showing that L. siceraria initially reached the Americas from Asia rather than Africa, and was being grown as a domesticated plant in the New World as early as 10,000 B.P., this study also provides a new perspective on a number of more general questions regarding the initial domestication of this “container crop” and its central role in the first efforts by humans to bring plants and animals under domestication. If the exocarp of wild bottle gourds is as thin and fragile as that documented for a range of wild cucurbit taxa, including Lagenaria (7, 19, 24, 26), for example, the probability that wild bottle gourd fruits drifted intact on surface ocean currents from Asia to the Americas is considerably reduced. Nor would the thinwalled fruits of wild bottle gourds have been as valued as containers, thus reducing any role that humans might have played as vectors of introduction of wild L. siceraria into the Americas. The fragile nature of wild bottle gourd fruits, and their resultant reduced utility to humans, along with the lack of evidence of wild bottle gourd ever having being present in the Americas thus substantially weakens the case for bottle gourd having diffused from Asia as a wild plant. In contrast to the thin-walled fruits of wild plants, however, thicker walled, domesticated bottle gourd fruits could potentially have been carried eastward from Asia to the Americas along the north Pacific current rapidly enough to reach landfall with still-viable seeds, based on recent drift and diffusion analyses of container ship spills of buoyant cargo (e.g., rubber bath toys, Nike shoes) in the North Pacific (35).
In contrast, any scenarios involving straight line, long-distance trans-Pacific transport of domesticated bottle gourds from Asia to the Americas by open-ocean seafaring vessels can be considered as having a close-to-zero probability, given the absence of evidence for watercraft capable of making such a voyage in the Late Pleistocene time frame required for bottle gourd to have reached the interior southern highlands of Mexico by 10,000 B.P. A human vector is still possible, however, as Paleoindian groups could have carried bottle gourds and still-viable seeds through the northern noncultivation zone along the south coast of Beringia, either on foot or in near-shore water craft, rapidly enough to have introduced domesticated L. siceraria to the New World along with the dog (Canis familiaris), another early utilitarian domesticate they brought to the Americas (36, 37). Although we favor a Paleoindian near-coast (land and/or water) introduction as representing the most plausible alternative, establishing the relative merits of these different possibilities will be a challenging process, with no easy or rapid resolution. Bottle gourd has yet to be recovered from any Paleoindian cultural contexts, for example, or from any early Holocene contexts along the western coast of North America.
Our documentation of the Late Pleistocene arrival of domesticated bottle gourd in the Americas, when combined with other available evidence, also strongly suggests that L. siceraria was independently domesticated at least twice. The significant degree of genetic separation between modern Asian and African subspecies of domesticated bottle gourd is paralleled by a very different archaeobotanical record for the plant in the two regions. The bottle gourd has been recovered from archaeological contexts in China and Japan dating to ca. 8,000–9,000 B.P. (12), whereas in Africa, despite decades of high-quality archaeobotanical research, the earliest record of its occurrence remains the 1884 report of a bottle gourd being recovered from a 12th Dynasty tomb at Thebes dating to ca. 4,000 B.P. (14, 38). When considered together, the genetic and archaeological information points toward L. siceraria being independently brought under domestication first in Asia, and more than 4,000 years later, in Africa.
In addition, the early arrival of L. siceraria in the New World also provides strong evidence for it being one of the first species brought under domestication worldwide. Because domesticated bottle gourd had reached the interior highlands of Mexico by 10,000 B.P., it is reasonable to look for evidence of its initial domestication in Asia perhaps as much as 2,000–3,000 years earlier. Such an estimated date of domestication for the bottle gourd in Asia (by 12,000–13,000 B.P.) would place it in the same general time frame as currently available archaeological evidence for the initial domestication of the dog somewhere in Eurasia (36). If this projected time frame of domestication for the bottle gourd proves to be correct, it would join the dog as a second “utilitarian species” brought under domestication by humans long before any plants or animals worldwide were targeted for domestication as food sources (1, 2), and that these two “first domesticates” subsequently crossed Beringia into the New World with Paleoindian populations (36, 37).
