prehistoric use of plants.pptx

The development of agriculture-‐based socie7es is presumed to have evolved from hunter-‐gatherer socie7es that gradually came to depend primarily on cereals as a major component of their food resource. This means that cereals were likely the first plants to be domes7cated. Domes7ca7on of cereals was a gradual transi7on from collec7ng wild plants to a greater reliance on cul7vated plants selected for specific proper7es. Plant domes7ca7on/agriculture required a convergence of socie7es at a stage of development ‘prepared' to adopt agriculture, i.e. possessing technologies for plant cul7va7on and prepara7on, storage of crops, the availability of the right kinds of food plants to support agricultural socie7es, and climate condi7ons favorable to plant cul7va7on.

Cereals and the Transi7on from Hunter-‐Gatherer to Agricultural Socie7es

People in many different parts of the world independently began to cul7vate and eventually domes7cate plants from 13,000 to 7000 years BP

Independent appearance of agriculture

Rela7vely cool-‐dry climate from about 25,000 to 15,500 years BP (the “older Dryas” period) favored the spread of many grass species through SW Asia (The Near East, present-‐day western Turkey, Syria, Israel), including the ancestors of barley and wheat. Grasses were abundant in the region and so would have been an available food source. How did the hunter-‐gatherer socie7es learn how to u7lize cereal grains?

Climate and Development of Agriculture

Earliest firm dates for domes7cated plants (11,000 -‐13,000 years BP) coincides with warm, wet, stable climate period.

Evidence for Paleolithic use of cereals

The Ohalo II archaeaological site on the Sea of Galilee in present-‐day Israel has provided the oldest evidence of human processing of wild cereal grains. The age of the site is es7mated to be between 22,500 and 23,500 years old and corresponds to the end of the Pleistocene glacial maximum. The site has evidence that humans were using cereal grains for food at the peak of the last ice age.

The Ohalo II site has the remains of a nomad camp of six brush huts, each about 12 m2 in size.

The huts had remains of plant material, seeds, bedding, grinding stones, and a stone hearth that appears to represent the oldest known oven, and hence the oldest evidence of baking.

Ohalo II hut

Ohalo II grinding stone Ohalo II stone hearth, the oldest known oven?

Plant Use at Ohalo II The plant remains found in the Ohalo II huts are remarkable for their diversity. Excava7ons at the site have yielded more than 90,000 different plant remains, belonging to 140 different species. Starch grains were extracted from grinding stones and could be iden7fied based on their characteris7c microscopic structure

Starch grains from representa7ve modern plants. a, Tri7cum dicoccoides (emmer wheat). The arrow on the leb points to dis7nc7ve crater-‐like depressions. b, Hordeum. spontaneum ( wild barley). The arrow on the leb points to lamellae; arrow on the right points to Hordeum-‐type surface depressions. c, Aegilops geniculata. The arrow points to the central protuberance with the pleat. d, Aegilops peregrina. The arrow points to the characteris7c lamellae present in this species. e, H. glaucum. f, B. pseudobrachystachys. g, Piptatherum holciforme. Scale bars, 10 microm.

Starch grains extracted from Ohalo grinding stone. a, Len7cular grain from the AHT (Aegilops-‐Hordeum-‐Tri7cum) taxa group. b, c, Len7cular grains with lamellae (right arrow in b and leb arrow in c) from Hordeum. They also have surface depressions (other arrows) characteris7c of the genus. d, Compound starch grain. Scale bars, 10 microm.

From Piperno et al. 2004. Nature 430: 670-‐673

Plant Use at Ohalo II The Ohalo II site provides evidence of extensive use of mul7ple grass species by a nomadic culture 23,000 years ago, more than 10,000 years older than previous evidence of use of wild grasses. The presence of grinding stones together with primi7ve hearths indicates that the grains were used to make flour and baked into seed cakes, foodstuffs that presumably could be stored for long periods. This is rela7vely sophis7cated food technology and it was being used well in advance of the earliest firm date for plant domes7ca7on, 13,000 to 11,000 years BP. Use of wild growing plant grains was well developed before cereals were domes7cated. The species used at the Ohalo II site are diverse and include ancestors of modern wheat and barley, as well as small grained wild grasses that were never domes7cated, indica7ng that a broad spectrum of plants were collected for food. Cereals and small grained grasses were apparently plen7ful and offered a reliable food source.

The primary Small Grained Grasses and cereals at Ohalo II

La7n name English name Quan7ty Small grained grasses Alopecurus utriculatus/arundinaceus Bladder/creeping foxtail 1,814 Bromus pseudobrachystachys/7gridis Brome 10,995 Hordeum glaucum Smooth barley 932 Hordeum marinum/hystrix Seaside/Mediterranean barley 574 Puccinellia cf. convoluta Alkali grass 1,853 Cereals Hordeum spontaneum Wild barley 2,503 Tri7cum dicoccoides Wild wheat 102 Total 18,773

A 23,000-‐year-‐old wheat fragment from Ohalo II.

Plant Use at Ohalo II

Use of plants by the inhabitants of Ohalo II was not restricted to food The hut sites at Ohalo II were preserved because they were submerged for most of the past 23,000 years . Several years of drought beginning in 1989 caused the lake level to fall and the site was exposed. The plant seeds and sob fibers were well preserved from being submerged.

Floors of the huts had bundles of grass arranged apparently for bedding.

Climate change

The cold/dry older Dryas ended with abrupt climate warming around 15,500 years ago. Along with climate warming the climate in the northern hemisphere became wemer, leading to increased forested areas and extensive grasslands. In addi7on, the concentra7on of CO2 in the atmosphere increased by about 40%, from 200 ppm to over 275 ppm. By 13,000 years BP the climate in SW Asia was mild and wet, and would have supported an abundance of wild fruit and seed plants, and likely abundant game animals. The availability of wild fruit and game would probably have reduced the need to use cereals as a main food source, and reduced any incen7ve to cul7vate grasses. Use of cereals during this period was more likely as a seasonal or supplementary food source.

CO2 concentra7on in the atmosphere has fluctuated during the history of the Earth, and is very closely linked to average temperature.

The Natufian Culture

The Natufian culture appeared at around 14,500 years BP and occupied the Jordan Valley to the Mediterranean coast.

The earliest Natufians were hunter gatherers who were familiar with, and used cereals such as emmer wheat and barley. Later Natufians were one of the oldest cultures to adopt permanent semlements, and this occurred before the widespread adop7on of agriculture. What allowed Natufians to adopt a sedentary lifestyle? Possibly the expansion of forest and grassland with climate warming and increased precipita7on at around 14,500 years ago made it possible to find sufficient food resources within a rela7vely small area, making nomadism unnecessary. A more sedentary culture would have had dis7nct advantages. Natufian culture developed dis7nc7ve ar7facts, rituals, architecture, art, tools, etc. and may have been the earliest culture to use farming in Eurasia.

The Younger Dryas

Natufian culture thrived for about 2000 years, from 14,500 un7l 12,800 when the climate again changed abruptly. The Younger Dryas period was a brief (ca. 1300 year) cold period. It is called “Dryas” because of the appearance of pollen of the circumboreal plant, Dryas octopetala, in sediments in more southern la7tudes, indica7ng subarc7c condi7ons.

The arrival of the prolonged cold and aridity of the younger Dryas was catastrophic for the Natufian culture. The Natufians who survived un7l the end of the younger Dryas and the return of milder temperatures during the Holocene are thought to be the innovators of agriculture in the Near East. However, whether widespread plant cul7va7on began during the younger Dryas ca 13,000 years ago, or later aber the return of warmer, wemer climate, ca. 11,500 years ago, is s7ll a subject of debate.

the rapidity of climate change at the beginning and at the end of the Younger Dryas Interval was one of the most extreme to occur in the last 100,000 years

End of the Younger Dryas

Very abruptly, at around 11,600 BP the climate entered another prolonged warming phase. Average temperatures increased by 8 -‐ 10° C in a single decade. With the increase in temperature came increased precipita7on, leading to condi7ons much more favorable to the advent of wide scale farming. The first evidence of systema7c selec-on and cul7va7on of plants by humans comes from the period during and immediately aber the Younger Dryas. This was probably not a coincidence. Why did agriculture in Eurasia begin at this par7cular 7me? A convergence of several factors was essen7al to the development of agriculture: 1.  Plants. Humans had been consuming wild plants for many millennia (e.g. Ohalo II), very likely contribu7ng

unconsciously to selec7on for certain desirable traits, “pre-‐agricultural selec7on”. The availability of starch-‐rich, high-‐yield, cul7va7on-‐ready plants was a prerequisite to agriculture. Humans also gained experience about condi7ons where the most desirable wild plants grew, and may have prac7ced ‘quasiagriculture’.

2.  People. People gained several thousand years of experience in collec7ng wild plants, and in developing tools and technologies for cul7va7ng, harves7ng, processing and storing crops. People learned what condi7ons favored the growth of which plants.

3.  Environment. The abrupt cooling at the beginning of the Younger Dryas Interval followed by the return to a rela7vely stable, warm period provided an impetus for the development of farming. The cold/drought during the Younger Dryas may also have made alterna7ve food sources scarce, forcing a greater dependence on wild cereals, and eventually, their cul7va7on. Prior to the Holocene clima7c condi7ons were unstable and highly unfavorable to sustainable agriculture.

4.  Cultural maturity. The Natufian culture was a prefarming culture, but the advantages of a sedentary society may have helped push the transi7on from a sedentary gatherer society to a sedentary agrarian society.

Preagricultural selec7on

Human use of wild cereals da7ng to ca. 23,000 years before present (Ohalo II) suggests that humans interacted with wild plants over thousands of years before the plants were actually domes7cated. But humans may have inadvertently caused changes in wild popula7ons through their ac7vi7es and affected the evolu7on of some wild plant popula7ons. What kinds of gene7c traits might have been uninten7onally selected by early hunter gatherers? 1.  Seed germina7on. Seeds that germinate uniformly and rapidly, loss of seed dormancy would

be one naturally occurring muta7on that would have been reinforced by human harves7ng ac7vity.

2.  Uniform flowering and seed reten7on (nonshamering). Humans would have collected more seed from synchronously flowering plants that retained their seeds. Some of these would then be planted, thereby uninten7onally increasing the propor7on of plants in the popula7on with these characteris7cs.

3.  Larger seeds. Clearly people would have conformed to the ecological ‘op7mal foraging hypothesis’ that the most favored food would be the one that provided the greatest caloric benefit for the smallest energy expenditure.

Later Natufians

From 12,800 to 11,600 BP the Younger Dryas Interval imposed harsh, inhospitable climate in the Northern Hemisphere. Popula7ons of animals such as gazelle, auroch (the ancestor of modern camle) and wild boar were pushed south. Natufian cultures that also moved south as more mobile hunter-‐gatherer groups disappeared from the archeological record, possibly because of the drier condi7ons, scarcity of food or compe77on from other socie7es. However the Natufian cultures that remained in the North (present day Anatolia region of Turkey) survived. Climate change during the Younger Dryas caused forests to be replaced by grasslands comprising the cereals that the Natufians were experienced with harves7ng. As forests receded so did forest dwelling game species, forcing an increasing reliance on wild grains, mainly barley and wheat. Increasing reliance on these cereals during a 7me of inhospitable climate would have strongly favored the prac7ce of plan7ng, tending and harves7ng crops. Together with wheat and barley, figs also were apparently domes7cated by later Natufians. The oldest domes7c figs (parthenocarpic) are known from the Jordan Valley 11,400 BP. The figs were effec7vely sterile and so had to be propagated by humans.

from Bar-‐Yosef 1998. Evolu7onary Anthropology 6:159-‐177

Later Natufians

from Bar-‐Yosef 1998. Evolu7onary Antrhopology 6:159-‐177

Distribu7on of wild cereals during the Younger Dryas

“In conclusion, the ‘‘Neolithic Revolu7on’’ cannot be understood without research into its origins in the Natufian culture. The emergence of farming communi7es is seen as a response to the effects of the Younger Dryas on the Late Natufian culture in the Levan7ne Corridor. The beginning of inten7onal widespread cul7va7on was the only solu7on for a popula7on for whom cereals had become a staple food. Domes7ca7on of a suite of founder crops came as the uninten7onal, unconscious result of this process.”

Later Natufians

Ofer Bar-‐Yosef. 1998. Evolu7onary Anthropology 6: 159-‐177

Bar-‐Yosef has expressed the opinion that agriculture was an inevitable consequence of the reliance on wild cereals by Natufians:

Abu Hureyra The Abu Hureyra site was contemporaneous with the late Natufian culture 14,000 – 11,000 BP and is at the northern 7p of the Natufian cultural zone. Prior to the onset of the Younger Dryas, Abu Hureyra was a semi-‐sedentary hunter gatherer semlement that subsisted on a diverse assemblage of plants: wild almond, acorns, cereals and wild len7ls. Most of the diversity in plant species disappeared from the site during the first decades aber the beginning of the Younger Dryas, reflec7ng an abruptly more arid climate. An increasingly dry climate caused forests to recede and only the most drought tolerate plants persisted. But wheat and barley were present at Abu Hureyra, ca. 12,000 BP, together with seeds of some drought intolerant weeds, sugges7ng that the inhabitants were prac7cing agriculture in loca7ons where natural water sources could be exploited.

from Bar-‐Yosef 1998. Evolu7onary Anthropology 6:159-‐177

Abu Hureyra

Domes7cated rye at Abu Hureyra, the first domes7cated cereal?

(a)  Grain of wild rye from Abu Hureyra (b) Grain of domes7cated rye from Abu Hureyra

At about 12,000 BP a new cereal appeared in the record at Abu Hureyra: rye (Secale cereale), with grains that were substan7ally larger than those of wild rye. The rye grains also had marks indica7ve of threshing to remove seeds from the culms, sugges7ng the plants had a nonshamering trait, and therefore ac7ve selec7on by humans. These characteris7cs have been interpreted as possibly represen7ng the oldest evidence of a domes7cated cereal and the earliest example of agriculture anywhere in the world. Furthermore, domes7cated rye has a strong tendency to interbreed with wild, small grained forms, so maintenance of large grained rye would have required inten7onal human selec7on for self-‐fer7le forms. Large seeded legumes (len7ls) also appeared at around 11,500 BP and increased, sugges7ng that rye and len7ls were the earliest plants cul7vated at the site (see Hillman et al. 2001). The large rye grains remained in the later record at Abu Hureyra, from 10,500 to 8000 BP, the period when agriculture began to thrive in the Mesopotamian region. Aber about 8500 BP einkorn wheat and barley replaced rye as the main crop at Abu Hureyra, but it remained a minor crop.

from Hillman et al. 2001. Holocene 11: 383-‐395

Abu Hureyra

What led to the domes7ca7on of rye at Abu Hureyra? 1.  A primarily sedentary foraging society was in place, with wild food sources sufficient to meet

demand. 2.  Wild cereals were ini7ally an important seasonal staple food 3.  Abrupt climate cooling, increasing aridity resulted in reduced forage foods 4.  Limited migra7on op7ons forced more intensive management of wild foods, preagricultural

selec7on, self fer7le rye, non shamering, large grains 5.  Cereals supplemented with some legumes became the dominant year-‐round staple food 6.  Deliberate selec7on, cul7va7on of specific cul7gens 7.  More stable climate from end of Younger Dryas allowed expansion and con7nued cul7va7on

of crops over a long period

Advantages of rye

Hillman et al (2001) suggest several characteris7cs of rye that may have led to its early domes7ca7on in an area where wild wheat was also present: 1.  Rye grains are more easily threshed and the chaff more readily winnowed than wheat 2.  Rye grains are more easily prepared as food, sober textured than wheat 3.  Rye starch is more slowly hydrolyzed, produces a lower insulin response, is more sustaining 4.  Local condi7ons probably contributed to ease of cul7va7on of rye at this site.

Transi7on from wild to domes7c wheat was gradual Although the first evidence of a domes7cated cereal from Abu Hureyra is that rye was the dominant cereal crop, wheat and barley gradually became the dominant cul7vated cereals aber the end of the Younger Dryas. The transi7on from wild forms to cul7vated ones was gradual, taking over 2500 years.

Tanno and Wilcox 2006. Science 311: 1886-‐1889

Early cul7va7on of wheat and barley

Different cereals were cul7vated at different sites in probably due to local soil condi7ons and probably the history of plant cul7va7on. Abu Hureyra (in the north) ini7ally grew rye, the southern Natufians grew barley and wheat (emmer or einkorn), together with pulses such as len7l and chickpea. Early farming in the eastern Mediterranean was a mosaic of different local species and types. Over about 4000 years (10,500 to 6000 BP) cereal and pulse farming spread across Eurasia west to Europe, south to the Nile Valley through trading of seeds and a gradual dissemina7on of knowledge. A cluster of crop plants, barley, wheat, len7ls, peas and flax, were all spread as a unit. By 5000 BP wheat and barley had reached China, although rice cul7va7on in eastern Asia had begun roughly about the same 7me as the Natufians were beginning to cul7vate cereals.

How did early agriculture spread?

A major ques7on in the prehistoric rise of agricultural socie7es is whether agriculture spread from centers of origin by trade or by migra7on or expansion? Did prehistoric farming cultures expand and replace hunter-‐gatherer cultures or did hunter-‐gatherers just acquire the tools, techniques, seeds etc from contact with farming cultures? Ancient DNA to the rescue! Farming was introduced to Europe from the Near East around 8500 years ago and reached northern Europe and the Atlan7c Coast by around 5000 years ago. In northern Europe it appears that farming was associated with genotypes in human remains (skeletons) that were more closely related to southern Europeans. This suggests that farming was brought to northern Europe by expansion/migra7on of a farming culture that originated in southern Europe.

Ancient Migrants Brought Farming Way of Life to Europe

Ancient DNA from human remains across Europe, such as this skeleton from Sweden, provide clues to the spread of agriculture.

Science 27 April 2012. vol. 336 no. 6080 400-‐401

Rice The origins of domes7cated (Asian) rice are not nearly as well understood as the temperate cereals because:

1. Asian wild rice grew over a much larger area than wild wheat and barley, greatly expanding the poten7al area of early cul7va7on

2. There are few surviving archaeological sites, and the climate in rice growing areas is less favorable to the preserva7on of archaeological ar7facts

Rice-‐ species and subspecies

African rice: Oryza glaberrima, domes7cated independently of Asian rice Asian wild rice: Oryza rufipogon, the wild progenitor of domes7cated rice Asian rice: Oryza sa-va ssp. indica: subspecies of rice from India, Thailand Asian rice: Oryza sa-va ssp. japonica: subspecies of rice from southern China

Rice was first grown as a crop around 10,000 BP, close to the first domes7cated wheat and barley. However rice did not become a dietary staple un7l several thousand years later. Rice became a major crop in Asia around 7,000 BP and by 3,000 BP was being grown in Korea and Japan. Today rice is the staple source of food for half the popula7on of the world, at least 2 billion people, and is an important supplementary food for many more.

Rice is by far the world’s most important food crop in terms of its contribu7on to the diet of the greatest number of people.

There are different interpreta7ons of gene7c data from the two subspecies (or varie7es) of Asian rice (O sa-va indica and O. sa-va japonica). Some argue that the gene7c data indicate that both subspecies of Asian rice had diverged gene7cally 1 – 2 million years ago, so they were already gene7cally dis7nct subspecies by the date of first domes7ca7on, around 10,000 BP. This would mean that rice was independently domes7cated in southern China (japonica) and India-‐Thailand (indica). However, a different interpreta7on is that both indica and japonica rice possess the same muta7on in an allele for the nonshamering trait. This fact supports a single origin and later divergence of japonica and indica rice. Advocates of this view argue that the gene7c divergence that appears to predate the domes7ca7on of rice may have resulted from a gene7c process called introgression. This would have resulted from gene exchange between the domes7cated forms and wild popula7ons and could affect interpreta7on of gene divergence data. Therefore there are at least two compe7ng hypotheses regarding the origins of domes7cated Asian rice.

Did contemporary Asian rice result from one domes7ca7on event, two, or more?

Unlike maize, wheat, barley or rye, the origins of domes7cated rice may have been mul7ple, widely separated loca7ons in southeast Asia –or may not, the data are s7ll inconclusive.

Domes7ca7on of rice

The origins of domes7c Asian rice are s7ll unclear, whether one, two or mul7ple centers of origin. Asian rice was first cul7vated about 10,000 BP but rice cul7va7on was slow to spread in Asia. Rice cul7va7on was widespread by 7000 BP, and reached Korea and Japan by 3000 BP. Rice was likely gathered as a wild crop to supplement the diet of Asia7c hunter gatherers prior to its cul7va7on, as occurred with cereals in the Near East.

Climate change and rice cul7va7on?

The first cul7va7on of rice occurred around 10,000 BP, well aber the end of the Younger Dryas Interval, so there is no correla7on between climate change and first rice cul7va7on. The effects of the Younger Dryas were less severe in subtropical Asia than in the eastern Mediterranean, so there is no clear clima7c s7mulus in the chain of events leading to rice domes7ca7on. But wild rice has several traits that would have been highly favorable in ‘pre-‐agricultural selec7on’ 1.  Under cul7va7on, domes7c-‐like traits like non-‐shamering and loss of seed dormancy appear

spontaneously within a few years under cul7va7on.

2.  Sequencing of the rice genome has revealed that several genes controlling domes7ca7on related traits are linked on the same chromosome. If early farmers selected plants for one desirable trait, such as seed size, several other desirable linked traits would also be obtained.

Maize, the first gene7cally engineered crop plant?

It is now widely accepted, based on mul7ple lines of evidence, that modern maize is derived from the wild Mexican grass teosinte. It has taken many years, and considerable research, to establish the connec7on between Z. mays and teosinte (previously classified in a completely different genus, Euclaena). Luther Burbank first suggested in 1907 that modern maize was derived from the teosinte, based on finding the two plants were interfer7le and hybrids had intermediate characteris7cs. Burbank’s hypothesis was not taken seriously and it took another 90 years for him to be proved correct. It is hard to imagine that two such dissimilar looking plants are actually members of the same species! Maize and mexican teosinte are now considered different subspecies of Zea mays, like the two subspecies of cul7vated rice; but maize and teosinte are more different from each other than indica and japonica rice. teosinte: Zea mays ssp. mexicana maize: Zea mays ssp. parviglumis

Teosinte

Maize

Hybrid

How did early farmers get maize from teosinte?

teosinte: has many branches, 7llers, mul7ple male infloresences, many small female infloresences (cobs) with few seeds maize: has a single stalk, single male flower, few large cobs with many seeds

Prehistoric gene7c modifica7on of teosinte It turns out that muta7ons in only 2 genes have major effects on the growth form and seed characteris7cs of maize

tga1: teosinte glume architecture tga1 gene controls the hard, stony fruitcase that encloses teosinte seeds. In teosinte the fruitcase allows seeds to pass through the diges7ve tract of herbivores. Maize kernels are not enclosed in a fruitcase, and have less silica and lignin in their glumes, so they are more palatable. The most cri7cal step in maize domes7ca7on was the libera7on of the kernel from the hardened, protec7ve casing that envelops the kernel in teosinte. This evolu7onary step exposed the kernel on the surface of the ear, such that it could readily be used by humans as a food source. This trait had to be ac7vely selected by humans because it would have reduced the reproduc7ve success of wild teosinte with a muta7on in this gene.

a, Maize ear showing the cob (cb) exposed at top. b, Teosinte ear with the rachis internode (in) and glume (gl) labelled. c, Teosinte ear from a plant with a maize allele of tga1 introgressed into it. d, Close-‐up view of a single teosinte fruitcase. e, Close-‐up view of a fruitcase from teosinte plant with a maize allele of tga1 introgressed into it. f, Ear of maize inbred W22 (Tga1-‐maize allele) with the cob exposed, showing the small white glumes at the base. g, Ear of maize inbred W22:tga1, which carries the teosinte allele, showing enlarged (white) glumes. h, Ear of maize inbred W22 carrying the tga1-‐ems1 allele, showing enlarged glumes.

Wang et al. 2005. Nature 436:714-‐719.

tb1: teosinte branching Muta7ons in tb1 convert the

mul7ple branching teosinte into a plant with a single main stalk with a single male infloresence (tassel). Muta7ons in this gene also lead to presence of female infloresences (cobs) on axillary branches rather than male flowers.

(a) A teosinte (Zea mays ss. mexicana) plant and (b) one of its primary lateral branches with terminal tassel (T). Silks (s7gmas, S) are shown emerging from teosinte ears hidden within the leaf sheaths. (c) A wild-‐type maize plant. (d) A wild-‐type maize ear shoot. (e) A tb1 mutant maize plant and (f) its primary lateral branches with terminal male inflorescences but no ears.

Prehistoric gene7c modifica7on of teosinte

White and Doebley 1998. Trends in Gene7cs 14:327-‐332

Domes7ca7on of maize

Muta7ons in tga1 that exposed the naked kernels and reduced the amount of silica and lignin, making the seeds more palatable, would have been highly deleterious to teosinte growing in the wild. Seeds would not survive passage through the diges7ve tract of an animal and so mutants would be inferior compe7tors with wild rela7ves and other species. This implies that humans ac7vely selected and propagated these seeds, and that humans had been harves7ng and consuming wild teosinte before the muta7ons occurred or were recognized. Humans probably also selected for other desirable traits in ‘pre-‐maize’ teosinte, such as cob size, kernel size, taste etc. A consequence of selec7on for desirable traits in crop plants is reduced gene7c diversity in crop selec7ons compared to the wild popula7ons. This is called a “gene7c bomleneck”. Subsequent gene7c diversity in domes7cants is limited by the gene7c diversity of the founder popula7on. ‘Domes7ca7on bomlenecks’ are a common feature of domes7cated plants, reflec7ng the small ini7al popula7ons selected for their desirable traits that became the progenitors of the contemporary crop species.


How many 7mes was maize domes7cated? Maize is very diverse in growth form, seed and cob characteris7cs, and this diversity has been considered circumstan7al evidence that contemporary maize was derived from teosinte more than once. But...

Global Crop Diversity Trust

Gene7c data comparing genotypes (DNA fingerprints) of maize from North America, Mexico, Caribbean islands, and South America with genotypes of Mexican teosinte have been used to determine whether modern maize has mul7ple origins. Matsuoka et al. (2002) analyzed DNA fingerprints of 190 maize samples represen7ng geographic races from Canada to Chile at 99 different gene loci. The results are unequivocal that all contemporary maize can be traced to a single common ancestor; maize was only domes7cated once.


Matsuoka et al. 2002. Proc Nat Acad Sci 99:6080-‐6084

All modern maize can be traced to a single common ancestor


But if contemporary maize is gene7cally diverse, how could it become so diverse following a domes7ca7on bomleneck from a single domes7ca7on event? Using computer models to compare the gene7c diversity of contemporary maize with that of teosinte, and using es7mated rates of gene7c change in maize aber its domes7ca7on, Eyre-‐Walker et al. (1998) determined that a very small popula7on of founder individuals (~20) could account for the gene7c diversity in contemporary maize. The number of individuals in the founding popula7on depends on the number of genera7ons that resulted in domes7ca7on. A domes7ca7on event of 10 genera7ons would require only 20 individuals. A domes7ca7on event of 300 years, similar to the domes7ca7on of wheat, would require about 600 individuals. What is important is that the computer simula7on shows gene7c diversity of contemporary maize could have arisen from a rela7vely small founder popula7on within a 7mespan consistent with ac7ve human selec7on resul7ng in a gene7c bomleneck.

Eyre-‐Walker et. al. 1998. Proc. Nat. Acad. Sci. 95:4441-‐4446

Gene7c data can also be used to iden7fy the loca7on and es7mate the date of maize domes7ca7on.


Popula7ons of teosinte from the central Balsas valley of southern Mexico are closest to the ancestral maize (asterisks in the previous slide).

Because maize had a single origin, the approximate date of maize domes7ca7on can also be es7mated from the gene7c data by assuming constant rates of gene muta7ons (based on known muta7on rates for certain genes). When this approach is applied to maize and teosinte, the origin of maize is es7mated to be 9,188 B.P. (95% confidence limits of 5,689–13,093 B.P.). This es7mate is consistent with the oldest known fossil domes7cated maize dated to be 6,250 B.P.


Physical archeological evidence for maize in southern Mexico

Piperno and Flannery 2001. Proc Nat Acad Sci 98:2101-‐2103 Benz 2001. Proc Nat Acad Sci 98: 2104-‐2106

The oldest physical evidence of prehistoric maize comes from the Guilá Naquitz cave (Oaxaca), dated by 14C, mass spectroscopy and phytolith analysis to 6,250 years ago. The fossil cobs bear similari7es to both teosinte and contemporary maize. Like teosinte, the infloresences are two-‐ranked. One of the specimens (c) is two-‐ ranked with two seeds per rank. Teosinte only has one seed per rank so this specimen resembles maize. Unlike teosinte and like contemporary maize, the ‘cobs’ of the Guilá Naquitz specimens are rigid, do not disar7culate to release seeds (i.e. nonshamering). Therefore human interven7on was necessary to disperse and propagate the seeds. The four-‐rowed specimen (c) suggests human selec7on for four or more rows of grain.

The Guilá Naquitz cave is also the site of the oldest evidence of domes7cated squash, Cucurbita pepo, about 10,000 years old, and suggests the inhabitants of the area prac7ced cul7va7on of both maize and squash. Squash is the oldest known domes7c crop from North America. As was the case with wheat, len7ls, peas early farmers in North America cul7vated a suite of food crops: squash, common bean and maize.

(A) Cucurbita pepo peduncle from zone B of Guila´ Naquitz that yielded an AMS 14C date of 7340 6 60 14C years B.P. (note diagnos7c alterna7ng large and small ridges). (B) Cucurbita pepo fruit end fragment from zone B of Guila´ Naquitz that retains orange rind color and yielded an AMS 14C date of 6980 6 50 14C years B.P. (C) A squash seed from zone C of Guila´ Naquitz 13.8 mm in length that exhibits marginal ridge and hair morphology diagnos7c of C. pepo and yielded an AMS 14C date of 8910 6 50 14C years B.P.

Smith 1997. Science 276:932-‐934

Map of Mexico showing the present-‐day geographical range of the wild progenitor popula7ons of the domes7cated common bean (red) (8) and maize (yellow) (7), as well as the area where wild pepo squash was likely ini7ally brought under domes7ca7on, based on archaeological evidence (orange) (14, 15). Also shown are the three areas (Tamaulipas, Tehuacań, Oaxaca) where dry caves have yielded much of the available evidence regarding the early pre-‐Columbian history of these three major crop plants. The associated chart indicates when domes7cated common bean, maize, and pepo squash ini7ally appear in the archaeobotanical sequences of Oaxaca, Tehuacań, and Tamaulipas, which along with the Southwest United States form a south to north transect (note degrees north la7tude designa7ons). Expressed in calibrated calendar years ago, the dates of ini7al appearance of these three major crop plants in these four regions are based on direct AMS radiocarbon age determina7ons (1, 3, 4, 13–15). The age determina7on for the ini7al appearance of pepo squash in Tehuacań is based on an AMS date of 7,100 6 50 14C yr B.P. (b123040)—about 7,900 calendar years ago, obtained on seed 201 from Coxcatlań Cave (square 148, level 11, zone XIV).

Smith 2001. Proc Nat Acad Sci 98:1324-‐1326

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