The Neolithic “Revolution”

The advent of plant and animal domestication profoundly altered the trajectory of human evolution and ushered in a series of cultural transformations that set the stage for the rise of cities and states and eventually the industrial revolution and all that has ensued since.  The processes of change from food acquisition based on what nature provided (wild foods) to food production based on domesticates was termed the Neolithic Revolution by V. Gordon Childe in 1935.  The consequences for human societies from the rise of food production were monumental in the long run, but they only look revolutionary when were compared to the gradual evolutionary changes that occurred during the previous 200,000+ years of modern humans.  The processes of domestication for any plant and animal species occurred across many hundreds if not several thousands of years, so hardly what looks like a revolution.  And the subsequent rise of cities and states also took hundreds if not thousands of years to transpire after agriculture was fully developed.

The the start of domestication and the origins of agriculture is one of the chief focuses of archaeological research.  Such issues lie in the realm of archaeology because they all occurred well before the advent of written records.  This is also true for the origins of social complexity characterized by cities and states—these too occurred before the advent of writing.  Coming to a scientific understanding of when, where, how and why domestication, food production, and social complexity occurred requires archaeological investigation.

Plant and animal domestication leading to food production occurred in the last 12,000 years, during the geologic interval known as the Holocene.  Most of human evolution occurred during the Pleistocene, a time of highly variable climates when there was a sequence of one ice-age after another interspersed by non-ice-ages (interglacial periods).  The Holocene is an interglacial period, but the impact of humans, seems destined to ensure that another glacial period will not occur.  Our impact on the world became noticeable by the rise of agricultural economies on most continents thousands of years ago and was greatly accelerated 200 years ago with the industrial revolution.  We are changing our environment so significantly that geologists have started designating the current interval starting around 1800 AD as the Anthropocene.

Food Gatherers vs. Food Producers

Gordon Childe coined the term food production over 70 years ago.  He contrasted food producers with the food gatherers.”  Food Gathering is what humans had been doing since the beginning.  Procuring what nature provides by hunting, fishing, and collecting available animals and plants.  This is done without the effort to enhance what occurs naturally.  But without such effort at some point foragers reach a point of diminishing returns.  All the easy to get food close to a camp is gone so you have to travel further and further to find food.  Essentially, people eat their way out of places and then have to relocate to a new location at some distance from their old home camp where the resources are not picked over.

The Penan of Borneo in their traditional lifestyle as foragers made 45 moves on average per year.  This means moving roughly every 8 days.  Imagine picking up and moving your family and belongings almost every week.

Food Production involves humans gaining control over their own food supply.  Such control varies in kind and degree, but the clearest expression of this involves the cultivation of domesticated plants and the herding of domesticated animals.  This is what Gordon Child emphasized.  Domestication involves human control over the reproductive cycles of plants & animals.  It usually entails the purposeful manipulation of the environment to greatly enhance the concentration and predictability of food resources.  The latter refers to the knowledge of where food should occur in both space and time; reasonable predictions as to which areas of land are likely to contain specific resources and when they will be are available for harvest.

Domesticated Land but No Domesticates

The first step towards domestication occurred when hunter-gatherers (food gatherers) started to alter their environments to improve the distribution and density of plants and animals.  Living hunter-gatherers around the world have been documented doing this to varying degrees, such as many, if not all, Aboriginal societies of Australia.  Their principal technique is burning, a practice given the name “fire-stick farming” by Rhys Jones.  This practice, which may have begun some 35,000 years in Australia, pushes local habitats back into early succession thereby enhancing biodiversity and most importantly, increasing potential food resources for humans.  Effective and controlled burning creates what are called anthropogenic landscapes that contain a greater diversity of successional stages than naturally burned lands (those caused by lightning strikes).  The importance of this to human foragers has been noted by many and demonstrated in a few careful studies, such as one by Stanford anthropologist Bliege Bird and coauthors.

In effect, Australian Aboriginal had effectively domesticated much of their environment by the time of European colonization, but not the species in that environment.  Australia stands out as a major exception when it comes to domestication since it is the one continent with sustained human occupation where plants and animals were not domesticated, where none show the obvious signs of phenotypic or genetic modification that comes with domestication.  Immediately to the north of Australia is the island of New Guinea that was populated by the same human population that migrated out of Africa some 75,000 years ago.  This island is one of the world’s centers of early plant domestication, including such important crops and sugar cane and banana but also taro and yams (true yams, Dioscorea, not the sweet potatoes, Ipomoea, that your mom serves at Thanksgiving).

Fire stick management of the environment was a widespread human practice.  Native Americans throughout much of North America did the same thing.  Even in places such as California where all natives were hunter-gatherers and none practiced agriculture, they manipulated the natural environment to ensure that it best suited their needs.  When Euroamerican explorers first entered Yosemite Valley, the beautiful black oak parkland they encountered wasn’t there by accident.  It was the product of careful management that involved controlled burns, pruning, tree and shrub planting.  Periodic burning eliminates brush and debris and encourages grass and weeds while leaving mature tress unharmed.  They also worked to eliminate nuisances such as poison oak.  Natives did not simply collect what nature provided, they worked to ensure that it offered the resources with the greatest value.

Low level food production is the term that the archaeologist Bruce Smith (Smithsonian Institution) used for the activities that humans have engaged in to enhance their food security.  This is something that humans started doing at differing times in many corners of the world.  In certain places these activities ultimately culminated in plant and animal domestication and eventually in agricultural lifestyles.  It is not a foregone conclusion that domestication will occur, as the continent of Australia well illustrates.  The nature of the species being manipulated make a difference.

Bruce Smith identified a vast and diverse middle ground that exists between societies depending solely on traditional food procurement: hunting, fishing, and gathering of wild species and those societies depending on agriculture.  He sees domestication as the central landmark of this middle ground, situated well away from the extremes of pure hunting–gathering and intensive agriculture.  He lists various societies from around the world according to the level of investment on food procurement vs. production.  Many existed as low-level food producers.  Some of these societies lacked domesticates such as the Owens Valley Paiute and Kumeyaay of southern California or various tribes of the NW coast, yet they all manipulated their environments to improve food resources.  Yet this manipulation never resulted in domesticated plants or animals.  In other cases, such as in the eastern woodlands of North America, such manipulation resulted in domestication. Even so, eastern woodland societies persisted for a long time with domesticates without becoming fully agricultural.  The Hopewell and many other groups of the eastern woodlands are good examples.  Its wasn’t until maize (corn) was introduced from Mexico via the Southwest that many Native groups of the woodlands started on the path for full agricultural dependence.

Horticulture vs. Agriculture

We also want to draw a distinction between horticulture and agriculture.  Sometimes you see these terms used interchangeably but they hold different meanings.  Horticulture is small-scale planting of domesticated species in house gardens or swidden plots.  The planting and tending of gradens is is usually combined with routine hunting and gathering of wild species to meet subsistence needs.  It is an example of low-level food production.

Swidden is a a common practice in tropical regions of the world and is commonly known as slash-and-burn farming or or fire–fallow cultivation.  This is where small parcels of dense vegetation get cut and burned down to prepare the ground for planting.  This burning not only clears the ground so that crops like maize can grow, but it adds forest nutrients to the soil.  A common problem to tropical habitats is that soil fertility rapidly decreases, especially when planting a crop such as maize, which rapidly depletes soil nutrients, such that after five years or so the farmer has to clear and burn the vegetation of a new area.  In this way, farm plots are constantly shifting around the landscape.  Swidden stands in marked contrast to the intensive forms of agriculture that are on display all around us here in Nebraska.

Agriculture is when domesticates account for the majority of subsistence, roughly 75% or more of the human diet.  Essentially this is full commitment to and reliance upon food production.  With agriculture, the full commitment to domesticates involves ever increasing investments of labor and capital to increase the yield that can be obtained per area of land.  This has often led to monocropping and dependency on one or two chief resources.  This can be a precarious existence.  Depending for most of your food on a single resource such as maize or potatoes is disastrous when they fail and there are few if any fallback resources.  Such disasters have occurred time and time again around the world and will continue to occur, although these days they tend to be mitigated by our degree of connectivity.  Still, famines from agricultural failures is an annual problem around the world.

There can be a gray ares in distinguishing between horticulture and agriculture and the line can be especially fuzzy when trying to figure this out for prehistoric people.  Archaeologists have various methods to try and reconstruct past diets, but these never provided such clear cut evidence as being able to observe people and actually measure the proportion of food from domesticated plants and animals vs. that from wild plants and animals.  These days there is also the “farming” of wild species such as elk, salmon, shrimp, scallops, and mushrooms.

Domestication

Domesticates are species of plants and animals that show recognizable changes in morphology away from wild phenotypes.  Any observable changes in phenotype may have an underlying change in genotype, especially with sufficient time, since are linked in a cause & effect relationship.  Domesticated plants and animals have been changed so much that in many cases they have lost their ability to survive in the wild.  Maize or corn is a good example.  This domesticate looks nothing like its wild ancestor and it is not capable of scattering its reproductive seed, which are tightly held to the ear and encased in husks.

The first part of domesticate is the word domestic, which as a noun refers to a servant who works in another’s house.  In this sense, maize is our servant.  Yet, because of us, maize has been spread around the world and into different environments, including northern latitudes where it never could have survived previous to human intervention.  This tropical grass is no longer confined to its place of origin in the hot and damp lowlands of western Mexico.  So who is the master and who the servant?  Moreover, we have become so dependent upon domesticates, such as maize, that human society cannot survive without them.

We have something of a mutualistic relationship with domesticates, but when crops fail the human toll can be considerable.  Take the Irish Potato famine that started in 1845 because of a blight that ravaged potato crops throughout Europe.  During the famine about 1 million people died.  Another million more emigrated elsewhere, mainly to America.  Ireland’s population fell by about 25%.  Of course, with this famine, and most natural disasters, human politics exacerbate the problem. As a commentator of the time John Mitchell said:  “The Almighty, indeed, sent the potato blight, but the English created the Famine.”

Domesticated plants & animals are human “creations,” although most were not intentional creations done with human foresight and planning, as in “I want a friendly and obedient carnivore who can guard my camp and help me hunt, so, I will turn this wolf into a dog.”   If this thought ever did occur to someone in the dim past, then the only way it could have come to fruition is if the concept and the procedural steps (recipe) for achievement were passed on through some 100 human generations.  And if you believe that is what occurred, then we just happen to have some magic beans that we would happily sell you.  The intentionality that we use today to modify the phenotype or genotype of plants and animals for our own benefit is not what occurred during the initial domestication process.

Unintended and Intended Consequences.

Most domesticated plants and animals arose as unintended consequences of our activities.  Humans determined the selective pressures, creating new rules for survival and reproductive success, but not in some conscious way.  Generations of response to these new survival rules change the phenotype and genotype.  Such incidental domesticates that reflect unintended consequences include all cereals such as wheat, rice, barley, and oats.  The incidental aspect is especially true during the early phases of the process for most domesticates.

The development of domesticated plants and animals followed by agriculture was a long-term gradualistic & incremental process.  It was not a rapid and broad scale transition between two steady states: procurement vs. production (not a revolution).  Given the gradual and incremental nature, unforeseeable end results cannot be reasons behind domestication and agriculture.  For example, no one could have predicted that trainable and loyal dogs could have resulted from wolves.

A Gradual and Incremental Process

Understanding how domestication occurs for each species constrains our ideas or hypotheses about why domestication occurred.  The process of transforming a wild species into a domesticated one, casts a bright light on the fallacy of thinking that domestication was a intentional “invention” to fix an adaptive problem such as too little food or about generating a surplus for social aggrandizing.  Both of these aspects have been put forward as explanatory theories to explain the “agricultural revolution.”

Eventually domestication allowed for the generation of huge food surpluses and the individuals that gained control of these were able to create indebtedness.  But that is after the fact,  after domestication had occurred and the productivity of the domesticates had increased.  The use of food surpluses to fund the political economy is a consequence not a cause.  This can be seen as another example of Romer’s rule introduced previously in the primate chapter.  Remember that the current utility or function of a trait (including behavior) might have nothing to do with why that trait or behavior appeared in the first place.

HOW? The Wheat Example

These are some of the beneficial effects that occurred after wild wheat had become domesticated wheat:

• larger seeds,
• thinner and smother coverings or husks,
• contraction and compaction of seeds, with more per stalk
• synchronicity in ripening, which means all plants can be harvested at the same time
• and non-shattering spikelets (grass flower part where seeds form)

It is important to recognize that in any wild wheat population there are always plant variants that have these traits or versions of these traits.  However, under conditions of natural selection these traits are never favored and become predominate.  The traits remains as low frequency or poorly expressed possibilities in those wild populations.

Lets look at the last of these.  The shift from shattering to nonshattering was important was important for human foragers.  Wild grasses, like wheat, naturally shed their seeds freely upon reaching maturation.  This provided the greatest reproductive success for the plants.  But free shattering limits the utility of the seed for humans because a considerable number get lost, falling free from the plant prior to our during harvesting.

What about synchronicity in ripening.  If there is nonsynchronous ripening then humans harvesting a field cannot get all the seeds, because some plants have already ripened and the seeds fallen whereas others are not ripe and so are not harvestable.  A field of plants that ripens all at the same time means a larger harvest.

As to having a thick and though seed coat, that too is adaptive in natural settings.  It is often beneficial to plants if their seeds have a prolonged dormancy period, which means the ability to survive in the ground for years if conditions are not right.   This was important for the wild ancestors of grasses such as wheat, barley, rye, and oats because the grasslands where they proliferated experienced drought years.  Domesticated seeds have a short dormancy period associated with thin seed coats.  Domesticated wheat has larger seeds, which goes along with quick sprouting and synchronous harvesting.

All of these changes occurred without humans being conscious that the changes were occurring, without humans trying to engineer such beneficial outcomes.  It all has to do with how the plants are harvested, stored and then some of the seed replanted for another harvest.  Human harvesting of wild wheat (or barley, rye, oats, etc.) will tend to preferentially collect seeds from those plants that have the nonshattering trait while seeds that fall freely will get lost.  When a portion of the harvested seeds are replanted during the next season there will an increase in the frequency of seeds with the nonshattering during the ensuing harvest.

So too that wild wheat plants that have trait variants for larger seeds, thinner seed coats, and more contracted and compacted seeds on grass stalk heads will also increase in frequency at the expense of those wild wheat plants that lack these traits.  Seeds with thinner seed coats will respond quickly to the first moisture after being sown in the ground and those with larger seeds will have more vigorous early growth (seedlings obtain their energy to germinate and break through the earth from their seeds).  This means that those plants that reach maturation first have a great chance or being harvested and forming the seed bank for the following year.

The human activities of harvesting wild wheat, storing the grain during the winter, then replanting some seed in fields for the following year’s harvest is what drove the domestication process.  These activities repeated across scores upon scores of generations, of harvest and replanting seasons, lead to the incidental domestication of many of our most important crops.  Key in this is not just the harvesting but also the replanting, which is what altered the selective force from nature alone to humans in nature.  Other human activities that played a key role in this was the harvesting of grass stalk with sickles and sowing the seeds into places that they would not have grown naturally, including area outside their normal habitat and range.  The latter might have been especially important since there was then less chance for genetic intermixing with wild types that lacked the traits beneficial to humans.

Long term field experiments with wild wheat, barley, oats, and other grasses by researchers such as Jack Harlan, Gordon Hillman, and Stuart Davies have demonstrated that the human activities mentioned above result in the incidental domestication of cereals.  When humans became the selective force rather than nature alone they can drive changes that result in a beneficial end result without being aware that this is occurring.

Archaeological Documentation of Cereal Domestication

Archaeologists who study plant remains are known as archaeobotanists.  They have documented the process or wheat, rye, barley and other cereal domestication at sites of the Fertile Crescent in the middle east.  At a 19,000 year old site in Israel (Ohalo) only four barley stem portions where seeds were once attached (the rachis) lacked a clean break, which means all but these fell freely from the plant.  This is fully representative of a wild barley where this trait has a low frequency of representation.  At a 10,000 year site also in Israel (Netiv Hagdud) over 100 barley rachis fragments out of a few thousand lack a clean break.  By 9000 years ago in Israel virtually all barley rachis fragments lack a clean break: barley domestication had occurred.

Similar sorts of changes are documented for wheat and other crops found at archaeological sites in the Fertile Crescent, although with the changes toward domestication often spread out across a much longer span of time.  The figure presented below from a paper presented in the journal Science in 2006 plots the frequency of wheat remains from sites in the middle east that are of different ages.  The age decreases from top to bottom.  This graph shows increasing evidence for domestication through time.  The carbonized wheat specimens include those that are clearly wild, the ones listed as dehiscent, which means the seeds freely separated from the rachis.  At the other extreme are specimens from clearly domesticated wheat, those listed as indehiscent, which means the seeds hard to be forcefully shattered from the plant by threshing.  Intermediate between these are those specimens from possible domesticated wheat.  This graph shows ever increasing frequencies of clearly domesticated wheat through time but it occurs across a span of some 4000 years.

Is a process of change that occurs across a millennial scale an adequate means to address a population “imbalance” or to deal with a climatic downturn?  A thousand years or more is the wrong time scale for responding to a food crisis.  Changes that transpire across such time spans do not reflect the day-to-day or even season-to-season decisions of foragers.

HOW? The Banana Example

The case for the banana provides a useful contrast to cereals like wheat, barley and oats.  Bananas are a hugely important crop worldwide for humans.  They rank next to rice, wheat, and maize in terms of their subsistence importance.  Americans are big banana eaters, consuming roughly 27 pounds of the fruit per person every year on average.

Banana traits that changed with the process of domestication include:

• suppression of seeds,
• enlargement of fruit,
• development of parthenocarpy (virgin fruit; fruiting without prior fertilization) &
• increased sweetness.

Bananas are not trees. They are the worlds largest flowering herb.  Wild bananas produce seeds (sexual reproduction) but they also propagate clonally (asexual reproduction) by sending out suckers.  This allows favored plants to be reproduced without using seeds and such favored plants are genetically identical to the mother plant.

Clonal propagation allows humans to selective cultivate and spread the most desirable banana plants, those that produced the sweetest fruit and had fewest seeds.  This is distinctly different from the nature of cereal reproduction, which is always sexual and one where individuals with favorable traits cannot be isolated from those that do not.  A single banana plant produces an abundance of fruit, unlike a single wheat or barley plant.  As such, the effort of trying to clone an individual banana plant with favorable qualities is worth it.  Additional favorable mutations when they occur at random or because of hybridization between similar species, can easily be perpetuated by the clonal propagation.  ones that reduce seed size and abundance, increase sweetness, or fruit number per bunch can easily be perpetuated by the clonal propagation.

This maps shows the distribution of the wild ancestors of cultivated bananas.  Two species and multiple subspecies seems to have been involved, ones that thrived from New Guinea in the east to Sri Lanka in the west.  Hybridization played a role with human movement of  plants from one region to another key in this, since it allowed genetic mixing unlikely to occur otherwise.  Random mutation also added to the complexity of the process, but the intentional role of humans is evident in the preferential propagation of those plants with the most desirable characteristics.

Unlike wheat and other cereals or seed crops, tracing banana domestication in the archaeological record s difficult.  With cereals there are carbonized seeds, but these are exceedingly rare occurrences with bananas.  Bananas were domesticated in the tropics where organic preservation in the archaeological record is exceptionally poor.  Moreover bananas do not produce many seeds and it would be even rarer that they might get carbonized because bananas do not need to be cooked.  Evidence from the archaeological record consists of what are called phytoliths (plant silica crystals).  The earliest examples so far were recovered from excavations at Kuk Swamp, in Papua New Guinea.  Unfortunately phytoliths are not diagnostic to the species level or subspecies level, which is critical for full understanding of the domestication process.

After domestication bananas spread from human migrations and also through interconnecting social networks.  Bananas reached Africa in prehistoric times.  European colonists ultimately spread bananas to the New World.

HOW? The Dog Example

Dogs are the first domesticate and appear to provide an excellent example of Romer’s Rule for animal domestication.  That dogs should be the first domesticate should perhaps come as no surprise.  Who understands us better? Who is more willing to put up with us? To love us unconditionally?

The latest genetic research indicates that wolves and dogs had a common origin with the split occurring perhaps as early as 40,000 years ago.  A 14,000-year-old jawbone is the oldest undisputed fossil from a domesticated dog, but dog-like remains date back as far as 35,000 years ago.  Genetic data indicate that the ancestors of all modern dogs split into two populations: one that gave rise to East Asian breeds & another that would become modern European, South Asian, Central Asian & African dogs.  Dogs came to the new world with people, but they probably did not look exactly like any of the modern dog varieties seen today.

How is it that wild wolves got transmuted into a domesticated dog?  With dogs it is hard not to think that there must have been a purpose, since they have so many uses both currently and in the documented past:

• Food (the stew pot)
• Hauling loads by pack or travois
• Hunting
• Guarding camp
• Herding sheep or other livestock.
• And many more

All of these were eventually true, but was there a reason initially?  Do current functions provide reasonable possibilities as to why people started interacting with wild wolves?  Wolves, the ancestors of dogs, were perhaps initially incorporated into human groups for no immediate purpose.  They may have been incorporated as camp followers, sort of like animal weeds.  Eventually it became a mutualistic relationship.  The domestication of wolves into dogs was probably a “passive” process.  This was not humans actively taming wild wolves.  It started with the animals approaching hunter-gatherer camps in search of food.   Those wolves willing to do so were perhaps less aggressive and less fearful of humans thus more successful at scrounging or begging around human camps and thus more likely to befriended by humans.  These were perhaps somewhat analogous to the village dogs that are seen hanging around human settlements without dog catchers.  Dogs that roamed freely and do not live in specific people’s homes.

Part of the process also likely involved capturing of baby animals especially after mothers have been killed.  Human hunter-gatherers everywhere show a proclivity for caring for baby animals—it seems to be part of the way that humans are wired: caring for the young and fragile.  The process of arriving at domesticated dogs from wolves was an especially lengthy one.  Only well after the domestication process had occurred and dogs had been living with humans for millennia did humans start to be more selective of specific traits well suited for specific tasks such as tracking, herding, or sitting on laps.  These aspects were deliberately caused by humans and truly represent intended consequences.  But the original processes of domestication from wolf to dog was an unintended consequence, something foreseen and not driven by purpose by conscious selection of favored traits.

Primary centers of domestication are those regions of the world where agriculture was developed—where certain plants & animals were domesticated.  Other regions of the world then adopted domesticated plants and animals from these primary centers and in some cases furthered the process.  Europe, for example, did not independently develop any domesticated plants or animals but benefited from those domesticated elsewhere.  Europe initially became a secondary center because of the expansion of farming populations out of the Middle East who brought their crops and livestock with them.  Europe continued to acquire plants domesticated elsewhere as the societies there started to interact with other societies around the world.  An additional large change happened with the accidental “discovery” of the new world and the immense diversity of the plants that Native Americans domesticated or were using.  This included such important items as tomatoes, potatoes, maize, chilies, squash (includes pumpkin), chocolate, vanilla, beans, peanuts, and more.  Its hard to imagine Italian cuisine without tomatoes, the Swiss without chocolate, or any of us without chilies.  An what would have the Huskers been called without maize?

There are at least 11 primary centers around the world.  Depending on spatial scale one could identify others.  Domestication occurred on all continents except Australia, the odd exception perhaps simply because the species there did not lend themselves to the domestication process.  If you map the current regions of greatest agricultural productivity in the world they do not reflect the centers of domestication.  The one exception is China and in this case the item domesticated there, rice, remains the chief crop grown.  The central US is one of the world leaders in agricultural production the key crops originated elsewhere: maize (corn) from Mexico, wheat from the Fertile Crescent, and soybeans from China.

WHY Agriculture?

Why humans ever started down the agricultural path has been a big question that archaeologists and others have tried to answer.  Most societies have myths about crops as divine gifts.  Some god or gods gave us corn, or grapes, or yams, or whatnot.  The bible provides a myth that stands as an exception: Genesis describes agriculture as our curse for being cast out of Eden.  Life was once good when we foraged for food, but now we are banned to a hard life of toil, working the ground to earn our daily bread, till we die and return to the ground.

The Victorian view of agriculture is that it was a one-off discovery or lucky accident, a Eureka moment.  This stemmed from the notion that domestication was such a complex and difficult process that it could only have occurred once for each species.  You still hear echoes of this even to this day, but it is simply not true.  This is not to say that the process was not complicated as the study of banana domestication reveals, but domestication happened in many different places around the world.  Moreover, plants such as cotton were domesticated at least twice on different continents from different ancestor species.  Squash was independently domesticated in all three primary centers of the Americas: eastern North America, Mesoamerica, and South America.  Chilies were domesticated in Mesoamerica and South America.  Wheat was independently domesticated in several different areas of the Fertile Crescent.  Millet was independently domesticated in Africa, India and China.

The record of domestication makes it is clear that all people of the world were talented naturalists.  People did not realize that they were transforming wild species into domesticates and in most cases were not aware that changes beneficial to them were occurring.  But people everywhere were smart, observant, and curious about nature and were certainly interested in providing a reliable and productive foods for their families.

Push & Pull Explanations

Explanations for the origins of agriculture can be differentiated into those representing pushes and those representing pulls.  Pushes are what force us to act.  Pulls are incentives to act.  These are the sticks and carrots of our behavior.  Push and Pull accounts extend to other complex questions examined by anthropologists/archaeologists such as origins of state level political organization.

In pull accounts, agriculture is seen as superior to hunting and gathering, so humans would seize the opportunity if presented.  Initially the Superiority of agriculture was a self-evident presumption.  Many early explanations were of this kind.  In push accounts agriculture was not seen as superior to hunting & gathering, so humans would not take to farming unless forced; there had to be an external cause.  Commonly identified external causes included stress from overcrowding (overpopulation) or famine from droughts.  Many explanations of the 1960s, 70s, and 80s were like this.

Pull accounts are now back in vogue but of a different sort.  The pull accounts these days have to do with using food surpluses to host feasts, to brew beer, and to supply other social rituals.  It is easy to envision the incentive behind working hard in order to get more supplies for brewing fermented beverages.  Such are almost universal in their use in social rituals or many varieties.

Niche Construction

These days archaeologists studying the origins of agriculture often refer to the modern biological concept of niche construction.  This occurs when an organism alters its environment, often (but not always) in a manner that increases its chances of survival.  In the standard biological model, the environment causes changes in species through natural selection—favored traits are spread.  In this standard model species adapt to live within environmental constraints over which they have no control, such as heat or moisture, its basically a “one-way street”.

But many species also cause changes in their environment.  This is especially true of species known as “ecosystem engineers,” such as beavers who build dams, ants, termites, and many other animals that build nests, or plants and microbes that release of toxic chemicals to inhibit the growth of competitors.  This creates a feedback between natural selection and environmental modification; its a “two-way street.”

Organisms can affect their environment to such an extent that this causes a shift in what traits are naturally favored.  If environmental alterations persist for several generations, this results in an ecological inheritance.  Offspring not only inherit genes and behaviors from their parents but also a modified environment.  This can result in what can be termed directional trends in evolution.

There are many species that can be characterized like this, but humans are a preeminent example:  we are the poster child for niche construction.  Part of this process for humans is the burning, or fire-stick management mentioned previously.  But there are a host of other cultural practices of societies around the world that purposefully altered their environments to improve plant and animal distributions, densities, or other traits.  Humans pass on these altered environments to their offspring along with the knowledge of the cultural practices for how to continue the process.

Fire and other human disturbances bring about and early successional plant community and many of the plants that humans depend on and ultimately domesticates are the early pioneers of disturbed habitats.  They are weeds in an ecological sense, which means aggressive colonizers of disturbed soil (note that the weed you can get in Colorado also qualifies as an ecological weed).  Weeds have evolved over millions of years to take advantage of natural disturbances such as wildfires, floods, volcanic eruptions, landslides, and animals.  Humans excel at land disturbance, creating habitats that weeds proliferate in.  We have been doing this in a significant way since at least 12,000 years ago.  Anywhere humans went in the world, except the extreme north, there were weedy species to colonize the disturbed habitats that we created.  Another name for these weeds is camp followers, because wherever humans camp they appear.

Weeds are usually annual plants, living for just a year, and most produce prodigious numbers of seeds.  This directly benefits humans since seeds that get produced in abundance for many species are good eating.  Some weeds also produce greens that are good to eat.  Bananas are also like this, pioneering disturbed soil.  Humans did not necessarily have to go searching for food since it was right there in the disturbed setting around camp, or the area that they burned three months ago, or the places they camped at the previous year.  If human foragers scattered seed in areas naturally disturbed such as those exposed by receding floodwaters of a river or those burned bare before the rains start then they could generate a substantial return with minimal investment.

A Coevolutionary Process

The sustained interaction with weedy species started humans down the path toward agriculture.  By manipulating the conditions of growth of various plants with the goal of increasing their relative abundance or predictability, humans initiated a coevolutionary process.  The coevolving mutualism between humans and their managed resources caused changes in them that further promoted this dependency.  A variety of cereals and pulses (legumes) were the direct result of this coevolutionary process, as where other crops such as bananas and squash.  In a process like this there is no single cause and what might appear to be a cause might actually be a consequence, such as population pressure.  It is rather clear the greatly accelerated growth in human populations occurred after the establishment of an agricultural lifestyle.