Bipedalism. Fire. Domestication.
Renowned geneticist Dr. Hugo Oliveira believes these are the three things that changed human civilisation most irreversibly, and he isn’t wrong. Bipedalism, or walking on two legs, freed up our hands; the invention of fire gave us cooking; and the domestication of plants and animals gave us civilisation. Domestication tethered us to land. By enabling us to settle down and take control of where our food came from, it nudged us to call places home, to specialise in crafts, and to form human-like bonds with distinctly non-human species.
Domesticate (verb)
To tame.
In animals, domestication manifests clearly. There's a lick, a wagging tail, a friendly tackle. But what is a domesticated plant, and how do we recognise the signs of domestication in it, as we recognise them in an approachable, affectionate dog?
In the agricultural context, domestication is when crops went from being wild plants humans foraged to consume as food and medicine, to being 'domestic' plants that humans cultivated through trial and error. The difficulty in spotting its signs comes from one key difference: in response to human interaction, the physiology and behaviour of animals were left altered. Plants, however, changed in biology, making the storied history and present of their ‘taming’, optimising, and adaptation a puzzle with a thousand pieces.
The blossoming of barley
Let's take the example of barley. One of the earliest domesticated plants, it also happens to be the best understood crop as far as the journey of its domestication is concerned. For over 10,000 years, barley was pre-domesticated, i.e. it was harvested from the wild. Then, around 10,000 years ago, the first signs of domesticated barley were found in modern day Syria and Palestine, part of the Fertile Crescent spanning much of West Asia, and referred to as the “cradle of civilisation”. The larger grain size led historians to believe that it was from a cultivated plant.
Each time barley changed its characteristics, it surrendered the very things that made it independent.
But an even more interesting transformation occurred around 9,000 years ago: the plant's rachis changed. Imagine rachis as the vertebrae of the grass, along which the kernels (i.e. seeds) are attached. In the wild, the rachis is brittle, to allow the kernel to fall and for the plant to propagate by itself. Barley’s vertebrae became like ours—flexible—and its parenthood, too: it started holding onto its seeds the way we hold onto our young. In doing so, it marked the first major sign of barley responding to artificial (human) selection rather than natural selection.
The lateral spikelet becoming as large as the central spikelet increased the yield per crop. Image from Universidad de Sevilla, Wikimedia Commons/Matt Lavin, and Wikimedia Commons/Matt LavinIn the wild, barley was originally two-row i.e. having two rows of kernels along its length. These kernels were composed of a central spikelet (spikelets are modified florets which grow into the grain), and two lateral spikelets flanking it on either side. Approximately 8,500 years ago–due to a genetic mutation–these lateral spikelets became as large as the central spikelet. So, the 2-row domesticate became a 6-row domesticate—a cultivar first found in parts of Egypt and Mesopotamia. This development allowed humans to get more grains from a single plant.
Slowly, the cereal began to migrate to the east of the Fertile Crescent. So far, the central spike had been the only fertile spikelet; now, the lateral spikes lost their sterility too. Soon after, it made its way to Iran where it truly warmed up to humans: it gave up its protective covering—its hull. This made it easier for humans to harvest it, and increased its beta-glucan content. Then, after over 2000 years of evolutionary lull, this hull-less barley began appearing in archaeological findings everywhere: Turkey, Europe, Scandinavia.
Also read: What it takes to feed India’s growing cities
Surrendering independence through adaptation
While barley continued to spread across the world thereafter, its evolutionary journey stagnated. It fluttered back into motion briefly around 60 years ago, with the emergence of dwarf varieties, but largely, barley’s metamorphosis ended with it losing its hull.
Each time barley changed its characteristics, it surrendered the very things that made it independent. In a sense, it entrusted its survival to humans, honouring a bond formed over generations of building trust and changing form. These changing characteristics that mark a plant’s shift from a wild plant to a domesticate are known as domestication traits. The non-shattering of seeds, loss of hull, and flexible stems are all domestication traits for barley.
But barley is only the first chapter of domestication. Even now, millenia later, we find that examining the life cycle of a plant—annual, biennial, perennial—can indicate the time period and geography of when it is most likely to have been domesticated over the last 10,000 years.
Time travel through history
Ten thousand years ago, you’d have to be on the eastern shores of the Mediterranean Sea and along the Horn of Africa to witness history, because that is where most annuals—the earliest crops to be domesticated—were first cultivated. Barley, for instance, is an annual, i.e. a plant which completes its growth and reproduction cycle (seed production) within a single year, at the end of which it dies. Most of our major cereal crops—wheat, barley, rice, corn—are annuals. Their rate of domestication peaked 8,000 years ago and plateaued around 4,000 years ago.
Most changes in annuals involve changes to seed morphology—a reduction in dormancy (or the seed’s instinct to ‘sleep’ rather than germinate in unfavourable situations), as well as seed coat thickness and impermeability. All these changes support faster germination during cultivation. Many of the domestication traits observed in barley—like non-shattering of seeds, loss of hull, and flexible stems—also appear in rice, corn, and wheat. These traits converge to make the seeds easier to collect, cultivate, and harvest. Early on in the domestication process, the effect of human technology is visible: the use of a sickle for harvesting was key to the development of the non-shattering trait in cereals, as visible in Asian rice. This ease of harvesting likely aided Asian rice in becoming the species that is predominantly cultivated across the world.
Early on in the domestication process, the effect of human technology is visible: the use of a sickle for harvesting was key to the development of the non-shattering trait in cereals
African rice, on the contrary, was harvested using swinging baskets—and it meant the seeds remained ‘wild’, falling off rather than holding on to the rachis. Some perennials (plants living for more than two years that go dormant in harsh weather) too were domesticated around this time, and cultivated as annuals. For instance, around 3,500 years ago, the Indian subcontinent witnessed one of its few cases of primary domestication: the pigeon pea (better known as toor dal), whose wild ancestor is native to southern Odisha and the adjacent Bastar area.
| Primary Domestication |
Cultivation started from wild ancestor local to region |
| Secondary Domestication |
Crops introduced into a region, not evolved from local wild ancestors |
Around 6,000 years ago, in the northern parts of Eurasia and North America in what is called the circumboreal or largest floristic region of the world, the first biennials—think carrots and beetroots—were domesticated. Biennials take two years to complete their reproductive cycle: roots and leaves establish in the first year, and seeds and flowers only come by the second year. In between, they undergo a short hibernation during the colder months, and this prolonged exposure to cold is often how the plants acquire their ability to flower. For us to cultivate them despite this extended growing cycle, biennials needed human civilisation to reach a stage where we could wait for the plant to give seeds. This explains why the wave of biennial domestication only peaked around 3,000 to 1,000 years ago. This time period also coincided with the Roman Empire’s trade activities in the Mediterranean, which allowed biennials to travel widely.
For us to cultivate them despite this extended growing cycle, biennials needed human civilisation to reach a stage where we could wait for the plant to give seeds.
The last to be domesticated were the perennials, which include both trees (like eucalyptus, mango, and coconut trees), and non-tree perennials (everything from tomatoes and strawberries to mint plants, and even dahlias). Unlike annuals, which die every year, perennials simply go dormant when the climate is harsh, and come back to life as the weather improves. Found across the globe, they were cultivated for a long time before being successfully domesticated, i.e. they were planted by humans for a long time before evolutionary changes initiated by human intervention started to manifest themselves. What delayed their domestication?
Two reasons have been hypothesised. The first is the life cycle of perennials: in the same 1000-year period, there are more generations of a rice plant (one every year) than of a walnut tree (one every 250 years). Each generation becomes an opportunity for mutations to take root, and for domestication traits to establish themselves. The longer lifespan of perennials inherently slows down their evolutionary journey.
The second reason is related to the reproductive strategy of the plant: the successful domestication of perennials has been linked to innovations in vegetative propagation, i.e. when the plant is bred not from its seed, but from the leaves, stems, or roots of the parent plant. The first wave of perennial domestication peaked around 4,000 years ago when vegetative cuttings were introduced, and the second wave around 2,000 years ago coincided with the rise of grafting.
Also read: Food fortification 101: Can foods built in with nutrients counter malnutrition, deficiencies?
Evolutionary give and take
As opposed to annual grains, where the seed modified itself for humans, in perennials (or indeed, in annuals and biennials with fruits) it tends to be the fruit that bends its nature. In many ways, this concept is a known one: animals (including humans) disperse seeds in exchange for something nutritious.
Did plants take down some of their shields because humans were protecting them from threats, or did the humans start protecting them because they reduced their bitter compounds to appeal to the human palette?
Potatoes, tomatoes, and cucumbers became less bitter, while grapes, apples, and maize enhanced their respective colours. Behind each of these modifications lie the chemical compounds that puppeteer them, otherwise known as secondary metabolites. A plant’s primary metabolites are those compounds involved in its growth and development, like chlorophylls. Secondary metabolites handle the rest—immune response, UV protection, and attracting pollinators, to name a few. As it happens, a lot of the compounds forming the armed forces of the plant (like tannins in tea) taste very bitter to the human tongue.
In a bit of a chicken-and-egg situation, we aren’t yet sure what happened first: did plants take down some of their shields because humans were protecting them from threats, or did the humans start protecting them because they reduced their bitter compounds to appeal to the human palette? One thing we do know is that across all regions and plant types, the most common domestication trait to be witnessed was this: changes to the presence and concentration of secondary metabolites.
Grapes enhanced their colour and reduced their tannins to become more appealing to humans. Image from Pierre Viala (1859-1936), Victor Vermorel Genetic fixations
While secondary metabolites bend the chemical composition of the plant, a trait called polyploidy tinkers with its genetic makeup in profound ways. Ploidy refers to the number of complete sets of chromosomes a somatic (non-reproductive) cell has, and most sexually reproducing organisms are diploid or greater (polyploid). Humans, for instance, are diploid since they have two sets of complete chromosomes—one from each parent. Plants have greater internal variation in ploidy: some like rice are diploid, while sugarcanes go up to octaploids. All in all, nearly a quarter of all current plant species are polyploid.
Domestication has had a tendency to initiate polyploidy in plants in one of two ways. One method is through abnormal genetic duplication within the same species (autopolyploidy), which is how vegetables like cauliflower evolved. This excess genetic material results in larger stems, roots, or leaves. So, the same parent species Brassica oleracea evolved into cabbage (larger leaves) and cauliflower (larger flower buds) when selected for certain features. These changes also make the plant more adaptive, and allow it to establish itself in regions where its ancestors could not survive.
Potatoes only exist because of a chance hybridisation between a wild potato and wild tomato. Image from Henry G. Gilbert Nursery and Seed Trade Catalog Collection;B.K. Bliss & Sons, No restrictions, via Wikimedia CommonsIf you relish potato-based dishes, you will love learning about the second kind of polyploidy (allopolyploidy) where the genetic material of two or more species is mixed to create a new variation. It is only because of a chance hybridisation between a wild potato Etuberosum (which was incapable of producing tubers), and a wild tomato (which has the gene that is the master switch for tuber formation) that we have the wildly popular modern-day potato! This kind of delightful development is at the heart of this method: by mixing genes from two distinct sources, it widens the pool of raw material for natural (or artificial) selection to choose from, resulting in a mix of desirable characteristics from both ancestors.
Changes in ploidy are central to the journey of wild plants differentiating into distinct species. This is simpler to observe in autopolyploidy: the same wild ancestor undergoes distinct domestication journeys at different geographic locations to evolve into cauliflower, cabbage, broccoli, etc. Allopolyploidy allows for something even more magical: it gives the plant ecological isolation even if it is geographically proximate to its wild ancestor.
When a polyploid plant is cross-bred with its diploid ancestors, the difference in chromosome numbers prevents the chromosomes from pairing and leaves the offspring sterile, essentially ensuring that it evolves independently. This allows it to retain and reproduce the characteristics it was chosen for, and solidify its own lineage. In short, it is what makes genetic changes stick.
Also read: What's lurking in our food?
The diversity discourse
Whether through polyploidy or otherwise, the process of domestication allows artificial selection to supersede natural selection. An unforgettable figure in revolutionising how artificial selection is deployed was Austrian biologist-mathematician Gregor Mendel. His work on plant genetics in the 1850s would be refined for over a century, and propel the breeding of varieties with more calorie-dense grains, and more yield per acre. By the 1960s, these developments would coalesce into the Green Revolution—an international programme aimed at battling hunger and poverty in Asia and Africa.
This marks an important shift in the prevailing mode of domestication. Earlier, small-scale farmers would select the grains which had the most starch, the trees with the best tasting fruit, and the plants with the fleshiest leaves. Now, dedicated organisations breed crops with the intention of mixing genes and creating hybrids with specific characteristics. Even when traditional domestication was intentional, the farmer's choice was limited to which crop they rewarded with propagation. The intentionality hybridisation offers is far more precise. Technology has made the process faster too: what used to take thousands of years can now be achieved in a decade or lesser.
Modern domestication is geared towards yield and ease of harvest, and often chooses genes that yield predictable, homogeneous crops. Along the way, we lose diversity.
When measured against a definition, both these practices count as domestication: they both involve a coevolutionary, mutualistic relationship where one species (humans) constructs an environment where it actively manages the survival and reproduction of another species (crops like rice and wheat) to provide itself with resources or services. Most scholastic work on the subject refers to hybridisation as a form of domestication, although there remains a strong counterargument to this nomenclature. If the meaning and implication of a word evolves so deeply that it births an entirely new practice, should they still share a name?
There is great risk in conflating traditional and modern domestication, given the diverse impacts they have had on human society. Traditional domestication has been instrumental in making plants digestible, and in turn, in the development of civilisation. Modern domestication is geared towards yield and ease of harvest, and often chooses genes that yield predictable, homogeneous crops. Along the way, we lose diversity. One way to fathom the scale of this loss is looking at the Food and Agriculture Organization’s data revealing that seventy-five percent of the global food supply comes from 12 crops, three of which—rice, maize, and wheat—make up 60% of the global calorie intake. A dozen crops are at the centre of global food grain demand—something farmers across the world respond to by growing these crops irrespective of geographic suitability, straining natural resources in the process.
Mendel’s experiments in plant genetics redefined modern-day domestication. Image from Daniel J. Fairbanks, CC BY-SA 4.0, via Wikimedia CommonsThis loss of plant diversity impacts not only biodiversity, but also food security and nutrition. Captured in the concept of genetic drift is the acknowledgement that multiple factors determine the fluctuations in the genetic diversity of any species. Humans have, however, developed a knack for being the factor with disproportionate influence. It is how we have driven animal after animal into extinction, and snuffed out over 600 (known) plant species over the past two and a half centuries.
Conservation efforts can mitigate extinctions, but do not always manage to address the problem of genetic diversity. Even in the case of the bearded vulture which was almost hunted to extinction (one of the best known wildlife comeback stories), the gene pool of the surviving vultures is limited, and biologists continue to worry about the vultures' capacity to withstand environmental change in the long term.
As certain easy-to-cultivate varieties become ubiquitous, we are only one plant disease or weather irregularity away from severely disrupting our food supply system. Optimising crops for yield has also resulted in calorie-dense starches replacing nutrient-dense crops, something that is at least partially responsible for the widespread micronutrient deficiencies we see today.
It is easy to understand the domestication of plants as a process where humans tamed plants. But in many ways, the plants tamed us too. They made us sedentary, put us on a diet of primarily 3 cereals, and got us well and truly hooked on starch and sugar. Continuing to feed ourselves this limited diet puts us at risk of fading away like the bearded vulture.
It is easy to understand the domestication of plants as a process where humans tamed plants. But in many ways, the plants tamed us too.
Modern-day domestication, driven by sophisticated science, offers its own solutions to these problems. It talks about the possibilities of selecting crops not for nutrition, but for ecosystem services like carbon sequestration, and using these intentionally bred species for ecological restoration.
Our escape route—surviving wild plants—does not lie in more petri dishes; they are hidden in roadsides and unplundered hills, passed on through oral traditions and aged guardians. Traditional modes of domestication are still open to us, and are still faster than earlier thanks to a better understanding of botany. Both kiwi and cranberry, domesticated only in the past 100-200 years, testify to this.
The world we have arrived into today isn’t irredeemable. But it is a tale of artificial rather than natural selection determining what plants are the fittest for survival. A little less meddling, and we may find the plants that escaped the calorie-dense transformations that human civilisation hammered their brethren into. In a world that is struggling, at once, with malnutrition due to hunger as well as due to overconsumption of high-calorie foods, we might find some answers in indigenous knowledge, and plants that still have diversity in genes and nutrients.
Cover image (desktop) from Henry G. Gilbert Nursery and Seed Trade Catalog Collection;B.K. Bliss & Sons, No restrictions, via Wikimedia Commons
Cover image (mobile) from Wellcome Library, London, CC BY 4.0, via Wikimedia Commons