In a world dominated by mass agriculture, chemical pest-control and crop fertilization, artificial growth stimulation and 5-storey farms stuffed with animals that have never seen a ray of sunlight, it is quite clear what is wild and what not. We humans have created a wide array of life forms that do not occur naturally in the wilderness: our crops and cattle obey different evolutionary rules than their wild relatives: they obey our rules. Darwin himself already recognized the distinction between naturally evolving lifeforms and the ones that were biologically changed by us as we domesticated them for our own use and consumption. While all wild species of plants and animals were moulded by the demands of their environment and the unconscious selection of the “fittest” individuals, our crops and cattle have arisen from thousands of years of conscious selective breeding by humans, who only allowed individuals with suitable characteristics, such as tame behaviour, small seeds or larger fruits, reproduce. Therefore, our crops and cattle did not evolve through natural, but artificial selection, and became domestic species that exist only thanks to us and for us.
This is a very anthropocentric idea of what domestication actually is. The process of domestication is as complex as any evolutionary phenomenon and consists of far more than the classical idea of humans selecting one generation of crops or animals after the other and consciously “creating” new species. Rather, we will see that there is much more to this process than meets the eye and that the lines of what we define as “natural” and “artificial” are less clear than most would think.
Humans have been domesticating plants and animals for thousands of years. The dog, the very first domesticated organism of all, has been by our side for 15 000 years, preceding the rise of agriculture. It is believed to have originated from packs of grey wolves that started scavenging on the carcasses of human prey between 20 and 40 thousand years ago. These wolves started to separate from other populations as they developed a migratory pattern of following human tribes. In the course of centuries or millennia, the relationship between wolf packs and human tribes tightened, with wolves and men cooperating during the hunt and defending their territory. This teamwork would have made both humans and wolves more successful ecologically, which would in turn lead to the maintenance of their relationship. After thousands of years of co-existing, both animal species would’ve become dependent on each other’s presence for their survival, and their relationship would’ve eventually reached a point-of-no-return, where it could not be reversed. Through a mixture of coevolution and selective breeding, the domestic wolves became a brand-new animal species: the dog.
Much later, hunter-gatherers domesticated the first plants in the Middle East by collecting their wild seeds and planting them. At the same time, sheep and goats ceased to be prey, and slowly became cattle. Through selective breeding, some of their traits became accentuated for our benefit, such as the growth of wool or the production of milk. These biological changes were everything except mild – you only have to compare the wild variant of a plant with its crop equivalent to see the dramatic effects of domestication.
According to the most recent genomic studies, the Basenji (left) and the Dingo (right) are the oldest still existing dogs. Their genetic lineage split off very early in the history of the dog, meaning that they are closely related to the first ancestral dogs, which descended directly from the grey wolf. The Basenji is a domestic African hunting dog that already appears depicted in 3000 year-old Egyptian art. The Dingo is a feral dog that has freely roamed the vast plains of Australia ever since it escaped human society about 3500 years ago. Ever since its return to the wild, the Dingo has barely changed morphologically, giving us a good idea of how the first dogs may have looked like. Because the Dingo descends from the domestic dog (Canis familiaris) but has undertaken its own evolutionary path by leaving mankind's side, its taxonomic position is heavily debated, with some experts arguing that it is still a domestic dog (or at most a subspecies, Canis familiaris dingo), others arguing it forms a species of its own (Canis dingo), and others proposing it to be a subspecies of the wolf (Canis lupus dingo). Photo credit Dingo: Australian Reptile Park.
As the example of the dog shows us, domestication is not a one-directional process. In truth, it is a very special form of a mutualistic symbiosis, where both partners benefit from and adapt to each other. What makes domestication different from "regular symbioses", is that eventually one of the two partners (the domesticator) assumes a considerable influence over the reproduction and care of the other (the domesticate). This doesn't mean, however, that the domesticator is the only one manipulating its partner. As in any symbiosis, the domesticator and domesticate coevolve. In other words, we humans are also somehow “domesticated” to depend on and serve our partners. This is of course hard to believe in a world of animal cruelty and mass agriculture, but purely on an ecological scale, domestic plant and animal species are infinitely more successful than their wild equivalents, and so are we.
So, if you think that this thousand-year-old relationship has only changed those plants and animals that we consume, you are deeply mistaken. First of all, a sedentary lifestyle and all its consequences, such as cities, nations, and other grand forms of human organization and development would have been impossible without the rise of agriculture and domesticated crops in the first place. Yet, domestication hasn’t only determined the course of early human societies and the rest of our history; it also changed us biologically. The best example of this, is the tolerance of lactose found in many European, Middle Eastern and African populations. In “natural” circumstances, mammals (including humans) can only drink milk during their infancy. After weaning, the lactase enzyme, which breaks down lactose into digestible sugars, decreases dramatically in activity, making most adult mammals lactose-intolerant. Yet, many humans can enjoy their milk well after reaching adulthood– a fact that long puzzled scientists.
Left: Comparison of the cobs the domestic Maize plant (right), an intermediate (middle) and Teosinte, its wild relative (left). Right: Comparison of the wild (left) and domestic (right) banana fruit.
Meanwhile, it has been shown that several mutations in the lactase gene occurred independently in European and African populations, which keep the lactase enzyme active after infancy and allow adults to digest dairy products throughout their entire lives. Additionally, genetic evidence indicates that these mutations became naturally selected in these populations about five to ten thousand years ago. This more or less coincides with the rise and spread of agriculture in human societies. Because of natural selection, the mutations spread through those populations and even became, at least in some cases, the most common variant of the lactase gene.
There are many theories as to why lactase persistence was evolutionarily beneficial for these human populations. One explanation is that there was a nutritional advantage to digesting milk once humans started to herd cattle that could provide it. Interestingly, traditional herding populations tend to have higher levels of lactase persistence, which supports this hypothesis. Another explanation is that the intake of milk helps the inner transportation of calcium in the absence of Vitamin D. The chemical reaction necessary to make this vitamin needs sunlight in order to work, meaning that drinking milk might’ve been extra advantageous in areas of low sunlight exposure. At least in Europe, a correlation exists between lactase persistence and latitude: populations living in higher latitudes (and being less exposed to sunlight) have higher levels of lactase persistence. This suggests that there could be some truth to this theory as well. Most probably, the independent spread of lactase persistence in several areas of the world arose from several advantages, but they all seem to indicate that our newly acquired habit to herd cattle played a key role. In other words, keeping domesticated cattle probably influenced the evolutionary course of some European and African populations.
It thus seems that, while we gave rise to entirely new species through our agricultural endeavours, we ourselves also evolved biologically to adapt to our new symbionts. This is an amazing example of what is called gene-culture coevolution, where cultural changes affect genetic evolution and vice versa. Although it is often thought that culture is unique to human beings, there are gene-culture coevolution cases found in other animals, such as killer whales. For example, distinct hunting strategies across killer whale pods – some more efficient to catch fish, others to catch mammals like seals – that mothers teach to their young, have led to genetic adaptations that optimize their metabolism for a fish-based or mammal-based diet respectively.
The same goes for domestication: other species do it too. Leafcutter ants cultivate a fungus in their nests, as humans do with plants. They tend to it, keep it free from disease and most importantly, feed it leaves that the ants themselves cannot digest. The fungus in its turn decomposes the plant material and leaves behind a pre-digested meal that the ants harvest and feed on. In many cases, the fungus has lost its ability to reproduce by itself, just as many plants did when they became domesticated by humans. The ants themselves have adapted so thoroughly to the presence of their symbiont that they too cannot survive without it. Other species of ants have mastered the art of herding. They protect and care for aphids, which feed on sugar-rich plant liquids and excrete the excess of it from their hind bodies – a secretion named honeydew. The ants feed on the honeydew, while in turn protecting the aphids against predators. In animals this is called a mutualistic symbiosis, but it is in fact very similar to the relationship we have with our domesticated crops and animals, and many scientists have pointed these examples out as cases of non-human domestication.
Left: Leafcutter ants feed freshly cut leaves to the fungus in their nest (Photo by Don Parsons). Right: A different species of ant harvests honeydew from the abdomen of an aphid (Photo by Charles Chien).
Still, there exists an essential difference between ant-fungus, ant-aphid and any other non-human "agricultural" relationship, and the human-crops-cattle symbiosis: the element of "conscious, selective breeding". That is what makes domestication stand out from any other natural symbiosis, making it often a much more rapid process of evolutionary change. Since we only have proof of humans consciously selecting a subset of crops and cattle for breeding, the domestication process is often seen as "unique to the human species". This encourages the idea that humankind is special, has broken itself loose of the chains of evolution, has ceased to obey its rules. Even more, it has taken over the reigns of its own biological fate and that of other species. Yet, it is unreasonable to categorize the gradual and complex processes of evolution in black-and-white boxes, and to argue that these processes (such as domestication and symbiosis) never meet and fade into each other.
It is, for example, very hard to say whether the first hunter-gatherers living with wolves or planting wild seeds were making conscious choices, rather than tumbling down the symbiotic rabbit hole, unaware as any other organism that becomes entangled in a relationship with other species, of the evolutionary effect their food choices were having on themselves and the animals and plants they ate. After all, all we know is that at some point in the history of humankind, we became aware of the possibility of manipulating our crops' and cattle's traits by limiting their reproduction to certain individuals, to make them more suited for their purpose in our society. Yet, there is no proof that these relationships started with selective breeding in the first place. In fact, as the story of wolves and humans - regarded nowadays as the most likely hypothesis for the origin of the domesticated dog - shows us, it is far more likely that all domestication processes we've conducted through selective breeding started off as any ordinary natural symbiosis: unconsciously. And who can tell, whether among those thousands of unknown or unstudied animal and plant species there isn't one, that makes "conscious" choices regarding the reproduction of their symbionts? But then again, how do you define conciousness and how do you measure it?
Experts still debate over a proper definition of domestication, and whether the process truly is limited to the human species or not. Yet these discussions are just another symptom of our desperate need to organize life in clear-cut boxes, while also suffering from the social construct which makes us believe that we stand above the world we live in and live outside its rules. Although we can't help but describing our world by placing everything in strict, non-overlapping categories, studies in biology and evolution have shown us over and over again that nature is not a discrete entity, but an endless continuum of life forms and processes, and that we are very much part of it.
The story of domestication is only a small part of the larger evolutionary epos that describes the trajectory of every species ever to have walked this earth. Although we often describe our species as special and unique, we are just as much part of this world as any other life form. Our own evolutionary story is entangled with those of many others through symbioses, some of which we started to steer consciously as we grew more aware of the effects of selective breeding, but many more of which we aren’t even aware. And certainly, there is no proof that any of these symbioses were consciously established by us in our early days. In my eyes, there exists a continuum between “natural” coevolution in a symbiosis and the “conscious” process of domestication. There’s little point in boxing each case in strict categories and separating us from all other living beings as the “special species”. After all, we are one more piece in the puzzle of life and we are chained to all other pieces through all kinds of wondrous processes, domestication just being one of them.
Further reading:
A scientific review on the most important questions in domestication research (among others, what it exactly is):
Zeder, M. A. (2015). Core questions in domestication research. Proceedings of the National Academy of Sciences, 112(11), 3191-3198.
A scientific review discussing plant domestication and exploring its differences and similarities with "plain natural selection".
Purugganan, M. D., & Fuller, D. Q. (2009). The nature of selection during plant domestication. Nature, 457(7231), 843.
A scientific paper reporting strong signals of natural selection in the human lactase gene: Bersaglieri, T. et al (2004). Genetic signatures of strong recent positive selection at the lactase gene. The American Journal of Human Genetics, 74(6), 1111-1120.
A scientific paper demonstrating the independent evolution of lactase persistence in African and European human populations:
Tishkoff, S. A. et al (2007). Convergent adaptation of human lactase persistence in Africa and Europe. Nature genetics, 39(1), 31.
A great book on the role of ecology in the rise and fall of human civilisations, with a very extensive look at the rise of agriculture and the process of human domestication:
Diamond, Jared (1997). Guns, Germs and Steel: The Fates of Humans Societies. New York: W. W. Norton.
