This pandemic has driven people to find companionship in animals, particularly dogs and cats. Outdoor wildlife pursuits have also increased, especially birdwatching and hunting. For example, the sale of hunting licenses went up in Pennsylvania in 2020, after dropping off for many years before [1]. Evolutionarily speaking, leaning on another living thing for companionship, food, or transport is hardly remarkable. In this article, I want to explore the earliest examples of domestication, trace it forward to our present day, and conclude with some thoughts on the intimate relationship between us and other animals.
First, get ready for a shock. The first example of domestication occurred about 2 billion years ago. To help put this in context, the Earth is about 4.6 billion years old now, so domestication is about half as old as the Earth itself. The first domestication led to the formation of the type of cell of which we are made, Eukaryotic cells. Scientists call this domestication “endosymbiosis”. How? Why? Well, about 3.8 billion years-ago, the Earthly environment was hot, sulfurous and, importantly, lacking oxygen. The two earliest basic groups of cells that had evolved from the primordial soup were both primitive cells: Bacteria and Archaea. Bacteria, then and now, are the go-getters, ready to change and adapt. A group of Bacteria called Cyanobacteria were the first organisms to start doing photosynthesis about 3.5 billion years ago: they stored the sun’s energy into organic molecules and released oxygen. But oxygen is very toxic to living cells. But the Bacteria evolved, and they adapted to detox oxygen by chemical reactions that reversed photosynthesis, releasing energy as a byproduct. That, in essence, was the Bacterial superpower – using oxygen to release energy from organic molecules – oxygen in, energy out.
But Archaea failed to adapt metabolically to oxygen like Bacteria. To escape the oxygen onslaught, some Archaea went to hide in places where it does not penetrate: the hot, the acid, the deep. Others, more desperate, took oxygen-tolerating and energy-producing Bacteria hostage, who became “mitochondria” and provided their “oxygen to energy” mojo to Archaea. To cut a long story short, these combination Archaea-Bacteria persisted and evolved into the Eukaryotes which compose the protists, plants, animals and fungi we see today. Consider what a turnabout for life this represented! And the more the Bacteria within each Archaean cell, the higher the energy production. Our cells have about 100 mitochondria per cell. Each of our cells is an energy powerhouse, but only as long as oxygen is available. The Archaea need Bacteria for energy; Bacteria need Archaea for protection. Mutual benefit. Win-win. Eukaryotes have thrived ever since because a long-ago Archaean domesticated Bacteria to live inside them.
As with Eukaryotes, relationships between organisms can be mutualistic, where both partners derive benefit. They can be commensal, where one organism derives a benefit, but not to the other partner’s detriment. [Aside: As a “dog” person, I feel that commensalism is encapsulated in the cat-human relationship]. Then there is parasitism, a simple concept to understand – it is the exploitation of one organism by another. Parasites can cause disease and death, and we are familiar and largely at war with many parasites.
Let’s go back to mutualism. This is extremely common, as every organism leans on partners to help it in a reciprocal manner, even if they are not obvious. For example, every plant has an invisible partner, a fungus. It may surprise you to know that fungi (and, for that matter animals), evolved about a billion years before plants. So, when algae moved to land to become plants, about 450 million years ago, they found fungi had evolved strategies and had already adapted to land. It was easier and simpler, evolutionarily speaking, for plants to develop a relationship with fungi than to evolve new strategies to deal with the hardships of land-dwelling. Theirs became a trading partnership, a symbiosis: plants provide nutrients to the fungi, and delicate fungal hyphae access water and minerals between soil grains which are challenging for the relatively large rootlets of plants. This successful domestication of fungi by plants has persisted until today. Look at the next batch of expensive potting soil you buy from a store. It will contain “mycorrhizae” to help your plants to grow.
Animals also have such long-term relationships, both inside their bodies and on the surface of their bodies, and these are called the “microbiome”, a collection of microscopic mutualists that include bacteria, fungi, and protists. The microbiome represents a symbiosis that helps animals digest food, fight off pathogens, train the immune system, and even influence mood. Termites and other wood-munching animals are famous for their cellulose-digesting partners. In ruminant animals such as cows, specialized chambers exist that contain microbes that digest cellulose in plant matter. In non-ruminant herbivores such as rabbits and horses the cecum, between the small and large intestine, provides a space for symbionts to aid in fermentation and extraction of nutrients. This long-term relationship can be called domestication.
But who domesticated who?
Parasites definitely domesticate their hosts, extracting benefit and leaving devastation. They snuggle into tissues to the host’s detriment, and some even manipulate their hosts’ behavior to maximize their survival prospects. Others develop relationships where both the partners benefit, some a little more, some a little less. Some ants corral aphids and drink the honeydew they exude. But the ants actively manage the aphids for productivity. When the aphids stop producing, they get eaten [2]. The behavior is as recognizable in ants as it is in humans tending their cows. You could argue ants benefit more. On the equal benefit side, birds are known to tuck spider egg cases into their nests; when the spiders hatch, they eat mites and other parasites that trouble the birds. The spiders obtain food, simultaneously ridding birds of pests.
But as we progress through examples, the argument about who domesticated who becomes more fretful [3]. Take the cereal-human relationship. Some scientists argue that plants have domesticated humans, not the other way around. Observe how humans exhaust themselves to till the ground and provide water and nutrition, protect plants from damage from pests, gather the harvest, and carefully propagate seeds which are the next generation of plants. This domestication by plants of humans forced humans to give up their leisure time and travel, in order to settle in one place to oversee their plant charges throughout the year. Even domesticated animals need humans to monitor and elevate their well-being, much like ants with their aphid cows.
There is a darker angle to domesticated animals. A few animals would have taken easily to being yoked by humans, but not all. The wild Auroch cattle of Europe and the Cape Buffalo of Africa have never been tamed. But Aurochs are now extinct – humans had no incentive to safeguard them. It is probably just the vastness of the continent of Africa that saved the Cape Buffalo from people. Because of our longstanding use of domesticated animals, our language is also peppered with idioms such as “strong as an ox” and “stubborn as a mule”. The phrase “lamb to the slaughter” comes from obedient lambs, particularly sought after as sacrificial offerings. And the word “scapegoat” came from the use of a goat to absolve people of their sins in early times. After “transferring” sins onto the goat, it was hounded off a cliff and to its death, conveniently taking the community’s sins with it.
But household pets evoke a different relationship, a one of choice and empathy. The empathy that manifests between dogs and cats and their owners is the pinnacle of this symbiosis. The emotional tug in this perfect empathy is hard to adjudicate. Millionaires have given money for their dogs and cats to be cloned; this is because they experienced a perfect relationship that they never wanted to end. This pet companionship has been particularly sought after during isolation from other social bonds during covid lockdowns.
We have no choice but to maintain a symbiosis with our mitochondria just to survive in the presence of oxygen, and the mitochondria inside us cannot survive without the protection the cell represents. Domesticated plants and animals could potentially survive without us. Some will certainly die, but many will become feral and adapt. But the relationships we cultivate with our pets are special because they enhance our lives and complete our families. And that is an invaluable relationship.
References:
1. Sutor, D., Virus a factor in hunting license sales increase in The Tribune-Democrat. 2020: Johnstown.
2. Ants farm subterranean aphids mostly in single clone groups – an example of prudent husbandry for carbohydrates and proteins? | | Full Text. 2021; Available from: https://bmcevolbiol.biomedcentral.com/articles/10.1186/1471-2148-12-106.
3. Diamond, J., Evolution, consequences and future of plant and animal domestication. Nature, 2002. 418(6898): p. 700-707.
