Figure 2: The early evolution of the domestic dog
The Domestication of the Dog and Fox
Modern breeds of dogs are only approximately 200 years old and so have spent most of their time as wolf-dog hybrids [Figure 2]. It is uncertain exactly when dogs were first domesticated, but it appears to be approximately 10,000 years ago. There appears to have been one or two primary domestication events which resulted in all the genes of the individuals being linked by a founder animal, later resulting in a genetic bottleneck to obtain the species of dogs that are present today. It used to be thought that their origin was Central Asia 20-30,000 years ago but highly recent developments have now disproved this. Instead, Druzhkova et al. (2013) found, through DNA analysis of a 33,000 year old dog, that “the unique haplotype of the Altai dog is more closely related to modern dogs and prehistoric New World canids than it is to contemporary wolves”. It is sure, however, that the Natufians kept dogs, selected for from tamed wolf cubs around 12000bp. A further uncertainty, as Morey (1994) pointed out, is that domestication may not have been the result of deliberate human choice and may have instead been due to self-domestication. This would means that new ecological niches were colonised by animals, thus they were unintentionally selected themselves.
10,000 years ago wolves were first domesticated to help hunt mega herbivores, changing their social structure so as to not live in packs. Humans did this because the mega herbivores were hard to capture due to their natural defensive mechanisms (e.g. horns). These mammalian mega herbivores occurred because, as Bergman’s rule states, in colder climates, animals have larger body sizes due to the temperature, due to reduced competition, greater food availability and reduced surface area: volume for heat exchange (Gingerich, 1985). These wolves were later used to hunt smaller prey, which were more difficult to track and required more precise senses. However, they were still of use when humans began to become reliant on crops, since dogs are highly protective and so were humans at that time because it was to a great deal of effort for the early humans to gather the food they stored within their villages. Thus, the dogs could act as a warning system, identifying when the village’s territory was being invaded, and chasing away any trespassers. They were additionally still of use 10-7000 years ago when livestock were used, since they were able to protect and control them, in the way that a sheepdog does today.
There has been much debate about how wolves were domesticated into dogs. It could be said that the changed features in the dog population may have been caused by mutations due to inbreeding, rather than any domestication events, therefore Belyaev (1969) did some experimentation of other species. He proved the hypothesis that many species have adapted to life among humans, through sexual selection, by emphasising tameness features. Due to genetic linkage, selecting for behavioural characteristics (such as tameness), causes many other changes, for example, physiological (in the individual's hormones and neurochemicals), behavioural and morphological changes. To test this idea, the process of domestication was replicated, doing selective breeding for tameness in silver foxes over 26 years. This animal was chosen because the silver fox (Vulpes vulpes) was trapped and then selectively bred in fur farms to sell the fur and so a small population was already available and caught.
At first, wild kits were made more tame by placing them in the hands of humans when they were a few month old, after weaning. They either responded to this human contact with an adaptive response (a traditional fight or flight hyper alert response such as being fearful, aggressive or curious) or a non-adaptive response (a response that would cause mortality to be more likely in a natural environment, for example, by being relaxed). The curious foxes were bred to recreate domestication in a completely wild animal. Other traits were also inadvertently selected for, due to genetic linkage, with the foxes becoming more pure black or pure white as more generations were produced. This may, however, be due to changes in the timing of embryonic development, since it usually occurs quicker in domesticated animals. One of the earliest traits found in the domesticated foxes was a loss of pigment in parts of the head and body. This ‘piebald’ pattern is caused by a gene called Star that affects the migration rate of melanoblasts (the precursors of the pigment cells that colour the fur), making it happen faster and killing the later migrators (Trut, 1996).
A further morphological example of an alteration that occurred by selecting for tameness was that of floppy ears. Darwin (1859), too, had noticed this feature in domestication much earlier, when he said “not a single domestic animal can be named which has not in some country drooping ears”. However, he believed it to be due to disuse of the muscles of the ear, linked with the domesticated animals being alarmed less frequently than wild animals. Although it is true that domesticated animals have less stress, modern understanding of genetics has allowed us to believe that drooping ears is a sign of domestication due to how close the gene for it and tameness are on the chromosomes. This also explains how ears are droopy in juveniles who have only just been born.
Another alteration in the foxes is that they began to vocalise for humans, for example cackling and panting, rather than just conspecific vocalisation (e.g. coughing, snorting). Both wild and domesticated foxes had the ability to produce these sounds but their use of them varied. Wild foxes associated human vocalization with aggression, such as before attacking, when domesticated, with friendliness, and as a method of attracting human attention. Dogs have adapted these behaviours too, in the form of whimpering and whining, formally just young animal vocalisations. The foxes lost their ‘musky’ sent too. They had more paedomorphic traits, meaning they appear to have an arrested development at a child-like stage. They had a child-like head structure and a flattened, shorter snout. These traits, too, are shared by dogs, in comparison with wolves. Both domesticated foxes and dogs also had less pronounced dimorphism due to the feminisation of the species. They appear morphologically to be more feminine, which could either relate to the paedomorphosis to better appeal to humans or it could be more due to the lack of aggression within the species (or a combination of the two). This is a similar occurrence to the craniofacial gracility of humans today. Modern humans are slender with thinner bones than their ancestors, making them faster but not as strong. To Neanderthals, modern humans would look more child-like. Thus, presenting the idea that humans, too, may be domesticated. This could, however, be due to self-selection and hormones, for example estradiol, and neurotransmitters, for example serotonin, which regulate behaviour and some aspects of development (such as facially). A Homo sapien who has a ‘friendly face’ is sought after because it is a marker of this desirable social trait, and so humans may be selected ourselves for beauty, causing beauty to ‘evolve’ over time (Elia, 2013).
To conclude, the study of foxes has been highly useful in understanding the potential domestication process of dog and what may have happened to the ancestors of modern dogs today. However, the extrapolation of domestication from foxes to dogs must be done carefully because foxes are not pack animals and this may have unintended consequences, causing variation in the domestication process. There are other differences between foxes and wolves too, for example, even after domestication, foxes could not be made to reproduce twice a year and all year round like dogs are now able to.
Figure 3: Cattle Domestication
Figure 4: Sheep Domestication
The Domestication of Cattle and Sheep
Cattle appear to have been domesticated in the Middle East [Figure 3]. Inbred populations, such as the taurine cattle (B. taurus), then moved to Europe, where there was refreshment of the gene pool by hybridising them with the wild European aurox (Bos primigenius). Aurochs were prized because of their meat, which was of a good quality. They were very large and so had a greater quantity of meat, containing large amounts of calories and a good source of protein. This process of domestication was found by locating the origin of their DNA, for example in the study completed by Park et al. (2013). This rejuvenation of the DNA seemed to be good for the health and the production of the animals, making this population have the best meat, milk and leather production. However, the hybridisation may have been unintentional because early domestic cattle were managed in a poorer manner than today and there is a lack of archaeological evidence that fencing and cattle barns were used (Clutton-Brook, 1999). Therefore it appears that the cattle may have been kept in free-roaming herds, meaning that unintentional mating was a possibility when there were times without close attention. One such time, as Götherström et al. (2005) said, is when the “herds [were being] driven between suitable pastures created by clearing and burning woodland”.
Cattle were selected for their genetic traits such as their docility and high meat and milk yield. The cattle with these traits were bred and any with undesirable traits were culled. They also were selected for a reduced horn size, since this would make them easier to control, for example, if they escaped, but this may just be a secondary trait, due to genetic linkage. Selection also caused paedomorphic mutation within the human population 5000-10000 years ago in northern European pastoralists. The availability of nutritious milk would cause humans with a mutation that made them able to tolerate liquid milk to be selected for, since before this only infants were able to do so. The advantages of this were that energy, protein and vitamin D could be obtained from the milk Thus, humans also adapted genetically to their animals.
Another unintended consequence of this process was a genetic bottleneck of the isolated populations due to inbreeding and lack of genetic diversity. This genetic uniformity had positive effects, such as all the animals being tame and negative effects, such as each animal having the same resistance to the same diseases. Thus, if one individual was susceptible to a disease, the entire herd would be prone to it. Thus it was important to refresh the gene pool by introducing wild individuals to the domesticated population, resulting in hybrids.
Cattle today are smaller than their ancestors, with reduced horns [Figure 3]. They are still strong but they are easier to control, as individuals, than their ancestors. They have now been moved around the globe. But cattle are now completely domesticated and it would be very difficult for them to revert to being a wild species. In the past, however, there is some evidence that cattle have reverted back to being feral, for example after the Texas-Mexico War and throughout the US Civil War, due to constant conflict between Anglos, Mexicanos and Native Americans (Everett, 1927).
Sheep (Ovis aries) seem to have been first domesticated at least three separate times in western Iran and Turkey, Syria and Iraq roughly 10500 years ago from different subspecies of the wild mouflon (Ovis gmelini) [Figure 4]. This has been deduced by studying multiple mitochondrial lineages (Chessa et al., 2009). These early sheep were fairly large with a primitive morphology and a coarse coat. They were also not as placid as modern sheep, making them more difficult to keep in herds but it did mean they were more resilient to harsh areas than modern sheep are. There are still species of sheep today that are similar to these early sheep species, for example, the Soay sheep and Jacob’s sheep, which have many characteristics of their ancestors. After these early sheep were domesticated in Southwest Asia, they spread to Europe in two major waves (with the second migration resulting in most modern sheep breeds today) (Lewin, 2009).
Sheep were the first animals domesticated purely with the intention of them being a source of food. Sheep are smaller and so have less meat so it may have been thought they were not as useful a food package as cattle. But it could be said that much of this small size may be due to domestication, since the domesticated sheep of modern species have a reduced body size, females without horns and the demographic profiles of juveniles. However, they were useful in that they are easier to manipulate because of this reduced size. Another advantage of owning sheep is that they are a more versatile animal. Cattle require rich, flat land, which is not always available in some areas and it may be used for planting crops instead, but sheep are able to survive on poorer grasslands.
These sheep were moved out of the Middle East and got exported into Europe 7000 years ago. Since Europe is a lot more cold than the Middle East, the secondary usage of sheep was soon realised; the availability of wool. This fine wool was an excellent insulator for human clothing and even houses. It was later seen as being even more important a resource than the meat, since the sheep could be shorn at multiple times over the year. Later, wool was exported around the UK and abroad (e.g. Holland), to produce fine clothes and cloths.
Sheep are also a more resilient species in terms of genetics. This is because sheep breeds have been formed in a ‘fluid’ way, when other livestock (such as cattle) have had very fixed moments of genetic refreshment. This repeated hybridisation and robust gene flow between sheep of the different breeds caused modern sheep species to have high levels of genetic diversity and low levels of genetic inbreeding (Kijas, 2012). This means that sheep breeders in the future will still have progresses in desirable traits, for example, increased meat production. It also means that if sheep were to be allowed to turn feral, they would be more likely to be able to survive in a natural environment. Thus, they may not be as domesticated as certain other species of animals and livestock (for example, the cow).
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