Plant and animal domestication has been one of the most significant developments to occur in the past thirteen thousand years, in human history. This development is of interest to all of us because it provides most of the food we consume today. It was required in the rise of civilization and was responsible for the transformation of global demography.
Domestication, however, had its own consequences such as the development of epidemic infectious diseases. The main human killers since the development of agriculture have mostly been acute, epidemic, and highly infectious diseases that are confined to human beings and will either kill their victim, and if the victim survives, it will immunize them for life. This type of diseases could not have been in existence before agriculture; therefore, they are referred to as ‘crowd diseases’. These studies demonstrated that the diseases evolved from similar epidemic diseases as that of our domesticated animals with which we started coming into contact with ten thousand years ago.
The development of these diseases, therefore, depended on two but separate roles of domestication: the creation of denser human population and permitting more frequent transmission of human disease from domestic animals than hunted wild animals. Tuberculosis for instance arose from cattle diseases and influenza from a disease of ducks. The measles virus is related to the virus causing rinderpest, a disease of cattle and many cud-chewing mammals. Rinderpest does not affect people; measles in turn does not affect cattle. The similarity of the rinderpest and measles viruses suggests that the rinderpest virus that was transferred from cattle to humans later became the measles virus. It changed its properties to adapt to humans. Considering our proximity to animals, we often are bombarded by animal microbes. These microbes are supported by natural selection; only a few, however, succeed in establishing themselves as human diseases.
Crowd disease paradoxically came to be agents of conquest. Exposed individuals gained immune resistance from childhood exposure, and populations that were exposed slowly evolved genetic resistance. Unexposed communities had neither types of resistance. There are four stages of disease specialization from animals to human beings. They include,
- At first, we acquire animal-borne microbes that are at an early stage in their evolution into becoming specialized human pathogens. They are not transmitted directly from person to person, and their transfer from animals to us as well is uncommon.
- A previously animal pathogen evolves to the point where it is not directly transmitted between people and cause epidemics. The epidemic, however, dies out for many reasons such as stopping when everybody has been infected and died, stopping when people have become infected and immune or, being cured by modern medicine.
- Former animal pathogens established in humans do not die out, until they do, whether they will become a major killer or not remains the unknown killer.
- Epidemic diseases become confined to humans. This could have occurred through many means such as an animal pathogen could have colonized humans recently and evolved to become a specialized human pathogen or an ancestral pathogen already present in humans could have co-speciated long ago when human lineages diverged five million years ago.
Most animal microbes are not passed on to human beings, meaning they do not pass from stage one to two. The probability of infection of a person of a new host species increases with the abundance of the existing host, with the fraction of the host people infected, with the frequency of the opportunities of transmission between the individual of the host existing and the new host. This probability also varies among microbes, for example, flaviviruses infect a large taxonomic range of hosts while simian foamy viruses and plasmodia only infect a narrow range. This variation is related to a characteristic in microbes such as the ability to overcome molecular barriers of new hosts such as cellular defense or they’re ability to create genetic variability these factors show different reasons why a certain animal host species may or fail to become a source of various infections to humans.
Although some of the stage two and three pathogens like Marburg and anthrax are feared and virulent, they claim few lives at present. If they would make it to the fourth stage; though, they would have devastating global impacts. Barriers between stages two and three, considering the rabies virus, involve differences between animal and human behaviors affecting the transmission such as animals often bite humans, but humans rarely bite other human beings. Barriers between stages three and four, considering Ebola virus, are those related to the size of the human population and the efficiency of transmission among humans. The emergence of novel pathogens is currently being facilitated by recent developments including blood transfusion (hepatitis C) intravenous drug use (HIV) and international travel (cholera).
Domestication of animals and plants was a great leap in the human evolution. Domestication, however, came with both positive and negative results. Increase in food production, which has led to new technological developments, increased human population and historical conquests were on the positive side while the introduction of new pathogens and diseases was some of the negative effects which have caused and continued to cause many deaths to date.