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The role of viruses, bacteria and infectious diseases on human evolution
on 12 August 2010
This is an excellent review of various infectious diseases that have shaped the history of human beings. Many cultures and the whole populations were impacted from the very beginning of our civilization or perhaps when Homo sapiens set foot on this planet. The author gives specific examples in our history and describes how diseases have played a role in the eventual determination of who we are today. One could see disease-caused human fatality as a tragedy to an individual or a family but it has long term advantage in evolution, if we apply Darwinism to human diseases. Genetic changes as a response to infectious disease makes us more resistant to infections. Such changes may also contribute to our physical characteristics, brain functions and development. The book is described in 11 chapters that include separate chapters on spread of virulent forms of bacteria and genetic resistance, origin of human disease, the decline of water supply and sewers that caused the fall of kingdoms and empires, pestilence and warfare, and emerging diseases of the future.
Many human diseases originated from animals, but not all bacteria are bad for health. The human gut provides a home for great number of bacteria. Majority of them are harmless and some are beneficial by aiding digestion, synthesis of certain vitamins, defending their habitat against more infectious forms of bacteria. Diseases like gonorrhea and syphilis (caused by bacteria), and measles, mumps, influenza (caused by viruses), and Ebola virus have evolved to become milder. Some diseases became extinct like the sweating sickness that erupted in London in 1845.
The author describes many historical facts hat makes the book even more interesting. The demise of Indus valley civilization around 1800 B.C., virtually without a trace is a great mystery and subjected many interpretations. One of them is the Aryan invasion from Europe. But the author suggests that cholera is more likely cause of human fatalities in Indus Valley. The diarrhea causing bacteria existed in India 3000 B.C to 2000 B.C., but they were present in non-virulent form, but gradually evolved into highly infectious form when drought hit the urban areas of the Harappa and Mohenjo-Daro. The local rivers were completely dried and the sewer system collapsed that may have led the spread of infections faster. This is certainly an interesting theory but genetic archeology has to answer these questions conclusively. The author also suggests that spread of malaria may be one of the main factors in the collapse of Roman Empire. After this, a decline in hygiene all over Europe resulted in the spread of diseases like typhoid, bacterial dysentery, and rotavirus all of which share diarrhea like symptoms that were spread by the contamination of water with sewer system. Early in the fifth century the Huns, led by Attila almost conquered the Roman Empire but withdrew because he and his army were apparently infected by virulent epidemic of dysentery. If this barbarian had succeeded in Rome, the history of Europe would have been different.
Cystic fibrosis mutation is common in north-western Europe, population genetics and mutation rates suggest that these mutations arose shortly after the collapse of Roman Empire when general hygiene was poor and water borne intestinal disease spread rapidly.
The history of smallpox is interesting in that the mortality rates in Asia and Europe dropped from 75% to about 20% over one thousand years illustrating that the development of genetic resistance. In 737 AD, a smallpox epidemic in Japan caused significant deaths. Measles was the Great Plague of Athens in 430 B.C. It is interesting to note that the history of humans would have been different if infectious diseases had not been present or if genetics resistance to virulence did not exist. Culturally and biologically we would have been different.