The startling role that disease played in human evolution – Sydney Morning Herald

The word pathogenesis means the ways in which a disease develops and progresses, and Jonathon Kennedys hopefully contagious thesis examines all manner of dreadful illnesses and their gruesome effects; not just on individuals, but on entire societies.

Most readers will be familiar with the calamitous results of the introduction of bacteria and viruses into populations with no prior immunity. British annexation of Australia, while a disaster for the Indigenous population, was made far worse by the introduction of smallpox and other diseases to which the colonists had, over centuries, developed immunity, or at least far lower fatality rates.

British annexation of Australia, while a disaster for the Indigenous population, was made far worse by the introduction of smallpox and other diseases. Detail from Taking of Colbee & Benalon, November 25, 1789 by William Bradley. State Library of NSW

The Aztecs and Incas of central and southern America, as well as the northern tribes of what was to become the USA and Canada, were all decimated by European pathogens far more deadly than swords and muskets. Indeed, it was much worse than that. The Latin word decimate means to kill one in 10. These cultures were not decimated they suffered casualty rates that a twisted Roman general could have only dreamed of.

Kennedy writes that the population of the whole of the Americas, estimated at more than 60 million in 1500, was at most 6 million a century later. The vastly outnumbered soldiers of Cortes and Pizarro did not defeat the massive armies of the Central American empires. No, they were beaten by an unintentional yet devastating form of germ warfare, invisible invaders that spread faster than the armies that followed them. Spanish soldiers would walk into deserted Andean villages, with only corpses to be found.

There was an exchange of diseases, with syphilis being brought back to Europe by conquistadors and rapacious slave traders, but it was essentially a one-sided affair. Kennedy concedes that venereal afflictions certainly caused a good deal of pain, irritation and embarrassment to the people of Europe, but points out that their impact cannot be compared to that of Old World pathogens in the Americas.

Europe had had its own catastrophic encounter with an imported pathogen two centuries earlier, the Black Death having made its way from eastern Asia aboard rats and fleas via the trade routes of the Silk Road. Although plague had been a regular and dreaded occurrence in Europe in the 6th and 7th centuries, the Black Death, which swept across Europe and the Mediterranean between 1346 and 1353, was probably the deadliest outbreak of contagious disease in history, killing between 75 million and 200 million people. Europe would never be the same.

So while Kennedys book is subtitled How Germs Made History, he has certainly not written a history of disease. The book is about disease as an essential driver of primate evolution, then the human story, from before we homo sapiens outlasted all the other hominids to become, for the time being at least, the dominant creatures on the planet.

He kicks off with microbiologys most perplexing debate is a virus alive? Well, yes and no. The lonely virus is both a dead collection of complex chemicals, and an incipient, bottled genie waiting patiently for the correct chemical caress to pop its cork.

A virus is both a dead collection of complex chemicals, and an incipient, bottled genie.

But COVID-19, AIDS and all the other blueprints for despair that wed rather had never evolved notwithstanding, Kennedy enlightens us on one viral infection so astonishing, so fundamental, that it explains how we came to be the creatures we are.

For billions of years, all creatures hatched from eggs. And we still do, in a way. A human ovum is an egg. Not much to look at, next to an ostrichs, but thats because it doesnt have to be. Until a fortunate retrovirus infected a shrew-like creature many millions of years ago, all eggs hatched outside their mothers body. Most eggs on earth still do.

Then mammals found themselves with an escape hatch, an airlock between umbilical babe and a hostile outside world.

Yes, the placenta, a temporary organ that allows all mammalian mums to hatch their eggs inside their own bodies. But the astonishing thing is that the evolution of our maternal engineering was less likely than an asteroid impact.

It was the result of an opportunistic viral infection, one thats been protecting us in the womb from being ousted by our own mothers immune systems. Then, when fully baked, we get shoved out, along with the placenta and any other odds and ends that mum doesnt need any more.

Not a bad effort for a humble virus, eh? But dont feel too grateful to this mindless sack of viral spare parts. Its done pretty well out of it, too.

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The startling role that disease played in human evolution - Sydney Morning Herald