The biological discovery that has benefitted mankind the most

It’s germ theory.

Since the advent of agriculture and the rise of cites (and thus civilization), infectious diseases were far and away the leading cause of death and disability. A reasonable guess would be that 75% of all deaths were due to infection, and these deaths were concentrated among the very young. Infectious diseases today, in all but the very poorest countries, account for no more than a few percent of deaths, and these are concentrated among the old. This chart pretty much tells the story:

From The Burden of Disease and the Changing Task of Medicine | NEJM. In 1900, kidney, cerebrovascular, and heart diseases were often sequelae of infections.

Note that by 1900 in the US, where these data are from, most cities had implemented clean water systems and had public health departments that promoted the new vaccines and other emerging scientific medicines developed in the wake of germ theory. The toll of infections was much higher just a few decades earlier.

Control of infectious disease had two world-changing consequences:

1. Women could bear far fewer children, and no longer needed to spend years of their lives desperately trying to nurse them through one illness after another. It’s no coincidence that women began entering civic life in significant numbers only after infectious disease control began. Female education, political rights and employment were pointless until bearing children and keeping them alive changed from a full-time to a part-time job. Without germ theory, this would have been impossible.
2. Large cities – the engines of innovation – became habitable. In 1800 the largest cities in the world numbered well under a million inhabitants, and deaths outpaced births. Their populations were maintained only by in-migration of impoverished peasants, who contributed little to intellectual or technological progress. The Age of Invention – in which we went from horseback to airplanes in just a few decades – began when germ theory allowed us to control infectious disease and keep educated city dwellers alive and interacting.

We all suffer from historical myopia, and tend to assume that the shape of the world in our lifetimes is normal and unremarkable. It is not.

Despite some significant technological advances – gunpowder, navigation, printing etc – the world of 1800 CE was not very different from the world of 1800 BCE, or for that matter, 4800 BCE. Nearly everyone was a subsistence farmer and nearly everyone lived in extreme poverty. Change was not a thing that happened, and when it did, it was in the form of disasters. Everybody was just barely hanging on and could aspire to no more.

The twin meta-innovations that make all other innovations possible are the demographic transition and the culture of continuous growth and improvement. They  make it possible for more people to live free from want and fear of an early death than at any time ever. And to become biologists and discover things like enzymes, insulin, antibiotics, vaccines, organ transplants, monoclonal antibodies, genomics and all the rest. You can’t have biological discoveries without biologists.

Of course, there is no guarantee that such a remarkable state of affairs will continue indefinitely. Our successes may fuel our own destruction. All previous advances in human affairs – although miniscule in comparison to those of the last two centuries – have ended in collapse. I see little evidence that we have the wisdom to avoid this fate.

And if the collapse comes, and if it leaves us worse off than we were before, then you can count germ theory as the discovery that harmed mankind the most.