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E. Paul Zehr is the author of Chasing Captain America: How Advances in Science, Engineering, and Biotechnology Will Produce a Superhuman. He is the director of the Centre for Biomedical Research at the University of Victoria.

The line between real science and comic-book fiction is blurring. Marvel’s superhero blockbuster team in Avengers: Infinity Wars includes the Incredible Hulk, the scientist Bruce Banner’s monstrous alter-ego; Spider-Man, a teen web-slinger; and Captain America, who was created by the legendary duo of Joe Simon and Jack Kirby back in 1941. In the Marvel film Captain America: The First Avenger, released 70 years later, his backstory involves a pharmacological cocktail “supersoldier serum” and “vita-ray” treatment that transforms a frail soldier reject named Steve Rogers into the “perfect” specimen of human development and condition – a real-life superhuman.

It’s all very dramatic – on page and screen. But what if you could really tap into and unleash some inner mechanisms inside your body that could transform you from “normal” human to superhuman? It’s not so outlandish: Thanks to science and biomedical engineering, humans have the ever-advancing capacity to alter our own biology. Are we prepared for what it means to be the first species with the ability to change its own biology? And if we could do that, should we – ethically and morally?

When I started thinking about these issues, I thought I’d wind up on the side of trepidation when considering alteration of human biology through gene editing, stem-cell treatments, technological implants and drugs. We should preserve the sanctity of humanity, in other words. Except, I kept thinking, why? What, if anything, was my “gut feeling” based on?

That’s what asked myself with each research study I read, each interview I did with scientists and engineers on the cutting edge of knowledge. Part of what I was experiencing was wrapped up in what we consider the acceptable limits of human ability. But are there any?

When thinking about all these things, I struggled to understand what my conflict was based on. I realized that a large part of my reluctance to endorse changing what it means to be human was based on thinking that is more commonly associated with sports rules and doping.

I remember being in an exercise physiology class as an undergraduate in 1988 when the Ben Johnson scandal broke. There was a lot of discussion about the physiology and morality of what was unfolding. This was the first really high-profile performance-enhancing drug story that I remember. And it resonated with me as something that was extremely negative. This has persisted, despite the fact that it seems that almost everyone was doping anyway. I, like many, conflated the ethics of sport for the rules of life.

The thing is, in sports there are usually clear rules. If someone steps outside the rules or bends or breaks those rules, there are penalties. In evolution, there are no rules about biological adaptation to environmental stresses that underlie the very process of life on Earth. There are simply stresses and functional evolutionary adaptations within biological systems that will persist if beneficial, and fade away if not. Viewed in that context, my limited view about what it means to be human wasn’t really based on anything very scientific or rational.

This led me to question and explore biomedical ethics and the issues of human enhancement more broadly. For example, most consider wearing glasses to assist with myopia as being perfectly acceptable and “normal” (ironic quotes intended). Assistive prosthetics to help people who have amputations are perfectly acceptable ways to help restore function.

But what about something that is done inside the body – a technological implant that improves function, or a gene deletion that allows for the expression of more strength or the implantation of a stimulator in the brain – to enhance abilities in ways that are “normally” outside our accepted range of human? Therapies, tools and treatments that can assist in “recovering” function can also be used to improve function that isn’t compromised.

Moving forward, of course, we’re going to have to be smashing the idea of “normal” in the first place. In truth, “normal” never existed except in our minds as a concept we use to categorize things into discrete units for simplicity. Biology functions on a continuum of performance from low level to extremely high level. There is no category that’s completely defined with hard boundaries. Rather, we move throughout our lives across a continuum of function.

What about changing the development of biological function in regular folks who are not injured, who don’t need “rehabilitation”? Growth hormone, a protein from the anterior pituitary, regulates cell production and growth. Low levels of growth hormone lead to weakness, poor energy levels, bone weakness and risk of cardiovascular disease. When there is a deficiency of this hormone during development, physical growth can be dramatically reduced and a person either won’t reach their potential size or will get there very slowly. In these cases, it’s often medically acceptable to use growth-hormone supplementation to return the person to a “normal” growth curve.

Because it can help stimulate growth and recovery across the lifespan, growth hormone is considered an “ergogenic aid” and is banned by the World Anti-Doping Agency and by most sports. What if a boy who was on a “normal” childhood growth trajectory, and who was projected to be about five-foot-seven, really wanted to be a foot taller so he could play professional basketball? Is it okay to give him growth hormone to help achieve his goal? Many might say it “feels” different to say yes in one case and no in another.

Many of the concerns around biomedical ethics in human enhancement have their origins in concern for their effect on future generations. Steve Rogers knew what he was getting into with the super-soldier procedure, and consented to participate. But that case may be the exception, and also it may depend on whether alterations affect only the individual or subsequent generations.

Eike-Henner Kluge, professor of philosophy at the University of Victoria, thinks “germ line alteration would be performed without the consent of those who are most affected: namely, future generations.” C.S. Lewis, writing in 1943’s The Abolition of Man, suggested that if a society gains power to make descendants “what it pleases, all men who live after it are patients of that power … the rule of a few hundreds of men over billions upon billions of men.”

Enhancing human ability carries with it ethical and societal implications. These need thinking about as we continue to move towards more technologically complex and integrated prosthetics such as brain-machine interface and other implanted stimulators. Clearly, technology and treatments initially and originally designed for cures and restoration of function can also be applied to enhance performance beyond the range of “natural” performance. Where is the boundary separating the range of acceptable human abilities and those that exceed them? And what happens to our perceptions of that boundary when we use technology to change the human inside as well as out?

Reminiscent to the other part of my life as a research scientist, my change in perspective is very similar to the idea of science and hypothesis testing. It only takes one observation that contradicts the predictions of hypotheses to render a given hypothesis moot. I found numerous examples where my thinking about the limits of what human means and where humans are headed to be wrong.

We need to be cautious, judicious and brave in our application of our biotechnological prowess, while at the same time tempering our desire to over reach. As Arkady and Boris Strugatsky wrote in the 1972 science-fiction book Roadside Picnic, scientists who know about the current state of things “… should be even more scared than the rest of us ordinary folks put together. Because we merely don’t understand a thing, but they at least understand how much they don’t understand.” And all of us need to have a basic understanding and participate in decisions about the future of our species.

We await those well-intentioned, targeted and regulated applications moving the biological, cultural and technological evolution of our species forward in helpful ways. Not just toward ameliorating the human condition by reducing the burden of disease, but also enabling adaptations to deal with environmental change on Earth and the rigours of long-term space flight.

Moving forward we need to heed the words of Stan Lee when he wrote, about a certain Spider-Man, that “with great power there must also come great responsibility.” Scientific enhancements formerly found only in the fiction of Captain America comic books and Avengers movies are now being assembled in real life.