It is now 13 years since Harry Parker stepped on an improvised explosive device in Afghanistan, creating a blast that would result in the loss of both legs. Alongside the physical pain of the subsequent weeks, months and years, he also had to cope with a profound change in his sense of self. He compares the experience to that of Gregor Samsa, the subject of Franz Kafka’s The Metamorphosis – “the strangeness of not being who you used to be, turned into something that sets you apart from those around you”. Equipped with two hi-tech prosthetic limbs, Parker can now walk holding hands with his wife and carry his children on his shoulders. From the outside, it would be easy to conclude that he has adapted extraordinarily well to the event – and he says that “being an amputee feels normal”. But he still considers himself to be a different person – a “new body with a new identity” who is “12% machine”. It is this transformation that forms the emotional and intellectual heart of Parker’s fascinating new book, Hybrid Humans, which examines the cutting edge of medical technology and the implications of these developments for our identities. The term “hybrid human” is personal: it sits more comfortably than other potential descriptions for his experience, Parker says – compared with “disabled” at the one extreme or “cyborg” at the other. The term may have the ring of futurism for many readers, but hybrid humans have been around since antiquity. Just consider Tabaketenmut, the daughter of an Egyptian priest who lived about 3,000 years ago. She seems to have lost her big toe to gangrene, which would have been a huge impediment to walking. (The big toe carries 40% of our body weight as we propel ourselves forward.) To solve this problem, she was equipped with an articulated prosthesis, made from wood and leather, with holes that could have laced the artificial toe to the foot. Ancient Romans, meanwhile, created prosthetic limbs from bronze and wood. Parker’s writing is elegant and often lyrical. He is particularly eloquent when describing the psychology of hybridity – how the mind melds with the machine. The brain, we hear, can quickly enlarge its body map to encompass a tool, creating the sense that it is a physical part of us. It is the reason that drivers duck when their car passes under a low barrier – in some sense, we have become the vehicle. Even an object such as a smartphone can become an extension of our mind – as we come to rely on it for memory storage and communication. This neural flexibility is common to all human beings. “We are set up for plug and play,” Parker says. But the emotional connection between an amputee such as Parker and the prostheses they wear is necessarily much more profound than most people’s experiences of technology. His legs may not be made of flesh, but they are critical for his humanity. “When I am without them, I feel less alive,” he writes. Awe-inspiring innovations have made Parker’s current life possible. His right knee, for example, is controlled by a microprocessor, with sophisticated sensors that can gauge the forces on the leg and guide its movements. It is, he says, like having another brain within his limb. And Parker gives us a taste of even more astonishing developments from the technological frontier, describing technologies such as exoskeletons – fitted over the body – that could allow people who are paralysed to walk again, and robotic skin, fitted with sensors, that could restore a sense of touch to people with spinal cord injuries. Such successes are often overhyped by the media, but Parker avoids this trap. Newspapers and websites may breathlessly celebrate the use of bionic eyes that can allow people with visual impairments to see again, for instance, but Parker points out that the resolution is very low. At best, someone using these devices can make out a few shapes that may help with navigation. And there are risks. Bionic eyes use metal electrodes implanted in the brain to communicate their signals. The electrical discharge can damage neural tissue, rendering them less effective over time. Scientists may find solutions to these problems – such as electrodes coated with stem cells that may sprout roots into the living tissue – but progress is made of small steps rather than giant leaps. Unlike many technology writers, Parker also recognises the social inequalities that are inherent in these innovations. Organisations such as the NHS cannot invest in the most up-to-date prostheses, meaning that the latest technology must be bought with private riches. For some, the necessary funds may come through insurance or compensation – but if you are born poor, with a congenital disorder, you may never make use of any of these technological developments within your lifetime. (My mother, an amputee, is still using the same basic design of prosthetic leg that she was given more than four decades ago.) Parker points out that it is often the very people who are most likely to suffer disability who are the least able to afford the best treatment – which, of course, makes it harder to find a way out of the poverty trap. His scepticism is particularly welcome when he meets some transhumanists, the followers of a philosophical movement that advocates the use of technology to overcome the limitations of our evolved brains and bodies. Among their many plans, they suggest that we all be fitted with brain implants that could give us instant access to vast stores of knowledge or link us up to other brains to create telepathic communication. Some transhumanists even hope that technology will allow us to beat death – by, for example, uploading our consciousness to an artificial intelligence, so that we are no longer reliant on our ageing and fragile wetware. For now, these are far-fetched dreams, and it’s not just the practical challenges of creating this technology that bothers Parker; the goals themselves seem undesirable. “I can’t imagine the pain, anxiety and frustrations of being a sentient being uploaded to a hard drive – what it would be like never again to feel rain on my face, or the visceral feeling of my family when we all bundle together on the sofa for a hug.” And doesn’t the possibility of death add meaning to our lives that would be lost with technological immortality? As someone who has lived as a “hybrid” for more than a decade, Parker never forgets the realities of everyday life, which encompass both pain and beauty. This may be a tour of the scientific avant garde, but the focus is always on the human heart and mind. David Robson is a science writer and author of The Expectation Effect: How Your Mindset Can Transform Your Life (Canongate)