1. Stephen Hawking

Born 8 January 1942 in Oxford, England

Stephen Hawking's parents were Frank Hawking, a medical researcher, and Isobel Eileen Hawking (née Walker), who held a secretarial job at a medical research institute. So you might think it inevitable that their offspring would grow up to be a scientific genius - but according to biographer Kitty Ferguson, young Stephen was "no prodigy".

In her book Stephen Hawking: His Life and Work, Ferguson writes: "Hawking remembers that he was just another ordinary English schoolboy, slow learning to read, his handwriting the despair of his teachers. He ranked no more than halfway up in his school class." One of Hawking's classmates, John McClenahan, supposedly bet a friend a bag of sweets that Stephen "would never come to anything".

However, young Stephen did have a keen interest in mathematics that was encouraged by a teacher named Dikran Tahta. He also enjoyed taking things apart to see how they worked - he and a group of friends even managed to assemble a primitive computer using a recycled telephone switchboard, some old clock parts, and an assortment of other bits and pieces.

Hawking's aptitude for scientific subjects eventually revealed itself, and he was awarded a scholarship at University College, Oxford. Stephen's father Frank wanted him to study medicine, but he ended up reading physics and chemistry before moving on to Trinity College, Cambridge for his graduate work. Despite being diagnosed with motor neurone disease and given only two years to live, Hawking persevered, obtained a PhD in applied mathematics and theoretical physics, and went on to become one of the most well-known scientists in the world.

2. Ada Lovelace

Born 10 December 1815 in London, England

The only child of Lord and Lady Byron, Augusta Ada King's beginnings were anything but humble. But even the most privileged women seldom became influential scientists in the early 1800s.

Ada's parents separated when she was still a baby, and Lord Byron - famously dubbed "mad, bad and dangerous to know" - died when his daughter was just 8 years old. Lady Byron, in an effort to stave off whatever mental disorders Ada might have inherited from her father, encouraged the girl's early interest in mathematics and technology, and she studied hard from an early age (perhaps because she was often ill and presumably had little else to do while recovering in bed.)

Later in Ada's all too short life, she would make the acquaintance of many leading scientists of the day, including Charles Babbage and Michael Faraday. But she embarked on her first major scientific project at just 12 years of age, when she decided she wanted to fly. That may sound childish, but young Ada attacked her goal with scientific rigorousness, studying the anatomy of birds, testing a variety of possible wing materials, and even recording her findings in a book titled Flyology.

By the time she reached adulthood, Ada was already noted for her "brilliant mind". Maths remained her primary interest for the remainder of her life; she was tutored by a number of notable scientists, and even though she died at the young age of 36, she did a lot of important work that remains highly relevant to our modern understanding of computers and algorithms. Some regard the Countess of Lovelace as the world's very first computer programmer.

3. Alexander Fleming

Born 6 August 1881 in Darvel, Scotland

Alexander Fleming was the man who, in 1928, discovered penicillin, the antibiotic that would save countless lives in the decades that followed. Fleming's gift to humanity has been recognised and honoured worldwide: there is a statue of him in Madrid, a school named after him in Sofia, and even a crater on the moon called the Fleming crater. And that's just the tip of the iceberg - check out the Awards and Legacy section of Fleming's Wikipedia page for more examples of the esteem in which he is held today.

Would anyone who knew the young Alexander Fleming have guessed that he was destined for such greatness? The son of a farmer, Fleming was born in a small town in East Ayrshire, studied at the Royal Polytechnic Institution in London, and spent two years working in a shipping office before he decided to follow in the footsteps of his older brother and pursue a career in medicine.

When reading about Fleming's first forays into the field that he would one day revolutionise, it's striking how many small details conspired to enable this one man to alter the course of human history. Alexander was only able to attend St Mary's Hospital Medical School in the first place because he had inherited some money from a deceased uncle, John Fleming, and it was the captain of the school's rifle club who suggested that Alexander join the research department at St Mary's (his ulterior motive being that this would enable Fleming to stay on the team after he had received his degree).

Fleming served in the Royal Army Medical Corps throughout the First World War, and during his time on the Western Front, he saw many soldiers die after contracting sepsis from infected wounds. He observed that the antiseptics of the day had an alarming tendency to make infections worse rather than better; his findings were ignored by most other army doctors, but Fleming would continue his study of bacteria and antibacterial substances after the war, and it was this interest (apparently assisted by a somewhat relaxed approach to lab tidiness) that would eventually lead him to one of the most important discoveries in the history of medicine.