Stephen Hawking, the world's most famous living scientist and a testament to the triumph of willpower over adversity, has celebrated his 70th birthday , revealing he did not learn to read properly until he was eight years old, and that his schoolfriends had made a bet he "would never come to anything".

A public symposium in Cambridge was told that, far from being top of the class, he was never more than half-way up. "My classwork was very untidy, and my handwriting was the despair of my teachers," he said. "But my classmates gave me the nickname Einstein, so presumably they saw signs of something better.

"When I was 12, one of my friends bet another friend a bag of sweets that I would never come to anything. I don't know if this bet was ever settled and, if so, which way it was decided."

Unable to attend the event in person due to ill health, Hawking sent a pre-prepared speech, which marked the culmination of four days of birthday celebrations organised by his colleagues at Cambridge University. The festivities were attended by the world's leading cosmologists, including Nobel laureates Frank Wilczek and Saul Perlmutter, and celebrities including Richard Branson.

Hawking's laidback approach to education continued during his years studying physics at the University of Oxford. "I did one exam before I went up, then had three years at Oxford with just the final exams at the end," he said. "I once calculated that I did about a thousand hours' work in the three years I was there, an average of an hour a day. I'm not proud of this."

In his half-century career as a researcher, H

awking has been at the forefront of the understanding of black holes and quantum cosmology. His fame has also brought his ideas to a vast audience outside academia. His first book, A Brief History of Time, has reportedly sold more than 10m copies worldwide and he has made guest appearances on The Simpsons and Star Trek.

In the speech, Hawking spoke of his early life growing up in St Albans and gave the highlights of his scientific career. But his main message was to "be curious" and never give up, however difficult things might seem. "Remember to look up at the stars and not down at your feet," he said. "Try to make sense of what you see and about what makes the universe exist. Be curious. And however difficult life may seem, there is always something you can do, and succeed at. It matters that you don't just give up."

Around the age of 21, as he started his PhD in Cambridge, Hawking was diagnosed with motor neurone disease. The first signs that something was wrong came after he fell over while ice-skating on the lake near his childhood home in St Albans. "At first I became depressed. I seemed to be getting worse pretty rapidly. There didn't seem any point in working on my PhD because I didn't know I would live long enough to finish it."

After his expectations had been reduced to zero, he said, he was only emboldened when he met his future wife, Jane, at a party. "Getting engaged lifted my spirits and I realised, if we were going to get married, I had to do a job and finish my PhD. I began to work hard and I enjoyed it."

Other speakers in the public seminar at Cambridge University included Lord Rees, the astronomer royal, and Perlmutter, who won the Nobel prize in physics last year for the co-discovery of dark energy, the mysterious substance said to drive the expansion of the universe. Professor Kip Thorne, formerly of the California Institute of Technology and a longtime collaborator of Hawking, spoke of a "new golden age" in the study of black holes.

Attending the birthday celebrations, Virgin chief executive Richard Branson said it was "wonderful to be celebrating his 70th birthday, which in itself is remarkable. He should have won the Nobel prize many times, he is somebody who has discovered many things in his lifetime and he has managed to do that through extreme disability. Stephen Hawking is very keen to go into space and one of his greatest desires is to find out if there is other life in space. We are hoping that by Christmas time we will have put our spaceship [Virgin Galactic] up into space, and hopefully soon it should be possible to get Stephen Hawking into space."

Hawking's finished his talk, to a standing ovation from the audience, by saying that it had been a glorious time to be alive and be a researcher in theoretical physics. "Our picture of the universe has changed a great deal in the past 40 years and I'm happy if I've made a small contribution. The fact that we humans – who are ourselves mere collections of fundamental particles of nature – have been able to come this close to an understanding of the laws governing us and our universe is a great triumph."