Elephants are the largest living land mammal and today our planet is home to three species: the African bush elephant, the African forest elephant, and the Asian elephant.

But a hundred thousand years ago, in the chilly depths of the Ice Age, multiple species of elephant roamed the earth: from dog-sized dwarf elephants to towering woolly mammoths.

These gentle giants' evolutionary story and its parallels with that of humankind has long fascinated Dr Tori Herridge, a senior lecturer in evolutionary biology at the University of Sheffield, where - as a seasoned science broadcaster - she's also responsible for their Masters course in Science Communication.

Tori has spent much of her life studying fossil elephants and the sites where they were excavated; trying to establish facts behind relics that are far beyond the reach of Radio Carbon Dating. To date she's discovered dwarf mammoths on Mediterranean islands, retraced the groundbreaking Greek expedition of a female palaeontologist in the early 1900s, and even held an ancient woolly mammoth’s liver. (Verdict: stinky.)

But as she tells Professor Jim Al-Khalili, this passion for fossil-hunting is not just about understanding the past: this information is what will help us protect present-day elephants and the world around them for future generations.

Presented by Jim Al-Khalili Produced for BBC Studios by Lucy Taylor Reversion for World Service by Minnie Harrop

What does it take to earn the nickname, ‘The Leonardo da Vinci of heart surgery’?

That's the moniker given to today's guest - a man who pioneered high-profile and often controversial procedures, but also helped drive huge medical progress; carrying out around 2,000 heart transplants and 400 dual heart-lung transplants during his 60-year career.

Sir Magdi Yacoub is Emeritus Professor of Cardiothoracic Surgery at Imperial College London, and Director of Research at Harefield Hospital’s Magdi Yacoub Institute. Inspired by a surgeon father and impacted by the tragic early death of his aunt from a heart condition, his medical career includes various surgical firsts alongside numerous research projects, to further our understanding of and ability to treat heart disease. He headed up the teams that discovered it is possible to reverse heart failure, and that successfully grew part of a human heart valve from stem cells for the first time.

But it hasn't always been plain sailing. At times, his work – such as early, unsuccessful transplant attempts, or using a baboon as a life-support system for a baby – attracted serious public criticism.

Speaking to Professor Jim Al-Khalili, Sir Magdi reflects on the highs and lows of his cardio career, and offers his advice to the next generation of surgeons and researchers hoping to make their mark in heart medicine.

Presented by Jim Al-Khalili Produced by Lucy Taylor Reversion for World Service by Minnie Harrop

Claudia de Rham has rather an unusual relationship with gravity.

While she has spent her career exploring its fundamental nature, much of her free time has involved trying to defy it - from scuba diving in the Indian Ocean to piloting small aircraft over the Canadian waterfalls. Her ultimate ambition was to escape gravity’s clutches altogether and become an astronaut, a dream that was snatched away by an unlikely twist of fate.

However, Claudia has no regrets - and says defying gravity for much of her life has helped her to truly understand it.

As Professor of theoretical physics at Imperial College London, she now grapples with deep mathematics, where the fields of particle physics, gravity and cosmology intersect, on a quest to understand how the universe really works. She is a pioneer of the theory of massive gravity, a theory which could take us beyond even Einstein’s theory of relativity and shed light on why the universe is expanding at an ever-increasing rate.

Presented by Jim Al-Khalili Produced for BBC Studios by Beth Eastwood Reversion for World Service by Minnie Harrop

When you think of Artificial Intelligence, does it inspire confidence, or concern?

Although it's now generally accepted that this technology will play a major role in our future, a lot of conversations around AI and machine learning come back to the argument over us losing control and robots taking over.

Happily, Neil Lawrence has a more optimistic view of the power of AI, and how we might navigate the potential pitfalls. Neil is the DeepMind Professor of Machine Learning at the University of Cambridge, and over the course of his career has been involved in deploying AI and machine learning in both academic and commercial scenarios, with a stint at Amazon as well as working across fields as varied as movie animation, Formula 1 strategy, and medical research.

Speaking with Professor Jim Al-Khalili, Neil says ultimately his efforts are all about making a difference to our everyday lives - and that we need to learn how to embrace AI, albeit with a healthy dollop of scepticism; not least when it comes to how our data is used, and the power of 'the digital oligarchy'...

Presented by Jim Al-Khalili Produced for BBC Studios by Lucy Taylor Reversion for World Service by Minnie Harrop