John Reilly talks to us about The Ascent of Mammals.
Could you tell us a little about your background and where your interest in evolution began?
My lifelong interest in evolution started while studying biology in high school. To this day, I remember being intrigued by Lamarck's and Darwin's different explanations for the evolution of the giraffe’s elongated neck. I was also beguiled by the simple, linear sequence of fossil species that purported to explain the stages of equine evolution. During the summer before university, I read John Maynard Smith’s influential book, The Theory of Evolution, a concise and informative read that delves into the scientific facts underpinning the theory. While a paradigm of popular science writing that kept me out of mischief during the summer holidays, I confess, I did not fully understand the section on mathematics and biology.
At medical school, human evolution formed part of the Anatomy course and sparked my interest in the early history of Homo sapiens; no doubt helped by the high-profile discovery of ‘Lucy’, a 3-million-year-old hominin unearthed in Ethiopia a few years earlier. Later, my career in academic medicine would keep me abreast of the latest genetic analysis techniques and the vast amounts of data generated through gene sequencing. Among these developments was the use of DNA sequencing to infer a creature’s evolutionary relationships, a field known as molecular phylogenetics. I can still recall the excitement of learning that this method had revealed an unexpected connection among several mammalian groups—aardvarks, hyraxes, sea cows, and elephants—showing that they are each other’s closest relatives. The finding was particularly striking because these relationships had not been anticipated by palaeontologists or comparative anatomists.
Evidence of my longstanding interest in evolutionary theory is reflected in the collection of books on the subject that adorn my shelves, many of which have phrases underlined in pencil and scribbled marginalia highlighting key facts to be committed to memory.
Despite having only seven cervical vertebrae like most mammals, the giraffe’s extreme neck elongation required numerous evolutionary adaptations.
What was the impetus behind The Ascent of Mammals?
I have always been fascinated by new and exciting scientific discoveries, particularly in molecular genetics, and I enjoy explaining their importance to others. I followed this approach in my previous book, The Ascent of Birds. Subsequently, I realised that while mammalian evolution has been extensively written about from a palaeontological perspective, a genetics-based account was missing.
The timing for such a project seemed ideal. Recent advances in whole-genome sequencing have produced a wave of new - and often surprising - insights into the evolution and speciation of many major mammalian families, including humans. Yet much of this information remains buried in specialised academic journals, where its significance is often obscured by dense scientific jargon. It was this fact that persuaded me to write The Ascent of Mammals to present these exciting breakthroughs in a way accessible to a wider audience.
You describe the book as a "tapestry of stories" involving monotremes, marsupials, and placentals. How do you manage to keep the complex science of genomics "accessibly written" for a general audience without losing the technical depth?
Explaining complex scientific ideas to the public is a key challenge for any writer. However, I firmly believe that if you struggle to explain a scientific concept to a non-specialist, then you probably do not fully understand the topic yourself! Not being deeply entrenched in the field may actually help, because acknowledged authorities often assume their audience knows more than they do and skip over details that seem ‘obvious’ to them. I was also fortunate to have a meticulous editor who wasn’t hesitant to point out when my explanations weren’t as lucid as I had assumed.
Tasmanian tiger © Tasmanian Museum and Art Gallery
What was the most surprising thing you learnt whilst conducting research for the book?
Palaeogenomics—the study of ancient DNA from extinct species—has revolutionised our understanding of evolution. By reconstructing genetic information from the distant past, scientists have revealed surprising patterns of genetic drift, historical population migrations, and the role of interbreeding and hybridisation in shaping new species. These insights are now recognised as key drivers in the evolution of numerous mammalian lineages, particularly primates, including humans.
The successful sequencing of extinct species' genomes has also opened the door to reversing extinction. Several research teams are actively exploring ways to bring back lost species, such as the iconic Tasmanian tiger and the woolly mammoth.
Your previous book, The Ascent of Birds, focused on avian evolution. How does the evolutionary story of mammals compare or contrast with that of birds?
While mammals and birds are obviously very different, their evolutionary stories are surprisingly similar. Both classes have been shaped by continental drift, multiple climatic upheavals, changing ocean currents and extinction events. In addition, the underlying genetic processes are often the same, including gene duplication, genomic mutations, and the roles of retrogenes, pseudogenes and gene loss.
Mountain gorilla – Its ancestors inhabited tropical Africa during the Pliocene, around 4 million years ago.
You have travelled the globe to see remarkable species, from the platypus to the mountain gorilla. Do any specific encounters stand out as favourite or most memorable?
One of my most unforgettable wildlife experiences was encountering a family of mountain gorillas in Rwanda. From a safe distance, I watched as the silverback kept a vigilant eye over his group, while the adolescents darted through the vegetation, swinging on bamboo shoots with boundless energy. Their human-like gestures, playful behaviour, and the way they communicated with each other were utterly mesmerising. At one point, a young gorilla approached surprisingly close, and I could see the curiosity and innocence reflected in its eyes. That humbling moment of connection left a lasting impression on me—an experience I will carry with me forever.
Order your copy of The Ascent of Mammals here.
