Peter Godfrey-Smith is the scuba-diving philosopher who took octopus off the menu for many readers of his bestselling book, Other Minds. It looked at the distinctive intelligence of cephalopods, rescuing this myth-laden eight-limbed creature from its most frequent setting of a seafood salad and recasting it as subaquatic hero of perception and understanding. Following up that literary success with 2020’s Metazoa (the word means multicellular animals), Godfrey-Smith is about to publish the final part of his trilogy on the roots of intelligence, Living on Earth: Life, Consciousness and the Making of the Natural World. It’s another wide-ranging book that swims back and forth across a multitude of disciplines, including philosophy, neurology, biology, chemistry, natural history and geology, as it explores how the various manifestations of life over billions of years have dramatically affected the planet. Godfrey-Smith begins with “the Great Oxygenation”, about 2.4bn years ago, when cyanobacteria – something like the components of the green algae often seen on lakes and ponds – became the Earth’s first photosynthesisers, taking water and the sun’s energy and releasing oxygen into the atmosphere. This process gradually increased oxygen levels until the chemical environment was one that could sustain life forms with brains and muscles. It is this arrival of oxygenic photosynthesis, says Godfrey-Smith on a video call from his home in Sydney, that he has come to see as the most remarkable stage of evolution. “You can imagine a situation in which life arises and has some effects on a planet, but remains a relatively minor part of the scene,” he says. “But oxygenic photosynthesis made life into a major player on Earth. It didn’t just change the living world, it changed the mineral composition and geological processes of the Earth.” The only evolutionary development that is remotely comparable in its impact is, of course, us, humanity. We live now in the Anthropocene, a geological epoch marked by human effects on climate, landscape, ecosystems and biodiversity. He believes research now shows the category of animals that experience pain is much larger than previously thought Among other things, Godfrey-Smith’s book is an unusual philosophical study of what he calls “a history of organisms as causes, rather than evolutionary products”. Because evolution by natural selection is a random process that leads to unpredictable developments, there is a tendency to see each new species of life it produces as an effect, an outcome, a product of nature. Godfrey-Smith is more intrigued by the other side of the coin, the manner in which these life forms shape the environment and very landscape around them – in the most basic sense, plant life can redirect the paths of rivers, which then reshape the topography of the land. He traces how various organisms – plants, animals, bacteria – became dependent on one another through coevolution or what he calls complementarity. It’s a dynamic story of action, but part of that action is the creation of minds that in turn determine other actions. As he writes: “Deliberate human action continues and extends a long tradition of organisms transforming nature.” The problem, in modern terms, is that there is growing evidence that deliberate human actions – such as the production of plastics, cutting down of forests and burning of carbon fuels – are destroying nature, wrecking ecosystems and leading to the extinction of many species. It’s a predicament that has led some environmentalists to argue that the planet would be better off without humanity. Does Godfrey-Smith agree? He looks towards his ceiling and offers up a long silence. Eventually, he says: “To some extent. I don’t discount it.” He appears uncomfortable with pat answers, wishing to consider the full meaning of a question before responding. He doesn’t rush to judgment but instead tries to put issues of human influence within contexts – and, in evolutionary terms, that context can be very large indeed. At some point, he notes, when the photosynthesis cycle runs its course, as it will inevitably do, then extinction awaits us all. That is a long time away, however, and there is plenty of opportunity for further animal suffering at the hands of humanity. “If I thought we were not going to start to do better,” he says, “and the idea that the malign side of our agency dominated all other sides, that would be a serious point for concern. And in that scenario, I don’t think the argument that it would be better if humanity went sooner rather than later is a crazy argument.” Doing better, in Godfrey-Smith’s mind, most urgently comes down to what he calls the “welfare arguments involving factory farming”. Human domination can be measured in many ways but one crude yet revealing metric is biomass, the total weight of a given species or organism. Humans amount to nine times the biomass of wild animals. And the livestock we rear makes up “more than 10 times the biomass of wild animals and birds”, Godfrey-Smith writes. The land required for this livestock is eating up vast areas of jungle and is almost certainly unsustainable, but it is the manner in which the livestock is treated that is the moral focus of Godfrey-Smith’s philosophical arguments. The book that really opened philosophy up to the issue of animal rights was Peter Singer’s Animal Liberation, which Godfrey-Smith cites as a major influence. However, Singer was arguing from a utilitarian basis, drawing on the work of Jeremy Bentham. A different argument comes from a Kantian perspective, via the work of the American philosopher Christine Korsgaard. The key issue here is not so much the suffering, as the fact that humans impose control and ignore the preference of animals. Godfrey-Smith seeks to shift the argument slightly by introducing the idea of “a life worth living”, which is a “life that’s better than no life at all”. To do this, he employs a reincarnation thought experiment that asks who, in the bargain for further life, would want to come back as, say, a factory farm pig, removed early from its mother and placed in crowded, stressful confinement for the rest of its short life? The easy answer to that is no one in their right mind. The excesses of factory farming are something that Godfrey-Smith believes we can address without too much cost to humanity, economically or in terms of nourishment. It would also reaffirm his belief in the human project by allowing farmed animals a worthwhile life. “I would feel fine about coming back as a cow on a humane farm,” he writes. Yet humans are beastly, as it were, to many animals, not just those that are farmed. Are all included in this reincarnation test? He says he recently signed a statement saying that research now shows the category of animals that experience pain and aversive experience is much larger than was previously thought, including arthropods – among them crustaceans and insects – as well as cephalopods. “One thing that bothered me,” says Godfrey-Smith, “is that most of the animals entering this new category of a reasonable chance of sentience we can make peace with. But we can’t make peace with the insects. Human interest and insects are often very strongly opposed – mosquitoes being the obvious example.” Godfrey-Smith, it turns out, is a long way from being an absolutist. He’s not vegan or vegetarian, although he’s tried both. For him, what matters most is the manner in which the animal is able to live and then how it is killed, not the fact that it is killed. “I do think death sometimes has too powerful a rhetorical role in these discussions, given that death is inevitable for everyone,” he says. He has little problem with sustainable, wild, caught seafood. So does that mean he eats octopus, which lives freely, is not endangered and could be killed humanely. “I don’t myself,” he says, “but I think that’s a sentimental response.” For such an evidence-based thinker, it’s a slightly surprising, if somehow reassuring, answer. As much as Godfrey-Smith sees death as the unavoidable end point of life, he nonetheless takes a look at the issue of immortality, a concept that increasingly features in the fantasies of tech billionaires and futurologists. In this version of life, there is some form of digitalisation, whereby we become effective simulations of life – some theorists, such as Nick Bostrom, have speculated that we already are simulations. In contemplating living for ever, or at least for a couple of million years, Godfrey-Smith cites the philosopher Thomas Nagel, who has written of death as “a great curse”. “[G]iven the simple choice between living for another week and dying in five minutes, I would always choose to live for another week: and by a version of mathematical induction, I would be glad to live for ever,” wrote Nagel. It is a version of a life that’s always better than no life at all. Godfrey-Smith, who doubts that such a development will happen in the foreseeable future, has practical and philosophical reservations about this argument, including what part, if any, a cohesive “self” would play over such an unimaginably long period of time. But if there was a near-zero environmental cost to an indefinitely extended life, and human flourishing would otherwise be under threat, he “might see the idea of turnover and endings differently”. He asks me what I think of the question, and I say that I can’t separate the meaning of life from the inevitability of death, the latter can’t help but determine our understanding and give meaning to the former. He disagrees. Or rather, he has a characteristically different and perhaps more subtle take on it all. It is the coming and going of life and death that he sees as an essential part of Earth’s history. As he writes: “I identify with that process, including turnover and renewal, the flow of new arrivals who then depart and leave room for more.” Godfrey-Smith ends his book back where it all began, both this trilogy and life itself: in the sea. It has been an extraordinary journey from bacteria to writing books. This philosopher, who cites David Attenborough’s Life on Earth (whose title his book echoes) and John Rawls’s A Theory of Justice as seminal influences, has done much to improve our thinking about what that evolution has involved. Is he, therefore, of the opinion that this is experience that has recurred elsewhere in the universe? He is encouraged by the fact that life arose early in the history of the planet, rather than after “a vast stretch of deadness”, which he thinks suggests an increased chance of life taking shape on planets unknown. “My guess,” he says, “is that life is not rare, but complex life is rare.” It’s fair to say that, in this case, it really is an educated guess. Living on Earth: Life, Consciousness and the Making of the Natural World by Peter Godfrey-Smith is published by William Collins (£22). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply