Ludwig Wittgenstein once observed, “If a lion could speak, we could not understand him.” But Ludo, mind if I ask how much time you’ve actually spent with lions? Thought not. Because that’s rubbish, at least in the sense that humans and lions couldn’t possibly have common ground for a conversation. Wittgenstein can beat me in any logico-philosophical contest of his or anyone else’s choosing, but he hasn’t spent as much time as I have hanging out in the bush with lions.

It was a few weeks ago in the Luangwa Valley in Zambia. Six lionesses had just slain an antelope and were avidly devouring it. From where I was, a quarter of a mile off, all I could see of this meal was a companionable rosette of tawny fur. Near me, a lone nomad male lion was also watching. He had picked up an injury and had been unable to hunt for a few days. He was very hungry; you could count the ribs. He had no pride of his own; he wasn’t yet big enough, or strong enough, or confident enough to attempt a takeover. He had to kill for himself and he couldn’t.

He was watching a vision of everything he wanted in the world: food, the blessed intimacies of pride life and the company of those six sexy lionesses. He wanted more than anything to join them. But something very powerful stopped him from doing so. They wouldn’t have welcomed him. They would have chased him off; it would have ended in violence; it was no good. But he couldn’t stop himself watching. He made a series of retreats, each time stopping and staring back longingly.

Eventually, like Andrew Lincoln fighting the pangs of his unrequited love for Keira Knightley in Love Actually, he pulled himself together and forced himself to leave the world of longing and get back to reality. He walked into the river and swam decisively across: enough! Had he stopped to talk about that experience, I would have understood. So would we all. Loneliness, longing, hunger, despair, desire: are these things so remote from our own experience?

But this is dangerous ground. Most of our science, philosophy and religion starts from the assumption that there are humans and there are animals – and there could never at any point be any common ground between them. To call someone an animal is as bad an insult as you can offer, and yet we’re all mammals. For centuries, the notion of human uniqueness was the most fundamental orthodoxy. Now it is being challenged. Book after book ventures into the no-man’s-land – the no-animal’s-land – that lies between our species and the other ten million or so in the animal kingdom. As often as not, they reveal more of ourselves than of our fellow animals.

With every page we turn, we can feel the resistance to any suggestion that non-human animals are even remotely like ourselves. Of course animals can’t think, can’t feel, can’t talk. We resist this not because such things are impossible but because they are unthinkable. Our lives would be horribly compromised if we accepted that we humans were just one more species of animal.

Lucy Cooke, in The Unexpected Truth About Animals, examines the way in which humans have looked to animals for moral lessons, often framing them as creatures to despise, thereby failing to perceive countless things worthy of wonder and admiration. And though she is very sound on the ancient notion that a beaver will escape capture by biting off its own testicles and offering them to his pursuers, she is still better on sloths.

This is an animal named for one of the seven deadly sins – it’s hard to write off a creature more completely. “I have never seen such an ugly animal or one that is more useless,” wrote Gonzalo Fernández de Oviedo y Valdés in his 50-volume encyclopaedia, published in 1526. Cooke reveals the conceptual beauty of the sloth and the way that it is exquisitely engineered for a low-energy lifestyle. She shows that a sloth is a survival machine every bit as elegant as a cheetah or, for that matter, a human.

Not inferior: different. But that’s a concept that humans have struggled with across the centuries, probably since before language began. “One scientific survey from the 1970s found that sloths ‘are the most numerically abundant large mammal’, accounting for almost a quarter of the mammalian biomass,” writes Cooke, “which is a sophisticated biological way of saying you can take your patronising looks and direct them at some other animal.”

For years, it was accepted that the issue was binary. You could be objective, or you could be sentimental. Scientific orthodoxy stated that animals had no emotions or personalities: even to consider such a matter was a sin. This was not something to be investigated or put to the test. It was an error that could be corrected with a single word: anthropomorphism.

Mary Midgley, the ethical philosopher, wrote about mahouts, elephant riders, and how, if they failed to take into account “the basic everyday feelings – about whether their elephant is pleased, annoyed, frightened, excited, tired, sore, suspicious or angry – they would not only be out of business, they would often simply be dead”. Anthropomorphise or die. Anyone who works with horses knows that.

Peter Wohlleben had a hit with his book The Hidden Life of Trees. In this, he described the fungal connections between tree and tree, which he interpreted as highways of communication and wittily called “the wood-wide web”. He made trees the sort of living thing that we humans can empathise with, rather than pieces of rural furniture.

His new book, The Inner Life of Animals, is less assured. He mixes science with his love of a nice story, knowing that nothing upsets scientists as much as anecdotal evidence. So when he writes about Barry, a rescue dog – a cocker spaniel – that had been through many homes before joining Wohlleben’s family, and asks if Barry felt gratitude, we’re back in that dangerous border country:

For the rest of his life he worried that there might be yet another handover, but other than that Barry was always happy and friendly. He was grateful. It was as simple as that – or was it?

Wohlleben also tells a story about two deer running from the dog he used in his forestry work. The fawn left its mother’s side and turned around to run straight at the dog, forcing it to run away. Had that fawn been human, we would have called it courage. We humans know what we’re supposed to do in dangerous circumstances, but we have no idea if we’re actually going to do it. Some do, some don’t. Those who do the right thing we call brave: so if the human is brave, is the fawn also brave?

These are not areas that have often been explored. That’s true in literature, as well as in science and philosophy. But in an odd, altogether unexpected book, Animals Strike Curious Poses, Elena Passarello writes of human-animal relationships with all the weight of mighty literature that she can bring to bear. That she attempts so bold a feat shows that this border country can be explored in unapologetically literary terms and it is worthy of deep seriousness of purpose.

In this series of essays, she includes a kind of love letter to Charles Darwin, ostensibly from a tortoise he collected on the Galápagos Islands, and thickens the mixture by writing it in the second person. “He will shortly thereafter name you ‘Harry’, but don’t doubt that some part of him knows that you’re all woman.”

She also writes with some elan about Mozart’s starling, a bird for whom he held a solemn funeral, one of those curious Mozartian episodes in which he seemed unable to decide where the joke began and ended. And that prompts me to ask: when a nightingale sings – with a vocabulary of 600 sound units put together into 250 phrases – is he merely responding to his annual urge to make more nightingales? Or is he (it’s always the cock that sings) lost in the music? It’s always the hen that makes the decision on the quality of the singer – so is she responding to mere biology? Or is she making some kind of aesthetic judgement? Your call, dear reader: but perhaps the question points us at an understanding of life that does not take human uniqueness as a compulsory starting point.

Does the real answer lie in objective science? No doubt it should. But traditional scientists don’t start with the hypothesis that a non-human animal has nothing in common with us humans: instead, they start from the stone-cold certainty that it couldn’t possibly have anything of the kind.

Carl Safina, a professor of nature and humanity at Stony Brook University, New York, wrote:

Suggesting that other animals can feel anything wasn’t just a conversation stopper; it was a career killer. In 1992, readers of the exclusive journal Science were warned by one academic writer that studying animal perceptions ‘isn’t

a project I’d recommend to anyone without tenure’.

It is odd that scientists, who claim to work only from data, and philosophers, who, like Wittgenstein, might speculate without anything as sordid as data but still love a good bit of logic, operate on the certainty that, while all placental mammals are put together in the same way physiologically, one of them is somehow completely different from all the other 4,000-odd – so different that we don’t even need evidence to prove it. Are we talking about the soul here? I ask only for information.

Throughout the years, people have sought to isolate and identify humanity’s USP, and every time they have done so, they discover that some animal – some non-human animal – has it, too. All the barriers we have erected between ourselves and other animals turn out to be frail and porous: emotion, thought, problem-solving, tool use, culture, an understanding of death, an awareness of the self, consciousness, language, syntax, sport, mercy, magnanimity, individuality, names, personality, reason, planning, insight, foresight, imagination, moral choice… even art, religion and jokes.

It’s all in Darwin, but we have spent getting on for two centuries ignoring or distorting the stuff he taught us. In The Descent of Man, he wrote: “The difference in mind between man and the higher animals, great as it is, certainly is one of degree and not of kind.” If you accept evolution by means of natural selection, that must be true.

Why, then, are humans so resistant to the idea? We can find the answer in human history. For many years it was important to uphold the notion of the moral and mental inferiority of non-white people, because without such a certainty colonialism and slavery would be immoral. And that would never do: they were so convenient.

To change our views on the uniqueness of human beings would require recalibrating 5,000 years or so of human thought, which would in turn require revolutionary changes in the way we live our lives and manage the planet we all live on.

And that would be highly inconvenient.

Simon Barnes’s most recent book is “The Meaning of Birds” (Head of Zeus)

Animals Strike Curious Poses

Elena Passarello

Jonathan Cape, 256pp, £12.99

The Inner Life of Animals: Surprising Observations of a Hidden World

Peter Wohlleben

Bodley Head, 288pp, £16.99

The Unexpected Truth About Animals: a Menagerie of the Misunderstood

Lucy Cooke

Doubleday, 400pp, £16.99