"Maybe that's what being human is about," said the presenter, at the end of a documentary that was primarily close-up footage of caterpillars, tadpoles and locusts. I believed him.

David Malone has pulled this trick off before. In 2011 he fronted the BBC4 classic The Secret Life of Waves, a film that began as a fascinating study of the physics of the ocean but ended as a treatise on mortality – how our lives are a process, a beautiful thing made by energy that must eventually dissipate, because moving and changing is living.

Metamorphosis: The Science of Change (Wednesday BBC4; iPlayer) looked at the transformations that take place within us as we live, and our longing for the past, by first observing the phenomenon of radical physical change in nature. Ovid and Kafka mingled easily with the bristling, creeping oddness of sea urchins ripping themselves apart and starting again; or green, solitary locusts acquiring dark battle camouflage and joining deadly swarms.

How does a caterpillar know that this moult isn't just another moult - it's the big one? What makes some tadpoles decide to turn into frogs more quickly? And what do you have to do to a nice green locust to make it nasty? "I've always thought that metamorphosis was... weirdly interesting," said Malone, and how right he was. That a caterpillar and a butterfly can be the same creature with the same genetic code is, when someone as engaged and adept as Malone makes you think about it, among the most wrigglingly bizarre things in the world.

To lay out the facts, Malone enlisted help from jolly nerdy entomologists and beautiful/horrifying micro-photography, but it was his own screen persona, as a teacher who has you hanging on every word, that made the programme sprout wings. Malone has the gift of appearing to have the thought there and then rather than reading it from a script. He often breaks eye contact to summon up the right phrase. At his best, Kevin McCloud does something similar, although Kevin hams up the act by dramatically walking out of shot at the end of every programme.

When we'd learnt that locusts' metamorphosis is primarily one of the mind – a behavioural shift, prompted by a gush of serotonin – and that this can be undone when swarming's no longer expedient, Malone was taken by the desire to muse. We change ourselves, we become new people, we effect changes in technology and society that then turn round and change us. Our metamorphoses are only in our mind but are still profound and frightening. For locusts, read soldiers.

Malone saw glory and tragedy. In Kafka's story, the bug-man Gregor Samsa feels more alive and human than ever when he listens to his sister play the violin, because in his new form he hears it anew. But the change is about to kill him. Being able to alter our identity and circumstances is an immeasurable gift, for individuals and for humanity – "We are the one creature that can redefine the nature of life," Malone's philosopher friend Raymond Tallis told him, as the two of them stood on a beach with an offshore wind farm in the background – but it's one we fear.

"We remember," Malone said. "We can't help but look back, remember the creature we used to be, and regret what we might have lost." The sucker punch was landed. Once again I found myself staring straight through the end credits of a David Malone film, not knowing whether I'd been lifted up or taken apart, but enlivened by it either way.

When I saw the premise for Channel 4's new comedy The Mimic (Wednesdays; 4oD), I was furious. It might banjax my long-nurtured plan to write a sitcom for Alistair McGowan, in which he plays a TV impressionist whose personal life is a disaster because of his inability to converse as himself. Scene one: Alistair resolutely embarks on his sixth marriage, but recites the vows in the voices of Peter Snow, Jim Bowen, and Orville. Later, the wedding night is ruined when Alistair does Dot Cotton in his new wife's ear.

Anyway, as it turns out The Mimic is sort of the opposite of that. Terry Mynott is the fabulously named Martin Hurdle, a gentle loser who has only one friend, a dowdy trouper called Jean (Jo Hartley), and no future prospects in his work maintaining the grounds of a faceless pharmaceutical firm. His secret, and his mental release valve, is that he's a brilliant impressionist.

The Mimic is by Russell Brand's old sidekick Matt Morgan, who worked with Mynott on The Morgana Show and VIP. Where they were crass and brash, this is slow, quiet and lovely. It has the vibe of an indie film, possibly one starring a big comedy name gambling their fame to prove they're human and can act.

Mynott has no fame to risk, yet there's still bravery in the way he makes Martin so uninhibitedly genuine and sad. In the first episode he was often filmed to accentuate his isolation. His little triumphs mostly weren't witnessed by anyone. He stopped doing his spot-on Alan Carr in the company car park when people walked into earshot, and his fantastic imagined conversation between Morgan Freeman and James Earl Jones faltered when Jean asked who he was talking to and told him to get some sleep.

The Mimic is a bit more than a sitcom. You wonder not only whether it will still be funny next week and the week after, but also where it will go – what will happen to the hero. Is he a talented man waiting to be discovered or just a lonely man waiting to be loved?

Scenes where Martin met his previously unknown 18-year-old son, and where he took revenge on a bad HR manager by being him on the office tannoy, hinted that his achingly small world is about to expand. We'll be rooting for him to survive the change.

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