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Your special on consciousness was fascinating and thought-provoking ( 10 July, p 34 ). The section on whether machines could ever be "conscious" and "self-aware" seemed to ignore the role played by the fierce will to live and reproduce exhibited by living creatures. Where does this come from and where does it reside? One might …

You report research suggesting that after land-dwelling tetrapods evolved from fish, some tetrapod species "surprisingly" evolved to live in the water again ( 19 June, p 21 ). But is this surprising? We know that cetaceans made the same transition, even after becoming far more adapted to life on land than early tetrapods. More broadly, …

The final line of the box on missing lithium in our feature on gold (24 July, p 46) also went missing. The sentence in full should have read: Gas from that star falls onto the white dwarf, causing an explosion that may produce large amounts of lithium.

What horrifying thoughts from geneticist Chris Mason, with his idea of spreading humans across the galaxy ( 17 July, p 44 ). We may be the most intellectual creatures on this planet, but are hardly a role model. We kill our fellow humans in wars and our self-centred attitudes have eliminated many other species. Positive …

Mason describes ambitious plans to ensure the long-term survival of humans. But to whom (or to what) does it matter if we become extinct? The very nature of the event means there will be nobody around to care. Indeed, I suspect that the other species on Earth would be glad to see the back of …

Further to your article on the problems with pavements, years ago I learned that a long day of off-track walking was less tough on my feet than a few kilometres on a smooth path or vehicle track ( 10 July, p 46 ). On uneven ground, every footfall is slightly different, requiring different muscles to …

While the hardness of the concrete in sidewalk pavement is certainly an issue, it does have an advantage over asphalt-based alternatives: its much lighter colour reflects sunlight better and so stays cooler than darker materials, a benefit my dogs appreciate during walks.

John Koster says the time is right to try a massive and untested interference with marine ecosystems by seeding waters with iron to encourage plankton growth to capture carbon (Letters, 17 and 24 July ). Yet such geoengineering risks perpetuating fossil-fuelled lifestyles. Koster proposes that this takes place in the Southern Ocean. However, some of …

Paul Whiteley says seeding the ocean with iron will simply allow fossil fuel companies to keep their business models and politicians to avoid decisions. I'm not really in favour of such seeding, but even if we do drastically reduce our use of fossil fuels, we will still have a problem of too much carbon dioxide …

Your article on chemical pollution made some good points, but there is often confusion about what a "chemical" actually is ( 24 July, p 36 ). All materials are chemicals as there is no physical thing that is not made of chemicals. The idea of synthetic and natural chemicals is an artificial construct and often …