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Colin Barras correctly concluded that the line between human and non-human tends to be as much philosophical as biological. I am reminded of a subtle behavioural distinction made by physicist Brian Greene when he noted that humans transcend their timeline. This parallels something a philosophy professor told me long ago: humans ask what came before …

When did we become human? When we got language. Everything distinctively human – art, philosophy, technology, religion and so on – is predicated upon it. But its arrival has always been a conundrum for human evolution. First, there is nothing close to our sophisticated language in the animal world. Second, why did we need such …

You report on a proposal to put a frozen backup of Earth's life on the moon. The idea of off-planet reserves for life, albeit not frozen, isn't new. In the 1972 movie Silent Running , forests are kept in giant greenhouses beyond the orbit of Saturn. In any event, any lunar repository could be financed …

Alex Wilkins reports that a slight curve on the surface of a rock helps it make the biggest splash. I am of a generation that received corporal punishment. It was well known that the more charitable disciplinarians used an absolutely flat instrument. We, the recipients, could feel the reason suggested in the research: the flat …

With reference to childhood exercise, an additional factor struck me. As a child in a car-less household, I was frequently presented with a purse and a shopping bag and told to run and fetch whatever was needed from the local shops, thus clocking up both aerobic and weight-bearing benefits. My own children, however, just pop …

I read your article on attempts to detect very early structures in the universe with interest, and was taken by the line "the further back into cosmic history we have looked, the more astounded we have been to see nigh-on fully formed galaxies and supermassive black holes that shouldn't exist because there hasn't been enough …

You report that butchered bones hint at earlier human arrival in South America. But could the butchered glyptodont, which has been dated to the last glacial maximum period, have become frozen after death, like a Siberian mammoth, and likewise thawed thousands of years later and been butchered then( 27 July, p 18 )?

As a bit of a poet and a bit of a mathematician (and no great shakes at either), I found Peter Rowlett's piece on mathematics and rhyme entertaining, especially the notion of the pi-ku, a haiku variant with syllables based on 3.14 for its three lines, i.e. 3, 1 and 4. Why stop there, when …

Modern palaeoanthropology and archaeology can be misleading, I would suggest, and classification of "post-primates" like us and "prehistoric man" is confused ( 3 August, p 32 ). As we devise post-primate species and genus from a scattering of bones and stones, we can't see speech or family structures and dynamics that may be more significant …

A vaccine to stop cows burping methane is very ingenious. But wouldn't it be simpler to farm animals that eat grass and don't burp or fart much methane ? They are called kangaroos ( 10 August, p 16 ).