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As Adam Vaughan writes, it is now clear that we have to reduce our greenhouse gas emissions to zero ( 27 April, p 20 ). But the question of when depends on who we mean by "we". The Intergovernmental Panel on Climate Change special report shows that global emissions should reach net zero by about …

Watching people, in particular young children, concentrating on using their hands for a fine motor skill while sticking out their tongue lends credence to the possibility that language began with hand movements ( 4 May, p 34 ). I wonder whether speech developed as our ancestors began to live in groups, with some members going …

Since gathering, rather than hunting, provided the majority of calories for early communities, why is such a focus for the origins of language placed on suggestions of what may have aided our ancestors to hunt, rather than to gather? Some propose singing for protection as the origin of language. Maybe this started when the gatherers …

If another human species has been discovered on the island of Luzon in the Philippines, how did it get there, and when? There were five or six periods of low sea level in the mid-to-late Pleistocene ( 13 April, p 9 ). In the two most recent of these, levels were about 120 metres below …

What is the motivation of the US in trying to stop Huawei selling 5G infrastructure to the UK and others? There is a body of opinion that it is to get US companies to dominate the market, and to enforce the spread of US spyware, not Chinese. The US National Security Agency has reportedly meddled …

Tom Stafford argues that the idea that free will doesn't exist is based on misguided intuitions of what it means to be a biological machine ( 6 April, p 34 ). Is it fair to assume that any scientific model of the mechanism of a free will decision should ultimately be expressible in mathematical form? …

Stafford confuses complexity with freedom. He admits that "our thoughts are caused by our brains, our environment and our history". But he maintains that, because this causal mix is unique to each individual at each moment, human behaviour is so difficult to predict that it is elevated above that of digger wasps. Yet so long …

Chris Baraniuk concludes that it is one thing to hack an artificial intelligence controlling a vehicle in a car park, and quite another to hack an AI controlling a weapon ( 27 April, p 34 ). But what about a weapon such as a driverless car that is doing 50 kilometres per hour on a …

Discussing whether cystic fibrosis carriers might recognise each other, Richard Harris says "it seems extremely unlikely that one mutated gene among tens of thousands would be detectable by smell" Letters, ( 20 April ). But there are examples of just this. For example, trimethylaminuria, also known as fish odour syndrome, is caused by a mutation …

• Wet, wet, wet: It is the class of space rocks that includes Itokawa that may have brought half as much water as is in Earth's oceans ( 11 May, p 18 ). • Mind like a sieve: The reason that Harald Helfgott's prime number-finding algorithm is purelytheoretical is that he hasn't optimised it ( …