36 | NewScientist | 21 April 2012

Decline and fallFrom Stephen WilsonLike many readers, I’m sure, I read the article on the decline of phytoplankton and the cost of carbon fixes with a sense of dread (7 April, p 42). It reminded me of historian Paul Kennedy’s theory of imperial overstretch.

In his 1987 book The Rise and Fall of the Great Powers, this Yale professor of economic history demonstrated that empires tend to expand until (basically) the amount that can be raised in taxes is lower than the cost of enforcing taxation, and the economy collapses.

Has the human empire reached the point at which the work needed to repair and maintain the ecology is already greater than our economic ability to do it?

And is this an inevitable result of the development of a technological culture?

If so, it would explain why SETI, the search for extraterrestrial intelligence, listens for signs of other such civilisations but hears only silence.London, UK

Salt! salt!From Peter UrbenReviewing the book Taste What You’re Missing, Catherine de Lange raises the question of why we do not crave salt as we do the equally essential water (24 March, p 54). But I do crave it, in exactly the same way; in fact, my cravings make no distinction.

When I go for a long walk on a hot day, the result is, not surprisingly, a salt-encrusted brow and a chronic thirst which persists for several days – long after I am rehydrated – unless I drink lightly salted water to stop the craving.

I conclude that my remote forebears evolved in an environment of brackish water.Kenilworth, Warwickshire, UK

If we’re so smart…From Martin van Raay“Why are humans the smartest animals on Earth?” you ask (24 March, p 3). Suppose any animal, in less than 2 million years, changed its environment

so thoroughly that not only did many other species die out, but the existence of the animal itself was endangered. Would we call that animal “the smartest on Earth”? Maybe what separates us from other animals is that we are too “smart” to see our own mistakes for what they are.Culemborg, The Netherlands

Wilful genesFrom Rhiannon MillerYour special report on God mentioned the human tendency to describe events in terms of agents, even when we know this is not truly the case (17 March, p 38). I have often thought that this tendency causes problems for people struggling to understand how evolution works.

We talk of antelopes evolving longer legs to escape from lions, of coloration evolving for camouflage, of evolutionary strategies and arms races, not to mention selfish genes. It is as if the antelopes deliberately set out to evolve longer legs; and it is not surprising that some people, realising that no creature actually sits down to figure out an evolutionary strategy, think evolution is nonsense.Perth, UK

Multiple moralsFrom Richard HarrisRobert Kurzban suggests we have at least two parallel systems for deciding right and wrong: one

based on kin selection and one prohibiting specific actions (18 February, p 10).

These moral systems often suggest outcomes that are in conflict. I don’t doubt that we have a utilitarian mode, but we are also concerned with ensuring justice for the individual.

For example, killing an individual might benefit the public good by harvesting their organs, so it would be right from a utilitarian point of view, but could be wrong because it is unjust for the individual. I suggest that we have evolved instincts to consider the consequences of acting in accordance with both of these ethical systems.

Being social animals, we also have an instinct for complying with socially sanctioned arbitrary rules, giving the potential for a three-way split.

For example, working on a Sunday should be allowed for the general public good; to protect the shop worker there is a legal provision in the UK to opt out; and the Bible requires Sunday workers to be put to death (Exodus, 35:2). Fortunately, that third option is not currently promoted.Corfe Mullen, Dorset, UK

No evidenceFrom Graham CoxCharles Darwin suggested humans first stood up to free their hands to use tools. Kate Douglas says we know this “cannot be right since the oldest tools are a mere 2.6 million years old” whereas bipedalism arose 4.2 million years ago (24 March, p 36).

Who can be certain there are no older relevant stone tool finds to be made? And what of other materials? Many areas have no stone and certainly no flint. We may have stood up to more easily jab animals with poisoned sticks that were sharpened on rocks; preservation of such tools would be very unlikely.Hothfield, Kent, UK

Enigma Number 1694

OPINION LETTERS

WIN £15 will be awarded to the sender of the first correct answer opened on Wednesday 23 May. The Editor’s decision is final. Please send entries to Enigma 1694, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, or to enigma@newscientist.com (please include your postal address). Answer to 1688 Eight prime factors: the two integers are 39 and 94 The winner David D. Peterson of Port Royal, South Carolina, US

IAN KAyOur town clock has a standard 12-hour face, divided into 60 minute marks. The hands do not move smoothly: the minute hand moves sharply on to the next minute mark at the end of each minute, and the hour hand moves at the end of each 12 minutes.

The other day, a prankster swapped the hands (an operation that only took a few seconds

between movements of the mechanism). Immediately after the swap, the clock showed the wrong time, but it was a valid time; in other words, the relative positions of the hands made sense. Within the next 10 minutes, the clock had shown the right time for exactly 2 minutes.

What time did the clock show immediately before the hands were swapped?

The town clock prank

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