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THE LAST WORD

Hot youthAll of the radioactive elements that made up the early Earth started out in the hot ash of ancient supernova explosions. This means we’ve been working through their half-lives for at least 5 billion years since our planet was born. How much hotter was Earth’s interior then? What would the heat from nuclear reactions mean for tectonic activity and the evolution of life in our world’s feverish youth?

n It would take a major study to calculate details of the isotopic make-up of the planet 5 billion years ago. I suspect that internal temperature would have been dictated by gravity making the planet contract and impactors hitting its surface, rather than isotopic decay or nuclear fission.

Our present concerns with nuclear decay vanish over such timescales. Consider that plutonium-242 has a half-life of more than 370,000 years. There are several thousand half-lives in a billion years, so even if our galaxy were made of solid

plutonium-242 at the start, none would now remain. With a half-life of over 710 million years even uranium-235 would by now be reduced to less than 1 per cent of the original. Strontium-90 would vanish in less than 1 million years

and would be gone before the planet had clumped together from the remnants of supernovae.

In short, we need not consider isotopes with lives much shorter than uranium-235’s. Having said that, self-sustaining nuclear chain reactions are possible. Natural nuclear fission reactions took place around 1.7 billion years ago in uranium-235 deposits in Oklo, Gabon. They ran for a few hundred thousand years, with an average power output of 100 kilowatts during that time.

We don’t know exactly when life started, but it was a good 500,000 years after Earth formed, when tectonic activity had calmed to something less than the free convection of molten rock. Jon RichfieldSomerset West, South Africa

If nuclear decay didn’t heat the planet, does anyone know what did? – Ed

Hidden evidenceWhy do cats usually bury their faeces but dogs don’t, even though they are good at digging?

n Dogs are pack animals. They evolved their sensitive noses for hunting and, helped by scent glands around the anus, use them to recognise odours that tell them if another dog is a friend from their pack, or an enemy.

Dogs leave their smelly faeces and urine around their territories to inform other dogs about who they are, and to warn off

unfamiliar dogs. This is called “marking”. They will smell another dog’s anus when they meet so they know who the other dog is. Some people think that a dog can use anal gland smell to recognise another dog just as humans recognise faces.

Cats are solitary animals. To a wild cat, any cat is a threat, never a friend. It is not advantageous for a cat to leave smelly faeces; better to hide it underground where it can rot down quickly. But cats are hunters too, have just as sensitive noses as dogs and sometimes use marking behaviour. When a cat is looking for a mate, it will use smelly urine to mark its territory and to tell other cats about it. Cats have scent glands in the skin of their faces and sides, and that’s why they like to rub themselves against you – they are marking you as their territory.John DaviesLancaster, UK

Pit problemsMy doctor says that skin tags – small growths on the skin’s surface – commonly form in the armpit. Why do they appear?

n Skin tags, the small projections of skin called cutaneous papillomas or acrochorda, are sometimes thought to be caused

by friction because they are most common in areas that receive most friction, such as armpits, the back of the knees, between the breasts and under them (especially where bras rub), and in the skin folds of obese people. However, friction makes skin tags worse but doesn’t cause them. People with type 2 diabetes also must put up with these annoying, unsightly but benign growths.

Most skin tags are small, but some can grow big enough to warrant removal. Doctors tend to “freeze” them off with liquid nitrogen. Cautery and excision are also performed. Regrowth is common unless the root is completely removed, but it is worth noting that the scar can be larger than the original tag.

Skin tags are more common in women than in men and, like liver spots, are a side effect of maturity. Toshi KnellNowra, New South Wales, Australia

n Skin tags and warts are quite different; warts are caused by human papillomavirus and are infectious; skin tags are not infectious and appear to arise spontaneously. Tags are very common but are nothing to worry about. They are harmless in themselves and if they do cause irritation they can be removed by freezing or by tying off to stop the blood supply. It is not advisable to cut them off because larger ones can bleed profusely.Chris WarmanSaltburn-by-the-Sea, North Yorkshire, UK

“Cats are solitary. To a cat, another cat is a threat, so there is no advantage to leaving smelly faeces”

“We don’t know exactly when life started, but it was a good 500,000 years after Earth formed”

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