When humans have vanished from the planet, one of the most enduring marks of our impact on Earth will be the sudden appearance in the fossil record of copious chicken bones.

Geologists have proposed that the age of humans constitutes a new epoch in Earth’s history, known as the Anthropocene. The explosion in chicken farming and the rapid changes in the form of chickens due to selective breeding make them an ideal sign of our time, reports newscientist.com.

‘We think they are a really important symbol and potential future fossil of this age, and man’s impact on the planet,’ says Carys Bennett at the University of Leicester, UK.

The 20th century saw an explosion in the numbers of domesticated chickens all over the world. The current population is now 21.4 billion – more than any other land vertebrate and an order of magnitude greater than any other bird. Over 60 billion are slaughtered every year – a rate of carcass accumulation that is unprecedented in the natural world.

The modern broiler chicken – the variety farmed for meat – is now unrecognisable from its wild ancestor, the red jungle fowl. Though chickens were domesticated around 8000 years ago, they have undergone especially marked changes since intensive farming took off in the middle of the 20th century. Today’s chickens grow to become four or five times as heavy as birds from 1957. The leg bone of a juvenile broiler is triple the width and double the length of a red jungle fowl equivalent.

Signs of our time

The timing of these changes in chickens coincides with other proposed signs of the Anthropocene, such as plastics, fertilisers, fossil fuels and radioactive deposits from nuclear weapons.

Most chicken carcasses are thrown into landfill, where the oxygen-free conditions tend to mummify organic matter. That means they have the potential to fossilise and remain preserved for millions of years. ‘You would find them with all the usual human detritus — tin cans, soft drinks, remains of plastic wrappings,’ says Bennett.

If future geochemists were to analyse the carbon and nitrogen isotopes in these fossils, they would see a striking difference compared to wild fowl, thanks to the chicken’s homogenised diet.

Chicken will soon overtake pork to become the meat humans eat most. The rise of chickens mirrors the decline of wild birds, meaning Earth’s avian population is dominated by one species more than ever before. Even if climate change and overpopulation force us to change our eating habits, our impact will be preserved in the rocks, says Bennett. ‘The signal of our civilisation is already being recorded.’