Young people are the ‘engine of society’ (Picture: Shutterstock/gob_cu)

‘Overpopulation’ is a scary word,bringing to mind a dystopian Earth where people live in grimy cities and fight over what little food is left.

But the reverse of this is just as terrifying – depopulation – if humans suddenly began having fewer and fewer babies.

And if that were to happen,it wouldn’t take long for humanity to go completely extinct.

While some people live beyond the age of 100,the human race would only be around for a few decades at a push,said Birmingham University anthropologist Professor Michael Little.

Writing in The Conversation,Professor Little said this is because society needs young people to care for the elderly and drive economic growth.

He wrote: ‘Eventually,civilisation would crumble. It’s likely that there would not be many people left within 70 or 80 years,rather than 100,due to shortages of food,clean water,prescription drugs and everything else that you can easily buy today and need to survive.’

The Handmaid’s Tale is set in a world where an increasing number of people are suffering from infertility (Picture: Jasper Savage/MGM/Hulu/Kobal/REX/Shutterstock)

After the final human is born,a countdown would begin as everyone simply grows older until everyone dies from old age.

Professor Little said: ‘Eventually,there would not be enough young people coming of age to do essential work,causing societies throughout the world to quickly fall apart.

‘Some of these breakdowns would be in humanity’s ability to produce food,provide health care and do everything else we all rely on.

‘Food would become scarce even though there would be fewer people to feed.’

There are many reasons why people could stop having children,Professor Little said,such as a disease making people infertile or a nuclear war.

While a few viruses,like HIV,the Zika virus and a few STIs such as HPV,can lead to infertility,they very rarely do so or only have very mild effects.

Without young people,society would grind to a halt,Professor Little said (Picture: Getty Images)

So a virus wiping out the world’s ability to have children is,for now,just science fiction,though male fertility rates are a worry among scientists.

But a rapidly ageing population and declining birth rate are very much real.

Earth is home to 8,200,000,000 human beings,with the global population increasing since the end of the Black Death around 1350.

And the number of humans will keep rising until about 2080,when the UN expects the size of humanity to peak at 10.3billion,before it drops slightly.

One reason for this inevitable slowdown is that people are already having fewer babies in some parts of the world,such as Japan and South Korea.

These countries are now facing a new issue,an ageing population,as they’re under the 2.1 children per woman rate with their population stable. In China,for example,the fertility rate is just 1.18.

Many nations,like Japan,are seeing their populations get greyer by the day as people have fewer children (Picture: Getty Images)

This is also happening in the UK,where the fertility rate fell to just 1.44 children per woman last year,down from 2.47 in 1946.

Ageing population is a problem,because young people are the ‘engines of society’ who keep new ideas flowing and work jobs that elderly people would struggle to do.

He likened it to how humans,otherwise called Homo sapiens,became the dominant species on Earth over the Neanderthal.

Neanderthals were humans like us,but were a distinct species that were around for about 350,000 years.

Dr Little said: ‘Some scientists have found evidence that modern humans were more successful at reproducing our numbers than the Neanderthal people.

‘This occurred when Homo sapiens became more successful at providing food for their families and also having more babies than the Neanderthals.’

Neanderthals are,in the world of human species and offshoots,like a weird,distant cousin you don’t want coming to the family reunion (Picture: Getty Images)

There would be,however,some perks to humanity going extinct,as some campaign groups have long dreamt of.

Amid climate change,caused by humans pumping out planet-warming gases,wildlife populations have fallen by 70%.

Professor Little said: ‘If humans were to go extinct,it could open up opportunities for other animals to flourish on Earth.

‘On the other hand,it would be sad for humans to go away because we would lose all of the great achievements people have made,including in the arts and science.’