Scientists blame agriculture, with restricted diets and urbanisation compromising health and leading to the spread of disease
And if that’s not depressing enough, our brains are also smaller.
The findings reverse perceived wisdom that humans have grown taller and larger, a belief which has grown from data on more recent physical development.
The earliest, from Ethiopia, date back 200,000 years, and were larger and ‘more robust’ than their modern-day counterparts, said Dr Marta Lahr, an expert in human evolution.
Fossils found in Israeli caves and dating from 120,000 to 100,000 years ago, reveal a people who were tall and muscular, a pattern that continued uninterrupted until relatively recent times.
An average person 10,000 years ago weighed between 12st 8lb and 13st 6lb. Today, the average is between 11st and 12st 8lb. Dr Lahr, who last week presented her findings to the Royal Society, Britain’s most prestigious scientific body, described the changes as ‘striking’.
Bigger brain: Six different views of a 160,000-year-old human skull of an adult male from Ethiopia
‘We can see that humans have continually evolved but in body size it is not until the last 10,000 years that they have changed substantially, so the question is why this should have happened.’
The timing points to the switch from a hunter-gatherer lifestyle to agriculture, which began 9,000 years ago. While farming would have made food plentiful, focussing on a smaller number of foodstuffs could have caused vitamin and mineral deficiencies that stunted growth.
In China, early farmers relied on cereals such as buckwheat, rice and maize, all of which lack niacin, a B vitamin vital for growth.
However, the rise of agriculture does not explain why brains are also shrinking.
The male brain of 20,000 years ago measured 1,500 cubic centimetres. Modern man’s brain averages just 1,350 cubic cm – a decrease equivalent to the size of a tennis ball. The female brain has shrunk by about the same proportion.
It doesn’t mean we are less intelligent – rather we have learnt to make the best use of our resources.
Dr Lahr said: ‘Over evolutionary time there would have been huge energy savings in making the brain smaller but more efficient – as we see today with computer processors.’
Robert Foley, a Cambridge University professor of human evolution, said: ‘Becoming human, in an evolutionary sense, is a continuous and gradual process. Our species, rather than being a fixed entity, is more like a piece of putty, changing shape and dimensions all the time.’