HARDSHIP, not helpfulness, caused the human brain to grow to its large size, research suggests.

Biologists have long struggled to unpick the secrets of what makes us who we are today.

Much of this has centred around our grey matter, a part of the body whose importance went unrecognised for ages.

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Greek philosopher Aristotle considered it a secondary organ that served the heart and, though they carefully preserved other organs, Ancient Egyptians discarded the grey matter when preparing bodies for mummification.

The present day human brain, which remains a key area of study, is three times larger than that of our ape-like Australopithecine ancestors, which lived in Africa between two and four million years ago.

Over the course of eons, it has become six times bigger than what would be expected for the average human-sized mammal.

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Until now, many scientists have posited that the organ developed thanks to co-operation and our complex social relationships, something known as the "social brain hypothesis".

But a paper published today by biologists at St Andrews University says working together by our ancestors actually made the brain smaller.

This is because the the mutual aid allowed individuals to rely on others, saving the body from investing resources in it.

They also dismiss suggestions that the change was driven by alterations in the diet which saw our distant relatives drew nutrients from meat at the expense of the gut.

Instead, they claim the grey matter got greater as a result of dealing with "ecological problem-solving".

Learning to stay alive in tough and dangerous surroundings, authors say, is responsible for 60% of brain evolution, with just 30% from co-operation and only 10% from inter-human conflict.

Dr Mauricio González-Forero and Dr Andy Gardner say their "intriguing" claim is based on a new model they have drawn up to explain our evolution.

It mechanistically models the energy costs of brain growth and maintenance and the organ’s ability to enable its bearer to solve environmental and social problems.

Using this method, the study found that human-sized brains and bodies can evolve in "tough environments" – as long as individuals continue improving their skills through their youth.

And sustained improvement of the individuals’ abilities as they age may be aided by "cultural processes" like learning things that previous generations have figured out, rather than figuring them out for themselves.

The results are published in the specialist journal Nature.

González-Forero, of the Fife university's School of Biology, said: “The findings are intriguing because they suggest that some aspects of social complexity are more likely to be consequences rather than causes of our large brain size, and that the large human brain is more likely to stem from ecological problem-solving and cumulative culture than it is from social manoeuvring.”