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It's long been known that DNA from so-called retroviruses make up around five per cent of our genetic makeup.

But for years, this was dubbed junk DNA with no real use, and was considered to be a side-effect of evolution - until now.

New research suggests that, over the course of evolution, the viruses took an 'increasingly firm hold' on how cells work, and they may have made brain cells in particular more active and dynamic, ultimately making us smarter.

DNA from retroviruses (HIV pictured) make up around five per cent of our genetic makeup, but for years, this was considered to be a side-effect of evolution. New research suggests that, over the course of evolution, the viruses took hold of how cells work, and may have made brain cells in particular more active and dynamic

In particular, the study from Lund University in Sweden claims that inherited viruses, which are millions of years old, play an important role in building up the complex networks that characterise our brains.

Endogenous retroviruses share three genes in common, known as 'gag', 'env' and 'pol'.

Gag creates a so-called 'inner shell' where the virus' genes are kept, env helps the virus attach and invade cells, and pol creates an enzyme that puts the viruses' genes into the person's DNA.

HIV, for example, belongs to a class of viruses called retroviruses.

Scientists have discovered more than 100,000 pieces of retrovirus DNA in the human genome.

It is thought that our ancestors became infected with retroviruses, and occasionally the viruses were able to infect an embryo.

The retrovirus DNA was implanted in the cells of the embryo, and was passed down through the generations.

Over time the virus was unable to create new viruses and infect the host, and mutations stopped the gag genes working properly. 

But the dying virus could still copy its genes, and became what's known as ancestral viruses.

For the current study, Johan Jakobsson and his colleagues discovered retroviruses seem to play a central role in the basic functions of the brain. 

Specifically, by regulating which genes are expressed, and when.

They looked at virus expression - or how the viruses behaved and spread - using molecular biology techniques. 

They noticed that the viruses in the brain - and tumours in particular - can't form in nerve cells in the same way they can in other tissues.

Mr Jakobsson told MailOnline that tumors never start in nerve cells.

'In this context, nerve cells may therefore be 'alllowed' to use different molecular mechanisms than other cells,' he explained.

This suggests they play a different role, and react differently in the brain, which means viruses similarly express themselves differently in the brain, too. 

'We have been able to observe that these viruses are activated specifically in the brain cells and have an important regulatory role,' said Johan Jakobsson, head of the research team for molecular neurogenetics at Lund University.

'We believe that the role of retroviruses can contribute to explaining why brain cells in particular are so dynamic and multifaceted in their function.

'It may also be the case that the viruses' more or less complex functions in various species can help us to understand why we are so different'

The article, based on studies of neural stem cells, shows that these cells use a particular molecular mechanism to control the activation processes of the retroviruses.

Mr Jakobsson continued that the findings 'provide us with a complex insight into the innermost workings of the most minimal functions of the nerve cells.'

Researchers found retroviruses in DNA (illustrated) play a central role in the basic functions of the brain. 'We have observes that these viruses are activated specifically in the brain cells and have an important regulatory role. [This] can contribute to explaining why brain cells in particular are so multifaceted,' said the experts

At the same time, the results open up potential for new research paths concerning brain diseases linked to genetic factors.

'I believe that this can lead to new, exciting studies on the diseases of the brain,' continued Mr Jakobsson

'Currently, when we look for genetic factors linked to various diseases, we usually look for the genes we are familiar with, which make up a mere two per cent of the genome.

'Now we are opening up the possibility of looking at a much larger part of the genetic material which was previously considered unimportant.

'The image of the brain becomes more complex, but the area in which to search for errors linked to diseases with a genetic component, such as neurodegenerative diseases, psychiatric illness and brain tumours, also increases'.