While life on Earth is constantly evolving, the genetic code which drives it all remains frozen, using the same components in the same way as it did billions of years ago.

At some point in the planet’s history, the evolution of our genetic machinery reached a plateau which put the brakes on any further development of the code.

Now scientists believe the stalled evolution may be down to limitations in the way DNA is translated to make proteins.

A team of geneticists in Spain focused on a molecule called transfer RNA (tRNA).

This shuttles the building blocks of proteins to the assembly line so they can be linked together in the correct order.

When the intricate system of the genetic code was described by Francis Crick in the 1960s, the Nobel Prize winning scientist called it a ‘frozen accident’.

This is because the genetic machinery evolved to account for 20 amino acids – the building blocks of proteins – reaching a point where it was unable to expand to include more, essentially remaining frozen in time for more than three billion years.

While the human body can jumble and link these available amino acids together to make proteins, it uses additional amino acids which aren’t involved in genetic machinery, relying instead on complicated chemical pathways.

In theory, the genetic code could have expanded to use 63 amino acids, but hit its ‘genetic road block’ billions of years ago.

But if the machinery that reads DNA and translates it to proteins had to include any more than 20, it is highly likely errors would constantly creep into production process.

This would lead to faulty proteins and an eventual meltdown of the biological system.

‘Protein synthesis based on the genetic code is the decisive feature of biological systems and it is crucial to ensure faithful translation of information,’ explained Professor Lluís Ribas de Pouplana, a geneticist from the Institute for Research in Biomedicine (IRB Barcelona) and senior author of the study.

Each tRNA has two key regions, linking to a specific amino acid at one end,

The other recognises a three-letter stamp of genetic code – but the multiple genetic stamps can code for the same amino acid. The combination of these regions give each tRNA an identity.

According to the team, the reason the genetic code wasn’t able to expand beyond the 20 is because it wasn’t possible to create any new tRNAs without the system getting confused.

    

Professor Ribas told MailOnline: ‘Our work shows that the central pieces of the genetic code, the transfer RNAs, can not house enough specific identity elements for the system to be able to distinguish 63 of them.

‘Since you need a new tRNA for each new amino acid, once the limit of tRNAs is reached that determines how many amino acids you can use. This limit happened to be at 20, and it hasn’t changed for 3 billion years.’

But the team is looking to the emerging field of synthetic biology to overcome the limitations of mother nature and extend the code further.

Professor Ribas explained: ‘It is thus unlikely that the limit will change naturally, and you could certainly call it a bottle neck for molecular diversity.

‘Artificially, however, we are able to increase the number of amino acids used by cells under controlled laboratory conditions. I think that our work adds to the notion that adding new amino acids in a natural context would require very dramatic engineering of the system.

‘Something that nature can’t do. How to go about this engineering is the main question that opens after our study.’

The findings are published in the journal Science Advances.