The ‘primordial soup’ on Earth a billion years ago, which led to first life on our planet, might have been teeming with primal precursors of proteins.
According to a new study, ancestors of the first protein molecules, which are key components of all cells, could have been plentiful on pre-life Earth.
Researchers formed hundreds of possible precursor molecules in the lab, finding that the molecules formed quickly and abundantly under conditions that would have been common on pre-life Earth.
New NASA-affiliated research conducted by the Georgia Institute of Technology suggests that the first molecules of life may have arisen in variations of daily processes still observed on Earth today – for example the repeated drying and refilling of pond water
The new NASA-affiliated research suggests that the first molecules of life may have arisen in variations of daily processes still observed on Earth today – for example the repeated drying and refilling of pond water.
The study, led by researchers at the Georgia Institute of Technology, involved forming hundreds of possible precursor molecules in the lab, and analyzing the molecules with new technology and algorithms.
The found that molecules called depsipeptides formed quickly and abundantly under conditions that would have been common on pre-life Earth, with ingredients that would have likely been abundant too.
Depsipeptides look a lot like regular peptides and can be found today in biological systems.
‘Many antibiotics, for example, are depsipeptides,’ said Dr Facundo Fernández, a professor in Georgia Tech’s School of Chemistry and Biochemistry, and one of the study’s authors.
Some of the depsipeptides evolved into new varieties in just a few days, an ability that – very long ago in Earth’s history – could have accelerated the birth of long molecules, called peptides, that make up proteins.
According to the researchers, the study adds to the growing body of evidence suggesting that the first polymers (molecules with a large number of similar units bonded together) of life may have formed in variations of daily processes still observed on Earth today, such as repeated drying and refilling of pond water.
These polymers may not have all ‘zapped’ into existence due to blazing cataclysms, which is an image often associated with the creation of the first chemicals of life.
‘We want to stay away from scenarios that are not readily possible,’ said Dr Fernández.’
‘Don’t deviate from conditions that would have been realistic and reasonably common on prebiotic Earth.
‘Don’t invoke any unreasonable chemistry.’
Researchers have for a long time been puzzled over how the first proteins formed, as the long-chain molecules that they consist of – called polypeptides – can be difficult to make in the lab.
Some researcher have tried to make tiny chains, or peptides, under more extreme scenarios that likely occurred less often on early Earth.
But the yields from these experiments have been modest, and the resulting peptides have had only a couple of component parts – whereas natural proteins have a large variety of them.
Martha Grover, a professor in Georgia Tech’s School of Chemical and Biomolecular Engineering, and Facundo Fernández, a professor in the School of Chemistry and Biochemistry, in Fernández’s lab
In addition, the complex molecules of life likely didn’t arise in one dramatic step – which is the idea that’s driving the research of Dr Fernandez and his colleagues at the NSF/NASA Center for Chemical Evolution headquartered at Georgia Tech, in collaboration with the Scripps Research Institute.
Instead, the researchers propose that multiple, easier chemical steps produced an abundance of in-between products that were useful in subsequent reactions that eventually led to the first biopolymers.
And, the depsipeptides in this study could have been been some of these in-between products.
The new study joins similar work about the formation of RNA precursors on pre-life Earth, and about possible scenarios for the formation of the first genes.
The collective insights may someday help explain how first life arose on Earth, and also help astrobiologists determine the probability of life existing on other planets.
To understand depsipeptides and their significance in the context of the researchers’ findings, it’s helpful to first understand what peptides are.
Peptides are chains of amino acids – and when these chains become very long, they are called polypeptides, and as they grow even longer and more complex, they’re called proteins.
Living cells have machinery that read codes in DNA on how to link up amino acids in a specific order to build very specific peptides and proteins that have functions in a living cell.
Researchers propose that multiple hemical steps produced an abundance of in-between products that were useful in subsequent reactions that eventually led to the first biopolymers
But making a protein with the correct sequence of amino acids isn’t the end of the job – for a protein to have a function in a cell, its polypeptide chains have to clump up in a specific way to form useful shapes.
Other bonds aside from peptides, called ester bonds, form more easily and can link up amino acids with very similar molecules called hydroxy acids.
Hydroxy acids are very similar to amino acids, and can in some cases function as their stand-in doubles.
For the study, the researchers mixed three amino acids with three hydroxy acids in a water solution and they formed depsipeptides – chains of amino acids and hydroxy acids held together by intermittent ester and peptide bonds.
The hydroxy acids acted as an enabler to put the chains together, that would have otherwise been difficult to form.
This primordial soup of depsipeptides may have ended up on rocks, where they dried out in the sun, and then rain or dew dissolved them back into the soup – with the cycle repeating itself.
The researchers recreated this cycle in the lab, watching as the depsipeptide chains further developed.
‘We call it an environmental cycling approach to making these early peptides,’ Dr said Fernández
Like nature, the researchers made the soup, dried it out, and repeated.
Nicholas Hud (pictured) researches the possible origins of life chemicals on early Earth, when many of them may have formed in puddles. He has produced good candidates for precursors of RNA in easy reactions and in plentiful quantities using barbituric acid and melamine
In the lab, the researchers set the drying temperature to 85 degrees Celsius (185 degrees Fahrenheit), although the reaction has been shown to work at temperatures of 55 and 65 degrees Celsius (131 to 149 degrees Fahrenheit).
‘If you think about early Earth having a lot of volcanic activity and an atmospheric mix that promoted warming, those temperatures are realistic on many parts of an early Earth,’ Dr Fernández said.
According to the researchers, early Earth took hundreds of millions of years to cool, and temperatures in the hundreds of degrees are thought to have been commonplace for a long time – even today, the hottest deserts can reach over 55 degrees Celsius.
Since ester bonds break more easily, in the experiment, the chains tended to come apart more at the hydroxy acids, while holding together between the amino acids, which were connected by the stronger peptide bonds.
As a result, chains could re-form and link up more and more amino acids with each other into sturdier peptides.
In what can be compared to a square dance, the hydroxy acids often left their amino acid partners in the chain, and new amino acids latches onto the chain in their place, where they held on tightly.
Some of the depsipeptides ended up being composed almost completely of amino acids – and had only remnants if hydroxy acids.
‘Now we know how peptides can form easily,’ Dr Fernández said.
A primordial soup of depsipeptides may have ended up on rocks, where they dried out in the sun, and then rain or dew dissolved them back into the soup – with the cycle repeating itself.
‘Next, we want to find out what’s needed to get to the level of a functional protein.’
In order to identify the more than 650 depsipeptides that formed, the researchers used mass spectrometry, combined with ion mobility – a type of ‘wind tunnel’ for molecules.
Along with the mass spectrometry, which measures the characteristics of individual molecules, the ion mobility measurement gave the researchers data on the shape of the depsipeptides.
Algorithms created by Georgia Tech researcher Dr Anton Petrov processed the data to finally identify the molecules.
To illustrate how potentially plentiful depsipeptides could have been on pre-biotic Earth – the researchers had to limit the number of amino acids and hydroxy acids to three each.
If they’d taken 10 each instead, the number of theoretical depsipeptides could have reached over 10,000,000,000,000.
‘Ease and bounty are key,’ Dr Fernández said.
‘Chemical evolution is more likely to progress when components it needs are plentiful and can join together under more ordinary conditions.’
In the lab, the researchers set the drying temperature to 85 degrees Celsius (185 degrees Fahrenheit). If you think about early Earth having a lot of volcanic activity and an atmospheric mix that promoted warming, those temperatures are realistic on many parts of an early Earth