During the second half of the 20th century researchers of the origin of life worked every in your camp. Each group insisted on its own version of events and tried to destroy competing hypotheses. This approach was certainly successful, as evidenced by the previous chapters, but every promising idea on the origin of life ultimately ran into a serious problem. So some researchers are now trying to find a more unified approach.
Part one: how to make a cage?
Part two: the split in the ranks of scientists
Part three: the search for the first Replicator
Part four: the energy of the protons
Part five: so how is it to create the jail?
A few years ago, this idea received a powerful impetus by the result of supporting the established theory of the “RNA world”.
By 2009, supporters of the RNA world was a big problem. They couldn’t make nucleotides, the building blocks of RNA as if it occurred in conditions of the early Earth. This led people to believe that the first life was not built on the RNA, as we explained in the third part.
Earth is the only place where there is life. Yet
John Sutherland was thinking about this issue since the 1980s. “I thought to demonstrate that RNA can samoubiytsy, it would be very cool,” he says.
Luckily for Sutherland, he got a job in the Laboratory of molecular biology (LMB) in Cambridge. Most research institutions encourage their employees to constantly generate new work, but LMB no. So Sutherland could think about it, why do the nucleotides of RNA is so difficult, and has spent years developing an alternative approach.
His decision led him to an entirely new idea about the origin of life: all the key components of life could be formed at the same time.
“In the chemistry of RNA were certain aspects that did not work,” says Sutherland. Each nucleotide of RNA consists of a sugar, base and phosphate. But to get sugar and the base connect. Molecule just is not the same shape.
So Sutherland started to try a completely different matter. Ultimately his team came to five simple molecules, including other sugar and cyanamide, are related to the cyanide. These chemicals pass through chain reactions and ultimately made two of the four nucleotides of RNA, making the individual sugars or bases.
It was a dazzling success that made the Sutherland name.
Many observers interpreted these results as further evidence in favor of the RNA world. But Sutherland himself did not think so.
The “classical” RNA world hypothesis asserts that the first organisms RNA was responsible for all the functions of life. But Sutherland says that’s hopelessly optimistic. He believes that RNA has been an important part, but all the wedge has not converged.
Instead, he was inspired by one of the last works of Shostak, who (as we found out in the fifth part) combined RNA-world “first reproduction” ideas “first of compartmentalization” pier Luigi Luisi.
Sutherland went even further. His approach was a “first”. He wanted a one-piece cage gathered itself from scratch. This brought him a strange detail in his synthesis of nucleotides, which at first seemed random.
You need a bold mixture of substances
The last step in the process, Sutherland was to throw the phosphate in the nucleotide. However, he found that it was best to turn the phosphate into the mix from the beginning, since it had accelerated the first reaction. It seemed that the inclusion of the phosphate before you will need it in fact was a little “dirty” effect, but Sutherland found out that this chaos is good.
And so he thought about how disorderly must be the mixture. In the early days of the Earth there had to be dozens or hundreds of chemicals floating along. The recipe for slime? Possible. But the disorder can be important.
The mixture that Stanley Miller made in the 1950-ies, which we discussed in the first part, was much dirtier saterlandic. They included biological molecules, but Sutherland says that they “were in small amounts and was accompanied by a huge number of other, non-biological compounds.”
Sutherland believed that the approach of Miller was not good enough. He was too dirty, so a good chemical substances are simply lost in the mix.
Therefore, Sutherland set out to find the “chemistry Goldilocks”: not too dirty to be useless but not too simple to be limited in scope. To a rather complicated mixture — all components of life can be formed at the same time and find each other.
In other words, four billion years ago the Earth was a pond. It existed years before it gathered the right chemicals. Then perhaps for some few minutes there was a first cell.
A handful of chemical substances, insufficient for life
This may seem completely implausible, like the statements of the medieval alchemists. But Sutherland only added evidence. In 2009, he showed that the same chemistry that allowed us to collect two nucleotide RNA, can also create many other molecules of life.
The obvious next step was to do more nucleotides of the RNA. While this failed, but in 2010 he collected a closely related molecule, which could potentially turn into nucleotides. Similarly, in 2013, he made the precursors of amino acids. This time he had to add the cyanide of copper to make the reaction proceed.
Associated with the cyanide chemicals turned out to be a common theme, and in 2015, Sutherland did with them even more. He showed that in the same pot with chemicals can occur and the precursors of lipids, molecules that comprise the cell wall. All of these reactions relied on ultraviolet light, included sulfur and copper as a catalyst.
Life has a veritable cornucopia of chemical substances
“All the building blocks came from a common core in chemical reactions,” says Shostak.
If Sutherland is right, then our whole approach to the origin of life over the past 40 years was wrong. Since then, as it became obvious complexity of the cell, scientists began to work with the assumption that the first cells had to be collected gradually, piece by piece.
Following a proposal Leslie Orgel that it was RNA came first, scientists tried “to put one before the other, and then to order,” says Sutherland. But he thinks it is best to do everything at once.
“We are, in fact, questioned the idea that to do all at once is too difficult,” he says. “You can definitely make the building blocks of all systems at once.”
Shostak now suspects that most attempts to make the molecules of life and collect them in living cells have failed for one reason: the experiments were too clean.
The researchers used a few chemicals that were of interest to them, and left all the rest, that is also probably present on the early Earth. But the work of Sutherland showed that adding more chemical to the mix of things, you can create more complex phenomena.
Shostak and he ran with it in 2005, when he tried to place the enzyme RNA in their protocells. The enzyme needed magnesium, which destroyed the membrane of protocells. The solution was surprisingly simple. Instead of making vesicles from only one fatty acid, they are made from a mixture of both substances. New, “dirty” vesicles coped with magnesium and could post a working enzyme RNA.
Moreover, Shostak says, the first genes could also include the mess.
DNA consists of small molecules, nucleotides
Modern organisms use naked DNA for gene transfer, but pure DNA, probably did not exist at first. What was needed was a mixture of nucleotides, RNA and DNA nucleotides.
In 2012, Szostak showed that such a mixture may be collected in a “mosaic” of molecules that looks and behaves almost like a pure RNA. These mixed strands of RNA/DNA can even be carefully folded.
Turns out, it doesn’t matter, could the first organisms to have pure RNA or pure DNA. “I even went back to the idea that the first polymer was very similar to RNA, a more dirty version of RNA,” says Shostak. Alternatives RNA could be even more, like TNCs and the NCP, which we have discussed in the third part. We don’t know they existed on Earth or not, but if so, the first organisms could use them as well.
It was no longer “the world of RNA”, and “mixed up world”.
The lesson of these studies is that the first cell can be was not as difficult as it seems. Yes, cells are complex machines. But it turns out that they continue to work, though not so good if they blind casually as a snowball.
It seems such a clumsy cage had no chance to survive on the early Earth. But they have virtually no competition, they were not threatened by any predators, so in many ways life was easier than it is now.
In his youth the Earth is constantly bombarded by meteorites
However, there is one problem that could not solve the Sutherland or Shostak, and this is a serious problem. The first organism had to have some kind of metabolism, metabolism. From the very beginning, life had to get energy or die.
At the same time Sutherland agrees with Mike Russell, bill Martin and other proponents of theories of “metabolism first” of the fourth part. “While RNA-guys butting metabolism guys, both sides had good arguments,” says Sutherland.
“Metabolism had somewhere to flow — echoes Shostak. — Source of chemical energy is a huge issue.”
Even if Martin and Russell are wrong on the fact that life began at deep springs, many elements of their theory are almost certainly correct. One of them is the value of the metals for the birth of life.
This enzyme is in the center of the metal
In nature, many enzymes have a metal atom at the core. This is often the “active” part of the enzyme; the rest of the molecule acts as a support structure. First life could not have such complex enzymes, so almost certainly used the “bare” metals as catalysts.
Gunter, Wachtershauser noticed this when he suggested that life was formed on the basis of iron pyrite. Similarly, Russell was stressed that water hydrothermal vents are rich in metals, which can act as catalysts and Martin study revealed a set of enzymes based on iron from the last universal common ancestor (LUCA).
In light of this, it makes sense that a chemical reaction Sutherland’s rely on copper (and — as stressed Wachtershauser — sulphur), and RNA protocells Shostak needs magnesium.
It may be true that hydrothermal vents would suddenly the most important elements of the puzzle. “If you look at modern metabolism, there are such eloquent things as gelaterie clusters,” says Shostak. This speaks in favour of the origin of life at the vents, where the water is rich in iron and sulfur.
But if Sutherland and Shostak are indeed on the right path, one aspect of the hydrothermal theory completely does not make sense: life could not appear in the deep sea.
Life could appear in shallow water
“Chemistry, to which we have come, highly dependent on UV light,” says Sutherland. The only source of ultraviolet radiation is the Sun, so his reactions can proceed only in sun illuminated areas. This eliminates the deep scenario.
Shostak agree that deep water was hardly the cradle of life. In addition, they are isolated from atmospheric chemistry, which is a source of high-energy starting materials such as cyanide.
But these problems do not rule out hydrothermal theory completely. Perhaps these sources were in shallow water, basking in the sunlight, and cyanide.
From Armen Mulkidjanian there is an alternative. Perhaps life appeared on earth, in volcanic pond.
Or volcanic pond
Mulkidzhanian, drew attention to the chemical composition of cells: in particular, what chemicals they let in and what not. It turned out that cells, regardless of organism carrier, contain a lot of phosphate, potassium and other metals — but not sodium.
Currently, cells get them, pumping the materials in themselves, but the first cell couldn’t do, because they do not possess the necessary machinery. Therefore, Mulkidzhanian suggested that the first cells formed somewhere that was roughly the same composition of chemicals, that of modern cells.
The ocean immediately disappears. In the cells a lot more potassium and phosphate than the ocean, and a lot less sodium. But come to mind are geothermal ponds near active volcanoes. These ponds have exactly the cocktail of metals found in cells.
The Shostak like this idea. “I think my favorite scenario at the moment would include a shallow lake or pond on the surface in a geothermal active area,’ he says. — Then we will have hydrothermal vents, but not those that in the depths of the ocean, and some-like sources in volcanically active areas and the type of Yellowstone”.
Chemistry Sutherland could work in such a place. These sources any chemical composition, the water level fluctuates, sometimes all dry, and ultraviolet radiation from the sun is enough.
Or in the hot springs
Moreover, Shostak says that such ponds would be his protocells.
“Protocells would have been relatively cool most of the time, which is good for copying RNA, and other types of simple metabolism,” says Shostak. “But they would need periodic heating,which would help the RNA chains to fall away for the next round of reproduction”.
Streams of cold or hot water would help the protocells to share.
Based on many of these arguments, Sutherland offers a third option: the place of the meteorite fall.
Meteorites fall to Earth constantly for her first half-billion years of existence — and since then, too, sometimes fall. A good strike would create conditions similar to ponds Mulkidjanian.
First, meteorites are mostly made of metal. Exposure usually rich in useful metals like iron, and sulfur. And most importantly, the meteorite impact melted the earth’s crust, leading to geothermal activity and hot water.
Sutherland represents small streams and rivers flowing down the slopes of an impact crater, the leaching of the chemicals on the basis of cyanide from rocks, while ultraviolet radiation is shed from above. Each thread brings a particular mixture of chemical substances that develop different reactions and produces a variety of organic chemicals.
Or in the crater of the meteorite
In the end, the streams flow in a volcanic pond on the crater floor. In this pond, perhaps all the pieces would have been together and formed the first protocells.
“It’s quite a specific scenario,” says Sutherland. But he chose it on the basis of chemical reactions encountered. “While this scenario is only compatible in terms of chemistry.”
Shostak is not so sure, but agree that the idea of Sutherland deserves attention. “I think the scenario with a great impact. I think the idea of the volcanic system could work. Both theories have good arguments.”
While that debate will continue. But the decision will depend on the chemistry of protocells. If it turns out that one of the scenarios is lacking important chemicals or something destroys the protocell, it would have to be abandoned.
But for the first time in history, we can gain a comprehensive explanation of how life began.
So far, the approach “all at once” Shostak and Sutherland provides only a sketchy narrative. But these steps were developed on the basis of decades of experiments. Also, this approach relies on all the other hypotheses of the origin of life. He tries to use all their good side, however, solving all their problems. For example, it does not destroy the hypothesis of Russell hydrothermal sources, but rather includes its best elements.
Of course, we can’t know for sure what happened four billion years ago. “Even if you built the reactor and it left the Escherichia coli… it doesn’t prove that it was,” said Martin.
The best we can do is to make a story that is consistent with all evidence: with experiments in chemistry, our knowledge about the early Earth, with what biology tells about the most ancient forms of life. Finally, after a century of strenuous efforts, the story begins to emerge.
And that means that we are approaching one of the most important turning points in human history: after which we learn the story of the emergence of life on Earth. All the people who died before Darwin published “Origin of species” in 1859, had no idea where it came from, because he knew nothing about evolution. But anyone living today can know the truth about our relationship with animals.
Similarly, anyone born after the release of Yuri Gagarin into Earth orbit in 1961, lived in a society that can go to other worlds. Space travel became a reality, even if we ourselves were not involved.
These facts change our view of the world. They make us wiser. Evolution teaches us to cherish every living thing, because we all descended from one ancestor. Space travel allows us to look at other worlds from afar, to see all their uniqueness and fragility.
Some of the people living today will be the first in history who will be able to say with confidence that we know exactly where they came from. They will know in what was our first ancestor and where he lived.
This knowledge will change us. On a purely scientific level, it tells us how likely the emergence of life in the Universe and where to look. And it tells us about the nature of life. But can we know what knowledge will be revealed to us once we learn the secret of life? Unlikely.
On materials BBC
The mystery of the emergence of life on Earth. Part six: the Grand unification
Ilya Hel