- Part one: How to make a cage -
- Part two: A split in the ranks of scientists -
- Part three: in search of the first replicator -
- Part five: so how do you create a cell? -
- Part Six: The Great Unification -
In chapter two, we learned how scholars split into three schools of thought, reflecting on the origins of life. One group was convinced that life began with an RNA molecule, but was unable to show how RNA or similar molecules could spontaneously form on the early Earth and then make copies of themselves. Their efforts were encouraging at first, but ultimately only disappointment remained. However, other origin-of-life researchers who have followed different paths have come up with some results.
The RNA world theory is based on a simple idea: the most important thing a living organism can do is reproduce itself. Many biologists would agree with this. From bacteria to blue whales, all living things strive to have offspring.
However, many origin-of-life researchers do not consider reproduction to be fundamental. Before an organism can reproduce, they say, it must become self-sufficient. He must keep himself alive. After all, you cannot have children if you die first.
We keep ourselves alive by consuming food; green plants do this by extracting energy from sunlight. At first glance, the person eating a juicy steak is very different from a leafy oak tree, but when you look at it, they both need energy.
This process is called metabolism. First you need to get energy; let's say from energy-rich chemicals like sugar. Then you must use this energy to build something useful, like cells.
This process of using energy is so important that many researchers consider it the first, from which life began.
Volcanic water is hot and rich in minerals
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What would these metabolic-only organisms look like? One of the most interesting assumptions was made in the late 1980s by Gunther Wachtershauser. He was not a full-time scientist, but rather a patent lawyer with little knowledge of chemistry.
Wachtershauser suggested that the first organisms were "radically different from anything we knew." They weren't made from cells. They didn't have enzymes, DNA or RNA. No, instead, Wachtershauser imagined a stream of hot water flowing out of a volcano. This water is rich in volcanic gases like ammonia and contains traces of minerals from the heart of the volcano.
Where water flowed through the rocks, chemical reactions began to take place. In particular, metals from water helped simple organic compounds merge into larger ones. The turning point was the creation of the first metabolic cycle. It is a process in which one chemical is converted to a number of other chemicals until the original is eventually recreated. In the process, the entire system builds up energy that can be used to restart the cycle - and for other things.
Everything else that makes up a modern organism - DNA, cells, brains - appeared later, on top of these chemical cycles. These metabolic cycles bear little resemblance to life at all. Wachtershauser called his invention "precursors of organisms" and wrote that "they can hardly be called alive."
But metabolic cycles like those described by Wachtershauser are at the core of all life. Your cells are essentially microscopic chemical factories, constantly distilling one substance into another. Metabolic cycles cannot be called life, but they are fundamental to life.
During the 1980s and 1990s, Wachtershauser worked on the details of his theory. He outlined which minerals would be most suitable and which chemical cycles might take place. His ideas began to attract supporters.
But all this was purely theoretical. Wachtershauser needed a real discovery to support his ideas. Fortunately, it had already been done ten years earlier.
Sources in the Pacific
In 1977, a team led by Jack Corliss of Oregon State University plunged 2.5 kilometers into the East Pacific. They studied the Galapagos hot springs in places where high ridges rose from the seabed. These ridges were volcanically active.
Corliss discovered that these ridges were literally dotted with hot springs. Hot, chemical-rich water rises from under the seabed and flows through holes in the rocks.
Incredibly, these hydrothermal vents were densely populated with strange animals. There were huge clams, mussels, and annelids. The water was also heavily saturated with bacteria. All of these organisms lived on the energy of hydrothermal vents.
The discovery of these sources gave Corliss a name. And it made me think. In 1981, he suggested that such vents existed on Earth four billion years ago and that they became the place of origin of life. He has devoted the lion's share of his career to studying this issue.
Hydrothermal vents have a strange life
Corliss suggested that hydrothermal vents could create cocktails of chemicals. Each source, he said, was a kind of spray of primordial broth.
As the hot water flowed through the rocks, heat and pressure caused simple organic compounds to fuse into more complex ones, such as amino acids, nucleotides, and sugars. Closer to the border with the ocean, where the water was not so hot, they began to link in chains - to form carbohydrates, proteins and nucleotides like DNA. Then, when the water approached the ocean and cooled down even more, these molecules gathered into simple cells.
It was interesting, the theory caught people's attention. But Stanley Miller, whose experiment we discussed in the first part, did not believe it. In 1988, he wrote that the deep vents were too hot.
Although intense heat can produce chemicals like amino acids, Miller's experiments showed that it can also destroy them. Basic compounds like sugars "could survive for a couple of seconds, no more." Moreover, these simple molecules are unlikely to bind in chains, since the surrounding water would instantly break them apart.
At this stage, geologist Mike Russell joined the battle. He believed that the theory of hydrothermal vents could be quite correct. Moreover, it seemed to him that these sources would be the ideal home for the precursors of the Wachtershauser organism. This inspiration led him to create one of the most widely accepted theories of the origins of life.
Geologist Michael Russell
Russell's career had many interesting things - he made aspirin looking for valuable minerals - and in one remarkable incident in the 1960s, he coordinated the response to a possible volcanic eruption, despite lack of preparation. But he was more interested in how the Earth's surface changed over the eons. This geological perspective gave rise to his ideas about the origin of life.
In the 1980s, he found fossil evidence of a less turbulent type of hydrothermal vein, where temperatures did not exceed 150 degrees Celsius. These mild temperatures, he said, could allow the molecules of life to live longer than Miller thought.
Moreover, the fossil remains of these "cool" vents contained something strange: the mineral pyrite, composed of iron and sulfur, had formed in tubes 1 mm in diameter. While working in the laboratory, Russell discovered that pyrite could also form spherical droplets. And he suggested that the first complex organic molecules could have formed inside these simple pyrite structures.
Iron pyrite
It was around this time that Wachtershauser began to publish his ideas, which were based on the flow of hot, chemically enriched water flowing through minerals. He even suggested that pyrite was involved.
Russell added two plus two. He suggested that hydrothermal vents deep in the sea, cold enough to allow pyrite structures to form, harbored precursors of Wachtershauser organisms. If Russell was right, life began at the bottom of the sea - and metabolism first appeared.
Russell put it all together in a paper published in 1993, 40 years after Miller's classic experiment. It didn't generate the same media buzz, but it was arguably more important. Russell has combined two seemingly separate ideas - the Wachtershauser metabolic cycles and the Corliss hydrothermal vents - into something truly compelling.
Russell even offered an explanation for how the first organisms got their energy. That is, he understood how their metabolism could work. His idea was based on the work of one of the forgotten geniuses of modern science.
Peter Mitchell, Nobel laureate
In the 1960s, biochemist Peter Mitchell fell ill and was forced to retire from the University of Edinburgh. Instead, he set up a private laboratory on a remote estate in Cornwall. Isolated from the scientific community, he financed his work with a herd of dairy cows. Many biochemists, including Leslie Orgel, whose work on RNA we discussed in part two, considered Mitchell's ideas completely ridiculous.
A few decades later, Mitchell was waiting for an absolute victory: the 1978 Nobel Prize in Chemistry. He did not become famous, but his ideas are in every biology textbook today. Mitchell spent his career figuring out what organisms do with the energy they get from food. Basically, he wondered how we all manage to stay alive every second.
He knew that all cells store their energy in one molecule: adenosine triphosphate (ATP). A chain of three phosphates is attached to adenosine. Adding a third phosphate requires a lot of energy, which is then locked into ATP.
When a cell needs energy - for example, when a muscle contracts - it breaks down a third phosphate into ATP. This converts ATP to adenosidiphosphate (ADP) and releases stored energy. Mitchell wanted to know how a cell makes ATP in general. How does it store enough energy in ADP to attach the third phosphate?
Mitchell knew that the enzyme that makes ATP was in the membrane. Therefore, I assumed that the cell pumps charged particles (protons) through the membrane, so many protons are on one side, but not on the other.
The protons then try to leak back through the membrane to balance the number of protons on each side - but the only place they can pass through is the enzyme. The flow of flowing protons thus provided the enzyme with the energy needed to create ATP.
Mitchell first presented his idea in 1961. He spent the next 15 years defending her from all sides, until the evidence was irrefutable. We now know that the Mitchell process is used by every living thing on Earth. Right now, it is flowing in your cells. Like DNA, it underlies the life we know.
Russell borrowed from Mitchell the idea of the proton gradient: there are many protons on one side of the membrane and few on the other. All cells need a proton gradient to store energy.
Modern cells create gradients by pumping protons across membranes, but this requires a complex molecular mechanism that simply could not appear on its own. So Russell took another logical step: life had to form somewhere with a natural proton gradient.
For example, somewhere near hydrothermal vents. But it must be a special type of source. When the Earth was young, the seas were acidic, and there are many protons in acidic water. To create a proton gradient, the source water must be low in protons: it must be alkaline.
Corliss's sources did not fit. Not only were they too hot, they were also sour. But in 2000, Deborah Kelly of the University of Washington discovered the first alkaline sources.
Lost City
Kelly had to work hard to become a scientist. Her father died while she was finishing high school and she was forced to work to stay in college. But she coped and chose underwater volcanoes and burning hot hydrothermal springs as the subject of her interest. This couple brought her to the center of the Atlantic Ocean. At this point, the earth's crust cracked and a ridge of mountains rose from the seabed.
On this ridge, Kelly discovered a field of hydrothermal vents, which she called "The Lost City." They didn't look like those found by Corliss. The water flowed out of them at a temperature of 40-75 degrees Celsius and was slightly alkaline. The carbonate minerals from this water clumped together into steep white "plumes of smoke" that rose from the seabed like organ pipes. They look creepy and ghostly, but they are not: they are home to many microorganisms.
These alkaline vents fit perfectly with Russell's ideas. He firmly believed that life appeared in such “lost cities”. But there was one problem. As a geologist, he did not know much about biological cells to present his theory convincingly.
A column of smoke from the "black smoking room"
So Russell teamed up with biologist William Martin. In 2003, they presented an improved version of Russell's earlier ideas. And this is probably the best theory of the emergence of life at the moment.
Thanks to Kelly, they now knew that the rocks of the alkaline springs were porous: they were dotted with tiny holes filled with water. These tiny pockets, they suggested, acted as "cells." Each pocket contained basic chemicals, including pyrite. Combined with the natural proton gradient from the sources, they were the perfect place to start metabolism.
After life learned to harness the energy of spring waters, Russell and Martin say, it began to create molecules like RNA. In the end, she created a membrane for herself and became a real cell, escaping from the porous rock into open water.
Such a plot is currently considered as one of the leading hypotheses about the origin of life.
Cells flee from hydrothermal vents
In July 2016, he received support when Martin published a study reconstructing some of the details of the "last universal common ancestor" (LUCA). It is an organism that lived billions of years ago and from which all existing life originated.
It is unlikely that we will ever find direct fossilized evidence of the existence of this organism, but nevertheless we can quite make educated guesses about what it looked like and what it did while studying microorganisms of our day. This is what Martin did.
He examined the DNA of 1930 modern microorganisms and identified 355 genes that almost everyone had. This is convincing evidence of the transfer of these 355 genes, through generations and generations, from a common ancestor - around the time when the last universal common ancestor lived.
These 355 genes turn on some to use the proton gradient, but not to generate it, as Russell and Martin predicted. What's more, LUCA appears to have been adapted to the presence of chemicals like methane, suggesting that it inhabited a volcanically active, vent-like environment.
Proponents of the "RNA world" hypothesis point to two problems with this theory. One can be fixed; the other may be fatal.
Hydrothermal springs
The first problem is that there is no experimental evidence for the processes described by Russell and Martin. They have a step-by-step history, but none of these steps have been observed in the laboratory.
“People who believe that it all began with reproduction are constantly finding new experimental data,” says Armen Mulkidzhanyan. "People who stand for metabolism don't."
But that could change, thanks to Martin's colleague Nick Lane of University College London. He built a "Origin of Life Reactor" that simulates the conditions inside an alkaline source. He hopes to see metabolic cycles, and maybe even molecules like RNA. But it’s too early.
The second problem is the location of sources in the deep sea. As Miller noted in 1988, long-chain molecules like RNA and proteins cannot form in water without auxiliary enzymes.
For many scientists, this is a fatal argument. “If you are good at chemistry, you will not be bribed with the idea of deep sea springs, because you know that the chemistry of all these molecules is incompatible with water,” says Mulkidzhanian.
Yet Russell and his allies remain optimistic.
It was only in the last decade that a third approach came to the fore, supported by a series of unusual experiments. It promises something that neither the RNA world nor hydrothermal vents have been able to achieve: a way to create an entire cell from scratch. More on this in the next part.
ILYA KHEL
- Part one: How to make a cage -
- Part two: A split in the ranks of scientists -
- Part three: in search of the first replicator -
- Part five: so how do you create a cell? -
- Part Six: The Great Unification -