Which came first? That is a basic question about the origins of life. Forget the chicken and the egg, though. Scientists are more interested in how and when the basic parts of a living cell first appeared. Ever.
Now chemists may have the answer. A set of chemical reactions on the surface of ancient Earth could have made the building blocks, researchers say. And they could have done it all at once.
The researchers described this online March 16 in Nature Chemistry.
At a minimum, a living cell must be able to do three things: encode information, produce proteins and form compartments. That information contains the instructions for what a cell is to do. Proteins carry out those tasks. And the membranes that form a cell’s compartments hold it all together. For a long time, scientists believed these three jobs were so different that each must have gotten a separate start.
But that’s not necessarily true, the new study finds. It provides the first experimental evidence that all three of these could have arisen together. That’s means in the same place, at the same time and from the same starting ingredients.
That place and time could have been a network of streams and pools on Earth’s surface nearly 4 billion years ago, the study’s authors say. That is about when life started.
The study is an impressive achievement, says Terry Kee. He’s a chemist at the University of Leeds in England. Finding that these three life components could share an origin is an exciting step forward, adds Key, who was not involved in the new study.
What may have happened
For decades, many chemists believed that the origin of each of life’s cellular components would have required different chemicals and conditions. That scenario is a lot like imagining the evolution of a human arm that’s been removed from the body, says John Sutherland, an author of the study. “It looks really crazy,” says this chemist, who works at MRC Laboratory of Molecular Biology. It’s in Cambridge, England.
Sutherland’s group built on earlier work that had looked at two chemicals likely to have been found on a lifeless Earth: hydrogen cyanide and hydrogen sulfide. These two chemicals could produce components of the genetic material RNA, the team discovered through a series of reactions.
Again starting with hydrogen cyanide and hydrogen sulfide, the researchers now sprinkled in phosphates and a few other ingredients. All were likely present on a lifeless Earth. The chemicals began to react with one another, creating what resembled a complex network. These reactions required occasional heat, radiation and the addition of a new chemical. Sometimes, the researchers added more of one of the two main ingredients.
This network may look complicated, Sutherland says. Still, what’s key is that “it’s all the same reactions.”
The experiment didn’t just produce some building blocks of RNA. The reactions also yielded amino acids. These are the building blocks of proteins. As well, the reactions created a precursor to fatty materials known as lipids. Lipids make up cellular membranes.
Sutherland’s group says these reactions could have taken place in streams of water that trickled over the cracked landscape of a young Earth. The water would have picked up molecules from the surrounding rocky landscape.
Meteorites that pummeled the planet would have contributed ingredients too. Those molecules would have triggered chemical reactions. Occasionally, the streams may have partially evaporated. What remained would have soaked up ultraviolet radiation from the sun. More reactions would have followed. Eventually, the chemical soup could have pooled in puddles. There, life may have assembled for the first time.
“This is one of the best, most complete and most thoughtful papers ever written about how the building blocks could have formed,” says Doron Lancet. He is a chemist at the Weizmann Institute of Science in Rehovot, Israel. Next up: figuring out how these components came together. It’s that step that lies at the heart of the mystery of the origins of life, he says. “The paper lays a solid foundation of the first step.” Now, he says, “we can free our minds to think about the second step.”
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amino acids Simple molecules that occur naturally in plant and animal tissues and that are the basic constituents of proteins.
cell The smallest structural and functional unit of an organism. Typically too small to see with the naked eye,it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size.
cell membrane Separates the inside of a cell from the outside of it. Some particles are permitted to pass through the membrane.
chemistry The field of science that deals with the composition, structure and properties of substances and how they interact with one another. Chemists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) The term is used to refer to the recipe of a compound, the way it’s produced or some of its properties.
hydrogen cyanide A chemical compound with the formula HCN (meaning it consists of a bound atom of hydrogen, carbon and nitrogen). It is a toxic liquid or colorless gas. It can have an almond-life odor.
hydrogen sulfide A chemical compound with the formula H2S. It is a colorless gas that is often produced when bacteria are active without the presence of oxygen. Hydrogen sulfide is the smell that most people associate with swamps or rotten eggs.
lipid A type of fat.
meteor A lump of rock or metal from space that hits the atmosphere of Earth. In space it is known as a meteoroid. When you see it in the sky it is a meteor. And when it hits the ground it is called a meteorite.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
phosphate A chemical containing one atom of phosphorus and four atoms of oxygen. It is a component of bones, hard white tooth enamel, and some minerals such as apatite.
proteins Compounds made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. The hemoglobin in blood and the antibodies that attempt to fight infections are among the better known, stand-alone proteins.Medicines frequently work by latching onto proteins.
radiation Energy, emitted by a source, that travels through space in waves or as moving subatomic particles. Examples include visible light, infrared energy and microwaves.
reactive (in chemistry) The tendency of a substance to take part in a chemical process, known as a reaction, that leads to new chemicals or changes in existing chemicals.
RNA A molecule that helps “read” the genetic information contained in DNA. A cell’s molecular machinery reads DNA to create RNA, and then reads RNA to create proteins.
ultraviolet (UV) A portion of the light spectrum that is close to violet but invisible to the human eye.