How life originated on Earth is among the most important mysteries in science. Although it’s still an open question in biology, scientists have some pretty promising theories about how a chemical soup eventually generated complex, cellular life around 3.7 billion years ago. But a theory is more than just a good guess – it must explain the available evidence, and it must also be testable.
Scientists believe that amino acids existed in great abundance in Earth’s first years, and that these bits helped create the first cell.
There is ample data to suggest that single-celled organisms slowly emerged from the random collection of chemicals floating around in water. For example, we can see signatures of ancient life in the form of microbial mats, which are sheets of dead, fossilized microbes that form in sediment or on the surface of rocks. We can estimate their age with radiometric dating and chemical analysis, which gives us a sense of when life began.
We can even test this theory using experiments that simulate our best estimates of what an ancient Earth was like. In 1952, for example, the Miller-Urey experiment simulated the conditions of Earth’s early atmosphere by passing electrical sparks through a mixture of gases (primarily methane, ammonia, and hydrogen). The experiment produced several organic compounds, including amino acids, which are the building blocks of proteins. Scientists actually believe that amino acids existed in great abundance in Earth’s early years, and that these bits helped create the first cell.
Amino acids are like the letters of an alphabet, while proteins are like words. With enough proteins, you can form “sentences”—in this case, biological pathways that are a bit like microscopic Rube Goldberg machines that allow life to do, well, everything.
Proteins are not considered living by most biologists. They are large, complex molecules consisting of long chains of amino acids, which are essential for the structure and function of all living organisms. So where do you draw the line between non-living protein and living organism? It comes down to these pathways, governed by the grammar of biology, which we call DNA.
In other words, understanding how life started means figuring out how DNA was formed in the first place. And understanding what the Earth looked like before the emergence of life – and what kind of amino acids and proteins swirled around – means we are one step closer to understanding how life arose.
Although hundreds of different amino acids may have been present on the early Earth, all living things depend on only about 20 of these compounds.
Now, a new study in the Journal of the American Chemical Society suggests that evolution began long before life appeared, and that proteins swirling around in the primordial soup selected for preferred properties. In other words, there was a kind of Darwinian selection effect that took place (even before the emergence of life) among the non-living proteins and amino acids in the primordial soup.
This idea explains why, although hundreds of different amino acids may have been present on the early Earth, all living things depend on only about 20 of these compounds. So why were these specific amino acids singled out?
“You see the same amino acids in every organism, from humans to bacteria to archaea, and that’s because all things on Earth are connected through this tree of life that has an origin, an organism that was the progenitor of all living things,” Stephen Stephen Fried , a Johns Hopkins chemist who led the research with researchers at Charles University in the Czech Republic, said in a statement. “We describe the events that shaped why that ancestor got the amino acids it got.”
Although there are 20 amino acids that are important, growing evidence suggests that ancient life first arose using just 10. To test this, the experimenters created several different libraries of different amino acid combinations and then screened them for two things: their solubility and secondary structure propensities . It sounds complicated (and it is), but these terms are simply related to the correct shapes of proteins.
Protein shapes and how they fold are quite crucial to their function. This determines how they interact with other molecules and their environment. Think of them as a sheet of origami paper that can be folded into many different patterns.
So by testing which libraries had the best solubility and structure, the researchers were able to conclude that proteins evolved and drove natural selection before they even formed living things. Over time, the proteins that were the best form of biochemical processes were incorporated into the basic cycles of life.
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“Protein folding basically allowed us to do evolution before there was even life on our planet,” Fried said. “You could have evolution before you had biology, you could have natural selection for the chemicals useful for life even before there was DNA.”
“To have evolution in the Darwinian sense, you have to have this whole sophisticated way of turning genetic molecules like DNA and RNA into proteins. But replicating DNA also requires proteins, so we have a chicken-and-egg problem,” added Fried to. “Our research shows that nature could have selected building blocks with useful properties before Darwinian evolution.”
Of course, the more we study how life formed on our planet, the better we can estimate how life on other planets might form. Amino acids are abundant in asteroids, suggesting that the conditions for alien life forms exist in other corners of the universe.
“The universe seems to love amino acids,” Fried said. “Maybe if we found life on another planet, it wouldn’t be so different.”
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