Every time one of the cells in your body divides, it has to double its quota of DNA so that each daughter cell gets a complete set. DNA is a replicator—a molecule that can be accurately duplicated, admittedly with some help from proteins. DNA has been doing this for billions of years, well before there were humans, before animals existed, and probably before the first cells evolved.
But what came before DNA? Probably RNA, a related molecule. Certain types of RNA can store genetic information, just like DNA. And much like proteins, they can fold into three-dimensional shapes to speed up chemical reactions, among other functions—these are called ribozymes.
The dominant theory is that an “RNA world” preceded the origin of life. It’s possible that the Earth’s first true replicators were RNA molecules that could fold up to speed up their own replication. They copied themselves. They did so imperfectly, creating daughter molecules with slightly different sequences. Some of them copied themselves more efficiently, and left more descendants than their peers. Gradually, the entire population evolved towards ever more efficient replication.
But there’s a problem with this story. The RNA molecule we’re talking about would have been long and folded into a complex ribozyme. But the ribozymes that scientists can make today are simple, and made from very short pieces of RNA. You can imagine a simple molecule gradually growing and evolving into a more complex one, but that idea has problems too. Mathematical models predict that this burgeoning replicator would be unable to copy itself accurately enough, and would start accumulating errors. After a while, it would face an “error catastrophe”, where the build-up of mistakes crippled it.
But what if there wasn’t just one RNA replicator copying itself? What if, instead, there was a whole network of them? This idea was originally floated in 1971 by Nobel-winning chemist Manfred Eigen. “He came to the conclusion that an individual replicator couldn’t persist for very long, and came up with the idea of a hypercycle,” says Niles Lehman from Portland State University. That is, molecule A helps B to copy itself. B helps C, C helps D and so on, eventually looping back to A.
Eigen predicted the existence of hypercycles using mathematics. Now, Lehman has created something similar in a test tube. It’s a contrived set-up, and it doesn’t confirm that such networks were genuinely involved in the origin of life, but it shows that they can form, and that they become more complex over time. As James Attwater and Philipp Holliger from the University of Cambridge write in an accompanying piece, the study makes “a persuasive case for the benefits of cooperation even at this nascent stage of life. The first genes may not have been so selfish, after all.”
The natural world is full of great partnerships. Bacteria give animals the guts to digest all manner of otherwise inedible foods. Algae allow corals to harness the power of the sun and construct mighty reefs. Ants cooperate to become mighty superorganisms. But the greatest partnership of all is far more ancient. It’s so old that we can only infer that it took place by looking for signals of history, embedded into the genomes of modern species. The details of how and when it happened are still the source of fierce debate but this was undoubtedly the most important merger in the history of life on Earth: a partnership between two simple cells that would underlie the rise of every living animal, plant, fungus and alga.