In the beginning was Ida, the initial Darwinian ancestor 鈥� the first material on Earth to transform from inert to, well, ert. Ida begat Luca, the last universal common ancestor, and the molecule that gave rise to every kind of life on Earth, from lowly bacteria to the giant panda.
But what were these molecules? How did they work?
We do know that Ida somehow popped into existence in a wet corner of our planet around 4 billion years ago. It soon evolved the ability to store information in the form of the genetic code, and so Luca was born.
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Ida and Luca existed so long ago that if any rocks ever harboured traces of their existence, they were long ago subsumed by tectonic forces and melted into Earth鈥檚 depths. But we can find clues to their nature in every living cell today, including our own. Cells use the same genetic code embodied in DNA. That would suggest that the ancestor of all living things, Luca, was made of DNA.
At first, that idea threw up a chicken-and-egg problem. All life uses proteins to carry out its essential functions, including making DNA and executing its code. But proteins themselves are made from DNA templates 鈥� so without DNA, no proteins. Which came first?
The solution appears to be neither. RNA is a close relative of DNA. It, too, is found in all living cells, and also carries the genetic code. Unlike DNA, RNA molecules come with their own toolbox, acting as enzymes and catalysing chemical reactions. Our current best theory of life鈥檚 origins, known as the RNA world hypothesis, says the genetic code was born out of an early soup of RNA molecules that eventually gave rise to DNA and the first cells. Indeed, not all life made the switch 鈥� some viruses are still RNA-based.
But this still leaves a problem: if Luca was made of RNA, where did RNA come from? In the 1950s, US chemists Stanley Miller and Harold Urey famously tried to find out by zapping a mixture of gases and water with electricity. They ended up with a handful of biotic molecules 鈥� and naturally concluded that with a bit of extra va-va-voom, dead stuff could turn into live building blocks.
Nowadays, though, this idea of a bolt from the blue has been overtaken by more nuanced ideas. Nick Lane at University College London, for example, thinks that warm vents on the ocean floor 鈥� 鈥渂lack smokers鈥� with their soup of methane, minerals and water聽鈥� would have provided the right conditions to form RNA. Michael Yarus of the University of Colorado in Boulder, meanwhile, favours the idea of a slushy pond. The continual freezing and thawing could have pushed chemicals together in just the right way, he says.
Or maybe it was some combination of both, maybe neither. Intriguingly, more recent experiments trying to coax RNA into existence have shown that when the chemistry is just right, many of the building blocks seem to form all by themselves. That widens the possibilities for how life started. Not just that: if the chemistry of life came naturally to our planet, why not elsewhere, too?
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