Mark Peplow, science writer
One of the biggest unanswered questions in science is how life began on Earth. One theory is that primitive chemicals developed the ability to copy themselves, and eventually evolved to encase themselves inside bubbles - effectively becoming cells. But where did those chemicals - which scientists think were early forms of the genetic letters - or bases - that make up our DNA - come from in the first place?
Well this week researchers have - quite literally - shed some light on the question by using a laser to recreate the conditions of a time when the early Earth was being brutally battered by droves of incoming asteroids and comets during a phase called the late heavy bombardment. The energy in these collisions, they’ve shown, could have driven the chemical reactions that created DNA-like molecules, as science writer Mark Peplow explains to Chris Smith...
Mark - A team of researchers from the Czech Republic has found that impacts from asteroids or comets billions of years ago could’ve created some of the key molecules necessary for life. As far as we know, all living things store their genetic blueprints in DNA. These blueprints were encoded in long sequences of just 4 different types of molecules known as bases which are peppered along DNA’s backbone. Now a related chemical called RNA which many scientists think was the very first molecule to encode genetic information, that also uses 4 bases. They're called adenine, guanine, cytosine, and uracil. But the big question is, when life appeared on Earth maybe about 4 billion years ago, where did they get these bases from?
Chris - So, what are the current theories then about where those molecules might have come from in the first place?
Mark - Well, people have looked at the timing of when life arose 3.5 to 4 billion years ago and they’ve noticed that around about the same time, the planet was being absolutely hammered by comets and asteroids. There have been suggestions that the space rocks could’ve delivered some of the molecules needed for life. What these Czech researchers have shown now is that the impact when a comet or asteroid landed could’ve synthesised the 4 crucial bases right here from ingredients that were already on the Earth.
Chris - Is that because as the impactor comes in and slams into the surface, it creates a whole bunch of conditions which are just right for the chemistry needed to make those molecules?
Mark - That’s right. You're talking about temperatures in the thousands of degrees, intense pressures, as well as some ultraviolet and x-ray radiation. The Czech researchers simulated that using a high power laser to blast samples of a simple molecule called formamide which is thought to be widespread on the early Earth. So, you hit it with those conditions, it rips apart the formamide to create ammonia and cyanide in highly reactive forms called radicals. These then react further with more formamide to generate the 4 bases in RNA and that’s what they picked out of this mixture that they created in the laser furnace.
Chris - Where does the formamide come from in the first place and what exactly is that?
Mark - Well, formamide is a very simple molecule and it contains carbon, hydrogen, oxygen, nitrogen – all those 4 key elements found in pretty much all the molecules of life. It’s what you would get when you're burning up even smaller things like cyanide and water.
Chris - And throughout in space because the Earth obviously formed from dust and gas in space. There must have been some of this already there.
Mark - Yeah, you can imagine that if you have atoms of hydrogen, oxygen, nitrogen, carbon, these things are being jumbled around as the Earth is forming. That is going to create simple organic molecules that are around the Earth at that time.
Chris - Do you think that a laser really can recreate what happens when a very big object hits the Earth?
Mark - So, with all these experiments, we can never really know how accurately they represent the conditions on the early Earth. There's always going to be a certain amount of speculation, but they can at least offer us clues about what are the most plausible explanations for the chemical origin of life. With this laser experiment, because of the temperatures and pressures that they're creating, it’s certainly going to – I mean, we know that temperatures and pressures like that were created during impacts, during the late heavy bombardment of this period about 4 billion years ago. So, we know those conditions were present on the early Earth. That’s what makes this such a plausible suggestion.