Where did life on Earth arise? It could have started in ponds on the planet's surface, after meteorites splashed down and infused them with the building blocks of life, a new Canadian-led study suggests.
Scientists had previously proposed that the earliest form of life may have been RNA, a molecule similar to DNA capable of self-replication. Now, a new study suggests there were enough meteorites carrying the raw ingredients for RNA splashing into enough ponds in the early Earth to produce ample opportunities for RNA to form.
"There's probably thousands of ponds from 4.5 to 3.7 billion years ago [where] you're actually getting chances for life to emerge," said Ben Pearce, a McMaster University PhD student in astrobiology and the lead author of the study published today in the journal Proceedings of the National Academy of Sciences.
The study, done in collaboration with researchers at the Max Planck Institute for Astronomy in Germany, suggests it was most probable for RNA to arise earlier than 4.17 billion years ago.
Scientists estimate that the Earth formed about 4.5 billion years ago. The oldest known fossils, found in Greenland, are 3.7 billion years old, although there is some evidence of life in Canadian rocks from as far back as 3.95 billion years ago.
How did life arise during Earth's first half billion years or so?
There are a few competing theories, but one was proposed by Charles Darwin, the English scientist known for his theory of natural selection as the driver of evolution. In a famous letter to his best friend, botanist Sir Joseph Dalton Hooker, in 1871, he suggested that life could have first arisen from chemistry "in some warm little pond."
Earliest life form
Since then, other scientists have suggested that the very first life forms were RNA molecules, because they're found in all organisms, contain genetic information, can act like a protein in some ways, and can self-replicate.
"They fit the most basic definition of life," Pearce said.
The chemical building blocks of RNA molecules, called nucleobases, have been found in carbon-rich meteorites. Those were raining down on the early Earth at a much higher rate than they are now, suggests evidence based on the age of craters on the moon.
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Using published data, Pearce and his colleagues estimated the number of ponds at the surface of the early Earth and the concentration of nucleobases in those ponds delivered by the meteorites splashing into them.
They propose that those ponds may have dried up at some times of the year, concentrating the building blocks of RNA and allowing them to link together — something that's been shown to happen in the lab. Rain would re-form the pond and mix up the building blocks, allowing them to link into longer chains the next time the pond dried up.
However, the RNA would have to have formed fairly quickly from its building blocks — within a few years. Otherwise, the ingredients would leak right out of the pond into the ground or be destroyed by the sun's ultraviolet rays or chemical reaction with the water itself before they could form RNA, the researchers found.
The team says this is the first time anyone has assembled together information about the conditions on the early Earth and experiments on chemically building RNA, filled the gaps by calculating the physics that they would have undergone through the process, and put all the puzzle pieces together to see whether it was feasible for life to get its start this way.
"Theres' a lot of different physics to consider," Pearce said. "Grabbing data from different aspects of science and putting them in one model is something that's not done very often."
While the study suggests life could indeed have started this way, there are some unfilled gaps in the picture. For example, there's an extra chemical step to get from nucleobases to RNA that scientists still haven't figured out, acknowledges Ralph Pudritz, an astrophysics professor and director of the Origins Institute at McMaster University, who co-authored the paper.
Scientists are also not sure they have an accurate picture of the rate of meteorites falling to Earth, how fast land was forming out of the oceans in the early Earth, and how much water there was at that time.
Pudritz said that's why it's important to test out the theory in the lab — something he plans to do after McMaster opens a new Origins of Life laboratory next year that will re-create the conditions on the early Earth in a sealed environment.
In the meantime, he said, understanding the origins of life is becoming pressing as we discover more and more habitable planets where we may find evidence for life.
"We need to start really asking the hard questions of how this works."
The research was funded by the Natural Sciences and Research Council of Canada.