Living things depend on water, but it breaks down DNA and other key molecules. So how did the earliest cells deal with the water paradox?


NASA’s Perseverance rover will search for signs of life in Jezero Crater on Mars.Credit: ESA/FU-Berlin

Narrowing down the location where life started will require understanding of the broader picture of prebiotic chemistry: how the many reactions fit together, and the ranges of conditions under which they occur. That mammoth task has been attempted by a group led by chemist Sara Szymkuć, president of the start-up firm Allchemy in Highland, Indiana. The team published a comprehensive study in September that used a computer algorithm to explore how a vast network of known prebiotic reactions could have produced many of the biological molecules used in life today17.

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The network was highly redundant, so key biological compounds could still form even if multiple reactions were blocked. For this reason, Szymkuć argues that it is too early to rule out any of the scenarios for where life originated. That will require systematically testing a range of different environments, to see which reactions occur where.

Beyond Earth

If experiments such as Sutherland’s do point the way to how life began on Earth, they can also help to explore where life might have started elsewhere in the cosmos.

Mars has attracted the most attention, because there is clear evidence it once had liquid water on its surface. The landing site for NASA’s Perseverance rover, the Jezero Crater, was chosen in part because it seems to have once been a lake — and could have hosted the chemistry Sutherland has studied. He helped to write a 2018 presentation to NASA led by Catling, which summarized the prebiotic chemistry findings and advised on where Perseverance should look. “We presented this chemistry and said this Jezero Crater, which is the one they eventually chose, is the one where there was the highest likelihood of this chemistry playing out,” says Sutherland.

It will be two months before Perseverance reaches Mars — and years before the samples it collects are returned to Earth by an as-yet-unnamed future mission. So, there is still a long wait before we find out whether Mars harbours life, or if it did so billions of years ago. But even if it did not, it might reveal traces of prebiotic chemistry.

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The best case, says Catling, is that Perseverance finds complicated carbon-based molecules in the layers of Martian sediment, such as lipids or proteins, or their degraded remains. He also hopes for evidence of wet–dry cycles. This might come in the form of carbonate layers that formed when a lake dried and refilled many times. He suspects that “life didn’t get particularly far on Mars”, because we haven’t seen any obvious signs of it, such as clear fossils or carbon-rich black shales. “What we’re looking for is pretty simple, maybe even to the point of being prebiotic rather than the actual cells themselves.”

It could be that Mars took only the first few chemical steps towards life, and did not go all the way. In that case, we might find fossils — not of life, but of pre-life.