How did life start here on Earth? Ever since Darwin, scientists have been struggling with that question. (Before him, there was nothing to explain — God did it.) When Darwin posited that species evolved from prior species, he implied there once was a single organism from which all species are descended. We can now say with some confidence that the original organism:
Was a single cell, the simplest possible body that can reproduce.
Lived about 4 billion years ago.
Used RNA (a DNA precursor) to reproduce.
Made use of the essential CHNOPS elements we see in all life today: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur.
Several steps were needed to get from these six constituent elements to that original living cell. First, the elements combined into simple molecules such as HCN (cyanide), CH4 (methane) and HCHO (formaldehyde). That's easy, and so is the next step, synthesizing these rudimentary molecules the "nucleotide bases" found in the double-helix structures of RNA and DNA: adenine, thymine, uracil, guanine, cytosine (A, T, U, G and C). In addition to these bases, DNA and RNA molecules consist of phosphates and sugars. With DNA, the sugar is deoxyribose (the D) with bases ATGC; with RNA, the sugar is ribose (the R) with bases AUGC.
So far, so good. Now comes the tricky part: getting from chemistry to biology. It's not quite miraculous but so highly improbable that creating strands of RNA out of "pre-biotic" molecules in just the right formulation and geometry may have happened just once. According to Dr. Steven Benner of the Foundation for Applied Molecular Evolution in Florida, planet Earth was an unlikely place for this to have occurred:
Problem: "Naked" nucleotide bases normally decompose spontaneously, before any further reactions can occur. Solution: Borate (roughly, boron plus oxygen) can keep the bases stable long enough for the assembly process to take place.
Problem: Ribose requires a catalyst to help its constituent elements self-assemble. Solution: An oxide of the element molybdenum can act in that helper role.
Problem: Ribose, phosphate and the nucleotide bases into RNA can't occur in the presence of water (the bonds aren't stable), so the assembly won't happen in, say, oceans. Solution: dry land.
Conclusion: RNA probably wasn't created here, because early Earth was all oceans, while the necessary borate and oxygen were in short supply. Mars is a much more likely candidate for life's cradle. Back then, Mars had far less water than early Earth, while borate minerals and oxygen were comparatively abundant, according to evidence from our rovers and orbiters, and from analysis of 30-odd meteorites we believe came from Mars.
Knowing that life can survive within the interiors of meteorites for eons, we can conjecture this sequence: RNA life began on Mars about 4 billion years ago. Asteroid impacts threw up showers of Martian dust and rocks, some of which escaped Mars' gravity — the planet has a low (5 km/sec) escape velocity. Said dust and rocks began a long, spiraling sunward journey. Perhaps millions of years later, Earth's gravity captured Martian rocks, which finally fell to Earth as meteorites, some containing the seeds of life.
Meaning that, as I discussed in a previous column ("Men (and Women) are from Mars ... Maybe," Jan. 30, 2014), when Earthlings finally walk on Mars, we won't be strangers in a strange land. We'll be home.
Barry Evans (firstname.lastname@example.org) considers Stranger in a Strange Land, Robert Heinlein's 1961 science fiction novel, required reading for everyone.