The Breakfast Table

Our Martian Heritage?

Dear George,

Thanks for drawing my attention to Nick Wade’s piece in today’s New York Times. Thanks to the Internet, I was able to read it from a point roughly 100 miles north of (old) York.

I, too, have been intrigued by the point Wade makes–that life seems to turn up embarrassingly early in the history of a hot planet. A few weeks ago, I did my own sketch of this for a Natural History magazine article I’ve just completed on the same subject and I was struck by how little time there was for the “RNA world” that most people think preceded the DNA world (this is pretty plain from the all-purpose roles that RNA can play, from the fact that even today we make our DNA components from RNA components and other lines of evidence). Just a few hundred million years after the place cools down, and–whoosh!--there are fossils already. What those fossils were is an open question. I’m convinced by the work of a French scientist called Patrick Forterre and a New Zealander, David Poole, that they probably were not bacteria or any kind of prokaryote, which look too streamlined, like later, simplified versions of life. The first cells might have been a bit more like our clunky “eukaryotic” cells with introns, telomeres, and all that jazz, just as the first computers had valves, but transistors came later.

But that aside, there’s another problem. As Wade points out, the early earth was hot. Yet RNA is much more thermally unstable than DNA. The RNA world must have been cool, not hot, or the organisms would have kept falling apart before they got started. Norm Sleep of Stanford has an answer to this: The RNA world was not on earth at all; it was on Mars, which was not subject to what he calls “ocean-boiling episodes” (apparently if you slam a big enough rock into the ocean, it boils). Life arrived here on meteorites after the invention of DNA. Even today meteorites come from Mars fairly regularly and they must have been more frequent in those days. It’s been tentatively shown, I believe, that some life forms could probably survive the trip. Then life died out on Mars when it dried up, but survived on the cooling earth.

It’s one of those ideas that crop up in science from time to time that is so beautiful, it ought to be true. Rather like time travel or the aquatic ape hypothesis: that the reason we have no body hair but lots of subcutaneous fat, are good at swimming, thrive on a diet of fish, and rush to the seaside for holidays is that our ancestors were a small group of apes marooned on an island in the Red Sea for a million years or so. I long for that theory to be true. Alas, it’s almost certainly bunk. But I’m keeping an open mind about Mars for the time being.