There are only a few places on the surface of Earth where you can find really old rocks–and by old, I mean 3.5 billion years old or older. The rest have gotten sucked down into the planet’s interior, cooked, scrambled with other rocks, and pushed back up to the growing margins of continental plates. The few formations that have survived are mere fragments, some the size of a football field, some a house. And generally they’re are mess, shot through with confusion such as intrusions of lava from more recent volcanoes. Paleontologists are drawn and repulsed by these rocks, because they may hold the oldest clues about life on Earth, or lifeless mirages that only look like clues.
In the past couple years, scientists have been putting the oldest evidence of life on Earth under tough scrutiny. The oldest fossils, 3.45 billion year old bacteria from Western Australia, have been attacked as mere crud. Life not only leaves fossils behind but also can create peculiar ratios of isotopes in rocks. The oldest isotopic evidence for life came from 3.8 billion year old rocks in Greenland. But that also came under attack by critics who questioned whether the rocks were actually sedimentary (and thus might contain biological material) or belched up from a very nonbiological volcano.
This does not mean that the fossil record has collapsed down to yesterday’s road kill. In other parts of Greenland, scientists have found slightly younger rocks (if you can call 3.7 billion year old rocks young) that are almost certainly sedimentary. And they contain a clear isotopic signature of life. The Danish geologist Minik Rosing, who has studied the rocks, argues that this particular fingerprint is so detailed he can tell what kind of life produced it: photosynthesizers. That’s tantalizing for several reasons. One is that photosynthesizers give off oxygen, and yet there’s no record of any significant levels of oxygen in the atmosphere for well over a billion years after Rosing’s rocks formed. Another is that the early Earth may not have been a very friendly place for photosynthesizers—the oceans were hot and loaded with nasty metals.
The controversy over ancient fossils has forced some paleontologists to look for new kinds of evidence of life. For example, some bacteria can eat through glass, leaving behind microscopic pits. Volcanoes form glassy rocks such as obsidian, and in recently formed volcanic rocks scientists have found tunnels that seem to have been created by hungry microbes. (They’re even slathered with DNA and other biological material.). Today in Science, researchers reported that 3.5 billion year old rocks from Zimbabwe bear the same sorts of tunnels. They’re also slathered in organic carbon with an isotopic fingerprint that looks like life. The evidence has impressed some researchers, but others are still skeptical. The possibility that these formations are formed without the help of microbes hasn’t been eliminated yet.
I find all this work fascinating, but in one fundamental way it’s a bit pedestrian. These scientists are looking for the earliest signs of organisms that resemble organisms alive today, looking for the traits that are common to both. But a photosynthetic or glass-chewing bacterium is already pretty nicely evolved. Someday, a clever paleontologist is going to figure out how to identify something that no longer exists on Earth, such as an RNA-based organism. That discovery will push the fossil record back to a different chapter altogether in the book of life.
Originally published April 22, 2004. Copyright 2004 Carl Zimmer.