When 3.4 billion years old you reach, look as good you will not

I finally got a chance to read the latest microbial news that is creating a buzz, the discovery of some tantalizing maybe-fossils of cells over 3.35 billion years old. (News reports here and here; a couple of blogs’ opinions here and here.) I’m not a geologist, so I can’t adequately judge everything about the paper, but as a microbiologist I can offer my two cents.

I’ll confess that I rolled my eyes a bit when I first heard the news, for several reasons. First, I’m not a fan of science by press release. Fortunately this is actually in the legit literature*. Second, the topic of ancient microfossils is one that leads to a lot of squinting at grainy pictures and seeing what one wishes to see. Third (and partly because of the very subjective nature of interpretation) this has been a field of heated, occasionally quite personal debate, and I am conflict-averse. Finally, I’m not sure it matters.

Here’s the problem. It would be nice to find a beautifully preserved, clearly cellular structure over 3 billion years old with unambiguous evidence of biological functions. It would also be nice if a unicorn came by and pooped golden bricks for me, and about as likely. First, it’s really hard to find nice sedimentary, fossil-bearing rocks that are that old—plate tectonics being what it is, any really old rocks are likely to have been recycled, squeezed and cooked so much that fossils wouldn’t exist. Second, any life present over 3 billion years ago would have been unicellular and very small. It’s hard to find small things. Finally, it’s hard to find something that old that is unambiguously biological. Non-biological processes can generate mineral patterns that look cell-ish, and even mimic some of the chemical processes of life. Remember the martian meteorite that was supposed to have evidence of life on Mars?

So, here’s this report from Martin Brasier and his associates at the University of Western Australia. It’s actually pretty good.

To solve the first problem, they found one of the few places on earth with really old, relatively un-metamorphosed rock—the boonies of Western Australia. These are sandstones, sedimentary rock that was laid down on top of some volcanic rock that’s about 3.5 billion years old, then buried by more volcanic rock 3.35 billion years ago. That this rock is sandwiched between these layers is nice—the volcanic stuff is relatively easy to date, and it helps to protect it from modern organisms (there are some bacteria that live in surprisingly deep hot basalts; this geological setting minimizes the chances of contamination by these organisms, but does not eliminate it).

To solve the problem of finding fossils, they looked in likely places—places where carbonaceous traces were present. Here’s some pictures of what they found:

These are, of course, not convincing on their own to the layman or the specialist. Which brings up the third problem, and the meat of the report. As the authors say with modest understatement, “determining the biogenicity of putative [3 billion year old] microfossils is notoriously difficult.”

The authors provide some pretty good structural arguments. The candidate microfossils are cell-shaped, cell-sized (unlike the supposed martian fossils), and (like modern cells, but unlike some geological artifacts) quite uniform in size. They are located in the sediments like normal cells would be in such an environment. Some pretty amazing work with spectrometry shows that the things that look like cell walls are actually where most of the carbon is found. So, the geology is good and the taphonomy (I love that word—it means the conditions of fossil formation) is pretty good.

Some more convincing evidence comes from the chemistry of the supposed fossils. The researchers determined not just that carbon and nitrogen and sulfur were present, but which isotopes of carbon and sulfur were present. (Isotopes are versions of atoms with different numbers of neutrons—the number of protons and electrons remains the same, but an atom may have more or fewer neutrons. So, most carbon has 6 neutrons per atom, but a very small amount of carbon has 7 neutrons per atom.)

Isotope fractionation is one of the more reliable fingerprints of life. When living things such as plants and microbes take carbon dioxide from the atmosphere and make it into part of a living thing, they preferentially take the certain isotopes of carbon. To “fix” carbon dioxide, a biological enzyme first has to catch a molecule of the gas. The enzyme is not all that great at this, so it tends to catch relatively more of the heavier, slower-moving molecules that have heavier carbon isotopes. So, it’s possible to examine a carbon sample’s isotope ratios and determine whether it’s of biological origin. (Not only that, but different carbon-fixing enzymes are better or worse at snatching carbon dioxide from the atmosphere, so with modern carbon samples it’s possible to determine what sort of organism actually did the job—we can even determine how much of you is made from corn-based carbon vs. carbon fixed by wheat and other crops!)

The carbon isotopes reported with these microfossils check out as biological, which is reassuring. There are some arcane ways to make slight isotope imbalances without using biological processes, but the carbon compounds that are made are not consistent with what was found.

The researchers also presented evidence of isotope fractionation with sulfur found in the samples. This is particularly suggestive about what may have occurred so long ago. One place you may think of sulfur in your own biology (especially after the posts from this last week!) is in flatulence—and in a very real way, what is being looked at here are 3.4 billion year old bacteria farts.

The way we get most of our energy is through what are called oxidation-reduction, or “redox” reactions. In these reactions, an electron goes from a high-energy situation to a low-energy situation. Electrons don’t really like to be attached to carbon, so that’s a high-energy situation; electrons love to be attached to oxygen, so that’s a low energy situation. So, we get energy by letting electrons move (accompanied by a proton, to make a hydrogen atom) from carbohydrates to oxygen. To a very rough degree, you can think of these reactions moving hydrogen atoms from carbon to oxygen:

C₆H₁₂O₆ + O₂ --> CO₂ + H₂O **

There’s plenty of evidence that 3 billion years ago, there was no oxygen in the atmosphere. However, something like this process probably went on, and goes on today in your intestines, where there is no oxygen available. Although electrons love to be on oxygen, sulfur is a pretty good second choice. So, the microbes in your gut can do this reaction:

C₆H₁₂O₆ + S --> CO₂ + H₂S

The cells won’t get as much energy, but they’ll get enough to live—and your farts will smell like rotten eggs.

Surrounding the supposed fossils, the researchers found grains of pyrite. If this metabolism (or one very like it, using hydrogen instead of carbohydrate and sulfate instead of sulfur) occurred, the resulting H₂S would react with iron in the environment and form pyrite. This reaction can happen without biological intervention, but if it’s done by life as we know it, you would again expect to see an enrichment of heavier isotopes of sulfur—and this is just what was found. The spatial relationship of the supposed fossils and the pyrite grains is also the same as what one sees with modern cells performing this type of metabolism.

So, Brasier and his colleagues present a very good (but I hesitate to say iron-clad) case for having discovered 3.4 billion year old evidence for life. Their geology and taphonomy are sound, and they provide a couple of lines of evidence consistent with life, especially life under the conditions of the young earth. How young was the earth when these maybe-cells maybe lived? There’s geological evidence that the young earth was pummeled by a massive barrage of meteorites. This “late heavy bombardment,” which would have made life impossible by vaporizing the oceans, ended about 3.8 billion years ago. So, as one news report has it, life may have emerged “very soon” after the late heavy bombardment—although I think of 400 million years as being a pretty long time.

This is some impressive work, and it may well be the “oldest evidence for cellular life.” However, as I mentioned, I’m not sure it matters beyond who gets bragging rights. Although it’s old, it doesn’t change anyone’s estimate about when life started. It doesn’t change our views of ancient biochemistry, as it is consistent with what has been predicted. It would be nice to know if the purported cells were Bacteria or Archaea, but that information can’t be recovered. Finally, unambiguous proof of cells that old (or for that matter, if we every find anything on Mars or Europa) is just about impossible.

On the bright side, I suppose it is nice not only to know that you had a great-great grandmother, but also to have a picture of her. It makes for a better lecture to show a slide of these pictures rather than a slide of isotope data.

*David Wacey, Matt R. Kilburn, Martin Saunders, John Cliff and Martin D. Brasier (2011). Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia. Nature Geoscience (Published online 21 August 2011 | DOI: 10.1038.

**Yes, I'm not balancing the equation; I'm just interested in the chemicals, not their amounts. If you're concerned, I encourage you to do the stoichiometry.