Tuesday, July 6, 2010

Colonial life--not 200 years ago, 2 billion years ago.

It’s kind of annoying how some things get hyped. There is no clear relationship between scientific significance and press. There is a weak correlation between “coolness” and press, witness the excited newspaper articles about the fossil giant predatory sperm whale with thirty-centimeter-long teeth. That is cool, so the press is justified. But there was another fossil find this week, published in the same issue of Nature, that some undue hype.

The hype was about “the oldest multicellular organism” or “2 billion year old multicellular life.” This would be interesting—there’s good fossil evidence of organized multicellular organisms going back about 600 million years. These fossils, the “Ediacaran fauna,” are mysterious. Unlike their geological successors in the Cambrian, at 500 million years, the Ediacaran biota don’t resemble any modern organisms. There are very few believable fossils of real, organized multicellular life before the Ediacaran—so the press blurbs about two billion-year-old multicellular organisms made me suspicious.

The discovery of these fossils in Gabon is interesting, but doesn’t quite justify the hype. The reason has to do with the definition of “multicellularity.” We are multicellular. Each of us is a colony of genetically identical cells that have specialized for particular tasks. The Ediacaran fauna were multicellular. There are recognizably differently shaped organs and appendages in each of the different fossil types. Even Fuligo, which spends some of its life as a giant multi-nucleate cell, is multicellular. At the critical point in its life cycle, it differentiates into individual cells with different shapes, dedicated to different tasks. The achievement of this sort of multicellularity is an evolutionary milestone; it asks individual cells to form a cooperative, and the majority of these cells will die without leaving any offspring. For us, really, only our sperm and ova will have any direct descendants a hundred years hence.

But where do we draw the line on multicellularity? There are plenty of Bacteria, and some Archaea, that achieve something like multicellularity. Some types of Cyanobacteria—pond-scum—form long chains of cells where every 10th cell or so is different, both metabolically and morphologically. These specialized cells, like most of the cells in our body, have sacrificed their own chances for reproduction for the "common good". This sort of multicellularity has been invented over a dozen times in the Bacterial and Archaeal world, and challenges the traditional view of multicellularity as being an exclusively Eukaryotic invention.

But to some biologists, even the biofilms that make the rocks in a stream slippery can be thought of as multicellular entities. A biofilm has many different types of cells embedded in a matrix of slime like raisins in a loaf of pannettone. The different types of cells in the biofilm, which may be of completely different species, all have different and specialized roles in the biofilm. However, a critical difference between biofilms and the other types of multicellularity is that there is no organization, no distinctive organs. A biofilm is just a layer of scunge that can get bigger and bigger without pattern.

This brings us to the Gabon fossils. They are big, a couple of centimeters across, so they are definitely multicellular. They have some distinctive shapes—ruffled edges, and a thicker blob in the center. Because of the taphonomy (I love saying that word, it just rolls off the tongue. It just means how the process of fossilizing replaced biological material with minerals, in this case pyrite) of these specimens, it is impossible to resolve individual cells. The researchers do a good job of establishing that they are really fossils of formerly living things, and not just some geological trick (they examined the carbon composition of the fossils; carbon that has been part of life has a different isotopic composition than carbon from, say, a volcano). They are also able to precisely date these fossils to 2.1 billion years ago, since they are in the vicinity of the well-studied Oklo formation (a natural nuclear reactor—very cool). They even make a case, though far from iron-clad, that the fossils are those of Eukaryotes (the same Oklo formation has some fossilized carbon compounds that only Eukaryotes can make—but these fossils are only near Oklo, not in it). So the fossils are big, old, eukaryotic—but are they multicellular like us, or multicellular like a biofilm?

The researchers, unlike the press managers, are conservative in their interpretation. They point out that the feature of these fossils that look like distinct organs—the fringes at the edge, the blob in the middle—can be achieved by modern bacterial cells grown in the unusual conditions of the laboratory. These modern cells don’t normally have a tendency towards multicellularity, but they do have mechanisms for communicating with each other and regulating their growth in response to neighboring cells. Some of these patterns are quite striking:

While these patterns are not the result of multicellularity with specialization, their production does require a well-evolved mechanism for signaling and responding between cells. Seeing such patterns in the Gabon fossils does at least indicate that life had evolved some form of social interactions by this time.

So, we have NOT discovered the fossils of our great-great-diddly-great grandparents. I want to find the publicists and press people who ginned this up, and slap them. But I’d also like to find the authors of this paper, and complement them on their admirable restraint. They conclude that they have found good evidence of colony-forming organisms that are very, very, very old, and that they had evolved some methods for interacting with their neighbors similar to those used by modern organisms. They note that the fossils have a shape that hasn’t been seen preserved before, that they are the oldest fossils of colonial organisms, and that they might be eukaryotic. And while that lacks the bite of a whale with thirty-centimeter-long teeth, it’s still really cool. To me.

Abderrazak El Albani, Stefan Bengtson, Donald E. Canfield, Andrey Bekker, Roberto Macchiarell, Arnaud Mazurier, Emma U. Hammarlund, Philippe Boulvais, Jean-Jacques Dupuy, Claude Fontaine, Franz T. Fursich, Francois Gauthier-Lafaye, Philippe Janvier, Emmanuelle Javaux, Frantz Ossa Ossa, Anne-Catherine Pierson-Wickmann, Armelle Riboulleau, Paul Sardini, Daniel Vachard, Martin Whitehouse & Alain Meunier (2010). "Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago." Nature 466: 100-104.

Eshel Ben-Jacob, Inon Cohen, and David L. Gutnick (1998). COOPERATIVE ORGANIZATION OF BACTERIAL COLONIES: From Genotype to Morphotype. Annual Review of Microbiology 52:779–806.

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