Sometimes, I really hate fossils. I hate them with the passion of a spurned lover, one who is consumed with desire but knows that he will never, ever be satisfied. They drive me mad.
Right now we’re at a point in our technology where we can take a small sample from a living organism and break it down into amazing detail — we can extract every gene, throw them into a computer, and compare them with every other gene that has been similarly sampled. We can look for the scars of evolution, we can analyze and figure out where on the tree of life this cell resides, we can even figure out what local populatons it lived in, who its ancestors bred with, and to a certain extent, what various alleles contributed to its form and physiology. We don’t know everything, but every time someone works out some new detail in a related species, it goes into the databases and presto, the information cascades through every other relative. I’d call it magic, but that would insult the science with cheap understatement.
We can’t do that with most fossils (with some recent exceptions). The cells are gone. Their contents are obliterated — DNA fragmented, dissolved, corrupted, lost. And the farther back in time we go, the less information we have, but the more interesting the problems become.
All organisms are built of cells — they’re like the Lego building blocks of biology, with specific features that snap them together. With Legos, of course, you can build all kinds of different forms: stick them together and build a Lego Triceratops or a Lego T. rex. Different on the outside, different in arrangement, different in pattern, but all fundamentally built of the same kinds of blocks. I can get into the coolness of digging up a Triceratops or a T. rex, but these are all variations on a theme of phylum Chordata, superclass Tetrapoda, and they’re all using the same building blocks, and all the really interesting stuff, the details in the genome that make one morphology different than another, have all been bled out on the sands of time and gnawed by all-devouring bacteria and reduced to at best a non-specific smear of carbon. That makes me frustrated.
Even worse, most familiar fossils are big bony animals — they’re all pretty much the same, deep down. If they’re built of Legos, there are whole other clades of multicellular organisms that are the equivalent of meccano, lincoln logs, Capsela, and tinkertoys. How were they put together? And how did they evolve these different patterns of connections? To know that, we have to go way back into deep time, and look at the unicellular organisms, the cells that first pioneered patterns of interactions and laid down the possible rules of development that enabled big clumsy multicellular to accumulate the bulk that made them more likely to be fossilized. Those pioneers are practically nonexistent in the fossil record.
What prompts my lament for lost cells is this recent amazing discovery: a collection of fossilized multicellular organisms unearthed in Gabon that are 2.1 billion years old. Keep in mind that in comparison, the Cambrian explosion, the event that was the root of familiar animal diversity, was a mere half billion years ago, so these are genuinely ancient. They’re also beautiful.
These small, flat, furrowed sheets lived at a kind of temporal boundary, a few hundred million years after a rise in atmospheric oxygen called the Great Oxygenation Event — a crisis in the history of life on earth which occured when the production of oxygen by photosynthetic organisms could no longer be buffered by reacting chemically with minerals, and began to build up in the atmosphere. This was catastrophic for most of the organisms living at that time, which were anaerobic and found oxygen to be a caustic poison. It was an advantage to a subset that adapted to use oxygen as a fuel in chemical reactions, though, so there was also the beginnings of new forms which exploited this newly oxygenated atmosphere. That’s where these mysterious blobs come in; they were found in formations that had a chemical signature indicating the presence of free oxygen.
These were almost certainly colonial organisms that took advantage of the higher concentration of oxygen to build denser mats on top of the sea floor. They probably weren’t true multi-cellular organisms; they were a step up from a colony of bacteria that you might see growing on a petri dish, but with additional molecular features that permitted greater coordination and the development of more elaborate spatial patterning.
We also know that these had to have been very different from organisms that exist now. Those are not animals, they are not plants, they are not fungi — they are something primeval and radically different, organisms that most likely do not have any living descendants. Those are real aliens in the photo above. There is no category in your experience which you can put them into.
It’s what we don’t know that inflames my curiousity. One of the other things that was going on during the Great Oxygenation Event was the steady loss of dissolved iron in the seas — it was all being oxidized, rusted out, and precipitating out, forming geological structures like the banded iron formations. It was also facilitating the preservation of these organisms by pyritizing them — all their soft gooey bits, the whole of creature, were being replaced by fool’s gold, iron pyrite. There are no cells left here. We don’t even know for sure that these are eukaryotic cells; they probably are, indicated by the presence of a sterane chemical signature in the rocks that is characteristic of eukaryotes, but there isn’t even enough fine detail to tell whether there was a nucleus in these cells. It just breaks my heart.
It’s a beautiful tease. We can see that life was exploring the edges of multicellularity over 2 billion years ago, but…the molecular sinews that stitched them together are all gone. The signals and receptors that enabled communication between them are all gone. The genes that drove their growth are all gone. There is nothing left but a blurry crystal-ruptured outline of what once was.
I have to shake an angry fist at you, fossils. I won’t go all Mel Gibson in incoherent rage at you because I like you too much, but still…you taunt me. I want your cells. Nothing less will do.
El Albani A, Bengtson S, Canfield DE, Bekker A, Macchiarelli R, Mazurier A, Hammarlund EU, Boulvais P, Dupuy JJ, Fontaine C, FÃ¼rsich FT, Gauthier-Lafaye F, Janvier P, Javaux E, Ossa FO, Pierson-Wickmann AC, Riboulleau A, Sardini P, Vachard D, Whitehouse M, Meunier A. (2010) Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago. Nature 466(7302):100-4.
Chris Nedin, who should know, does not think these fossils represent multicellular organisms at all — they are fossilized, folded microbial mats. Which is fine by me — 2 billion year old microbial mats are also exceedingly cool, and I still want their cells.
You do know that if you want to know more about anything pre-Cambrian, you should be reading Ediacaran, right?