Two billion year-old fossils may rewrite natural history books

If you took biology and natural history years ago, you learned a fascinating tale of a microbe dominated primeval earth. In this story, one-celled critters swarmed in primeval oceans, blissfully free of larger predators, for billions of years. Then along came snowball earth, followed by the blooming Cambrian Explosion over half a billion years ago that changed everything.

Shallow oceans were suddenly teeming with giant flesh-eating shrimp like the Animalocaris, massive snails -- some armored in chain-mail -- on steroids, and other predators of tentacle and chitin patrolling a bizarre watery sea bottom populated with strange frond-like animals, mud-sucking hallucigenia, and of course everyone's favorite Cambrian critter, trilobites such as the reconstruction illustrated right. It's a great story of the tenacity of the simplest creatures, the ones that nobly ended a sci-fi Martian attack in H. G. Wells' classic War of the Worlds. It is also, as we're learning lately, debatable:

The discovery in Gabon of more than 250 fossils in an excellent state of conservation has provided proof, for the first time, of the existence of multicellular organisms 2.1 billion years ago. This finding represents a major breakthrough: until now, the first complex life forms (made up of several cells) dated from around 600 million years ago.

It's not clear exactly what these are, or were. Plants, animals, or do those modern distinctions even apply? They may be cousins of later trisymmetrical biota that have no living analogue today. Or they could be related to modern day Xenophyphora which would still make them one-celled creatures, just exceptionally large ones. But they could also represent something like a primitive Cnidaria, specifically a coral-like colony of individual polyps that pooled resources and went into a cooperative business for themselves similar to siphonophores today.

Cue the "Scientists Stunned" headlines -- and forgive me for my own spin on that old trope above. Because the truth is paleobiologists have long suspected, and recent fossil evidence strongly suggests, that the Cambrian Explosion is more an artifact of random fossilization and geology than the Start of Life as We Sort of Know It. Most ancient ocean strata laid down that long ago has disappeared under the grinding edge of tectonic plates, or been eroded away into dust. Much of what gave rise to the idea of the classic explosion decades ago comes from a single small site of rare, fossil bearing marine sediment in Canada that preserved a snapshot of Cambrian plants and animals in exquisite detail. Newer discoveries have since pushed the likely start of complex life back over a hundred million years. But that headline hype might, for once, actually apply assuming this holds up: two billion year-old multicellular critters are pretty surprising.

We're lucky to have fossils at all. If not for a quirk of geochemistry they would not exist, and all paleobiology would be reliant on inference, comparative anatomy, and genetic analysis. This new find is luckier still, and adds yet more support -- and no small amount of wonder and mystery -- to the idea that our planet's biological history is far more intricate than we may ever fully understand.

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