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The Cambrian Explosion

The Great Cambrian Explosion is when geologists and palaeontologists used to think that life first emerged on the planet. It was a perfectly reasonable interpretation at the time. Many rocks of ‘Cambrian’ age and younger were teeming with fossils. All rocks older than this appeared lifeless and dead.

They couldn’t date rocks at the time, but geologists were already very good at deciding which rocks were older than which, and which rocks in different parts of the world were the same age. The name Cambrian Period comes from Wales, where the oldest rocks with fossils in were first found. Now of course they’re found all over the World. Everything older was lumped together and called ‘Precambrian’. This illustration is by Zdenek Burian. Note that it shows jellyfish, likely direct descendents of the earlier and mysterious Ediacara fauna (more).

Trilobites

The Cambrian Period appeared to be populated almost entirely with trilobites, I think!. These were clearly fully-evolved creatures, as it were. And if the theory of Evolution was right, they could not have been the earliest life-forms.

Darwin was seriously worried by this. He knew that if some precursor to trilobites wasn’t eventually found then his theory was wrong.

Microscopic life

However he needn’t have worried. We now know that earlier rocks are packed with signs of life. It was a bit small that’s all. And you need a microscope to see it. These wee beasties come from William Schopf’s “Cradle of Life”. They are not the oldest ever found (for these click here). They are ‘only’ some 900 million years old. The little black bars represent 10μm, or a hundredth of a millimetre. Apparently many of the bacteria that Schopf has found are the spitting image of particular modern bacteria. And he has given them names similar to the modern bacterium that each resembles.

We now also know that there were large creatures that preceded the trilobites, the Ediacara fauna that we mentioned earlier. But they needed special conditions to be preserved, so again it’s not surprising that these early geologists and palaeontologists didn’t find any.