the cambrian explosion


Charles Darwin’s On the Origin of Species contains only one illustration, and a rather dull one at that – a simple image of the tree-like branching relations between hypothetical species, with the present at the top (not all branches reach the top), and common ancestors deep in the past. In fact, the drawing does not look much like a tree – it is more like some kind of spindly weed. Although it might not seem impressive, this figure was a revolutionary way of representing life, summing up Darwin’s central idea of evolution by natural selection. This image was not the first that Darwin chose to represent his hypothesis. Shortly after his return from the voyage of the Beagle, Darwin drew a coral-like diagram and wrote “I think” alongside it. In his notebooks he later mused that “The tree of life should perhaps be called the coral of life”. Over the decades, however, the “tree” image and terminology gradually predominated. They were given particular and literal force by Ernst Haeckel, who at the end of the nineteenth century drew a sturdy oak-like tree with the names of organisms scattered around its branches. Down at the bottom were the monera (single-celled organisms without a nucleus), while at the very top – literally the pinnacle of evolution – were humans. We now know that Haeckel’s representation was wrong in so many ways. Not only are humans not at the top of the tree – we are no more or less “evolved” than the monera Haeckel put down at the bottom – if the tree of life were to be drawn to scale, in terms of either the number of organisms, or species, or the duration of their existence on the planet, then monera would take up almost all the space. Life on earth began 4 billion years ago, a mere 500 million years after the planet formed. If you represent our common history as lasting sixty seconds, life is mainly composed of monera, before proliferating in the last seven seconds, following the massive diversification of animal life that occurred with the “Cambrian Explosion” around 542 million years ago. On this scale, the appearance of our species 100,000 years ago is subliminal.

more from Matthew Cobb at the TLS here.