James Le Fanu

‘For every problem there is a solution: neat, plausible and wrong’. H.L.Mencken

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Life’s Big Bang: Part 2

The world, and our understanding of it, is happily full of surprises—though few quite as surprising as the recent scientific revelations of the distant past. The notion of a ‘Cambrian explosion’ is certainly familiar enough as suggested by the sudden appearance of the fossilized remains of diverse forms of life in ancient rocks laid down 540 million years ago. But who could have imagined that when palaeontologists had finished categorising those first simple (though in fact very complex) creatures they would encompass the distinctive body plans on which all subsequent forms of life are modelled—insects, crabs and spiders, legless worms, segmented starfish and the ‘chordates’, the millions of species of fish, reptiles, birds and mammals with which we share our planet.

There have been surprises too of a rather different, but similarly challenging, sort from studying the life and history of the most successful of those early creatures—the famed trilobite. The joy of the trilobite is that its armoured carapace fossilizes so well, as indeed do the several intermediates it must discard as it increases in size to adulthood.

This near limitless supply of fossilized remains found in successive strata of rock provides the unique opportunity to observe the entire evolutionary history of a single organism from its first appearance to its final extinction 250 million years later.

First their general structure provides an opportunity to reflect briefly on what is entailed in the sudden emergence of a novel form of life where, as paleontologist Richard Fortey describes, “There is not a sniff of a trilobite as you work your way up from one (geological) bed to another till quite suddenly one as big as a crab will pop into your waiting hand as you split a rock”.

The trilobite’s carapace consists of three segments, the head together with a pair of antennae and two prominent eyes (the trilobite was amongst the first creatures to see), a segmented body and tail. The paired legs are arranged beneath together with its mouth, anus and digestive tract, a simple circulatory system and a brain to coordinate its activities and perceive the external world. It is not necessary to be a creationist to be deeply puzzled as to how these many specialised parts and functions ‘came together’ seemingly from nowhere to form the immensely successful creature the trilobite will become.

Then, reverting to the geological search for this fossilized remains Robert Fortey describes how in the rocks a foot or so higher up, that first trilobite is joined by others “half a dozen or so different species all individually quite distinctive”. A bit further up and there are more still and it becomes clear that the cardinal feature of the trilobite is its creative exuberance: its 17,000 (or more) species each a readily recognisable variation on the same simple theme, “As odd a parade as any carnival could offer—giants and dwarves, pop eyed popinjays, blind grovellers, flat as pancakes or puffy as profiteroles.”

Some bristle with dangerous,fantastically shaped spikes while others are covered with ridges ‘as complex as a fingerprint’. And that variation extends beyond the merely anatomical to whole scale specialised adaptations as trilobites diversified to become variously predators tearing their prey apart with their legs and spines, peaceful grazers on beds of algae, or (like whales) particle feeders with filter chambers to extract the nutrients from the sea.

All this however is no more than a triviality when trying to conceive the processes behind that evolutionary leap to becoming a sighted creature requiring the simultaneous development of an eye composed of thousands of perfectly angled lenses with a photosensitive cell at the base of each and the neural networks in its brain to construct a visual image of the external world.

And how they endured! Each of those thousands of species persisting virtually unchanged for tens of millions of years. Then towards the end of their 250 million year reign, the trilobites life force seems to have exhausted itself, its numbers dropping precipitously till finally vanishing as suddenly as it had arrived—leaving behind a stunning affirmation, immortalised forever, of the creativity of life.

Life’s Big Bang: Part 1