The Oldie: Profitable Wonders – October 2011
Life’s Big Bang: Part 1
Science’s ability to penetrate deep into the unimaginably distant past never ceases to amaze. It is impressive enough that we can trace our human lineage all the way back to our earliest ancestors on the plains of the African Savannah three million years ago. But traversing the aeons of preceding time, palaeontologists can now provide a comprehensive account of the whole range of complex life forms that emerged during the Cambrian period 530 million years ago—so long ago that in the interim, as it were, the glacial movement of the tectonic plates beneath the earth’s surface have had time enough to elevate the depths of the oceans to the soaring peaks of the Rocky Mountains.
This notion of the Cambrian ‘explosion’ of life is scarcely novel. Victorian geologists tapping away at ancient rocks with their hammers were forcibly impressed by the dramatic transition between those strata empty and devoid of life only to be filled, from apparently nowhere, by the sudden influx of billions upon billions of fossilised remains.
Still it was not till the first decade of the twentieth century that the opportunity arose to examine those fossils in the detail necessary to recognise their unique and striking characteristics. In 1909 Charles Walcott of the Smithsonian Institute in Washington was returning from an expedition in the Rocky Mountains when his (pregnant) wife’s horse stumbled on a rock—prompting him to split the offending boulder with his hammer, revealing a profusion of the most perfectly preserved fossils ever encountered. Since then palaeontologists have dug out, scrutinised and categorised 70,000 specimens from the Burgess Shale, as it is known, (and similar more recently discovered sites in Siberia and China)—the highlights of which Professor Simon Conway Morris describes in his book The Crucible of Creation.
We start with the’mud dwellers’ in the ocean floor featuring the ‘efficient and dangerous’ worm-like predator Ottoia whose retractable proboscis sucks its prey towards its mouth where a formidable array of sharp teeth, pointing inwards and downwards extends astonishingly into the upper line of the digestive tract —ensuring there can be no escape. Then we encounter the remarkable variety of sponge-like ‘mud-stickers’ fixed to the ocean floor such as Dimonischus that resembles a daisy with a long stalk topped by a goblet-shaped body formed from a palisade of plates each covered by numerous miniscule hair-like cilia that strain the sea water from food. Next we meet this extraordinary bestiary of ‘strollers walkers and crawlers’ including the famed trilobite with its armoured carapace and Hallucigenia, so called because of its ‘bizarre and dream-like appearance’ propelling itself on seven sets of stilts echoed by seven tentacles protruding from its back. And finally there are the ‘swimmers and floaters’ such as the darting lancet-like Pikaia that moves by flicking its body in a series of rapid side to side undulations.
This exhilarating exuberance of the these fossils would seem to contradict the common perception of the Tree of Life starting off simply enough before diversifying into ever more sophisticated and complex forms. And indeed it does—but its true significance is much profounder still.
The main virtue of the scientific method is its ability to reveal the hidden and unifying reality behind appearances—and no more so perhaps than in recognising that the millions of species with which we share the planet, and the vastly greater number long since extinct, can all be categorised as belonging to just one or other of a limited number of basic ‘body plans’. Thus while the diverse forms of insects (butterflies, beetles, flies, ants and so on), crustaceans (crabs, lobsters, shrimps) and the arachnids (notably spiders) could scarcely be more distinct, they are all arthropods built on the same plan with segmented bodies consisting of a head, thorax and abdomen, six or more legs, held in place by an external exoskeleton. This is in marked contrast to, for example, the very different body plan of the legless worms with their long cylindrical tube-like bodies, or the echinoderms such as the star fish or sea urchin defined by the radial symmetry of their five or more similar parts. And then there are the chordates with a backbone and spinal cord and complex circulatory and nervous systems that encompasses the millions of species of fish, reptiles, birds and mammals (including ourselves).
The most astonishing of all the extraordinary observations—which has only recently become apparent—to emerge from categorising these fossilised forms of life that arrive ‘from apparently nowhere’ during the Cambrian explosion is that (bar one) each of these basic ‘body plans’ is represented. “Let us seek to fathom those things that are fathomable", observed the great poet and naturalist Johann Goethe, “and reserve those things that are unfathomable for reverence and quietude”.