Yorkshire Philosophical Society

Event Information

‘John Phillips and the Cambrian Explosion’

Professor Rachel Wood, Professor of Carbonate Geoscience, University of Edinburgh

In 1841, John Phillips published the first global geological time scale, based on the correlation of fossils in rock strata. This fundamental concept has underpinned our understanding of the ebb and flow of life on earth through time.  The Cambrian Explosion, 540-520 million years ago marks the appearance and rise of animals on Earth. During this time we saw the rapid emergence of all major modern groups, modern-style food webs, as well as a substantial rise of animal abundance and biodiversity.  This talk will explore the processes that may have driven this revolutionary event. Were both internal (genetic) or external (physicochemical) processes important? This radiation took place on a totally different Earth – with many continental land masses clustering around the tropics, no polar ice caps, and much lower atmospheric oxygen levels compared to today. Life had not colonised land and the modern carbon cycle was yet to form. While we are starting to understand how new forms of developmental gene regulatory networks, pulses of oxygenation, and ecological feedbacks played key roles, unpicking the drivers of the Cambrian Explosion remain a profound puzzle in the history of life.

A John Phillips 150th Anniversary Lecture

Lecture to be held in the Tempest Anderson Lecture Theatre, Yorkshire Museum,
YO1 7DR at 7pm.

Photo: Geological strata that record the root of the Cambrian Explosion. Nama Group, Namibia. Photo: Rachel Wood

Member’s report

This YPS talk was  given in celebration of the Yorkshire Museum’s first keeper, John Phillips, on the 150th anniversary of his death, 24th April 1874. (200th anniversary of his coming to York to assist his uncle William Smith, who gave a series of lectures to the YPS on geology.)

Professor Wood showed John Phillips’ global geological time diagram of life on earth and his naming of the three eras of life: Palæozoic, Mesozoic and Cænozoic which he constructed from the fossil record in rock strata. The two low points on the diagram are two major mass extinction events for life. She used this as a place from which to explore current thinking on the Cambrian Explosion of c. 538 million years ago.

Although Phillips did not have current methods of measuring the age of the Earth, he recognised a long pre-human history and estimated that one thousand million years would be needed for the formation and erosion of rocks. He maintained that basic body plans had persisted throughout the fossil record, though certain species became more common and others became less common. It is now accepted that most of the animal kingdom’s basic phyla evolved at the beginning of the Palæozoic-The Cambrian Explosion.

Professor Wood gave an account of how life and the Earth evolved together from the global glaciation of the Cryogenian and Ediacaran to the diversity of the Cambrian: a world so different from today. Deep oceans and atmosphere with much lower oxygen levels. No land plants, the only photosynthesis from bacteria and algae. Land inhospitable to life. No ice at the poles and large areas of shallow continental seas.

From enigmatic, soft-bodied Ediacaran life, evolved and radiated metazoans/animals with so many recognisable features: clonality, surface locomotion with complex tracks and traces, skeletal parts, appendages, limbs, vision, thought-guts and predation – over a forty-million-year period. Evidence for hard and soft body preservation has been found in the Burgess Shales of Canada and Aldan River Siberia.

This adaptive radiation came about when a single or small group of ancestral species rapidly diversified into a large number of descendant species.

She discussed results from her field work in Namibia where, following life from shallow to deep, gave evidence of anoxic and oxic sites and allowed measurement of increases in amount and types of oxygen. Cycles of surface simple life would bring pulses of oxygenation.

Plate tectonics, mountain building and global sea level rises, the position and erosion of land masses, cyclic alteration in the rotation and orbit of the Earth, global climate changes, interactions between the planet’s biota and their differences in need for oxygen all dance around one another.

So much of life is a cyclical response to physical and chemical changes in the environment. In lineage populations, evolution tends to increase body size over geological time, but surviving species grow small (even toy size) at the time of mass extinctions.

The question remains as to whether the ‘Explosion’ could be driven by internal genetic processes that could be separated from environmental ones. Many aspects of this would not have been known to Phillips, but his insight has inspired so much that followed.

Paul Thornley