One of my pleasures is to remind people that reading any part of the fossil record is rather like taking part in a game of cards. Consider, for example, the following four competing hands: Q,Q,Q,Q / 6,7,8,9 / A,2,3,4 / 2,3,5,8. Our task here is to guess which is the winning hand. But have we got the right number of cards? And what game is actually being played? Our challenge has been to guess the name of the game.
Questioning our assumptions
So when have we been provided with the wrong cards or guessed at the wrong game? Where might we have been going wrong over the last few decades? Happily, my own research field is ripe with charming examples. Here is a list of some of my favourite seriously mistaken concepts relating to the history of life in deep time. I am happy to confess to being adjunct to some of these debates myself over the decades (Brasier 2009, 2012). So moving backwards in the manner of a Time Traveller from modern to ancient, here are some questionable guesses at gambits in life’s great game:
- Were mass extinctions caused by large environmental perturbations? This was popular in the 1980s but now looks highly misleading. Large perturbations could conceivably have caused the timing, but arguably not the magnitude of extinctions. Magnitude is likely related to the state and connectedness of the ecosystem itself. That means it is the conditions before a mass extinction — and the interconnectedness of the ecosystem — that now deserve our understanding. In short, we need a systems approach to the fossil record.
- Was the Cambrian explosion an explosion of fossils rather than of body plans? This idea has been popular in recent decades, but no longer looks sustainable on geological evidence: the high quality of preservation in the Neoproterozoic — almost wherever we look — fails to show fossil signs of a very deep ancestry. We are starting to realize that fossilization has evolved. And the Cambrian explosion was real.
- Does the Ediacara biota actually preserve the ancestors of Cambrian animals? This was orthodoxy until the 1990s, but few now accept this — except for the last few million years, where some precursors may have emerged. Instead, we must accept a much more interesting concept — that the rules for functional morphology (and hence for morphospace) were different before the advent of the through-gut circa 545 Ma. My hunch is that the evolution of the gut plus symbiotic microbes revolutionized ecosystems and changed the nature of biogeochemical cycling. We therefore need to explore co-evolutionary processes in deep time.
- Did eukaryotes largely evolve in the sea? Maybe. Maybe not. We know so little about the non-marine fossil record in the Boring Billion. But we are now discovering a surprisingly diverse biota, including eukaryotic algae, in one-billion-year-old lakes. Indeed, there are biological reasons for suspecting much of the evolution of green algae, cyanobacteria, and maybe even life itself could have taken place away from the sea. There is therefore a need to check out all manner of unexplored habitats, to fill in our missing fossil record. We need to challenge our long-entrenched assumptions about evolutionary habitat.
- Was oxygenic photosynthesis present in the early Archaean? Few are willing to accept this now. The Apex chert ‘microfossils’ (3.46 Ga) are now sadly dismissed (by myself and other researchers) as inorganic mineral growths that formed over eons in a hydrothermal vein system. Cyanobacteria are now thought to have evolved up to a billion years later, and even then they barely take on their familiar modern forms. This reminds us of the wider point that we must not shoehorn ancient structures into modern groups. Early organisms often differ from those we would expect to find. Think of Jurassic dinosaurs, Ordovician graptolites, Cambrian anomalocardids, or Ediacaran rangeomorphs. Fossils do it differently.
A final word of advice. Remember that science is not a belief system. Science is a unique system for the measurement of doubt. Nothing is sacred. And certainly not the words of an Oxford professor. Oh, and I nearly forgot. The winning hand was, of course, 2,3,5,8. (Fibonacci series: self-similarity, growth stability, multicellularity.)
Brasier, M (2009). Darwin’s Lost World: the hidden history of animal life. Oxford University Press. 304 p.
Brasier, M (2012). Secret Chambers: the inside story of complex life. Oxford University Press. 256 p.