Our universe, it seems, came into existence in the Big Bang 13.7bn years ago. Ten billion years later, according to the giant-impact hypothesis, the newly formed Earth collided with a sister planet, resulting in our moon.
During the last 540m years, at least five mass extinction events decimated plant and animal species. The ‘Great Dying’ of 252m years ago wiped out 96% of life-forms. The dinosaurs met their end when a 10km-wide asteroid struck the earth 66m years ago.
The current human population explosion is causing another extinction. We are on the threshold of catastrophic change.
But upheavals and explosions aren’t always destructive. Evolution itself may not be the painfully slow process of popular imagination. According to the theory of punctuated equilibrium, it moves in fits and starts. Around 540m years ago an irruption, known as the Cambrian explosion, took place.
Adam Sedgwick, an English clergyman and contemporary of Darwin, was a pioneering geologist. Working on rocks in Wales, he coined the term ‘Cambrian’ to describe the era in which the deposits were laid down; ‘Cymru’ is Welsh for Wales.
At the start of the Cambrian, 570m years ago, the areas we now call Europe and America had a wide ocean between them. Each side of the divide had its own characteristic marine life. The plates on which the continents rested moved slowly towards each other and collided. This brought the two landmasses together forming, among other things, what would eventually become Ireland; ancient American fossils are found west of a line from the Shannon to Dundalk while European ones tend to occur east of it.
There were no land plants or animals at the start of the Cambrian era but soft-bodied creatures lived in the sea. Then all kinds of marine animals suddenly appeared. The oldest jellyfish fossils, for example, date from this time. From 1909 to 1924, the remains of 65,000 “weird wonders” were discovered in the Burgess Shale, a fossil-rich Cambrian deposit in the Canadian Rockies.
New types of animal had arrived, but was there really what might be called an explosion? Creatures started to grow shells and create external skeletons from calcium salts at around that time. These remains fossilised well, whereas their soft-bodied predecessors didn’t and left few traces.
Tiny blue-green algae use sunshine water and carbon dioxide to create sugars, giving off oxygen in the process. As levels of oxygen rose during the early Cambrian, a tipping-point was reached at which whole new life-forms became possible.
In a paper just published, Guangyi Wei of Yale University, and colleagues from Nanjng University, challenge this gradualist theory. By measuring changes in uranium isotope ratios of carbonate deposits found in China, they detected patterns of oxygen level changes in the early Cambrian. The results show that oxygen levels fluctuated widely over periods lasting 2m to 10m years. There were times when almost no oxygen was present at some locations. These changes, the researchers claim, “could have destabilised ecosystems, fragmented habitats and triggered an explosion of changing life forms”.
As always in science, the answer to one question spawns a host of new ones. What, for example, caused oxygen levels to fluctuate in the first place? Did plate tectonics earthquakes and volcanoes have a role in them? Could climate changes have altered plant and animal densities?
‘It would be great to get more data from other regions and time intervals’, says lead author Wei More data from other regions may help provide an answer.