About 4.5 billion years ago, our planet formed. Around a billion years later, the first life forms appeared, which were mostly single-celled microbes, and this is pretty much how life was on our planet for billions of years. Then, about 540 million years ago (or some three billion years after those microbes first appeared), multicellular life finally took off during the Cambrian explosion. What took it so long?
A recently published study suggests that not only were oxygen levels relatively low before this point (which has been thought to have limited evolution during Precambrian times – limited oxygen limits the size an oxygen-dependent organism can be), but the levels of other important chemical elements (certain trace metals) were also reduced, possibly because of the lack of oxygen. The researchers figured out these intricate chemical changes by analyzing thousands of ancient rock samples using a novel, sensitive lab technique they developed.
Specifically, the study revealed that right before the Cambrian explosion, oxygen levels significantly increased, as did the levels of several trace metals (molybdenum, selenium, uranium, copper, and nickel). The authors propose that the change in the levels of trace metals may be tied to the changes in oxygen levels. So what caused the oxygen levels to increase? This is thought to be due to the actions of photosynthetic (oxygen-producing) microbes over the course of billions of years.
It is worth noting that large organisms – the Ediacara biota — did exist before the Cambrian explosion, though little is known of them (for example, it’s unclear whether they should be classified as plants, animals, or something completely different). The Precambrian explosion brought with it an amazing range of multicellular diversity that’d never been seen on our planet before.
It’s always fascinating to see how the future brings us tools that help us better understand the past, even as it becomes increasingly distant (though when you’re dealing with billions of years, a few years spent developing a new technology is completely insignificant!).
For further reading:
- Ross R. Large et al.’s article “Trace element content of sedimentary pyrite as a new proxy for deep-time ocean-atmosphere evolution“
- ScienceDaily’s article “Evolution stuck in slime for a billion years”
- Teisha J. Rowland’s book Biology Bytes: Digestible Essays on Animals Both Commonplace and Bizarre