By Amara Muller ’29
What if one of evolution’s most famous events didn’t happen when scientists thought it did? For decades, most scientists believed that complex life on Earth arose through the Cambrian Explosion, a sudden burst of rapid evolution beginning around 535 million years ago; this diversification of life led to many of the organism groups we see today. However, recent fossil discoveries in China are challenging this narrative. If these new findings prove to be accurate, complex life’s origins may stretch back millions of years before previously thought, and the ‘explosion’ theory may require more nuance than before.
The Transitions Heralded by the Cambrian Period
Before the Cambrian Period spanned the Ediacaran Period, lasting from 635 to 541 million years ago. The majority of the species at this time were simple life forms, most being unicellular or very small multicellular organisms. The larger species that existed were soft-bodied creatures resembling modern-day plants or extremely flat leaf-like organisms that lived only on the sea floor.
Scientists have previously thought that these simple organisms rapidly diversified during the Cambrian Explosion. One key event of the explosion was the evolution of bilateral animals; unlike the creatures present in the Ediacaran, Cambrian creatures possessed symmetrical right and left sides, and distinct front, back, top, and bottom ends. Additionally, all bilateral creatures had three germ layers (which would subsequently differentiate into bodily components) in their embryos. The bilaterians would eventually take over the animal kingdom as both adaptations allowed them to travel in sediment, move up and down in the water, and to develop a nervous system.
The evolution of creatures capable of burrowing and producing hard exoskeletons was also a major development. These hard-shelled trilobites, along with other arthropods, became one of the most common organisms. Simultaneously, the evolution of the first burrowing species also occurred, allowing organisms to exploit the wealth of nutrients below the sediment, and supporting the growth of new plankton species and food webs.
Another Heading
While this rapid explosion of life during the Cambrian was thought to begin 535 million years ago, new discoveries have brought that timeline into question. A team of researchers recently unearthed a collection of 700 fossils in China’s Yunnan province dating from 539 to 554 million years ago. The fossils show complex, three-dimensional organisms with tentacles and feeding apparatuses which ascended the water levels to feed. One key discovery is fossils appearing to be deuterostomes, a group that today includes vertebrates. This revelation would push the evolution of deuterostomes back into the Ediacaran period for the first time, pushing back the start of the period of rapid evolution by millions of years.
Many unearthed species also simultaneously displayed characteristics thought to belong exclusively in either the Cambrian and Ediacaran period. This crucial discovery of features thought only to emerge in later years shows that complex life evolved earlier than previously thought. The Yunnan fossils also provide the first concrete evidence that rather than an abrupt shift, the Cambrian Explosion was more like a part of a longer, more gradual transformation. With this breakthrough, previously established evolutionary timelines have been altered. Future research should continue to investigate this incredible time period of transition, and dig deep into whether more fossils like those in Yunnan can be found. Who knows? The findings might shake some of the foundations of evolution.
