The earliest signs of complex life first emerged about 538 million years ago. Fossils from this time, like traces made by worm-like creatures known as Treptichnus, reveal a sudden explosion of diversity, including ancestors of crabs, mollusks, and sea urchins. This rapid appearance puzzled Charles Darwin and continues to challenge scientists today.
In 1859, Darwin reflected on the scarcity of early life fossils in his book On the Origin of Species, admitting he couldn’t explain why evidence from these primordial times was missing. Today, experts still debate when complex animals actually evolved, a discussion that began with the “molecular clock” concept in the late 20th century. This theory suggests that genetic changes accumulate at a steady rate, allowing scientists to estimate when species diverged.
For example, humans and chimpanzees separated about 6 million years ago. If a gene shows six differences between the two, this implies one difference equals one million years, assuming the molecular clock ticks consistently. Early studies even suggested that the common ancestor of all complex animals existed over 1.2 billion years ago. However, more recent estimates suggest it was closer to 570 million years—still before the first fossils appeared.
This gap of around 30 million years presents a challenge. How could such ancient creatures exist without leaving behind fossils? One theory posits that during this time, these animals were small and soft-bodied, making fossilization difficult. An increase in oxygen levels later allowed these creatures to grow larger and evolve into the more complex forms we recognize today.
Recent research by paleontologist Graham Budd and mathematician Richard Mann offers a new perspective. They propose that whenever a significant group of organisms emerged, evolution may have sped up. In this case, the molecular clock might actually tick faster than previously thought, making it seem like more time had passed than it actually did.
This could solve the inconsistency between the molecular clock and the fossil record. For instance, it might help explain why early flowering plants existed for millions of years before showing up in fossils. It could also clarify whether early mammals coexisted with dinosaurs, a hot topic in evolutionary studies.
The modern understanding of evolution is much more complex than Darwin’s time, but these developments would likely interest him. As we dig deeper into our planet’s history, we unravel more mysteries of how life evolved on Earth.
For more insights on evolutionary biology, you can check out Nature or explore recent research on these topics.
