By Lewis Loflin | Published May 9, 2025
The Cambrian period, starting around 541 million years ago, is often called an "explosion" of life. But it’s better seen as the culmination of earlier life, building on simple multicellular organisms and leading to an explosion of modern life forms. Fossils like Hallucigenia in the Burgess Shale show how the Cambrian surpassed earlier life in complexity and diversity, setting the stage for animals we recognize today.
Long before the Cambrian, simple multicellular life existed, though it was rare and less complex. Fossils from billions of years ago show the first steps:
The Cambrian period took these early steps and turned them into a huge leap, creating modern life forms. The Burgess Shale in Canada, from about 515 million years ago, captures this change with fossils like Hallucigenia, a tiny worm-like creature with legs and spines. Here’s how the Cambrian went beyond earlier life:
| Feature | Pre-Cambrian Life | Cambrian Life |
|---|---|---|
| Complexity | Simple, like Dickinsonia with no clear organs | More complex, like Hallucigenia with legs, spines, and a gut |
| Numbers | Few species, about 100 known | Thousands of species in the Burgess Shale alone |
| Spread | Mostly in shallow oceans | All over—reefs, deep oceans, muddy seafloors |
| Ecosystems | Passive, absorbing food from water | Active, with predators like Anomalocaris hunting smaller animals |
Hallucigenia, found in the Burgess Shale, shows how Cambrian life connects to animals today. It was small, about the size of a paperclip, with seven pairs of legs and spines on its back for protection. It likely crawled on the ocean floor, eating sponges or dead animals. Scientists now think it’s related to velvet worms, which live in jungles today and look like caterpillars. Other Cambrian creatures, like trilobites, are ancestors of crabs and insects, while early fish-like animals lead to humans. This makes the Cambrian an explosion of modern life forms, not just a random burst of strange creatures.
The idea of a “Cambrian explosion” makes it sound like life appeared out of nowhere, but it built on earlier life. Ediacaran fossils like Kimberella, which moved and ate, and worm trails from 565 million years ago, show multicellular life was already evolving. The Cambrian added more oxygen in the oceans, letting animals grow bigger and more active. It also brought hard parts, like trilobite shells, which fossilized better than soft Ediacaran creatures, making the Cambrian look more dramatic than it was.
The Cambrian left earlier life far behind. Ediacaran creatures were mostly still, soaking up nutrients from the water. In the Cambrian, animals like Hallucigenia moved, hunted, or defended themselves from predators like Anomalocaris, a giant shrimp-like creature. This created a busy ocean with food chains, something earlier life didn’t have. The variety and spread of Cambrian life—thousands of species across the globe—made it a new era, one that shaped the animals we see today.
In *The History of the Earth*, William K. Hartmann describes the Cambrian as a major step in life’s story. He likely highlights fossils like Hallucigenia to show how life became more complex, building on earlier forms like those in the Ediacaran. But he might overplay the “explosion” idea, when it’s really a culmination of millions of years of slow change.
The Cambrian period, starting 541 million years ago, was the culmination of earlier multicellular life, like simple algae and Ediacaran creatures. It led to an explosion of modern life forms, with fossils like Hallucigenia in the Burgess Shale showing the rise of complex animals—ancestors of insects, crabs, and even humans. By surpassing earlier life in variety, spread, and activity, the Cambrian set the stage for life as we know it, proving it wasn’t a sudden start but a big step forward.
Acknowledgment: Thanks to Grok, created by xAI, for drafting assistance. Final edits and views are mine.