Cambrian Explosion: An Astonishing Surge in Biodiversity

Fossils discovered in Canada and worldwide point to a remarkable surge in biodiversity approximately 539 million years ago, known as the Cambrian explosion. During this period, a variety of complex animal forms and functions emerged in the fossil record.

Fossil evidence indicates the presence of skeletons, shells, sensory organs, and innovative locomotion methods. This transformation would have significantly altered the ecosystems on our planet by introducing behaviors such as predation and defense.

The central question remains: “What triggered this explosion of life?” A key challenge in utilizing the fossil record to unravel animal origins is a phenomenon termed "exceptional preservation."

The Challenge of Fossilization

Most animal remains do not become fossils. The process of fossilization demands very specific environmental conditions to inhibit, or at least slow, decomposition, allowing sediment to preserve the animal remains as it solidifies into rock.

Scientists are divided over whether the Cambrian explosion signifies a sudden emergence of new species or merely improved conditions that resulted in a higher number of preserved fossils. Even Charles Darwin found the abundance of animal fossils from the Cambrian period perplexing, especially given their absence in older Precambrian strata.

Dr. Ross Anderson, a Royal Society University Research Fellow at Oxford University's Department of Earth Sciences, has spent the last decade using fossils to trace the evolution of multicellular life, examining how cells aggregate to create more complex structures and exploring the origins of animal life.

Connecting Fossil Diversity to Environmental Changes

Professor Anderson aims to decipher the relationship between fossil diversity and environmental shifts, focusing on the era that set the stage for the Cambrian explosion.

Recently, the journal Trends in Ecology and Evolution published a study led by Professor Anderson, which advances our understanding of when and how animals emerged on Earth.

One reason for the ongoing debate about the timeline of animal origins is the concept of the "molecular clock." By comparing the genes of related species to determine the time elapsed since they diverged from a common ancestor, scientists estimate that animals originated around 800 million years ago. Yet, the oldest known animal fossils date back only 539 million years.

Explaining the Discrepancy

This gap has prompted biologists to seek explanations. Is the molecular clock inaccurate in estimating evolutionary timelines, or did the fossil record fail to capture animal remains from earlier epochs?

To explore this question, Professor Anderson and his team conducted a comprehensive evaluation of the conditions necessary for the earliest animal remains to be fossilized. “The first animals likely lacked mineralized shells or skeletons, necessitating exceptional conditions for fossilization,” Professor Anderson states. “However, certain Cambrian mudstone deposits demonstrate remarkable preservation, even of delicate animal tissues.”

These favorable preservation conditions are referred to as Burgess Shale-Type (BST) preservation. The researchers examined samples from about 20 sites of Cambrian mudstone deposits.

Key Clays in Fossil Preservation

The research revealed that fossils exhibiting exceptional BST preservation were abundant in berthierine, a type of antibacterial clay. A similar phenomenon occurred with another clay known as kaolinite.

“The presence of these clays was the primary indicator of whether rocks would contain BST fossils,” Professor Anderson noted. “This implies that the clay particles act as an antibacterial barrier, preventing microorganisms from decomposing organic materials.”

Equipped with this insight, the team evaluated precambrian mudstone deposits recognized for their fossil content. Most of these deposits lacked the appropriate clays for BST preservation.

Despite favorable conditions, three locations—Nunavut in Canada, Siberia, and Svalbard in Norway—contained deposits at least 789 million years old without any animal fossils.

As Professor Anderson elaborates, “This serves as the first evidence for absence, supporting the perspective that animals had not evolved by the early Neoproterozoic era, contrary to some molecular clock predictions.”

Thus, the Cambrian explosion appears to be more than just a case of improved preservation conditions; it likely signifies a genuine surge in biodiversity and evolutionary innovation during that period.

Delving Deeper: Why the Cambrian Explosion?

The deeper inquiry is: what led to this explosion? Was it the culmination of a prolonged, subtle evolutionary process? Did ecological or geological factors suddenly create new opportunities or challenges for the emergence of animals? This remains a key focus for future research.

Humanity continually seeks a scientifically grounded narrative to explain the world and our role in it. Pinpointing the moment in Earth's history when animal origins began—and understanding the reasons behind it—is a critical chapter in that ongoing story.

A Broader Perspective on Biodiversity

Professor Anderson's findings elucidate the mechanisms that contributed to our intricate web of biodiversity. This research aligns with a global resurgence in recognizing our connection to the natural world.

In their research paper, the team suggests, “Future investigations could enhance our understanding of the conditions conducive to fossilization across different times and spaces, and how such conditions might facilitate the preservation of diverse animal groups.”

There is always more to uncover if we are willing to explore.

Learn more:

• New Oxford study sheds light on the origin of animals
• Fossilization processes and our reading of animal antiquity
• First Animals: Oxford Museum of Natural History
• Beaver Fossil Sheds Light on Mammal Evolution
• Bird Evolution Can Be Surprisingly Fast
• Life Began Even Earlier Than Thought