ANIMAL ORIGINS REVEALED BY RETHINKING FOSSIL RECORD

As Canada Day approaches, I’m planning my usual trip to the family cottage. One of the activities I’ve enjoyed there since childhood has been fossil hunting on the Bruce Peninsula’s limestone beaches.

The rocks there are teeming with fossils, marine animals beautifully preserved in their sedimentary layers. Most of the fossils record the shapes of invertebrates with bivalve shells called brachiopods.

We keep some of the best samples we’ve collected on a shelf in the den as reminders of our summer home. Glancing at them now reminds me of something that’s always puzzled me about the fossil record.

CAMBRIAN EXPLOSION – REMARKABLE BURST OF BIODIVERSITY

Based on the animal remains preserved in Canada and around the globe, there was a remarkable burst of biodiversity around 539 million years ago. Scientists call it the Cambrian explosion, because a wide range of complex animal forms and functions suddenly appear in fossils from that period.

Fossil traces reveal skeletons, shells, sensory organs and new ways of moving. This would have changed our planet’s ecosystems by introducing animal behaviours like predation and defence.

The riddle is, “What caused this burst of life?” The trouble with relying on the fossil record to understand animal origins is something scientists call “exceptional preservation.”

MOST ANIMAL REMAINS DON’T END UP AS FOSSILS

Most animal remains don’t end up as fossils. Preserving a fossil calls for very specific environmental conditions to prevent, or at least slow down, decomposition so the sediment records the animal remains as it hardens into rock.

Scientists are divided on whether the Cambrian explosion reflects a sudden blooming of new species or simply better conditions that preserved more fossils. Even Charles Darwin felt stumped by the wealth of animal fossils in Cambrian samples and their absence from Precambrian rock.

Dr. Ross Anderson is a Royal Society University Research Fellow in the Department of Earth Sciences at Oxford University. He’s been using fossils to map the development of multicellular life, the way cells combine to form more complex structures, and animal origins for the past decade.

FOSSIL DIVERSITY AND ENVIRONMENTAL CHANGES

Professor Anderson strives to understand the connection between fossil diversity and environmental changes. He focuses on the time period that laid the foundation for the Cambrian explosion.

This week, the journal Trends in Ecology and Evolution published a study Professor Anderson led. Its findings take us a step closer to understanding how and when animals first arose on Earth.

One reason scientists disagree about the timing of animal origins is what they call the “molecular clock.” When they compare the genes of related species to the elapsed time since they evolved from a common ancestor, they calculate that animals appeared about 800 million years ago, yet the oldest animal fossils are only 539 million years old.

BIOLOGISTS DETERMINED TO EXPLAIN DISCREPANCY

That’s quite a discrepancy, and biologists are determined to explain it. Does the molecular clock overstate the time it takes for species to evolve, or did the fossil record simply not preserve animal remains in the distant past?

To that end, Professor Anderson and his team carried out the most thorough evaluation so far of the conditions causing the earliest animal remains to fossilize. “The first animals presumably lacked mineral-based shells or skeletons, and would have required exceptional conditions to be fossilized,” Professor Anderson explains. “But certain Cambrian mudstone deposits demonstrate exceptional preservation, even of soft and fragile animal tissues.”

These favourable conditions are called Burgess Shale-Type (BST) preservation. The researchers analyzed samples of Cambrian mudstone deposits from about 20 sites.

TWO TYPES OF CLAY DRIVE FOSSIL PRESERVATION

The team’s findings show that fossils with exceptional BST-type preservation were rich in berthierine, an anti-bacterial clay. Something similar happened with another type of clay called kaolinite.

“The presence of these clays was the main predictor of whether rocks would harbour BST fossils,” Professor Anderson said. “This suggests that the clay particles act as an antibacterial barrier that prevents bacteria and other microorganisms from breaking down organic materials.”

Armed with this understanding, the team analyzed deposits from a range of precambrian mudstone deposits known for their fossil content. It turned out that most didn’t have the right clays for BST preservation.

NO ANIMAL FOSSILS DESPITE FAVOURABLE CONDITIONS

There were three that did however, in the Canadian territory of Nunavut, in Siberia and in Svalbard, Norway. Even so, these deposits, which were at least 789 million years old, contained no animal fossils despite their favourable conditions.

As Professor Anderson explains, “This provides the first “evidence for absence” and supports the view that animals had not evolved by the early Neoproterozoic era, contrary to some molecular clock estimates.”

So it looks like the Cambrian explosion wasn’t just a matter of better preservation conditions. There seems to have been a genuine burst of biodiversity and innovation during that era.

GENUINE BURST OF BIODIVERSITY AND INNOVATION

The deeper question is why? Is the Cambrian explosion the climax of a longer, subtler evolutionary process? Did ecological or geological factors suddenly offer new opportunities or challenges for animal origins? This is the focus going forward.

Humanity keeps searching for a new, science-based story explaining the world around us and our place in it. Pinning down the point in Earth’s deep history where animal origins began, and why, is an important chapter in that narrative.

AND ANOTHER THING…

Professor Anderson’s discovery helps explain the processes by which our web of biodiversity emerged. It’s part of a global reawakening to our relationship with our natural world.

In their research paper, the team suggests, “Future research could lead to a better understanding of the availability of of favourable fossilization conditions in space and time, and how such conditions might promote the preservation of different animal groups.”

We always have more to learn if we dare to know.

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