567-Million-Year-Old Fossils Suggest Animals Evolved Earlier Than We Thought

The story of Earth's first animals might need to change. New fossils, 567 million years old, suggest that complex life started much earlier than we thought.

Earth is home to many different animals, from butterflies to whales. Understanding how these animals evolved from simpler forms is a key part of life's history on our planet.

Scott Evans, a curator at the American Museum of Natural History, led a new study. It uses these rare fossils to show that animal evolution may have begun much earlier. The findings were published in Science Advances.

Long before fish or land animals, large and complex creatures lived on the ocean floor.

The Ediacaran World

Some of these early animals were soft-bodied and strange. They looked like pancakes, soft tubes, or spirals pressed into the mud.

This period, from about 635 to 538 million years ago, is called the Ediacaran Period. Scientists wonder if these animals are our ancient ancestors or if they were failed evolutionary experiments. The Cambrian explosion, which happened later, created most of the animal groups we know today.

To help answer these questions, the Ediacaran fossil record is split into three main parts: the Avalon, White Sea, and Nama assemblages. Each represents a different group of Ediacaran animals found at different times and in different places.

These chapters help scientists see how early animal life changed. It went from mostly deep-water organisms stuck in mud to more varied shallow-water communities that included animals.

Ancient Fossils Change the Timeline

The Avalon assemblage is the oldest, with simple, strange deep-water organisms. The White Sea assemblage is the middle chapter. It has larger, more varied animals, like the famous Dickinsonia. This ribbed, oval organism looks a bit like a quilted placemat.

The Nama assemblage is the most recent. It includes some of the first animals with hard, shell-like parts.

The research team combined fossil hunting with geology. They collected and photographed fossil rocks from the remote Mackenzie Mountains in Canada. They compared these fossils to other Ediacaran organisms. They also studied nearby rocks to figure out when and where these animals lived.

Many of the fossils looked like those from the White Sea assemblage. They had frond-like shapes and segmented, quilted bodies. This is important because the White Sea animal community was mostly known from sites in Russia and Australia.

The new fossils show that similar communities also lived in the deep waters of Laurentia. This was an ancient continent that included much of North America.

Deep Oceans: A Cradle for Animal Evolution?

In early animal evolution, a few million years can make a big difference. The rocks with fossils seem to match nearby layers that are about 567–566 million years old.

If this is correct, it means this community is much older than the classic White Sea assemblage. That assemblage is usually dated to about 560–550 million years ago. This discovery pushes back the timeline for some important early animals, including mobile forms like Dickinsonia.

It also changes our understanding of their environment. White Sea-type fossils are usually found in shallower marine areas. But these Canadian rocks suggest the animals lived in a deep-water slope environment. This means these early animal communities were more widespread and could adapt to more environments than previously thought.

A More Gradual Story of Early Evolution

This raises an interesting question: Did early animal ecosystems first develop far offshore in deeper, more stable marine settings? Did they then become common in shallower seas later?

This discovery is important because it blurs the lines between the classic Ediacaran "chapters." The Avalon and White Sea assemblages might not have been a clean switch, where one world disappeared and another suddenly took its place.

Instead, the new Canadian fossils suggest an overlap. Avalon-style frond-like organisms and more diverse White Sea-style animals may have lived together in similar deep-water environments.

This makes early animal evolution seem less like a sudden change and more like a gradual expansion. Animals were trying out new body shapes, new ways of living on the seafloor, and perhaps new ways of moving and feeding.

How Environments Shaped Early Animal Life

The roots of modern animal diversity might come from a long, uneven process. This process began in deeper marine environments, far from the Ediacaran sun's warmth. It happened before many animal groups became common in shallower seas.

The study also brings up a broader idea about evolution. Environments help shape life. A soft-bodied animal in a quiet, deep seafloor faced different challenges than one in shallow water with waves, light, currents, and shifting sand. These pressures can influence which body shapes and behaviors are useful and get passed on.

This is where convergent evolution comes in. Convergent evolution happens when unrelated organisms develop similar solutions to similar problems. Examples include wings in birds, bats, and insects, or streamlined bodies in fish, dolphins, and extinct marine reptiles.

Evolution is like nature's problem-solver. It repeatedly finds solutions under changing environmental rules over billions of years.

The same general solutions, like tubes, fronds, and flattened bodies, may have been tried many times as early animals explored the seafloor. Over vast periods, life can seem incredibly inventive. But it is always shaped by Earth's constant testing ground.

Deep Dive & References

• Scientists can’t agree on when the first animals evolved – our research hopes to end the debate - The Conversation, 2023
• Avalon-style frond-like organisms - PNAS, 2014
• Fronds - PLOS ONE, 2016