From butterflies to blue whales, corals and worms, Earth is home to an incredible diversity of animals. How all of these animals evolved from earlier, simpler ancestors is one of the most exciting stories in the history book of life on our 4.5 billion-year-old planet.
A new study, published today in Science Advances, adds crucial information to this story. Led by Scott Evans, assistant curator of invertebrate palaeontology at the American Museum of Natural History, it draws on rare 567-million-year-old fossils to show animal evolution may have started far earlier than previously thought.
Alex Boersma
Ancient life on the seafloor
Long before life on land or even fish, Earth’s seafloor was home to large and complex animals.
Some of these soft-bodied and strange animals were shaped like pancakes. Others were more like soft tubes or spirals that pressed into the mud.
We call this time, from about 635 to 538 million years ago, the Ediacaran Period. Do animals from this period represent our ancient ancestors before the Cambrian explosion, which produced most of the basic groups of animals we know today? Or are they failed evolutionary experiments?
To help us answer these questions, we divide the Ediacaran fossil record into three broad chapters: the Avalon, White Sea and Nama assemblages. Each represents a distinctive community of Ediacaran animals that tend to appear in different times and environments.
These chapters help scientists track how early animal life changed from mostly deep-water organisms that were stuck in mud to more diverse shallow-water communities that included animals.
The Avalon assemblage is the oldest chapter, dominated by simple yet strange deeper water organisms. The White Sea assemblage is the middle chapter. It is characterised by larger, more varied animals, including forms such as the famous Dickinsonia, a ribbed, oval organism a bit like a quilted placemat. The Nama assemblage comes last and includes some of the earliest animals with hard shell-like parts.
Ryan Schwark/Wikimedia, CC BY
Combining fossil hunting with geological detective work
The team behind the new study combined fossil hunting with geological detective work. They collected and photographed fossil-bearing rocks from the remote Mackenzie Mountains in Canada, compared the fossils with other Ediacaran organisms, and studied nearby rocks to reconstruct where and when these animals lived.
Remarkably, several of the fossils, with frond-like forms, segmented and quilted bodies resembled those from the White Sea assemblage. That matters because the White Sea animal community was previously best known from famous sites in Russia and Australia.
The new fossils show that similar communities had also reached the deep waters of Laurentia, the ancient continent that included much of present-day North America.
In early animal evolution, a few million years can matter. The fossil-bearing rocks appear to correlate with nearby layers dated at about 567–566 million years old.
If that correlation is correct, this makes the community considerably older than the classic White Sea assemblage, which is usually placed at about 560–550 million years ago. Their discovery pushes back the timing of some important early animals, including mobile forms such as Dickinsonia.
It also dramatically changes the environmental picture.
White Sea-type fossils are usually associated with shallower marine settings. But these rocks suggest the Canadian animals lived in a deep-water slope environment. Together, that implies these early animal communities were both more geographically widespread and more environmentally flexible than previously recognised.
That raises an intriguing question. Did early animal ecosystems first develop far offshore, in deeper and perhaps more stable marine settings, before later becoming common in shallower seas?
Scott Evans
Blurring the boundaries
The discovery matters because it blurs the boundaries between the classic Ediacaran “chapters”. The Avalon and White Sea assemblages may not represent a clean handover, with one world disappearing and another suddenly replacing it.
Instead, the new Canadian fossils suggest overlap: Avalon-style frond-like organisms and more diverse White Sea-style animals may have shared the darkness and lived together in similar deep-water settings.
That makes early animal evolution look less like a sudden switch and more like a gradual ecological expansion. Animals were experimenting with new body shapes, new ways of living on the seafloor, and perhaps new ways of moving and feeding.
The roots of modern animal diversity may therefore lie in a long, uneven process that began in deeper marine environments far from the warmth of the Ediacaran sun, and before many animal groups became common in shallower seas.
A broader evolutionary idea
The study also raises a broader evolutionary idea.
Environments help shape life. A soft-bodied animal living on a quiet, deeper seafloor faced different challenges from one living in shallow water affected by waves, light, currents and shifting sediment. Those pressures can influence which body shapes and behaviours are useful and are passed on.
This is where the idea of convergent evolution can become helpful. Convergent evolution is when unrelated organisms evolve similar solutions to similar problems like wings in birds, bats and insects, or streamlined bodies in fish, dolphins and extinct marine reptiles.
In this sense evolution is repeated problem-solving under changing environmental rules over billions of years.
The same broad solutions, tubes, fronds, flattened bodies, may have been tried repeatedly as early animals explored the seafloor.
Over deep time, life can look uncannily inventive. But it’s shaped by the relentless testing ground of Earth itself.
