For the longest time, it was believed all complex animal life could be traced to the Cambrian explosion that occurred over 500 million years ago. However, the discovery of Dickinsonia, a now-iconic member of the Ediacaran biota and possibly the oldest macroscopic animal fossil recorded to date, challenges these notions and our understanding of evolution on Earth.
Living approximately 558 million years ago — well before the rapid diversification of animal body plans in the Cambrian — Dickinsonia raised fundamental questions about growth, movement and the evolution of early developmental strategies. Its fossilized imprints challenged conventional narratives on the origin of complex life and provided direct clues about early animal physiology and ecology.
Furthermore, the 2018 discovery of distinctive molecular signatures in fossils, particularly cholesterol derivatives, has transformed our view of these soft-bodied organisms and provided strong evidence for their classification as some of the earliest animals in evolutionary history.
The Discovery Of Dickinsonia
Dickinsonia was first discovered in the late 1940s in South Australia’s Flinders Ranges. Paleontologist Reg Sprigg initially identified its distinctive quilted pattern and named it in honor of Ben Dickinson, a government official with the South Australian Mines Department.
Over the ensuing decades, fossils of Dickinsonia were reported from several continents, including sites in Russia and Ukraine, suggesting a cosmopolitan distribution during the Ediacaran period. These organisms were preserved only as impressions or casts in quartz sandstones — typical of soft-bodied organisms that left little by way of hard skeletal parts.
The unique traits of Dickinsonia include its bilaterally symmetric, oval shape with a pronounced anterior–posterior axis. Repeating modules or isomeres run along its length, arranged in an alternating fashion that some researchers interpret as a form of glide reflection symmetry rather than strict bilateral segmentation.
Studies on Dickinsonia have thus steadily shifted its classification due to how difficult it has been for scientists to peg it to a particular species. Once hypothesized as a jellyfish, fungus or even a giant protist, science now favors an animal interpretation.
Clues To Dickinsonia’s Classification As Animal
For decades, Dickinsonia’s placement in the Tree of Life was hotly debated. Early interpretations ranged from it being a member of the Cnidaria, a group including jellyfish and sea anemones, to suggestions that it was a fungus or even a giant single-celled protist.
Morphological features alone proved insufficient to resolve its systematic position since Dickinsonia exhibits few structures similar to modern animals. However, with the emergence of innovative geochemical techniques, researchers began to look at the molecules trapped within the fossil tissues, moving beyond mere morphology.
The discovery of animal-specific cholesterol derivatives in Dickinsonia fundamentally altered its classification and strongly argued against non-metazoan alternatives. This is because cholesterol and its degradation product cholestane are found exclusively in animal cell membranes, thus providing molecular evidence of an animal affinity.
Researchers suspect it likely lived on shallow marine substrates where it “crawled” slowly, feeding on microbial mats by external digestion — a behavior somewhat analogous to modern placozoans, the phylum of free-living, non-parasitic marine invertebrates.
The current consensus, therefore, leans toward classifying Dickinsonia as an animal. More specifically, it may have been an early relative of simple animals like placozoans or part of a completely new branch of early animals that later gave rise to creatures with bilateral symmetry.
Studies of its growth patterns have shown consistent evidence for a regulated developmental program, a signature trait of animal life. These findings have not only provided the positive evidence needed to affirm Dickinsonia’s animal status but have also reshaped our understanding of the Precambrian evolutionary landscape.
From Ediacaran Precursors To The Cambrian Explosion
The discovery and classification of Dickinsonia have had profound implications for our understanding of the Cambrian explosion. This rapid burst of evolutionary innovation, beginning around 500 million years ago, gave rise to nearly all the modern animal phyla.
Dickinsonia and other Ediacaran organisms were once viewed as evolutionary dead ends — enigmatic forms with no descendants. However, the mounting evidence that Dickinsonia was an animal forces us to reframe these early organisms not as isolated experiments but as integral steps in the progression toward more complex body plans.
Recent research has demonstrated that Dickinsonia’s biological features — their regulated modular growth, evidence of animal-specific biomolecules and locomotive trace fossils — establish them as a credible evolutionary bridge between simple, soft-bodied organisms and the complex hard-bodied animals that dominate the Cambrian fossil record.
In this light, the Ediacaran biota, far from representing failed experiments, appears to have set the stage for the diversification of animal life as we know it today.
Does reading about how all life on Earth possibly traces back half a billion years fill you with appreciation for nature’s guiding hand? Take the Connectedness To Nature Scale and find out how deep your connection is with the natural world.
