Cambrian explosion: Fossils push back the rise of complex life

Climate, geology, and biology together gave rise to one of Earth’s most dramatic turning points: the Cambrian explosion. About 530 million years ago, most of the basic body plans of organisms that have survived until now emerged.

Yet a new study argues that key elements of this revolutionary event actually took shape earlier – perhaps ten-plus million years before the formal start of the Cambrian Period.

The research relies not on bones or shells, but on trace fossils – the tracks, burrows, and other imprints left by organisms as they crawled through ancient sediments.

Co-authors Olmo Miguez Salas of the University of Barcelona and Zekun Wang of London’s Natural History Museum analyzed these subtle signatures.

The goal was to reconstruct early animal anatomy and behavior during the Ediacaran-Cambrian transition, roughly 545 million years ago.

Fossil traces reveal ancient life

Traditional fossil studies skew toward creatures with hard parts, because bones and shells preserve most readily in rock. Trace fossils capture a different story: the activities of soft-bodied animals that rarely leave skeletal remains.

“The trace fossil record provides valuable information about evolutionary periods when soft-bodied fauna were dominant,” Miguez Salas said.

“Fossil traces reflect the behavior of the organism that generates them, which is determined by habitat and responses to environmental stimuli. Therefore, they are an indicator of the palaeoecological conditions in which the organisms that generated them lived.”

That focus is especially useful for the Ediacaran-Cambrian boundary, a time when soft-bodied organisms still outnumbered shelly ones and ecological dynamics were in rapid flux.

Early burrowers and crawlers disturbed microbial mats on the seafloor. This paved the way for new feeding strategies, novel body plans, and ultimately the hard-part explosion more familiar to paleontologists.

Fossils show how life moved

By quantifying the widths, lengths, and winding patterns of fossil trackways such as Archaeonassa, Gordia, Helminthopsis, and Parapsammichnites, the team derived scaling laws that link track size to the animals’ own proportions.

The results show that organisms with streamlined, elongated profiles were already active 545 million years ago.

“These organisms probably possessed coelomic hydrostatic bodies, with an anteroposterior axis, muscles, and possibly segmentation,” Miguez Salas said.

“Furthermore, these organisms could move in a specific direction (directional locomotion) and probably possessed sensory capabilities to move and feed on heterogeneous substrates in a habitat dominated by microbial mats.”

Traits predate Cambrian explosion

Such abilities – directional crawling, muscular movement, possible segmentation – echo the hallmarks of bilaterian animals that dominate the modern biosphere.

If those traits existed well before 530 million years ago, then the lineage-splitting events that scientists historically assign to the Cambrian must have roots in the late Ediacaran.

“Therefore, the so-called Cambrian explosion and its evolutionary implications may have occurred much earlier than estimated,” Miguez Salas added.

Instead of a sudden burst, the data point toward a protracted interval of experimentation. Ecological engineering by active burrowers created niches that later organisms exploited.

Math reveals ancient mobility

Until now, suggestions of pre-Cambrian complexity leaned heavily on qualitative observations – isolated sites showing possible bilateral tracks or ambiguous body impressions. What sets the new paper apart is its mathematical approach.

The authors modeled modern track size to body length, then applied that scaling framework to ancient trace fossils. The outcome is a statistically grounded estimate of early animal width, length, and locomotion speed.

“This new discovery opens the door to quantitatively study future Ediacara trace fossils and to corroborate that the explosion did not happen in the Cambrian, but many millions of years earlier,” Miguez Salas said.

The same scaling rules could probe other critical transitions, such as the Great Ordovician Biodiversification Event, offering a consistent yardstick for comparing evolutionary shifts across geologic time.

Fossils shift life’s timeline

If complex bilaterians were shaping seafloor habitats 545 million years ago, paleontologists must rethink long-held assumptions. Environmental and genetic triggers like oxygen rise, ocean shifts, or gene changes may need rethinking for the Cambrian proper.

Those factors may have been in motion earlier, with the visible “explosion” marking only the point when hard parts made diversity easier to preserve.

The study also underscores how behaviors – burrowing, mat-grazing, sediment mixing – can serve as evolutionary catalysts. Mobile animals turning over sediment could have unleashed nutrients, altered microbial communities, and spurred arms races that drove further innovation.

What fossils may still reveal

Future expeditions into late Ediacaran strata may uncover additional trace fossil assemblages to test the new timeline.

High-resolution 3D scans, geochemical proxies, and refined radiometric dating can link track-making horizons to precise ages and environmental conditions.

Meanwhile, the mathematical framework introduced by Miguez Salas and Wang equips researchers worldwide to translate faint surface scribbles into vivid portraits of Earth’s earliest movers and shakers.

In shifting the Cambrian explosion’s fuse back several million years, the fossil record reminds us that life’s history often blurs textbook boundaries. Evolution, like geology itself, tends to roll out gradually – until we gather the evidence to see its subtler steps.

The study is published in the journal Geology.

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