First complex ecosystems formed in Ediacaran – millions of years earlier than thought

New research coming out of the University of Cambridge, UK, suggests that complex ecosystems emerged earlier than we thought, in a period of Earth’s history called the Ediacaran.

The Ediacaran period began 635 million years ago and marked the development of the first complex, multicellular organisms branching out into the first animal species around 580 million years ago. Many bizarre species with no clear lineage in the fossil record emerged out of the blue in Ediacaran oceans only to disappear without a trace. This period of Earth’s history is marked by intense evolutionary experimentation as life on Earth diversified for the first time.

But the Ediacaran ended about 540 million years ago after a sharp drop in biological diversity. The end of the Ediacaran coincides with the beginning of what is called the “Cambrian explosion” – the dramatic burgeoning of life leading to many of the recognisable major branches in the tree of life, including the first arthropods (leading to insects and spiders) and first vertebrates.

The researchers were initially looking for evidence of a theorised mass extinction event 550 million years ago which led to the sudden decline in biodiversity in the dying days of the Ediacaran.

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Published today in PLOS Biology, the team’s analysis of the timespan 575 to 543 million years ago looked at three fossil assemblages from published paleoenvironmental data. The data included ocean depth and rock characteristics, and fossils.

Looking for metacommunity structure, the palaeontologists found increasingly complex community structure in the fossils from later in the Ediacaran period. The results suggest species specialisation and interspecies interactions.

Ecological complexity has previously been associated with the Cambrian explosion. The dynamics observed in the fossil record suggest that it was not mass extinction which led to the drop off in diversity toward the Ediacaran’s end, but “competitive exclusion“. Competitive exclusion is the evolutionary process by which different species occupying the same ecological niche compete for dominance until there is a preeminent genus which wins out.

The research suggests that specialisation and niche contraction were not first observed in the Cambrian explosion, but millions of years earlier in the Ediacaran. “We found that the factors behind that explosion, namely community complexity and niche adaptation, actually started during the Ediacaran, much earlier than previously thought. The Ediacaran was the fuse that lit the Cambrian explosion,” says co-lead author of the paper Emily Mitchell, of the University of Cambridge.