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A team at NUI Galway reckon they've cracked 'Darwin's Dilemma'

The great naturalist could not explain an apparent gap in animal evolution.

Charles Darwin
Charles Darwin
Image: Shutterstock/Everett Historical

ANIMALS MAY HAVE existed on earth for hundreds of millions of years before they first appear in the fossil record, according to new research from NUI Galway.

In the mid 19th century, naturalist Charles Darwin (1809-1882) puzzled over an “apparently sudden appearance” of different types of complex animals around half a billion years ago, says Dr Breandán MacGabhann who conducted the research for his PhD degree. 

Known as ‘Darwin’s dilemma’, this observation – based on fossil record evidence – did not fit with Darwin’s proposal that animals slowly and gradually evolved from much simpler ancestors over a very long period of time.

Darwin decided that the earliest record of animal life on Earth was, therefore, not represented in the fossil record.

NUI’s MacGabhann explains that, for most of the fossil record shells and skeletons are evident but there is little evidence for jellyfish and worms, for instance, creatures that make up much of early marine animal life.

The oldest animal fossils – impressions left behind by soft-bodied creatures on the seafloor – are from far before animals first developed the ability to make shells.

“We have never known how well these fossils represent…early animal communities,” MacGabhann says. 

That’s because these creatures were quite different from modern animals. “But also due to the fact that we didn’t understand precisely how they came to be preserved as fossils.”

But when the team shipped over thousands of fossils of the disc-shaped creature called ‘eldonids’ – collected from the edge of the Sahara Desert in Morocco – they had their “eureka moment”, says MacGabhann. 

“The key [was] coming up with an explanation for how these different tissues can or can’t get fossilised.” 

Examining the anatomy and preservation of ‘eldonids’ fossils at NUI Galway and in labs in the USA, the research team realised that the earliest and most primitive animals to appear on Earth would not have had body tissues capable of becoming fossilised.

Therefore, these don’t represent the oldest animals, thus confirming ‘Darwin’s dilemma’. 

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Dr John Murray from Earth and Ocean Sciences at NUI Galway has said that the new research confirms a “suspicion that scientists have long-held but which they had struggled to conclusively prove.”

That is, that the earliest animal ancestors most likely evolved during a protracted and cryptic time interval, long before more advanced creatures began to become preserved as fossils.”

This research provides that extra time Darwin needed to account for the early stages of the evolution, solving “a seemingly intractable problem”, Murray has said.

The research involved using concepts and techniques from soil science, microbiology, and environmental science.

“It took a while to actually get the proof we needed, though,” says MacGabhann. “It was a couple of years of work.”

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