When was the First Animal?

A correspondent has sent me THIS LINK. The first fossils recognised as animals appeared in the Cambrian but were already complex beasties. Then animals earlier than the Cambrian were recognised even though they did not have hard parts - the Ediacaran fossils.

But how far back can we trace animals? There are ancient deposits which have all the signs of being a great place for animals to thrive but do not have animal fossils. But there are similar deposits of the same age which do! All very confusing. 

For several years the Weng'An biota (590 MA) was considered the base line - it should have fossils but didn't. But then a younger, similar, rock was found which should have fossils but also didn't. But there were undoubted fossils around at that time. So the absence of evidence should not be used as evidence of absence.

So deciding that the Weng'An was no longer the baseline the authors looked again at the molecular clock and decided that animal life could start earlier. So now animal life started, not at about 574MA, but between 800 to 720MA.

The link above is based on THIS ACADEMIC PAPER. This is much more detailed than the first link above and quickly becomes more complicated than I am comfortable with. So I asked Google Gemini to summarise it for me. This is given below.

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Summary of the Research

A study published in Science Advances led by researchers from the University of Oxford—alongside scientists from Berkeley, Yale, and ETH Zurich—suggests that the first animals on Earth may have evolved between 800 and 700 million years ago. This timeline pushes back the estimated origin of animal life by roughly 126 to 226 million years compared to traditional estimates based on physical fossils.

How Scientists Estimate Evolutionary Timelines

To pinpoint when ancient animal groups originated, evolutionary biologists use a tool called a "molecular clock." This method compares genetic differences in the DNA of living species to estimate how long ago their shared ancestors split.

However, genetic clocks require calibration using the fossil record:

• Minimum age limits: Set by the oldest undisputed animal fossil found in rock layers.
  
  
• Maximum age limits (ceilings): Set by rock formations known to preserve microscopic structures exceptionally well, but which completely lack any evidence of animal life. The assumption was that if animals had existed at that time, they would have been preserved.
  
  

The Breakthrough

For years, scientists used China’s Weng'an Biota (dated to ~590 million years ago) as a maximum age ceiling, assuming animal life could not have existed prior to it because no animal fossils were found there.

The researchers tested this assumption by comparing the Weng'an deposits to a rock formation in Mongolia called the Kheseen Biota (~550 to 526 million years old). The Kheseen rock preserved microscopic fossils under the exact same fine environmental conditions as Weng'an, but it contained no definitive animal fossils either—even though animals were unquestionably alive on Earth during that period.

Key Findings & Implications

• Absence of Evidence isn't Evidence of Absence: Because animals were absent in Kheseen despite existing at the time, the lack of animal fossils at Weng'an no longer serves as proof that animals hadn't evolved yet.
  
  
• Removing the Evolutionary "Ceiling": By dismantling the Weng'an maximum age restriction, calibrated molecular clocks allow animal origins to be placed much earlier, between 700 and 800 million years ago.
  
  
• Understanding Early Animals: Early animals were likely tiny, soft-bodied organisms that easily escaped preservation, explaining the gap between early genetic origins and later, larger physical fossils.
  
  
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