Understanding when the first animals arrived on Earth is a bit like trying to find a small needle in a huge, ancient haystack.
Scientists have been scratching their heads over this question for ages.
Some believe that based on the evolution of genetic differences among various animal groups, the first animals might have appeared at least 800 million years ago.
But there’s a catch. The oldest known animal fossils are only 574 million years old, which creates a big gap in the timeline.
Even the famous scientist Charles Darwin was puzzled about why animal fossils suddenly appeared in rocks from the Cambrian period (about 539 to 485 million years ago), but none could be found in older rocks.
In a new study published in the journal ‘Trends in Ecology and Evolution’, a group of scientists, led by Professor Derek Briggs from Yale, tried a new approach to solve this mystery.
They wondered if the reason why they couldn’t find animal fossils in rocks older than 600 million years was that the conditions weren’t right for preserving these fossils.
For this investigation, they turned their attention to a type of sedimentary rock similar to what is found in the Burgess Shale, a famous fossil site in British Columbia, Canada.
These rocks are about 508 million years old and are known for preserving fossils of soft-bodied animals, even those without shells or mineralized skeletons.
There are a few reasons why these rocks are so good at preserving fossils. For one, the muddy sediment had less oxygen and sulphate, which might have helped to slow down decay.
Also, the composition of the clay in these rocks seems to have prevented bacteria that promote decay from doing their job.
On top of this, certain mineralization processes helped to preserve the soft tissues, and the rapid hardening of the sediment helped to lock in potential fossils early on.
The team looked at areas worldwide with similar rock conditions, especially older deposits. Despite their extensive research, they found no animal fossils in rocks that are older than 789 million years.
This led them to conclude that animals probably hadn’t evolved yet at that time.
While this doesn’t fully answer the question of when the first animals appeared on Earth, it certainly helps to narrow down the timeline.
As Professor Briggs noted, this estimate will undoubtedly be refined as future discoveries come to light. But for now, it’s a step closer to understanding our planet’s fascinating history.
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