The common concept is that the first animals evolved from the sea.
Now scientists studying ancient stone specimens in southern China have discovered that the first animal fossils were preserved in the sediment of an ancient lake. not in marine sediment as previously thought.
These new findings not only raise questions about the habitat of the earliest animals, but also these factors lead to the evolution of animals.
For about 3 billion years, unicellular life forms such as bacteria dominate our planet. Then, about 600 million years ago , the first multicellular animals appeared and rapidly diversified.
The oldest known fossils are in the Doushantuo Formation in southern China. These fossils do not have mature specimens but instead many fossils appear to be very small embryos.
Researcher Tom Bristown, now at the California Institute of Technology in Pasadena, said: 'Our first unusual discovery in this area was the abundance of a clay mineral called smectile. In rocks with a long age like this, smectite is often transformed into another form of clay. However, smectite in rocks in southern China does not undergo that transformation and has a chemical nature that requires certain conditions for this type of smectite to form - conditions commonly found in lakes. salty and alkaline '.
Researchers collected hundreds of rock samples from several locations in southern China. All analyzes show that these specimens are not marine sediments.
Bristow said: 'In addition, we only detected smectite in certain locations in southern China, and they are not quite the same as in marine sediments. Put together the evidence that these animals live in a lake environment '.
This finding raises questions about how and why these animals appear.
Researcher Martin Kennedy, a geologist at the University of California, Riverside, said: 'It is surprising that these first fossils did not come from marine sediments'.
He explained: 'Lakes are often shorter and unstable sea habitats on the Earth's surface. Therefore it is surprising that the first evidence of the animals we have discovered is linked to lakes, the environment is more variable than sea. Scientists have always assumed that the first appearance of animals must be in the most stable and mildest environment. '
It is still possible that animal fossils with a similar or even long-lived date of marine origin have not been discovered. However, at least this study confirms that 'animals have been able to cope with changes in the lake environment. This suggests that their evolutionary reactions are much faster than predictions, and that the earliest animals have greater diversification than we imagined. '
If animals actually develop in the first lake, one aspect of the lake environment may have prompted their evolution to be that air can easily penetrate the lake because they are often much more shallow than sea.
Kennedy told LiveScience: 'The most common explanation for animal evolution is related to the increase in oxygen in the Earth's atmosphere. It is very likely that the lake is the first habitat to benefit from this increase in oxygen. '
The study is published in detail in the Proceedings of the National Academy of Sciences magazine on July 27.