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The idea that there was a “Cambrian explosion” is a myth

While we often speak of the “Cambrian explosion” to describe the major phase of biodiversification that occurred 540 million years ago, a new study shows that this event cannot be seen as a brutal episode and registered on the contrary in a phase of slow and gradual diversification spread over more than 100 million years.

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If life appeared very early in the history of the Earth (the first living organisms are listed around 3.8 billion years ago), it nevertheless remained for a long time at a very primitive stage – 3.4 billion years to be exact. It was only around 540 million years ago that the situation drastically changed and that terrestrial life really took off, with unprecedented diversification in the history of the planet. Occurring at the beginning of the Cambrian, this major stage which marks a turning point in the appearance and evolution of very many species is most of the time referenced under the name “Cambrian explosion”, suggesting that it is a rather brutal event.

For a team of researchers, however, this idea would be erroneous and would only serve to satisfy our need to limit the evolution of terrestrial life by several spectacular and well-defined “big events” in time. Too simplistic a vision for these scientists, who present their argument in an article published in the journal Palaeo3.

An explosion that is not one

If the fossil archives attest that the first animals did indeed experience a very rapid diversification around -541 to -485 million years ago, this fundamental evolution of living things could not all the same be qualified as an “explosion”. A term that appeared in the 1960s and has since become well established within the scientific community and the general public. It was joined in the 1990s by another terminology, that of the “Great Ordovician biodiversification”, which describes a second stage of massive diversification of species during the Ordovician, i.e. between -485 and -443 million years ago. . Here too, reference is made to an “event” with a rather restricted duration on the scale of geological time.

For Thomas Servais, of the University of Lille, and his colleagues, these two evolutionary “Big bangs” could not in reality be clearly individualized and would quite simply be part of a single and unique phase of diversification, certainly major, but s spanning the long term, from the end of the Precambrian to the end of the Silurian. This phase would thus have lasted at least 100 million years. According to their study on this development of biodiversity, no particular “event” stands out during this period.

A bias in the data

This demarcation which prevailed until now would only result from a bias in the available data. Databases in paleobiology would indeed be incomplete for the period marking in particular the end of the Cambrian, leading to a fictitious separation of two events which would in reality only belong to the same evolutionary trend. While some fossil groups are indeed highly studied, others, on the contrary, are the subject of only a few studies. The same goes for geographies. These regional studies or studies focused on a limited number of species would thus have given the impression of observing two distinct events. An impression that disappears when the evolution of biodiversity is considered in a more global way.

Instead of two peaks of diversification, there would therefore have been only one, long and slow evolution spreading out at the beginning of the Paleozoic era. To describe this pivotal period in the history of life on Earth, scientists propose the use of more measured and less sensational terms, in particular that of “radiation” or quite simply “biodiversification”.

Written by Emilie Grenaud

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