A quick flip through any high school biology textbook will inevitably turn up a reference to the Cambrian Explosion, a period about 540 million to 520 million years ago when many groups of animals first arose and diversified. The event is frequently described as rapid and prolific, sparking a chaotic moment in early evolutionary history.
But has there really been a dramatic explosion of biodiversity on Earth during this time?
Thomas Servicepaleontologist and director of research at the French National Center for Scientific Research (CNRS) and colleagues published a 2023 paper in Paleogeography, Paleoclimatology, Paleoecology arguing that the Cambrian Explosion did not happen the way it is generally portrayed. It wasn’t an actual explosion, he told Live Science, but rather a gradual increase in biodiversity that occurred throughout the early Paleozoic era (541 million to 251.9 million years ago). He said that the appearance of an “explosion” is really an artifact of the biases that scientists have when they study the past.
The process of locating, excavating, and cataloging fossils is expensive and arduous, so researchers often add their samples to large databases to make it easier to compare finds. Two of these databases, namely Paleontology database and the Geographic diversity databasecollectively contain nearly 2 million entries and have been used to investigate global patterns in biodiversity, including trends emerging during the Cambrian period.
The authors stress that these resources are not truly universal. The Paleobiology database consists largely of fossils found in Europe and North America, while the Geographical Diversity database includes mostly fossils from China. These regions host some of the world’s most famous Cambrian deposits – including Canada’s Burgess Shale and the Qinjiang Fossil Basin in China’s Yunnan Province – which draw the majority of the funding. But at best, they can provide “a regional assessment of patterns in diversity, and then only those species that maintain enough health to survive in the fossil record,” Service said.
The databases also include samples from another period, called the Great Ordovician Biodiversity Event (GOBE), which is thought to have occurred roughly 40 million to 50 million years after the Cambrian Explosion. The period between the two events is relatively unstudied and appears to lack the same pattern of biodiversity blooms. But Servais said this is also the result of scientists’ bias. If they had put the same effort into studying this period, he said, the existence of two single events would probably vanish.
Related: Why do Cambrian creatures look so strange?
Karma Nanglo, a Harvard paleontologist who studies Cambrian and Ordovician fossils, told Live Science that he understands why Servais and his colleagues want to downplay terms like “explosion” and “event,” and said it’s well accepted in the field that estimates of biodiversity may be affected by bias in research. take samples. “But in my opinion, I still think there is really good evidence for the Cambrian Explosion, as we usually call it,” he said.
Regardless of whether the databases are biased towards particular groups or regions, there is a general trend of increased complexity that can be seen in the animals themselves.
“It’s not just that two species are equivalent in terms of what they contribute to diversity, it’s that species A and type B are fundamentally different from each other in terms of the way their bodies are organized, how they evolve, and what the role might be,” Nanglo said. environmental how they live.” “And up to this point, I think there is direct evidence that you can read directly from the rocks.
the The reasons for this biodiversity are not fully knownBut scientists have some ideas. During the Precambrian period, the supercontinent Rodinia split into parts, including Gondwana (modern-day Antarctica, South America, Africa, Australia, India and New Zealand) and Laurentia (most of North America). During this time, oxygen levels in the ocean increased, and there was a greater proportion of warm, shallow tropical coastlines—ideal conditions for new species to evolve and later fossilize. A similar hypothesis of the disintegration of supercontinents Pannotia and Pangea Much later, researchers identified a link between rifting and animal diversity in the Paleozoic Eon (541 million years ago to the present).
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