Until now, paleontologists have assumed that the first algae—like all other major groups in the animal kingdom—originated during rapid speciation during the Cambrian explosion. The nearly 500-million-year-old fossils of Briozoa species seem to prove this. But new, better-preserved finds of the same species show that this is wrong. Because the supposed algae turned out to be early green algae upon closer analysis. Thus bryozoans could be the only group of animals that did not actually evolve during the Cambrian Explosion.
More than 500 million years ago, at the beginning of the Cambrian period, biodiversity on Earth increased rapidly. In the course of this “Cambrian explosion”, the schemes of almost all groups of animals known today were developed. These include, for example, the mollusks that have relatives such as snails and mussels, but also the group of arthropods, which includes crabs and insects, among others. Until now, paleontologists have also assumed that the first bryozoans, also called bryozoans, evolved in the Cambrian period. Bryozoans are microscopic tentacled animals that live in “skyscraper-like” underwater colonies.
New fossils reveal botanical features
The oldest evidence for primitive algae were fossils more than 500 million years old, which were assigned to the bryozoan species Protomelession gateshousei. But new finds from the Hongjingshao Formation near the Chinese city of Kunming now challenge that assumption. Unlike the remnants of earlier proto-fission, these newly discovered fossils also contain preserved soft tissues. This allowed paleontologists led by Ji Yang of Yunnan University to closely examine the thin, fragile tissue using powerful microscopes and fluorescence imaging. They also compared the fossil tissue samples to other fossil finds to determine its exact place in the tree of life.
In their analyzes, the researchers discovered many characteristics that did not match bryozoans and animal organisms in general. “Instead of the tentacles we expect in bryozoans, we detected simple, plate-like edges,” says Yang’s colleague Zhang Xiguang. From these and other characteristics, paleontologists have deduced that these must not be animal remains, but plant remains. This means that the oldest evidence of algae in the world is actually nothing at all. Instead, Yang’s team placed the primary fission fossils in a group of early green algae called Dasycladales.
This correction is also important for the chronological classification of bryozoan evolution. Because what is now the oldest fossil of these animals is about 40 million years younger than the incorrectly designated Protomylation remains. According to paleontologists, this indicates that algae evolved after the Cambrian period. This makes them the only group of fossil animals that were not formed during the Cambrian Explosion. This knowledge has far-reaching consequences for reconstructing Earth’s history. Until now it was assumed that all animal life blueprints dated back to this stage of intense speciation and then simply modified over and over again. “If microorganisms did indeed evolve after the Cambrian period, this indicates that evolution remained ingenious even after this critical period of innovation–perhaps half a billion years ago, the course of life was indefinite,” explains co-author Martin Smith. .
Ji Yang (Yunnan University, China) et al., Nature, Available here. doi: 10.1038/s41586-023-05775-5
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