After 8 years of "soaking" fossils, they found that the identity of "magnificent green algae" has a great background!

Text | "China Science News" reporter Shen Chunlei

What were the first multicellular eukaryotes on Earth?

On January 25, Science Progress reported on the fossils of multicellular eukaryotes found by Zhu Maoyan, a researcher at the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences, in the 1.63 billion-year-old strata in the Yanshan area of North China. These microfossils, with their exquisite cellular structures, are believed to be the earliest fossil records of multicellular eukaryotes ever discovered in the world.

The first author, Miao Lanyun, is a member of the early evolution team of the Earth-life system led by Zhu Maoyan, and she "soaked" the fossils for 8 years, did countless experiments, and spent two years repeatedly demonstrating and revising the paper to achieve today's results.

This is another important breakthrough made by Zhu Maoyan's team in the field of early life evolution after the discovery of the world's earliest macro-multicellular eukaryotic fossil 1.56 billion years ago in Yanshan area in 2016, which advanced the emergence of multicellular eukaryotes by 70 million years.

When does eukaryotes become multicellular?

Most of the complex life on Earth today, such as animals, terrestrial plants, fungi and macro-body algae, are multicellular eukaryotes. Therefore, the multicellularization of eukaryotes is a necessary condition for the evolution of life to complexity and large-scale, and is considered to be one of the major key events in the history of life evolution.

But there has always been a question in the academic community: When did eukaryotes first become multicellular?

Known fossil evidence suggests that simple microsomal multicellular eukaryotes, including fossils of red algae, green algae, and fungi, have emerged and begun to diversify in strata around 1 billion years ago. However, the reported multicellular eukaryotic fossils in older strata are generally questioned due to the lack of reliable biological evidence, such as multicellular structure and complex morphology.

In 2016, Zhu Maoyan's team, together with Zhu Shixing, a researcher at the Tianjin Geological Survey Center of the China Geological Survey, and other domestic and foreign colleagues, reported the discovery of 1.56 billion-year-old macroscopic multicellular eukaryotic fossils in the Yanshan area in Nature Communications.

"This discovery breaks through the previous understanding of the scientific community, not only advances the emergence of large multicellular eukaryotes on Earth by nearly 1 billion years from the previously thought 600 million years ago, but also infers that the time of multicellularization of eukaryotes should be earlier," Zhu said. ”

In order to demonstrate this inference, in 2015, Miao Lanyun began a research project as a doctoral student to find a fossil record of multicellular eukaryotes in the Late Paleoproterozoic "Great Wall" strata in the Yanshan area, which predates 1.6 billion years ago.

In the past eight years, Miao Lanyun and others collected hundreds of Great Wall shale samples at multiple sections in the Yanshan area, and obtained a large number of microfossil specimens by soaking them in hydrofluoric acid and hydrochloric acid for experimental processing. While discovering a large number of fossils of various types of single-celled eukaryotes, Miao Lanyun et al. finally discovered microsomatic multicellular eukaryotic fossils in the upper part of the Chuanlinggou Formation of the Great Wall system in the Wengjiazhuang section of Kuancheng County, Hebei Province.

According to Zhu Maoyan, a layer of volcanic tuff has been reported at the top of the strata containing the fossil, in which the zircon uranium-lead isotope dating results are 1.635 billion years, which provides a direct age limit for the newly discovered fossils.

Magnificent green algae is a multicellular eukaryotic organism

There are a total of 278 fossil specimens found by Miao Lanyun and others, which are unbranched filaments composed of a single row of cells.

"Because the fossils we found are similar in shape and size to the fossils of 'Magnificent Qingshan Algae' found in the shale slices of the Chuanlinggou Formation in the Jixian area of central Yanshan, we classified them into the same genus. Miao Lanyun said.

An important new discovery in this study is that some cells of Magnificent Aoyama contain circular structures with a diameter of about 15~20 microns. The round structure has a complete and regular morphology, uniform texture, and can be compared in size and morphology with the asexual spores of some living eukaryotic algae, which is interpreted as a kind of vegetative cell. The research team speculated that the Magnificent Green Algae is an organism that reproduces through spores.

Among living organisms, there are many kinds of filamentous organisms consisting of a single row of cells, which are widely found in both prokaryotic and eukaryotic organisms. "We compared the morphological complexity of filaments, cell size, and reproduction patterns, and there is no type of prokaryotes that can be compared to the magnificent green algae. Miao Lanyun said.

According to statistics, the vast majority of known prokaryotic filaments are very small, with a diameter of about 1~3 microns, and only a few giant cyanobacteria and sulfur bacteria have a diameter of 200 microns, which is similar to the diameter of Magnificent Qingshan algae. But the cells of these giant bacteria are all disc-shaped, and there is no complexity in their morphology. There are many filamentous organisms in eukaryotes that resemble magnificent green algae, especially most eukaryotic algae contain filamentous bodies, such as brown algae, yellow algae, green algae, etc. Therefore, the research team believes that Macrophyllum magnificus is a multicellular eukaryotic organism.

Helps to uncover the origins of complex life

Detailed comparative studies have shown that the morphology, cell size distribution and reproduction mode of some surviving green algae are the closest to those of Magnificent Qingshan algae. As a result, the research team believes that Magnificent Qingshan Algae is not only a multicellular eukaryotic organism, but also may belong to a multicellular algae with the metabolic ability of photosynthesis.

In order to further verify the eukaryotic properties of Algae magnificia, the research team used laser Raman spectroscopy to analyze the organic matter composition of Algae magnificia, and used three cyanobacterial fossils produced in the same horizon as a comparison group. The results showed that the organic matter composition of Macrophyllum magnificus was significantly different from that of cyanobacterial fossils, supporting the explanation that it was classified as a multicellular eukaryotic.

At present, the earliest fossil record of eukaryotes is generally accepted by the academic community to be found in the Upper Paleoproterozoic strata about 1.65 billion years ago in northern China and northern Australia. "The emergence of Magnificent Qingshan Algae only slightly later than these oldest fossils of single-celled eukaryotes suggests that the emergence of eukaryotes was followed by a rapid and complex multicellular evolution. Zhu Maoyan said.

This study also proves that since eukaryotic algae belong to a clade of crown eukaryotes, if Magnificent Qingqing algae can be confirmed to be eukaryotic algae that are engaged in photosynthesis, then the last common ancestor of eukaryotes should be no later than the Late Paleoproterozoic period 1.63 billion years ago, 600 million years earlier than the current generally accepted scientific consensus of 1 billion years ago, which is basically consistent with the time estimated by the molecular clock.

Zhu Maoyan believes that this study provides new thinking for further revealing the mystery of the origin and early evolution of complex life, as well as the evolution of the Earth's environment in the Proterozoic.

Related Paper Information:

https://doi.org/10.1126/sciadv.adk3208