We once shared a common ancestor with bananas, mushrooms, and kelp! How did complex life emerge?

If you were asked to give a simple description of the life on the surface of the earth today, how would you answer? Just look at our dining table, poultry, livestock, eggs, milk, vegetables, fruits, grains, nuts, fungi, seaweed... From a more academic perspective, it can be described as animals, fungi, terrestrial plants, and multicellular eukaryotic algae. If you know something about biology, you will know that these organisms that we can see with our daily naked eyes are basically eukaryotic organisms with complex multicellular structures, also known as complex organisms , or "higher life." They are the real masters of the earth today. If an alien landed on the surface of the earth, the first life form he would see would most likely be complex organisms, rather than those bacteria, archaea, and single-celled eukaryotic organisms that are difficult to distinguish with the naked eye.

Large red mushrooms photographed in Wuliang Mountain, Dali Bai Autonomous Prefecture, Yunnan Province (Photo source: Xinhua News Agency)

Ripe nuts in Yingjiang, Yunnan (Photo source: Xinhua News Agency)

Enteromorpha fished out of the sea (Photo source: Xinhua News Agency)

01 The origin of eukaryotes

When did eukaryotes originate? There are many different opinions, ranging from 2.7 billion to 1.8 billion years ago. But it is generally believed that the origin process involved the endosymbiosis between the ancestor of mitochondria, the energy factory in eukaryotic cells, an alpha proteobacteria , and the main ancestor of eukaryotes, an Asgard archaeon . The combination of these two prokaryotes gave rise to the ancestor of eukaryotes.

However, the specific process of endosymbiosis is still controversial today, and it may also involve gene transfer from other archaea and bacteria, as well as gene creation and multiplication of eukaryotes themselves. When the first eukaryote was born, its cellular complexity and energy utilization efficiency were several orders of magnitude higher than those of its prokaryotic ancestors. This laid the foundation for the future generation of complex multicellularity in eukaryotes.

Schematic diagram of the cell structure of animals (eukaryotic organisms) (Source: People's Education Press High School Biology Textbook - Compulsory Course 1)

Schematic diagram of the cell structure of Escherichia coli and cyanobacteria (prokaryotes) (Image source: People's Education Press High School Biology Textbook - Compulsory 1)

In addition to endosymbiosis with mitochondria, some cyanobacteria capable of oxygenic photosynthesis had endosymbiosis with early eukaryotes. This is the source of chloroplasts in eukaryotic cells. This also allows eukaryotes to use light energy, water and carbon dioxide to synthesize organic matter . From then on, the producers on Earth were no longer just prokaryotes. These eukaryotes capable of photosynthesis later evolved into terrestrial plants and various eukaryotic algae, eventually replacing prokaryotes to become the most important primary producers on Earth.

02 Early diversification of eukaryotes

The origin of eukaryotes does not mean the birth of crown-group eukaryotes (the most recent common ancestor of all eukaryotes today and its descendants), just as humans may have originated about 6 million years ago, but our modern human species only appeared about 300,000 years ago. Current research suggests that the origin time of the most recent common ancestor of all living eukaryotes ranges from 1.8 billion to 1.2 billion years . From the origin of eukaryotes to the origin of crown-group eukaryotes, a large number of long-extinct stem-group eukaryote groups may have been produced in the meantime.

Australian researchers have discovered the earliest known eukaryotic fossils (Image source: Weibo screenshot)

The earliest confirmed eukaryotic fossils were found in strata in China and Australia about 1.64 billion years ago . These fossils are very small, invisible to the naked eye, and their biological classification attributes are unknown, so they are classified as ambiguus . But despite their small size, they are epoch-making giants in evolution. The protrusions and decorations on the surface of their fossils show that they are undoubtedly eukaryotic, but it is difficult to determine whether they belong to crown group eukaryotes. Interestingly, a study last year believed that stem group eukaryotes that could produce primitive sterols were widely present on Earth 1.64-800 million years ago , and some eukaryotic fossils that could not be classified as crown groups during this period may belong to stem group eukaryotes.

03 The Origin of Complex Life

The period of about 1.8-8 billion years in the history of the Earth is called the boring billion years , because in the past scientists believed that the environment and biological features of the Earth did not change much during this period. But as mentioned above, not only did the earliest fossils of eukaryotes appear at this time, but many of the earliest crown group eukaryotic fossils that can be identified at this time also appeared at this time, including the earliest green algae fossil Proterocladus , the earliest red algae fossil Bangiomorpha , the earliest fungal fossil Ourasphaira , the earliest animal kingdom fossil Bicellum , and the suspected earliest yellow algae fossil Palaeovaucheria . All of these fossils are about 1 billion years old , and the appearance of these fossils also means that the multicellularization of complex organisms may have occurred earlier than 1 billion years ago. This multicellularization is not a simple aggregation of cells, but involves the differentiation of cells with different functions, which lays the foundation for the future generation of complex biological tissues, organs, systems and complex structures .

04 Early attempts to enlarge eukaryotic organisms

Although the corresponding fossil records of red algae, green algae, animal kingdom and fungi can be found as early as about 1 billion years ago, the marine ecosystem at that time was probably still dominated by prokaryotes and stem eukaryotes. The early multicellular eukaryotes were very small, only micrometers in size. Some eukaryotes with unknown classification positions , such as Chuaria , Tawuia , Longfengshania , Pararenicola , Protoarenicola , Horodyskia , etc. , can reach the millimeter level. They may be some algae with multiple nuclei in a single cell. They are widely distributed in strata about 100-800 million years ago and were also important producers in the ocean at that time. Some of them even evolved simple fixation structures that could fix on the seabed instead of floating in the seawater . This shows that eukaryotes had begun to try to occupy benthic niches at that time, and a three-dimensional eukaryotic marine community with a layered structure of the surface and bottom layers began to form.

05 The Test of Snowball Earth

How did tiny, inconspicuous multicellular eukaryotes eventually begin to officially shine on the Earth's stage?

During the Cryogenian Period (720-635 million years ago), the Earth experienced two global glacial events - the Sturtian Glacial Age (717-659 million years ago) and the Marino Glacial Age (645-635 million years ago), also known as the Snowball Earth event . These two glacial events were not only a test but also an opportunity for complex organisms. The cold environment of the Snowball Earth may have caused the decline of prokaryotes in the shallow sea, creating more ecological niches for complex organisms, and also formed a strict natural selection for complex organisms, causing them to develop in the direction of large-scale and complex .

After the end of the Snowball Earth, the global warm and humid environment led to the strengthening of chemical weathering on land, and a large amount of nutrients entered the ocean, which promoted the improvement of marine productivity and thus increased the oxygen concentration in the atmosphere and ocean. Due to the limited global sedimentary records that can preserve fossils during this period, scientists know very little about the appearance and evolution of life during the Snowball Earth period.

Scientists propose a new model of the "Snowball Earth" 635 million years ago (Image source: Weibo screenshot)

06 The Rise of Complex Life

In the Ediacaran Period (635-538.8 million years ago) after the end of the Snowball Earth, the complexity and size of complex organisms increased significantly compared to before the Snowball Earth. The Lantian Biota in China about 600 million years ago perfectly recorded this process. A large number of macroscopic algae fossils with complex shapes such as clusters, cones, and fans were discovered in the Lantian Biota, and even five possible animal fossils were discovered.

At the same time, the discovery of terrestrial zygomycetes in China's 635 million-year-old Gaiman dolomite and lichen fossils in the 600-million-year-old Weng'an Biota also indicates that the adaptive radiation and terrestrialization of fungi began before the Ediacaran period. The early landing of fungi may have provided a prerequisite for the landing of terrestrial plants about 500 million years ago.

Between 580 and 540 million years ago, Ediacaran fauna and trace fossils were widespread on Earth, which means that animals began to have an important impact on Earth's marine ecosystems at this time. Along with the disturbance of seafloor sediments by animals, algae began to evolve from simple discs and spheres to pseudo-roots to adapt to softer substrates.

At the end of the Ediacaran period about 540 million years ago, although we still cannot see the common animal phyla and terrestrial plants today, multicellular algae including red algae, green algae and brown algae and early animals represented by Ediacarans have officially announced the arrival of the era of complex life.

Author: Niu Changtai, PhD candidate at Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences

Reviewer: Yuan Xunlai, Researcher, Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences

Produced by: Science Popularization China

References:

[1] Yuan Xunlai, Pang Ke, Tang Qing, et al. The origin and early evolution of complex organisms[J]. Chinese Science Bulletin, 2023, 68(Z1): 169-187.

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[3]Brocks JJ, Nettersheim BJ, Adam P, et al. Lost world of complex life and the late rise of the eukaryotic crown[J]. Nature, 2023, 618(7966): 767-773.

[4]Vosseberg J, van Hooff JJE, Köstlbacher S, et al. The emerging view on the origin and early evolution of eukaryotic cells[J]. Nature, 2024, 633(8029): 295-305.