Science

Where did life on Earth come from? Scientist commissioner takes you on a quest to find the tree of life

From the birth of the first living creature on Earth 3.8 billion years ago to the tree of life flourishing today, this planet has undergone a grand transformation of the seas and oceans, and the evolution of life has gone through a long journey of trial and error, innovation and change. Studying this long history and revealing the mystery of biological evolution is of great significance for us to understand the meaning of life and to maintain the Earth's ecosystem together.

Recently, in the reading group activity of "Science into Life" organized by the Committee of Textbook, Health and Sports of the National Committee of the Chinese People's Political Consultative Conference (CPPCC), members carried out study and discussion around "Science in China - Biology and Evolution". A number of renowned scientists in related research fields unveiled the mystery of "evolution of life" for us.

In order to let readers know more about this knowledge, some of the members' statements are excerpted.

From a few to tens of millions

Xu Xing, member of the National Committee of the Chinese People's Political Consultative Conference

Researcher, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences

The approximately 10 million species that exist on Earth today all originated from a common ancestor some 4 billion years ago, and this ancestral species has continued to diverge to form new species. On the one hand, new species continue to emerge; on the other hand, existing species continue to become extinct, sometimes on such a scale that most of the species that existed on Earth disappeared in a relatively short period of time. The five largest extinction events are known as "mass extinctions," the last of which was the extinction of the dinosaurs 66 million years ago. Some research suggests that the world is now entering its sixth mass extinction. Overall, however, the diversity of species on Earth has been increasing, and the number of species has gone from just a few 4 billion years ago to tens of millions today.

The process of species diversity formation is accompanied by an increase in the complexity of morphological, physiological and behavioral traits. A variety of familiar biological structures, physiological traits, and behavioral phenomena, like eyes and limbs, cold-blooded and hot-blooded, flight and lactation, emerged during this long evolutionary process.

At the same time, many clades of organisms have remained simple and comparable to their ancestral species billions of years ago, and many clades have even been reduced to simplicity and degradation. For example, many organisms living in caves have significantly degraded their eyesight, and some species have even degraded their eyes; and parasitic organisms, including the intestinal flora that help us digest, have shown significant structural simplification. My understanding is that this is related to adaptation, that is, ecological selection among natural selection. But conversely, some degraded structures can reappear if function requires it, highlighting the complexity of evolution.

Where did life on Earth come from? Scientist commissioner takes you on a quest to find the tree of life

The core of evolutionary biology is the reconstruction of the tree of life, which means discovering as many species as possible that have ever appeared on Earth, clarifying the genealogical relationships among them, identifying the main characteristics of each clade, recovering the spatial and temporal history of the emergence of each clade, and speculating on the ecological relationships between different clades.

At present, we have a general understanding of the formation history of the tree of life through molecular data such as DNA and phenotypic data such as macroscopic morphology, based on modern phylogenetic methods and combining data and methods from multiple disciplines, and have established a theory of evolution from microscopic to macroscopic.

But many questions remain: there are no clear answers to some of the framing questions of the Tree of Life, controversy over the location of some important groups of organisms on the Tree of Life, and even important questions like what the relationship is between viruses and other forms of life. In addition, there is growing evidence in recent years that there may be other driving mechanisms for species formation besides natural selection and genetic drift, and the roles and relationships of these different driving mechanisms are questions that need further study.

From the "Microbial Ecology Moon Landing"

Huang Li, member of the National Committee of the Chinese People's Political Consultative Conference

Researcher, Institute of Microbiology, Chinese Academy of Sciences

In the fall of 2018, I went to Busan, Korea, to attend a small international symposium on deep-sea microorganisms. That morning, a middle-aged Japanese scholar named Imachi gave a presentation. Unlike others, he came on stage with a bunch of props, which Imachi proudly told us were experimental equipment he had made by cutting squares of dishwashing sponges from his kitchen and threading them into skewers (which looked like candy canes).

His team placed the sponge strings vertically in a glass tube filled with methane gas, and then allowed sterile seawater to slowly flow through the strings from top to bottom. Using this device, they simulated the undersea environment and allowed the sponge strings to adsorb sediment (sediment in which all kinds of microorganisms naturally live).

After spending 12 years and using up 4,000 liters of seawater, they managed to obtain a peculiar microorganism destined to cause a sensation, Asgard archaea.Science magazine named the discovery one of the top 10 scientific breakthroughs of 2019, declaring that Asgard archaea "appears to reveal the ultimate ancestor of us all." The results have also been called "the moonrise of microbial ecology.

Where did life on Earth come from? Scientist commissioner takes you on a quest to find the tree of life

Microorganisms are the oldest life on Earth, and in the 1930s, some people divided the organisms on Earth (strictly speaking, they should be cellular organisms) into two major groups, namely, eukaryotes and prokaryotes. Eukaryotes include humans, animals, and plants, which have a nucleus in their cells, while prokaryotes are bacteria among microorganisms, which do not have a nucleus in their cells. By the 1970s, it was also discovered that what was originally thought of as bacteria actually contained two major groups of organisms, one part still being bacteria and the other part archaea (so named because these organisms are often found living in extreme conditions similar to early Earth environments).

Thus, according to the current understanding, three forms of life exist on Earth, namely eukaryotes, bacteria and archaea. Among these three life forms, bacteria and archaea, as microorganisms, are the most widely distributed and biodiverse, playing key driving roles in the Earth's vital element cycle and maintaining the Earth's ecosystem.

However, there is a problem: the majority of microorganisms (>99%) in nature are currently difficult to obtain in living, purified form (termed pure culture), and these microorganisms have been compared to "dark matter", and can only be inferred by collecting environmental samples, extracting fragments of microbial DNA, and analyzing the sequence characteristics of these DNA fragments The study found that environmental samples often contain microorganisms. It has been found that environmental samples often contain many different groups of microorganisms. Imachi et al.'s experiment is actually a step-by-step process to remove the slow-growing microbial taxa from the samples and eventually obtain the pure dominant microbial taxa in vivo. Therefore, it is easy to understand why the publication of the results has caused such a huge response from the academic community.

Imachi's report is reminiscent of the fact that this feat by Japanese scientists relies not on sophisticated equipment and large financial investments, but on imagination and perseverance. What is particularly remarkable is that they have persisted in an attempt that has been unproductive for a long time and may ultimately lead to nothing.

In recent years, our scientists have been fruitful in the study of Asgard Archaea, discovering several new members of the family and naming one of them "Goku". However, there are not many people who are willing to try to obtain purified living cells. Obviously, we need to further promote the spirit of "sitting on the bench" and cultivate a scientific culture that is compatible with this, so that such significant milestone results can emerge frequently in China.

Of course, the search for eukaryotic ancestors cannot end with the acquisition of Asgard Archaea, and there are still many unsolved mysteries. Only by finding and recognizing more currently undiscovered archaea that are closer to eukaryotes will it be possible to unravel the mysteries of eukaryotic origins.

1/2 of the atmospheric oxygen comes from marine algae

Xu Xudong, member of the National Committee of the Chinese People's Political Consultative Conference

Researcher, Institute of Aquatic Biology, Chinese Academy of Sciences

Algal organisms may sound a bit far away from us, but there are actually some that are well known to everyone. For example, in Taihu Lake, Chaohu Lake, Dianchi caused by different kinds of cyanobacteria, and then we eat seaweed, nori, caused by Qingdao "green tide" of marsh moss, respectively belong to brown algae, red algae, green algae.

Algal organisms come in all shapes and sizes, and are divided into more than a dozen large phyla. Some algae have only one to a few hundred cells and require a microscope to see, while the largest algae, such as the giant algae of the ocean, can reach hundreds of meters in length.

Algae have contributed greatly to the formation of the human habitat.

The first photosynthetic oxygen releasing organisms to appear on Earth were cyanobacteria, which accumulated oxygen in the atmosphere about 2.3 billion years ago due to their flourishing in the oceans. In other words, without the appearance of algae, there would have been no oxygen in the atmosphere and no aerobic respiratory organisms would have emerged.

However, we all have the impression that oxygen is not released by plants through photosynthesis? In fact, about half of the oxygen in the atmosphere today comes from plants on land and half from algae in the ocean, half of which comes from cyanobacteria and half from diatoms. The point is that both diatoms in the ocean and plants on land evolved their chloroplasts for photosynthesis from cyanobacteria.

Where did life on Earth come from? Scientist commissioner takes you on a quest to find the tree of life

About 1.5 billion years ago, a eukaryotic cell engulfed a cyanobacterium, and the engulfed cyanobacterium settled in this cell and became a "symbiotic" inhabitant of the cell. In this way, the eukaryotic cell acquired the ability to photosynthesize. Later, this eukaryotic cell with cyanobacterial "endosymbionts" evolved into three clades, one becoming green algae, one becoming red algae, and one becoming gray algae.

Later on, the green algae diverged into several clades, one of which landed about 500 million years ago, and diverged into two branches of mosses and ferns, from which the ferns evolved successively into gymnosperms and angiosperms, which became the protagonists of the forests and grasslands on the earth today.

Meanwhile, the single-celled red algae are engulfed by another eukaryotic cell and evolve into a new endosymbiont in the host cell. From this secondary endosymbiotic cell, taxa such as methanogens, diatoms, and brown algae evolved, respectively. Among them, diatoms are also one of the major contributors of oxygen in the present-day Earth environment.

Photosynthesis in algae and higher plants plays a key role in stabilizing atmospheric carbon dioxide concentrations and global climate by releasing oxygen while also fixing carbon dioxide and converting it into organic matter.

There are many other ways in which algae affect the Earth's environment. For example, algae release dimethyl sulfide, which is oxidized to sulfate in the atmosphere, promoting the generation of cloud condensation nodules, blocking sunlight through the reflection and refraction of clouds, and lowering the surface temperature; for example, diatoms absorb silicic acid to participate in forming their cell walls, reproduce and deposit in the ocean, forming diatomaceous earth, which is a key driver of the circulation and homing of elemental silicon in the ocean; furthermore, terrestrial cyanobacteria crust and fix sand in desert areas, greatly accelerates the recovery of vegetation.

In short, the earth's environment has changed dramatically, giving birth to algal organisms, and the evolution and prosperity of algae have in turn had a great impact on the earth's environment, creating suitable conditions for the survival and reproduction of human beings.

Every creature is worth learning from

Member of the National Committee of the Chinese People's Political Consultative Conference Tong Jinan

Professor, School of Earth Sciences, China University of Geosciences (Wuhan)

Every living thing on Earth has its temporal properties. It is recorded that about 1.8 million species of organisms have been discovered and named on Earth today, but there are still a large number of undiscovered or unidentified species, and the total number is estimated to be 3-10 million. And there may be more than 5 billion species of ancient organisms that once lived on Earth, but 99.9% of them have become extinct. It is evident that the history of biological development is also the history of rapid biological turnover, so paleontologists are in the best position to appreciate and understand the evolutionary features of organisms.

Regarding the dynamics and patterns of biological evolution, three levels of variability are generally considered, namely, microevolution at the population level, speciation at the species level, and macroevolution above species.

Microevolution is driven by a balanced selection between variation and genetics, where variation is a creative factor and genetics is a stability factor. Without variation, organisms can only produce the same species and there is no evolution; without heredity, organisms have no relative stability, variation cannot be preserved, and there can be no evolution. Natural selection, on the other hand, determines the direction of biological evolution, so that variation that is conducive to survival is preserved by heredity.

Generally speaking, natural selection results in changes in the genetic material within a population in a direction that is more favorable to adaptation to the external environment. The accumulation of microevolution within a population will lead to the formation of new species, while the way species are selected and evolve is expressed in macroevolution.

In the teaching of paleontology, the content of biological evolution is one of the best materials to enlighten students' minds, because every living creature living on the earth today is the product of more than 3 billion years of life and death and continuous evolution, so they can survive until now, there must be something worth learning and learning from us, even the simplest microorganisms (even the new coronavirus we are experiencing), are worthy of our treasure and to seriously study and learn from (so I think "bionomics" is the most important field that we should pay attention to).

Where did life on Earth come from? Scientist commissioner takes you on a quest to find the tree of life

On a geological time scale, we can assume that organisms and their environment not only interact continuously but also have a synergistic evolutionary relationship. On this point, some modern biologists may disagree, because from the observable time scale of people today, it is obvious that organisms are controlled by the environment, and the influence and modification effect of organisms on the environment is very limited, even negligible on short time scale. But if we look at the history of the development of the Earth, our planet would certainly not have the appearance it has today without living things. Not only is today's green Earth a biological creation, but today's "habitable Earth" environment, such as the Earth's atmosphere, hydrosphere and surface lithosphere, is the work of Earth's organisms.

Therefore, we say that organisms and the environment are not in a fully reciprocal co-evolutionary relationship, i.e., the effect of the environment on organisms is direct and short-term, while the effect of organisms on the environment is long-term and indirect; especially on long time scales, they still have significant "synergism". Recognizing this is of great importance to our contemporary ecological management and ecological civilization construction, especially sustainable development.

We can understand "ecology" and "civilization" as two systems formed by biological and human evolution, namely, the "ecosystem" of nature and the "civilization system" of human society, respectively. "civilizational system". Ecosystem is a dynamic and balanced system resulting from the evolution of organisms and the environment. It is a highly harmonious unity of organisms and the environment, and therefore it is superior to conventional non-living physical and chemical systems. Civilizational systems, on the other hand, are rule-based unities formed by the self-regulated activities of human society, which are subject to human intellectual and social activities. If we can draw on the dynamic balance law of natural ecosystem to scientifically guide the construction of human civilization system, establish harmonious human-earth relationship and build a community of human destiny, it will help the harmonious symbiotic development of all human beings on earth.

How humans evolved from "fish" to "human"

Wang Yuanqing, member of the National Committee of the Chinese People's Political Consultative Conference

Deputy Director of Academic Committee, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences

Nowadays, there are about 1.5 million species of animals and about 380,000 species of plants living together with us humans on the Earth. These organisms have not existed on our peculiar planet since the day of its birth, but are the result of the long evolution (or evolution) of life on Earth for about 3.8 billion years.

Of course, of all the complexities of biological evolution, it is the evolution of vertebrates, which is related to our own origins, that is of most interest to humans.

In the evolutionary history of "from fish to man", a series of key (or revolutionary) events took place.

1. The origin of vertebrates: the earliest vertebrates were fish-like animals represented by the Kunming fish and the Haikou fish 540 million years ago (early Cambrian period), and their appearance laid down the essential characteristics of later vertebrates - with a spine, which is the material basis for our human beings to be able to hold up their backs.

2. Emergence of jaws: The most primitive vertebrates did not have upper and lower jaws, and their mouths could not open and close as common vertebrates do. The mouth could not be closed and was called a round mouth, and the round mouth phylum, represented by the seven-gill eel and the blind eel, still belongs to this type. About 420 million years ago (late Silurian), fish had upper and lower jaws, which brought about a revolution in the way vertebrates fed and made it possible to develop verbal communication in the future.

3. Landing from water: 365 million years ago (late Devonian), amphibians represented by fish salamanders appeared, and the toe limbs replaced the fins of fish, becoming the first vertebrates on land. This change caused a fundamental change in the way vertebrates moved, expanding their living space and laying the foundation for the later dominance of dinosaurs and mammals in terrestrial ecosystems.

Where did life on Earth come from? Scientist commissioner takes you on a quest to find the tree of life

4. The emergence of amniotic eggs with shells: Fish and amphibians (frogs, etc.) need to lay their eggs in water in order to complete reproduction. In contrast, turtles, crocodiles and birds were able to lay their eggs on land, with the amniotic fluid providing the watery environment for embryonic development and the eggshell effectively preventing water loss, freeing the reproductive process from dependence on water. This revolutionary event occurred about 315 million years ago in the Late Carboniferous (late Carboniferous), and amniotes (animals that lay amniotic eggs) thus became truly terrestrial.

5. The rise of mammals: Mammals appeared on Earth at roughly the same time as dinosaurs. During the Mesozoic era when dinosaurs and other reptiles ruled the earth, mammals underwent a series of physiological and morphological changes, including from oviparity to fecundity and the formation of the mammalian middle ear, and had the advantage of adapting to environmental changes, waiting for the right time to explode. It can be said that everything is ready, only the east wind is missing. This "east wind" is the extinction event that occurred 66 million years ago, resulting in the extinction of dinosaurs and other large reptiles, allowing mammals to dominate the terrestrial ecosystem in the Cenozoic.

6. Emergence of primates: About 56 million years ago, the group of mammals to which we humans belong, the primates, appeared on Earth. These animals have developed brains, stereoscopic vision with converging eyes and other unique features, which really started the evolutionary journey of vertebrates to humans.

7. About more than 6 million years ago, the animal that could be called human appeared in Africa, and although it is still somewhat different from our present human, it has crossed the threshold of the ape-human boundary.

Some of the most critical events in the evolutionary process of "from fish to man" have been listed above, but in fact, different groups of organisms have their own unique evolutionary history.

A major manifestation of biological evolution in geological history is the turnover of biological taxa, of which the turnover of biological taxa caused by large extinction events is of particular interest. These are usually associated with environmental changes.

In the case of the extinction of the dinosaurs, for example, although there are various hypotheses such as asteroid impact and mega-volcanic eruptions, they all boil down to the fact that changes in the Earth's environment caused creatures such as dinosaurs to fail to adapt before they eventually disappeared. This can be used as an example to corroborate the view that the essence of biological evolution is the interaction between organisms and their environment.

On the other hand, the ability of mammals to dominate terrestrial ecosystems after the extinction of the dinosaurs is inextricably linked to the physiological and functional advantages that mammals developed during their long evolutionary history in the Mesozoic.

Comrade Mao Zedong pointed out in "Contradiction" in 1937: "Materialistic dialectics holds that external causes are the conditions of change, internal causes are the basis of change, and external causes act through internal causes", which should be appropriate here as well.


Originally published in the People's Political Consultative Conference Daily, May 12, 2022, page 6, Eco Weekly