When we eat fish, are we eating our ancestors?

It is the dream of many researchers to be able to publish academic papers in top journals, especially journals such as Nature and Science that enjoy a high reputation in the field of natural sciences.

On September 29, 2022, the academic paper on ancient fishes by the team of Academician Zhu Min from the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, was not only published in the journal Nature, but also published as a cover article. Not only was it published as a cover article, but it was published four times in a row! The importance of these research results can be seen.

The cover of Nature shows a 3D artistic restoration of the five newly discovered Silurian ancient fish species. Graphics by Heming Zhang

Perhaps because the words "from fish to man" and "filling the gaps" appeared repeatedly in news reports, this basic scientific research was fortunate enough to attract everyone's attention and became a hot topic on multiple media platforms, and also spawned interesting memes such as "Fishing is strictly speaking a phenomenon of atavism" and "No wonder I'm a salted fish!"

As a member of the scientific research team, I have received a lot of attention and curiosity about this achievement, and I feel honored. Today, I will first answer some questions that everyone is interested in on the Internet, and then try to combine your questions to talk about how to understand this achievement without the need for professional knowledge. Now, let's "eat melons" together!

01

Answers to some interesting questions

First, I collected some questions with high likes in the comment section and tried to answer them based on my personal understanding.

ask

When we eat fish, are we eating our ancestors?

answer

The answer is of course no. The relationship between humans and living fish is more like that of very distant relatives who separated 400 million years ago. We must have had a common ancestor at some point in the distant past, but humans and living fish chose two completely different paths and evolved to this day . Our "relative relationship" with the fish we eat daily is already very thin. In terms of "generation", it is not an exaggeration to say that we are of the same generation.

ask

Why do paleontologists often work on the side of the road in the news? This is different from what I expected!

answer

First of all, of course, it is convenient. Modern field work generally uses vehicles as the main means of transportation. Driving on the road and quickly "scanning" the strata is a common skill for many geologists. Secondly, there is an old saying in geological work called "every ditch will be broken", which means that the strata that are generally horizontally deposited will always be damaged in the deep ditch and are prone to developing faults. Although this statement may not be correct in actual work, the existence of the cross section does make it easier for us to study the strata . This is why paleontologists often appear in places where rock strata are exposed, such as construction sites and roads.

ask

In addition to the 5 fish discovered this time, have any other fossils been unearthed?

answer

In addition to the five new species published this time, there are more than ten new fish species discovered during the same period that are currently being sorted out and studied . In the foreseeable future, I believe that there will be more research results from South China to further fill in the gaps in the history of vertebrate evolution.

02

Why is this research said to “fill the gap”?

About 200 years ago, geologist Roderick Murchison named a set of rock formations on the border of Wales and England the Silurian System. He found in these formations some fish fossil fragments, including jaws, spines and scales, which he considered to be "the oldest of their kind" and "completely different" from other fossils found in overlying strata.

Geologist Roderick Murchison, Image source: Wikipedia

With the development of geology, people have found that the rare fish fossils in the Silurian strata are in sharp contrast to the abundant fish fossils in the overlying Devonian strata, which is known as the "Age of Fishes". The scarcity of Silurian fossils has left a large blank in the evolutionary map of life pursued by paleontologists. This has also caused many people to question the theory of evolution.

Modern paleontologists generally believe that the evolution of vertebrates can be traced back to the Cambrian explosion of life about 539 million to 520 million years ago. Including us humans, 99.8% of the vertebrates on Earth have jaws (upper jaw and lower jaw), collectively known as jawed vertebrates or jawed animals . Many of the human organs and body structures can be traced back to the beginning of the appearance of jawed animals.

However, there has always been a lack of complete fossil evidence for the early evolution of jawed vertebrates. Scientists have used molecular biological methods such as molecular clocks to measure the genetic differences of living jawed vertebrates and concluded that the earliest differentiation node of jawed vertebrates was no later than the late Ordovician period (about 450 million years ago). However, as mentioned above, jawed vertebrates are very rare in the Silurian system worldwide, and complete fossils are even fewer.

The situation was not greatly improved until 2009, mainly due to the long-term field survey conducted by the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences in South China, and the discovery of a large number of Late Silurian (about 425 million years old) fossils in the Xiaoxiang Fauna.

The four academic papers this time focus on reporting a batch of new fossil specimens from the border of Chongqing and Guizhou, making this once vague period clearer and indisputably pushing the origin of jawed vertebrates to deeper and older strata, while also clarifying how some iconic features evolved.

Therefore, the statement "filling a key gap" that you will see in the report is, to be precise, filling "a stubborn major gap in the history of paleontology" in the global Silurian early jawed fossil record, providing for the first time conclusive evidence for the rise and earliest radiation differentiation of jawed fish .

03

Research subjects of the 4 papers

They are 5 ancient fish that are more than 400 million years old.

As mentioned earlier, the four academic papers reported a batch of new fossil specimens from the border between Chongqing and Guizhou, to be precise, 5 ancient fish fossils that are more than 400 million years old.

List of species published in this collection, pictures from reference [1]

First up is the Qianodus duplicis from Shiqian, Guizhou Province, named for the pair of main dentition rows it has on its tooth whorl.

Although only one tooth whorl has been preserved of the Qiantooth fish, its clearly arranged, zigzag-shaped, unidirectionally growing main tooth units and staggered lateral tooth units are clearly different from the imbricate pharyngeal teeth of jawless fish such as the croaker, making it an undisputed jawed fish.

When it comes to tooth spiral, many students who often read biology popular science and game subculture may first think of the popular Helicoprion shark. Through phylogenetic analysis, the Qiantooth fish is indeed at the basal position of cartilaginous fish. Whether it has a closer evolutionary relationship with species such as Helicoprion shark that also have tooth spiral needs further study. The discovery of Qiantooth fish pushed the earliest tooth fossil record of jawed fish back to 14 million years in history.

The tooth whorl fossil of the Qiantooth fish clearly shows two rows of main tooth units (orange and green) and lateral tooth units (yellow) growing in a zigzag pattern. The scale bar is 0.5 mm. Image from reference [2]

Unlike the isolated specimens of Qiantooth fish, the newly discovered Fanjingshania renovata has thousands of fragmentary specimens, including scales, periorbital bones, incomplete branchial bones, pectoral spines, prothorax spines, ventral spines, dorsal spines, anal spines and intermediate spines. Through meticulous and long-term preliminary processing and microscopic work, the researchers collected and classified the specimens belonging to Fanjingshania from tens of thousands of microfossils smaller than sesame seeds through common histological characteristics, surface patterns, developmental characteristics and partial associated preservation.

The research team processed the microfossils. A: The collected rock specimens need to be rinsed and soaked; B: The fragments obtained after the preliminary processing are sorted under a microscope. Image from reference [3]

Fanjingshan fish has the histological characteristics of plate-like dentition, which was previously only seen in early cartilaginous fish. In addition, Fanjingshan fish has developed iconic anatomical features such as shoulder girdle and intermediate spines. According to the results of phylogenetic analysis, Fanjingshan fish belongs to the genus of cartilaginous fish-spined fish-grissaccharids. Its discovery pushed the earliest appearance of the grissaccharids back to the Landoverian period of the Silurian period, and also revealed the morphological structure of the oldest sharks .

In addition, the complete and mature body structure of Fanjingshan fish also indirectly indicates that the stem group of bony fish should have appeared in the Early Silurian, which is consistent with the time of morphological differentiation between cartilaginous fish and bony fish.

The research team identified different parts of the Fanjingshan fish based on common histological and surface pattern features. Scale bar is 1 mm. Image from reference [4]

As mentioned earlier, the gap caused by the scarce preservation of Silurian vertebrate fossils is a steep mountain that researchers must overcome, and the discovery of the "Chongqing Specific Buried Fossil Repository" in Xiushan County, Chongqing is tantamount to finding a shortcut for us.

The continental shelf east of Chongqing, located on the edge of the South China continent, was a warm coastal and shallow sea environment called the "Yangtze Ancient Sea" in the early Silurian period. It was a paradise for organisms, with a large number of fish and arthropods thriving here. Until 436 million years ago, active plate movements caused frequent earthquakes, and the raised mud and sand quickly buried and covered smaller waters such as lagoons and rivers. The struggling fish turned into lifelike fossils after hundreds of millions of years, and such rapidly buried and well-preserved fossil reservoirs are called "special burials" by researchers. The famous "Chengjiang Biota" and other fossil reservoirs with rich scientific research results that you may have heard of are all special burials. In this concentrated report, there are also two academic papers from this newly discovered "treasure house".

The large number of fossils found in the "Chongqing Special Buried Fossil Repository" reflects a thriving animal community. A: Shen's stickleback (1a, 1b) and Xiushan fish (2a, 2b); B: A large number of preserved fish fossils; C, D: Hughmilleria wangi fossils. Scale bar is 5 mm. Image from reference [5]

For a long time, researchers have always believed that cartilaginous fish are more primitive than bony fish and closer to the original form of vertebrate evolution, and that both bony fish and placoderms originated from some kind of cartilaginous fish. However, some studies in recent years have also raised many objections, and the discovery of Shenacanthus vermiformis has completely resolved this debate.

Shen's stickleback not only has similar anatomical features to typical cartilaginous fish-sticklebacks, but also has large bone plates covering the shoulder girdle and back that are unique to placoderms. This counterintuitive discovery completely rewrites people's impression of the evolutionary tree of life in the past century, meaning that the ancestors of cartilaginous fish came from placoderms with hard armor.

Although Shen's stickleback has a typical stickleback body structure, it has large bone plates on its shoulder girdle and back, a feature that links it to the ancestors of placoderms. Scale bar is 5 mm. Image from reference [5]

The miracle Xiushan fish (Xiushanosteus mirabilis) from the same report gives us a deeper understanding of the early evolution of placoderms.

When it comes to placoderms, the first thing that comes to mind for most people is Dunkleosteus, which lived about 20 million years after Xiushanichthys. Indeed, placoderms represented by Dunkleosteus gave rise to a large family with a large population in the Devonian Period, but the origin of placoderms is still hidden in a thick curtain.

Comparison of the size of a restored Dunkleosteus and a human, Image source: Wikipedia

In terms of age, Xiushanichthys is undoubtedly more primitive than the Devonian placoderms. From an anatomical point of view, Xiushanichthys is a mixture of characteristics of several different placoderms and cannot be classified into any currently known placoderms. If we list the characteristics of Xiushanichthys and Devonian placoderms in a graph, then their intersection will inevitably include more primitive placoderms and even primitive characteristics of jawed fish. Therefore, the discovery of Xiushanichthys is undoubtedly a lantern for us to grope in the dark for the origin of jawed vertebrates. Although its light cannot illuminate the scenery in the distance, it is enough to illuminate the road under our feet.

Xiushanichthys is the oldest complete armored fish fossil discovered so far, with many primitive features of jawed fish. Scale bar is 5 mm. Image from reference [5]

If the contributions to phylogenetics listed above seem a bit obscure, the lively Tujia fish (Tujiaaspis vividus), also from the "Chongqing Special Buried Fossil Repository", has promoted a more specific scientific question: where do our limbs come from?

Unlike the jawed protagonists that appeared before, the armored fish, which Tujia fish belongs to, are jawless vertebrates and do not have jaw structures. The armored fish belongs to the armored fish, which is characterized by a complete and huge head armor. It is also a group with a strong "indigenous color". It is mainly found in southern China and Vietnam and other regions around the world.

As their name suggests, the vast majority of armored fish fossils discovered by scientists are solid and easy to preserve heads, while the soft torso is very rare. Thanks to the formation mechanism of the special burial, the smart Tujia fish is the world's first armored fish fossil with a complete body preservation.

Many science enthusiasts know that the limbs of terrestrial organisms originated from the paired fins (pectoral fins and anal fins) of lobe-finned fish , but where do the paired fins of fish come from?

Through research in molecular biology and embryology in recent years, researchers have discovered that the odd fin (dorsal fin) and the paired fin have the same gene regulation mechanism, and gradually refined a hypothesis that the origin of the paired fin is similar to that of the odd fin, and both evolved from a fold that runs through the body. This is the "fin fold theory."

In the abdomen of the Tujia fish, we found a pair of ventral fin folds that run through the whole body, which undoubtedly provides key evidence for the "fin fold theory" of the origin of appendages that has been discussed in academia for a long time. The research team further used fluid mechanics to analyze it. Computational fluid dynamics showed that in the absence of the iconic differentiated paired fins, the fin folds can use the water flow to generate lift to offset their own gravity. This discovery proposed a new hypothesis about the origin of paired appendages in vertebrates from the functional mechanism.

In addition, it should be noted that although they belong to the jawless class, armored fish represented by armored fish are more closely related to jawed fish in evolution than living jawless fish (such as hagfish and lampreys), and living jawless fish are by no means the evolutionary descendants of armored fish.

Tujia fish is a rare armored fish fossil with preserved body parts. The wrinkles running through its body provide scientists with clear evidence for the origin of its appendages. Scale bar is 4 mm. Image from reference [6]

Finally, please allow me to end this article by quoting the commentary from this issue of Nature, by Professor Matt Friedman of the University of Michigan: "In terms of both theoretical development and timeline improvement, an exciting new era of early jawed evolution research has arrived, and we are at its starting point."

References:

[1] Fossils reveal the deep roots of jawed vertebrateshttps://www.nature.com/articles/d41586-022-02973-5【2】

[2] Andreev, PS, Sansom, IJ, Li, Q. et al. The oldest gnathostome teeth. Nature 609, 964–968 (2022). https://doi.org/10.1038/s41586-022-05166-2

[3] Natural History and Culture Research Center of Qujing Normal University. https://www.qjnu.edu.cn/

[4] Andreev, PS, Sansom, IJ, Li, Q. et al. Spiny chondrichthyan from the lower Silurian of South China. Nature 609, 969–974 (2022). https://doi.org/10.1038/s41586-022-05233-8

[5] Zhu, Ya., Li, Q., Lu, J. et al. The oldest complete jawed vertebrates from the early Silurian of China. Nature 609, 954–958 (2022). https://doi.org/10.1038/s41586-022-05136-8

[6] Gai, Z., Li, Q., Ferrón, HG et al. Galeaspid anatomy and the origin of vertebrate paired appendages. Nature 609, 959–963 (2022). https://doi.org/10.1038/s41586-022-04897-6

Produced by | Science Popularization China

Author: Luo Yanchao, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences

Producer｜China Science Expo

Submitted by: Computer Information Network Center, Chinese Academy of Sciences

The cover image and the images in this article are from the copyright library

Reproduction of image content is not authorized