Why are most organisms "symmetrical"?

Today, the biodiversity of the earth is very rich, and the appearance of organisms is colorful. However, have you noticed that most organisms are bilaterally symmetrical , such as humans, who are typical bilaterally symmetrical animals.

In the animal kingdom, whether it is the large mammals such as lions, tigers, and elephants among vertebrates, the reptiles such as crocodiles and snakes, the amphibians such as frogs and toads, or the mollusks such as clams and clams among invertebrates, and the arthropods such as insects and centipedes, there are many animals with bilaterally symmetrical shapes.

A butterfly with bilateral symmetry. Copyright image, no permission to reproduce

You may ask, what's so strange about this? This is how the biological world is! Indeed, this is the mainstream appearance of modern biological world.

However, even in today's biological world, there are other forms of organisms, such as asymmetrical gastropods (commonly known as snails), pentaradially symmetrical starfish, and irregular amoebas .

Five radially symmetrical starfish. Copyright image, no permission to reproduce

Interestingly, in the long evolutionary history of life on Earth, bilaterally symmetrical organisms did not emerge like a kaleidoscope until the Cambrian period 541 million years ago .

Although the fossil record of the oldest bilaterally symmetrical organisms can be traced back to 580 million years ago, the multicellular organisms of the Ediacaran period of the Late Proterozoic Era were mainly radially symmetrical , rather than the bilaterally symmetrical morphological characteristics that we are more familiar with.

So, what is the evolutionary relationship between radial symmetry and bilateral symmetry? What roles did they play in the history of biological evolution, and what is their relationship with their own functions and environment?

01

Let's see what forms of organisms there are.

Generally speaking, in the long history of life on Earth, the historical evolution of biological modeling has experienced morphological changes from asymmetry or instability to radial symmetry and then to bilateral symmetry .

The changes in the morphology of organisms are closely related to the development of their systems and structures. Especially when the earth's environment and biological evolution entered a period of major changes, the morphology of organisms showed revolutionary changes, which just represented a major evolution of biological evolution and had a profound impact on the radiation evolution of organisms.

Life begins with asymmetry

The oldest organisms in the history of life were primitive and tiny, and their shapes were asymmetrical or asymmetrical. Single-celled organisms such as amoeba, which are familiar to people, have irregular and amorphous shapes , and they can change shape at any time. Some primitive multicellular animals, such as some species of sponges and cnidarians, are also irregular. Many biological individuals aggregate into groups and often produce asymmetrical shapes.

Sponge. Image source: Wikipedia

Radial symmetry

Radial symmetry can be divided into spherical radial symmetry and axial radial symmetry .

Spherical radial symmetry is isometric symmetry, which can divide the body into two equal halves, infinite or finite, through the center, such as heliozoa and most radiolarians . They mostly live suspended in water, with the same environment above, below, left and right. Except for the difference from the center to the surface, these animals have no characteristic decreasing rate in one direction.

Axial radial symmetry is a uniaxial heteropolar symmetry , which is a fixed main axis that divides the body into two equal halves, such as scale insects, bell insects, sponges and cnidarians , adapted to fixed or floating life. Axial radial symmetry can be traced back to the Ediacaran period of the late Proterozoic era, when the mainstream form of the biological world was axial radial symmetry.

Spherical radiating radiolarians (provided by Luo Hui)

The Ediacaran biota, which existed between 575 million and 541 million years ago, has a variety of axially radially symmetrical fossils , including three, four, five, six, and eight radials. The spiral arms or walls of the radiations are not only straight, but also curved, such as the eight-armed celestial fossils found in Australia and the Weng'an biota in China.

Since the Cambrian period of the Phanerozoic Eon, although the axial radial symmetry has become a non-mainstream form, it has appeared in the form of secondary axial radial symmetry in the newly emerged echinoderms. For example, the starfish that everyone is familiar with is a marine echinoderm with five radial symmetry.

Colorful bilaterally symmetrical forms

Bilateral symmetry refers to the division of an animal's body into two equal parts through a central axis, i.e., a symmetry plane (or section), so bilateral symmetry is also called bilateral symmetry. Bilateral symmetry has been around since the emergence of flatworms . This type of organism characterized by bilateral symmetry is adapted to reptile life.

Bilateral symmetry means that animals can move from irregular to controlled, that they can distinguish front and back, left and right, and that they have a clear directional and forward tendency during movement, which is extremely beneficial for the formation of nerves and brain .

Bilateral symmetry is an important condition for the development of animals from aquatic to terrestrial, and is an important feature of animal evolution .

Bilaterian animals first appeared in the Weng'an Biota 600 million years ago. The famous spring worm is a representative of early bilaterian animals, but before the Cambrian period, bilaterian forms were a minority in the biological world. Bilaterian organisms are more advanced than radially symmetrical organisms. They completely replaced radially symmetrical organisms in the Cambrian period and became the most dominant form since the Phanerozoic, dominating the biological world.

The spring worm is the earliest bilaterian. Image credit: Bottjer

02

The history of morphological evolution is the history of life evolution

The Cambrian Explosion is a milestone event in the history of life on Earth. It not only had the strongest biological speciation effect, but also had the most significant plasticity in biological modeling. The 38 animal phyla on Earth today are mainly bilaterally symmetrical organisms, with various shapes and forms, all of which originated from the early Cambrian explosion. The emergence of a large number of bilaterally symmetrical organisms is undoubtedly a very important and significant evolutionary phenomenon in the Cambrian Explosion .

The bilaterally symmetrical biological patterns of the Cambrian explosion have shown a trend of diversification, such as fish-shaped bilateral symmetry , arthropod-shaped bilateral symmetry , and shell-shaped bilateral symmetry . Representative animals include Haikou fish, Anomalocaris, trilobites, and tongue-shaped shells. In the late Paleozoic era, the Mesozoic era, and the Cenozoic era, the bilaterally symmetrical patterns of amphibians, reptiles, birds, and mammals appeared successively.

Bilaterally symmetrical trilobite. Image source: Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences

So many animal phyla have formed morphological shapes characterized by bilateral symmetry, and in the biological shapes of different levels such as classes, orders, families, genera and species within the framework of their phyla, various morphological changes have occurred, ultimately forming the rich and colorful biological landscape of the earth today.

Of course, there is no lack of alternatives in the biological world. The asymmetrical spiral shape of gastropods is unique, and the newly emerged echinoderms in the Cambrian explosion reproduced the new glory of this ancient morphology with secondary radial symmetry. It is these unique biological morphological shapes that have become an important basis for scientists to study biological categories and establish biological taxonomy.

03

Which is better, radial symmetry or bilateral symmetry?

Radial symmetry differs from bilateral symmetry in many ways:

① Different number of symmetry axes

Radial symmetry has multiple axes of symmetry, while bilateral symmetry has only one axis of symmetry . For example, starfish have five radial axes, while butterflies have only one (the central axis).

② Different animal shapes

The radially symmetrical shape only has the difference between up and down , and there is no distinction between left and right. The bilaterally symmetrical animals have the difference between front and back, left and right, and dorsal and ventral.

③Different athletic abilities

Radially symmetrical animals have weak motor abilities , while bilaterally symmetrical animals have stronger motor abilities and can respond to the external environment more quickly and accurately.

④Different levels of evolution

From radial symmetry to bilateral symmetry is an evolution, bilateral symmetry is more advanced than radial symmetry . Radial symmetry is a primitive form of symmetry, represented by sponges and cnidarians. Starting from flatworms, animals began to have bilateral symmetry.

⑤Adapt to different environments

Radially symmetrical animals are adapted to a fixed or floating life , and bilateral symmetry promotes the formation of head in animals, allowing them to adapt to a more complex and changing environment.

After reading these differences, you will know which symmetry is more powerful.

Starfish fossil. Copyright image, no permission to reprint

04

Why does biological morphology evolve?

In the process of biological evolution on Earth, the morphological modeling of organisms is adapted to and related to the changes in the relevant structures of the biological body. Radial symmetry and bilateral symmetry animals have each taken the lead in different geological historical stages, and have jointly presented a new diversity prosperity in the biosphere of Earth today.

1) Changes in biological morphology are closely related to the biological germ layer system

Single-celled animals do not have the concept of germ layers, even Volvox has only one layer of cells. True multicellular animals have differentiated germ layers, and the emergence of three germ layers is of great significance in animal evolution.

Micrograph of a teratoma, a typical tumor with three germ layers of tissue. The image shows tissues derived from the mesoderm (immature cartilage - upper left corner of the image), endoderm (gastrointestinal glands - bottom center of the image), and ectoderm (epidermis - right side of the image). Image source: wikipedia

At the end of the Precambrian, in addition to the extinct "Vendaceae" animals, common radially symmetrical animals were sponges, cnidarians, and extinct animals that could not be classified into known animal phyla. Although they were all multicellular animals, they only developed ectoderm and endoderm, lacked mesoderm, and therefore could not form dermis, coelom membranes, and mesentery, etc. Therefore, they did not have the mouths and bones that modern animals have, and lacked the functional organs that animals usually have, such as movement, feeding, and digestion. Therefore, animals with radially symmetrical shapes generally live in fixed bottom dwellings or floating lives .

Most of the Ediacaran biota are radially symmetrical, and this morphological system determines that the nutrition of the organisms is mostly through contact between the body surface and seawater, and osmotic absorption of nutrients. So, an interesting scene appeared: in order to obtain more osmotic nutrients, the animals of the Ediacaran biota continued to expand and expand their body surface area, forming various strange shapes such as tubes and fans. The excessive expansion of the body surface area became one of the causes of their extinction .

Ediacaran biota. Illustration by Yang Dinghua

Bilaterian animals have a three-germ system, and representative animals include flatworms, annelids, molluscs, arthropods, echinoderms and hemichordates . Vertebrates in particular have an advanced three-germ system, which provides the necessary material basis for the formation and development of various tissues and organs in the animal body.

The muscle system developed from germ layer tissue strengthens the movement function and complicates the contact between animals and the environment, thereby promoting the development of sensory organs and nervous systems, and improving animals' response to stimuli and foraging efficiency.

Efficient foraging increases the nutrition of animals, promotes their metabolism, and strengthens their excretion function, which in turn "affects the whole body" and leads to a strong differentiation in the animal's morphological structure.

At the same time, the mesoderm not only has the ability to regenerate, but also can store water and nutrients , which greatly improves the animal's adaptability to drought and hunger, and provides the necessary material conditions for animals to break away from aquatic life and enter the terrestrial environment.

The emergence of the mesoderm completed the three-germ system of animals, and then produced two branches of animals: one is the protostomes and the other is the deuterostomes . Deuterostomes are the main line of evolution and represent the most important evolutionary force of the Cambrian explosion. As a vertebrate, we humans have gradually evolved from primitive deuterostomes .

Protostomes. Image source: wikipedia

Deuterostome. Image source: wikipedia

2) The evolutionary significance of drastic changes in biological morphology

In the approximately 4 billion years of life on Earth, the world has changed dramatically, and with the changes in the natural environment, organisms have also continued to evolve in constant adaptation. The innovation of biological morphology represents a series of breakthroughs and evolutions in biological evolution. They are major adaptive responses to the environment and are the product of a certain stage of geological history, indicating that biological evolution has entered a new stage of development.

The Ediacaran period was a turning point in the earth's environment , with the atmospheric oxygen content rising sharply for the second time. The Rodinia supercontinent was still falling apart, and the large-scale upwelling brought a large amount of phosphorus and other trace elements, providing rich nutrition for the prosperity of shallow-sea creatures. The biological world was also incubating major breakthroughs, presenting the eve of the dawn of animals. More than 90% of the embryonic fossils found in the Weng'an Biota indicate that life is ushering in a critical moment . It is under this geological background that the radially symmetrical organisms flourished.

However, as the evolution of the Ediacaran biota reached a dead end, new biological groups continued to emerge. In the Cambrian period, the competition for survival in the biological world continued to intensify, and animals opened up an increasingly larger living space. The biological world not only formed an interconnected food chain, but also made continuous breakthroughs in the expansion of ecological space.

In order to adapt to the new environment and new competition, a large number of organisms with bilateral symmetry emerged in the Cambrian period. Since then, the metazoans with bilateral symmetry of hard skeletons have become the main force of evolution to date , and eventually formed the most influential, widely distributed and diverse biological features in the biological world today.

Various symmetrical forms of organisms. Image source: Wikipedia

You see, the wonderful nature also loves to learn mathematics, and it has played with the mathematical concept of symmetry. It gives different organisms different forms. Symmetry seems to be natural, but it contains important nodes in biological evolution .

Produced by | Science Popularization China

Author: Feng Weimin (Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences)

Producer｜China Science Expo

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