2024-07-17
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In the long history of life evolution, the evolution of sponges has always been a fascinating but difficult puzzle. Sponges are considered to be one of the earliest multicellular animals on Earth and an important bridge between single-celled organisms and complex animals. However, there has always been a huge gap in the scientific community's knowledge of the origin and early evolution of sponges.
Until recently, Chinese scientists made a major breakthrough, providing a key clue to solve this mystery. An international cooperation team led by Yuan Xunlai, a researcher at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, discovered a sponge fossil about 550 million years old in Yichang, Hubei, which was named "spiral grid sponge".
This discovery not only fills an important gap in the early evolution of sponges, but also provides us with a new perspective to re-examine the entire process of life evolution.
Restoration of spiral grid sponge (Image source: Reference 2)
Sponges, this seemingly simple yet mysterious group of organisms, occupy a unique position in the animal kingdom.
Sponges have a simple body structure and no obvious organ system, but they have amazing survival abilities. Their bodies act like a huge filter, constantly sucking in and expelling water to obtain nutrients and oxygen. This simple and efficient way of survival has enabled sponges to thrive on Earth for hundreds of millions of years.
In modern oceans, there are many types of sponges with different shapes. Some sponges are like delicate vases, while others are like huge barrels or fans. They also have a wide variety of colors. These colorful sponges not only add brilliant colors to the underwater world, but also make important contributions to the balance of the marine ecosystem.
Various sponges (Image source: Wikipedia)
Despite their prevalence today, sponges have long been missing from geological history, a phenomenon that has baffled scientists and remains a mystery.
Imagine if you were a paleontologist searching for the ancestors of sponges. You excitedly dug through rock layers from all geological ages, only to find that before the Cambrian period, about 539 million years ago, the fossil record of sponges was almost blank. It's like looking at a thick family history book and the pages of the first few chapters are all missing.
Scientists have speculated through molecular biology research and molecular clocks that sponges should have originated about 700 million years ago. However, in the actual fossil record, confirmed sponge fossils did not appear in large numbers until the Cambrian period. This means that there is a huge gap of 160 million years in the fossil record of sponges, a period that scientists have vividly called the "lost years" in the early evolution of sponges.
Why does this happen? Scientists have proposed two main hypotheses:
The first hypothesis is that the common ancestor of sponges may have had mineralized spicules (the supporting structures of sponges), but the environment before the Cambrian may not have been conducive to the preservation of these spicules, making them difficult to find fossils.
The second hypothesis is that early sponges may not have mineralized spicules at all, but are made of soft tissue. This makes them more easily destroyed in the fossilization process and more difficult for us to identify. It's like painting on a rock with water, which will quickly evaporate and disappear, leaving no lasting traces.
This mystery has puzzled scientists until this recent study provided key clues to solving it.
Various types of spicules of modern sponges of the class Calcarea (Image source: Wikipedia)
In Yichang, Hubei, scientists discovered an exciting new fossil in the Shibantan Biota, which is about 550 million years old. The fossil is named "Spiral Grid Sponge". Its discovery is like finding a connection point in the long river of history, filling the gap in the early evolution of sponges.
The spiral grid sponge has amazing morphological features. Its body presents an elegant cone shape with a disk-like structure at the bottom, which may be used to anchor itself to the seafloor. The most striking thing is that its surface structure is composed of regular squares, which can be subdivided into smaller squares, forming a complex and orderly network structure.
Spiral grid sponge fossil photo (a) and laser scanning elevation map (b) (Image source: Reference
The structure is reminiscent of modern hexapod sponges, but with one key difference: The hexapod sponge's grid is composed of organic matter, not mineralized spicules.
This discovery provides us with a completely new perspective on the early evolution of sponges. It tells us that in the late Ediacaran period, there were already sponges without mineralized spicules. It is like we have discovered a "primitive version" of sponges, which have not yet developed hard spicules but already have the basic characteristics of sponges.
Scientists have conducted a detailed study and analysis of the spiral mesh sponge. They compared this fossil with various living animals and other fossils, constructed a complex data matrix, and conducted a rigorous phylogenetic analysis. The results showed that the spiral mesh sponge belongs to the crown group sponge and is closely related to the six-release sponge. This discovery further confirms the important position of the spiral mesh sponge in the evolution of sponges.
A certain class of sponge, which has siliceous spicules, some of which can aggregate into a network of skeletons, and no sponge in the bones. Therefore, this class is also called glass sponge. (Image source: Wikipedia)
The discovery of the spiral mesh sponge provides key information for our understanding of the early evolution of animals. It tells us that when looking for Precambrian sponge fossils, we cannot just use modern sponges as a standard. Early sponges may not have mineralized spicules or some of the features we are familiar with. This discovery broadens our horizons and points out new directions for future research.
Sponges and the Explosion of Life: Reexamining the Evolution of Life
The discovery of the spiral grid sponge also provides us with a new perspective on the evolution of life. Scientists have long believed that the biological communities of the Precambrian and Cambrian periods were very different.
The Ediacaran period is considered to be dominated by strange creatures with unknown relationships. In the Cambrian period, modern animal phyla suddenly appeared in large numbers, which is the famous "Cambrian explosion". This view is like describing two completely different worlds: one is an ancient world full of mysterious creatures, and the other is the prototype of the modern marine ecosystem we are familiar with.
However, the discovery of the spiral grid sponge has built a bridge between these two seemingly separated worlds. It tells us that in the late Ediacaran period, there were already organisms that had a direct evolutionary relationship with modern animals. In other words, a key picture in the ancient family history connecting ancient ancestors and modern descendants has been found.
This discovery also makes us rethink the nature of the "Cambrian Explosion". Perhaps the evolution of life is not a sudden "explosion", but a gradual process. Before the Cambrian period, various animal phyla may have begun to differentiate, but they had not yet developed hard shells or bones, so it is difficult to preserve them in the fossil record.
This new discovery also brings us more questions: Are there earlier sponges before spiral mesh sponges? How did the ancestors of sponges acquire the ability to mineralize spicules? The answers to these questions may be hidden in ancient rock formations, waiting for future scientists to discover.
The evolutionary relationship between spiral mesh sponges and other animals (Image source: Reference 2)
Conclusion
The evolution of sponges is like an epic spanning hundreds of millions of years. From the initial soft body structure to the later development of complex mineralized spicules, each step records the secrets of life evolution. The discovery of the spiral grid sponge adds a rich and colorful stroke to this epic, allowing us to glimpse the mysterious appearance of the ancient ocean.
In the future, with the advancement of technology and the application of new methods, we may be able to discover more early animal fossils and further improve the picture of life evolution. The story of sponges will continue to inspire us to think about the nature of life, inspire us to explore unknown areas, and pursue the mysteries of life evolution.
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Produced by: Science Popularization China
Author: Li Jinjuan (Waseda University)
Producer: China Science Expo
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