Where did humans come from? This "landing pioneer fish" gives a scientific answer to the question

"Where do humans come from?" This philosophical question has puzzled many people, but now there may be a scientific answer to this question.

Two years ago, a team led by Academician Zhu Min from the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, published a paper confirming that humans evolved from fish.

So, which "fish" was the first to land on land? Today we will talk about the evolution and landing process of this pioneer - Ichthyostega .

Species Name: Ichthyostega

Classification: Ichthyostega, Ichthyostega, Ichthyostega

Fossil distribution area: Greenland

Shanghai Natural History Museum "The Way of Evolution" Exhibition Area - Life Landing Exhibits

In order to achieve such an earth-shaking event, Ichthyostega spent nearly 100 million years , from the appearance of jawless fish, spanning the Cambrian Period to the Devonian Period, upgrading its body structure, just to take a big step towards the landing of vertebrates.

Jaw dropping

Fish, the oldest vertebrates, appeared in the early Cambrian period 530 million years ago. Believe it or not, they didn’t have chins ! So their mouths were disc-shaped, and they belonged to the jawless class . They could only passively inhale or filter microorganisms in seawater sediment for food.

The existing lamprey (Petromyzontiformes) is a typical jawless vertebrate fish-like animal .

As evolution continued, the nasopharyngeal complex of fish separated into the nasal sac and pituitary gland, providing space for the development of the jaw.

Source: References

In Chongqing's special buried fossil repository, Zhu Youan and others discovered the miracle Xiushan fish (Xiushanosteus mirabilis), which lived in the early Silurian period 436 million years ago. It is the earliest known jawed fish , proving that a group of fish represented by placoderms evolved by moving their gill arches originally used for breathing forward to form upper and lower jaws, allowing them to actively attack prey and facilitate biting and chewing prey.

Source: "The Emergence of the Jaw" (Shanghai Natural History Museum Darwin Center - Evolutionary Leap Area Video)

Miracle Xiushan fish restoration | Source: References

Miraculous Xiushan fish fossils and analysis | Source: References

Since then, fish have developed rapidly, increasing in size from tens of centimeters to several meters in 10 million years. Among them, Dunkleosteus reached a body length of 11 meters, and the bite force of its jaws far exceeded that of Tyrannosaurus Rex!

With this "advanced" equipment, fish have driven invertebrates off the throne and become the new overlords of the ocean.

Rise of the Bone

After fish dominated the ocean, they immediately began to compete more fiercely. Placoderms gradually evolved into two major groups, cartilaginous fish and bony fish .

Source: References

Cartilaginous fish have a low calcium content in their bones and are the ancestors of modern aquatic animals such as sharks and rays.

Plastinated specimen of the stingray | Photo by Cherry

The skeleton of bony fish has a high calcium content , which hardens the bones throughout the body. If you eat it, you will get stuck and be sent to the hospital. Therefore, the skeleton structure of bony fish can better support the weight, providing an important foundation for future landing.

Carp skeleton specimen | Photo by Cherry

Get on all fours

In the late Devonian period about 365 million years ago, the second mass extinction event caused the Earth's climate to cool, the oceans to retreat, the land to expand, and most fish died due to lack of water. Therefore, landing became the only hope for fish to survive.

There is buoyancy in water, but on land, it is affected by gravity. To overcome this difficulty, fish must adapt to changes in the environment and evolve the fins used for paddling into limbs to support their bodies.

The "top stream" of modern bony fish are the ray-finned fish, whose fins are membranes supported by fin rays that are attached directly to the shoulder girdle bones like the spokes on a bicycle wheel.

Comparison of skeletal structure between lobe-finned fish and ray-finned fish | Source: References

The lobe-finned fish were the ones that succeeded in conquering the land . The specimen of Latimeria chalumnae in our museum is a living lobe-finned fish. Its fins have well-developed muscle stalks and are supported by bone structures inside. The limbs of Ichthyostega evolved in this way.

Latimeria chalumnae | Source: Fingertip Museum

The evolution of fish fins to toes | Photo by Cherry

In appearance, Ichthyostega is very similar to the Chinese giant salamander (Andrias davidianus). It is about 1 meter long, with strong shoulder and waist girdle bones, and its limbs can support its body to crawl slowly on the ground. It has about 6 to 8 toes on its front and back feet , which is obviously different from the common five fingers (toes) of most animals.

Perspective view of the forelimb and hindlimb skeleton of Ichthyostega | Photo by Cherry

Although it has the trunk, legs and toes of a terrestrial animal, its head still looks like a fish. Its skin is covered with small scales to maintain body fluids and prevent dehydration. Its tail is also similar to the tail fin of a fish. It has the dual characteristics of fish and amphibians and is the earliest amphibian known to humans .

Ichthyostega restoration image | Source: Cherry

Swim bladder and lung are homologous

Landing also presents another challenge: how to absorb oxygen from the air?

In addition to using gills to breathe, early fish also used tissues such as the skin, throat, and intestinal mucosa to assist in breathing.

Frogs use their skin to help them breathe | Photo by Cherry

Later, some fish expanded part of their digestive tract to store air, which became the primitive lungs . During the course of evolution, ray-finned fish retained some of the characteristics of the primitive lungs and evolved into swim bladders, while lobe-finned fish completely retained and continuously strengthened this breathing skill, eventually forming lungs.

Source: References

In order to send air into the lungs, relevant channels need to be formed. This is just like we rely on our mouths to breathe when we swim in the water, and rely on our noses when we get on the shore. When fish land, the nostrils and lungs must be connected.

At the beginning, ancient fish only had two external nostrils that served as olfactory receptors. With continuous evolution, some bones in the mouth of lobe-finned fish formed a passage between the nasal cavity and the oral cavity - the internal nostrils .

Chinese scientists Zhang Yiman and Zhu Min's team found the earliest internal nostrils in the fossil of Kennethnis 390 million years ago. When analyzing the fossil of the true palm-finned fish 380 million years ago, they found that its internal nostrils were connected to the primitive lungs, successfully forming a "terrestrial respiratory system" . Therefore, Ichthyostega was able to completely get rid of the constraints of water and eat, sleep and breathe on land.

Evolution of the internal nostril | Source: References

The internal nostril of Kennet's fish | Photo by Cherry

The internal nostrils of true palm-finned fish are connected to primitive lungs. Source: "From Water to Land: The Story of Ichthyostega" (video of the Life Landing Theme Area of ​​Shanghai Natural History Museum)

“We came from the water.” Only by looking back to the distant past can we more clearly understand who we are, and more accurately predict our future and know where we are heading.

References

[1] Zhu YA, Li Q, Lu J, Chen Y, Wang JH, Gai ZK, Zhao WJ, Wei GB, Yu YL, Ahlberg PE*, Zhu M. * 2022. The oldest complete jawed vertebrates from the early Silurian of China. Nature. https://doi.org/10.1038/s41586-022-05166-2

[2] Gai ZK, Li Q, Ferrón HG, Keating JN, Wang JQ, Donoghue PCJ*, Zhu M.* 2022. Galeaspid anatomy and origin of vertebrate paired appendages. Nature. https://doi.org/10.1038/s41586-022-05136-8

[3] Andreev PS, Sansom IJ, Li Q, Zhao WJ, Wang JH, Wang CC, Peng LJ, Jia LT, Qiao T, Zhu M*. 2022a. The oldest gnathostome teeth. Nature. https://doi.org/10.1038/s41586-022-04897-6

[4] Andreev PS, Sansom IJ, Li Q, Zhao WJ, Wang JH, Wang CC, Peng LJ, Jia LT, Qiao T, Zhu M*. 2022b. Spiny chondrichthyan from the lower Silurian of South China. Nature. https://doi.org/10.1038/s41586-022-05233-8

[5]https://mp.weixin.qq.com/s/MB9wM67THal8LJFc4sXf_g

Planning and production

Source: Shanghai Natural History Museum (ID: snhm01)

Author: Song Ying (Cherry) Shanghai Natural History Museum Exhibition and Education Center

Editor: Zhong Yanping

Proofread by Xu Lailinlin

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