Finding the missing link in land vertebrates' emergence from sea

Mar. 24, 2018 00:04:00

图7a 新疆 野外 2011

Chang Meemann (center) collects fish fossils in northwest China's Xinjiang Uygur Autonomous Region in 2011. Photo provided by Chang Meemann 

By Quan Xiaoshu, Qu Ting

After half a century of studying prehistoric fish fossils, Chinese paleontologist Chang Meemann (张弥曼) has a favorite: Youngolepis.

Living about 400 million years ago, Youngolepis was first thought to be very close to a type of rhipidistians, a possible link in the long evolution from fish to tetrapods, or in other words land vertebrates, including humans.

图1a

Life restoration shows Youngolepis (top) swimming in the sea in the Devonian Period. Drawing by courtesy of Brian Choo

But after careful study of its small cranium in the 1980s, Chang concluded that Youngolepis had no internal nostrils, a key adaption to allow land vertebrates to breathe out of water.

The discovery overturned mainstream views at the time and led to a decade-long debate and re-consideration on phylogeny of lobe-finned fishes, whose descendants left the water and conquered the land.

图3a

Life restoration of the sea in the Devonian Period. Drawing provided by Zhu Min

Chang is still lauded for her academic contribution and her courage to challenge the dominant view more than 30 years later. The 82-year-old was named the 2018 L'Oreal-UNESCO For Women in Science Awards laureate "for her pioneering work on fossil records leading to insights on how aquatic vertebrates adapted to live on land".

The awards, jointly founded by the United Nations Educational, Scientific and Cultural Organization (UNESCO) and the L'Oreal Foundation, began 20 years ago and are given each year to five outstanding women scientists for their accomplishments in scientific research and commitment.

"Of course, I am happy. It's a great encouragement. But I don't think I am well qualified," Chang told Xinhua before she went to Paris for Thursday's ceremony.

SET GOALS

Born in Nanjing, east China's Jiangsu Province, in 1936, Chang wanted to become a doctor early on, due to the influence of her father, who taught human physiology at a medical college. "There were many diligent and well-educated doctors at my father's workplace," Chang recalls.

Her father often took her and her little brother to net shrimps, catch worms and observe ants. "We could express opinions freely to our father or even argue with him. He was always amiable."

But at 17, Chang changed her mind and chose to study geology at college.

"Everyone had a zeal to serve the country," she recalls.

About 200 other women students enrolled the same year and most went on remote geological surveys after graduation. Chang was selected to do scientific research and went to Moscow to study paleontology.

图4a 苏联留学

Chang Meemann (1st from left of front row) studied paleontology in Moscow in the 1950s. Photo provided by Chang Meemann

At the suggestion of Wu Hsienwen, a leading Chinese ichthyologist who was then visiting the Soviet Union, she focused on fish fossils. She often wandered along the rivers in Moscow to collect fossils from Holecene sediments and compare them with modern fish.

In 1960, she returned to China and entered the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP). She spent three months each year accompanying geological prospecting teams to collect fossils in the field, a practice she maintained till the age of 80.

Walking 20 kilometers a day was routine, and every member of the team would eat like a wolf. Chang set a personal record of eating 500 grams of rice in a meal.

"Once I set a goal, I never give up," she says.  

RAISE QUESTIONS

How did our primitive aquatic ancestors invade land? The change of body plan from breathing and navigating in water to moving on land is one of the most profound evolutionary changes. It's more than just turning fins into limbs; it's a comprehensive transformation of all body structures, including the respiratory system.

In 1980, Chang visited the Swedish Museum of Natural History in Stockholm and met with Erik Jarvik, famous for his study on rhipidistians, a type of lobe-finned fish. He believed that porolepiforms, a group of rhipidistians, had three pairs of nostrils: two external like modern fish, and one pair internal for breathing on land.

Three pairs of nostrils in the same fish made it impossible for another promising projection that the internal nostrils actually evolved from a pair of external ones.

图5a

Chang Meemann managed to work on the model of Youngolepis. Photo provided by Chang Meemann

图6a

The three-dimensional scaled-up model of Youngolepis restored with the serial-section technique. Photo provided by Chang Meemann

Chang overturned Jarvik's conclusion using the serial-section technique that she learnt from him.

She studied Youngolepis, a fossil fish found in Qujing City, southwest China's Yunnan Province. She first ground off a very thin section from the 2.8-centimeter cranium, drew a picture of the section with the aid of a microscope and repeated the process through the whole fossil. In total, she drew over 540 pictures, from which she made thin wax plates. These sticky wax plates were assembled to form a three-dimensional scaled-up model, with clear internal structures.

Jarvik took 25 years to restore a fossil by this method, but Chang took only two years.

Her hard work was soon repaid. She discovered there were no internal nostrils in Youngolepis, which was thought to be very close to porolepiforms. This led her to recheck porolepiforms, which were then proved to have no internal nostrils either.

图2a

In the late Devonian period about 365 million years ago, Ichthyostega began migrating to land. It is among the first tetrapods, intermediate between lobe-finned fishes and those fully capable of moving on land. Drawing provided by Zhu Min

The discovery made her peers reconsider the innovation of internal nostrils. Her student, Zhu Min, later found evidence from a small lobe-finned fish called Kenichthys that a pair of external nostrils was moving into the mouth.

"She raised a crucial question, so we could advance the building of the evolutionary tree of early vertebrates," says Zhu, the former director of IVPP.

NEW DIRECTION

In the 1990s, she handed over the fruitful study of fish fossils in the Devonian Period to Zhu Min and others and turned her focus to cyprinid fish fossils in the Cenozoic age.

Cyprinid fish fossils are widespread and common, and it's hard to publish high-quality research papers about them. "I did struggle. But how can we make really big discoveries without lonely and dreary basic work?" she says.

图8a

Chang Meemann receives an exclusive interview with China Features in her office on Feb. 27, 2018. Photo by Qu Ting

In recent years, her team has found well-preserved fossils in the Qinghai-Tibet Plateau. Some had extremely fattened bones, a possible result of increasing calcium salt in water.

"If we want to tell a story about the regional aridity trend, what else could be more vivid than these fossils?" she says.

The fish fossils carry the code of climatic and water changes in ancient times, and will help rewrite the history of the Earth, she says. (BEIJING 2018-03-23 21:39:57)

Editor: Miao Hong

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