Where did this common-sense discovery come from?

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For a long time, the paleontological community and textbooks have generally believed that amber originates from seed plants (gymnosperms or angiosperms). The oldest definite record is set in the late Carboniferous period in the United States about 320 million years ago. However, a scientific research team from the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences discovered the oldest known amber fossil so far in the coal seams of the Hujierst Formation (about 385 million years ago) in the Middle Devonian system of the Junggar Basin in Xinjiang. This result significantly pushes back the first record of amber by 65 million years, confirming that some primitive vascular plants in the Middle Devonian may have had the ability to synthesize complex terpene resins long before the origin and radiation of seed plants.

How was this common-sense discovery unearthed from the inconspicuous black coal? Faced with the interference of ultra-trace samples and hundreds of millions of years of thermal evolution, how does the team crack the identity code? In the Devonian period, when there were no large-scale gnawing by arthropods, why did ancient plants secrete resin? Focusing on these issues, the reporter talked with Wang Bo, a researcher at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, and postdoctoral fellow Luo Cihang, to restore the scientific exploration process that broke the existing cognition.

Microscopic hand-picking of samples to find the oldest amber

In traditional paleontological understanding, amber is often imagined as a large, crystal-clear gem enclosing ancient insects. Looking for amber in Devonian strata without seed plants has always been regarded as an idea that "goes against common sense." When 10 kilograms of pitch-black coal samples were placed on the laboratory table, facing extremely sparsely distributed particles with a diameter of only a few tenths of a millimeter, how did the scientific research team break their thinking and complete this extremely boring microscopic "gold panning"?

Reporter: People think that amber could not have existed in the Devonian period without seed plants. The team's deep coal mining in the Hujierst Formation was a planned and targeted exploration or an "unexpected harvest"?

Wang Bo: This is actually an unplanned attempt. Our initial trip to Xinjiang was mainly to collect fossil leaves and rhizomes of ancient plants. We did not set up a special project to find the oldest amber. However, previous literature has mentioned sporadically that resin-like compounds may exist in ancient strata, but there has been a lack of conclusive fossil verification. With the mentality of giving it a try, we conducted this research. It can be said that the idea of ​​searching for it came by chance, but the subsequent systematic research was the result of completely conscious advancement.

Reporter: The public has the impression that amber is a large gemstone containing insects, but the particle size of the amber discovered this time is generally only 0.1 to 0.5 millimeters. Facing the extremely complex black coal matrix, how did the team avoid damaging the sample and extract it accurately?

Luo Cihang: The distribution of tiny amber in coal seams is extremely uneven. If traditional heavy liquid separation or strong acid and alkali soaking methods are used, the sparsely distributed particles can easily be destroyed or missed as impurities. The key to breaking the deadlock is UV fluorescence technology. We took advantage of the fact that amber emits strong blue fluorescence under 365-nanometer ultraviolet light to quickly scan coal samples and locate abnormal areas with blue fluorescence, greatly narrowing the search scope.

Wang Bo: After locking the area, since the particles are much smaller than rice grains, no automated equipment can do anything. We can only use the "stupidest" method: manually picking each particle with a fine probe and tweezers under a microscope. This process is extremely demanding on patience. Working 10 hours a day can often only pick out a sample smaller than 1 mm. For two full years, the team overcame a deep sense of boredom and finally selected more than 200 tiny amber particles, which laid the sample foundation for subsequent hard-core identification.

The ability to synthesize complex resins predates the origin of seed plants

The microscopic appearance and weak blue fluorescence alone are not enough to confirm the true identity of amber, because the stratigraphic background of the coal measures that has experienced hundreds of millions of years of high-temperature, high-pressure thermal evolution is extremely complex. Faced with an extremely small amount of highly thermally mature fossil samples, the research team not only needed to issue an impeccable "chemical diagnosis" at the molecular level, but also had to face the profound paradox between the chemical characteristics and the known laws of plant evolution - in the Devonian period when there were no coniferous plants, how could the typical chemical composition of conifer resin be measured?

Reporter: Coal-measure strata have experienced hundreds of millions of years of thermal evolution at high temperatures and high pressures, and the organic matter background is extremely complex. Did the academic community initially suspect that these particles were organic pollution from later generations? How does the team build an irrefutable chain of chemical evidence?

Luo Cihang: For ultra-trace fossils that are 385 million years old, microscopic morphology and blue fluorescence alone are indeed not enough to eliminate interference. In order to eliminate geological pollution and complex background noise, we established a closed-loop evidence chain of "microscopic observation-infrared spectroscopy-organic geochemistry".

First, the sample was scanned 256 times using attenuated total reflection Fourier transform infrared spectroscopy. The spectrum clearly shows strong characteristic absorption peaks of resins such as aliphatic carbon-hydrogen bonds and carbonyl groups. Secondly, triple quadrupole gas chromatography-mass spectrometry was used to extract key signals from the background noise with high selectivity. A variety of sesquiterpenes and diterpenes were detected, and their chemical compositions were comparable to coniferous plant resins. At the same time, no related components such as dicadane and triterpenoids were detected, so the source of angiosperms can be excluded. The two analyzes echoed each other and completely confirmed that these particles were genuine amber.

Reporter: Analysis shows that the spectral type of these ambers is closer to gymnosperms (such as Coniferous), but seed plants have not yet evolved in the Middle Devonian. How to explain this "dislocation of chemical characteristics and evolutionary stages"?

Luo Cihang: Similar chemical characteristics do not equate to close genetic relationships. In the Middle Devonian, seed plants, including gymnosperms, did not appear, and they could never have been the "factories" for synthetic resin. Combined with the fossil record of the Hujirst flora, the most reasonable candidate parent species are progymnosperms and dendritic lycopods. This indicates that some early primitive vascular plants may have evolved or acquired biosynthetic pathways capable of synthesizing complex terpenoids. This proves that the biochemical ability of plants to synthesize complex resins is much older than the origin of seed plants.

Revisiting the ecological drivers of early resin production

The conclusive evidence of Hugelst amber not only significantly refreshes the history of plant-based resin synthesis, but also forces the academic community to re-examine the profound ecological driving force of resin evolution. In the Devonian period, when the feeding pressure of arthropods had not yet appeared on a large scale, why did ancient plants secrete resin? How will this "standardized technology package" that combines microscopic sample selection and molecular testing change the future paleontological research paradigm?

Reporter: The traditional theory is that plants secrete resin mainly to resist insect bites. Why does this study subversively propose that Devonian resin was used to "repair wounds" rather than "fight insects"?

Luo Cihang: This stems from the profound "dislocation" of fossil evidence on the timeline. Fossil records show that arthropods' extensive use of roots, leaves, wood, and seeds, as well as clear evidence of carnivorous feeding, became common mainly from the Carboniferous Period. Such evidence in the Devonian Period is extremely rare and is not enough to force plants to evolve complex resin defense systems. On the contrary, land plants increased significantly in the Middle and Late Devonian, forest fires occurred frequently, and parasitic fungi were widespread. Therefore, we speculate that the core driving force in the early stages of resin evolution was the "liquid Band-Aid" used by plants to deal with wildfire burns and physical trauma, as well as a biological barrier to resist fungal invasion.

Reporter: Some scholars are also looking for indirect evidence through molecular fossils in rocks. In contrast, what implications does the direct search for tiny amber entities have for the paleontological research paradigm?

Wang Bo: Molecular fossils in rocks are indirect evidence. Many other plant secretions will produce similar molecules under geological processes, so the convincing evidence is relatively weak. Paleontology is essentially a science based on first-hand discoveries. Only by directly finding fossil entities can we truly break the textbook mindset. This study revolutionizes past "size bias" and demonstrates that ancient coal seams may hold overlooked microscopic records. The standardized technology package we have established will guide scholars around the world in re-scanning ancient strata that have been overlooked in the past.

Reporter: From the oldest amber, 385 million years ago, to the second oldest amber, 320 million years ago, there is a gap of 65 million years in between. With the help of new technologies, what other breakthroughs will be made in paleontological research in the future?

Wang Bo: There must be more undiscovered amber hidden in the older strata of 385 million years ago and in the long faults of 65 million years. In the future, with the in-depth integration of new technologies such as synchrotron radiation, in-situ micro-area analysis, and artificial intelligence spectral big data interpretation, the efficiency and accuracy of interpretation of microscopic fossils will usher in a qualitative leap. But no matter how technology evolves, the curiosity to crack open rocks in the wild, the curiosity to discover things under the microscope, and first-hand physical discoveries will always be the soul that opens new doors for paleontology.

(Our reporter Cui Xingyi and our correspondent Shi Jingyi)