All life found on Earth today is made up of cells, with the vast majority of organisms still being unicellular; it is generally presumed that the earliest forms of life would have been single-celled Prokaryotes (organisms with cells which lack internal divisions and organelles). Multicellularity has arisen numerous times within both Prokaryotic and Eukaryotic groups, although complex multicellularity, with cells differentiated into specialist forms and organised communication between cells, has only arisen six or seven times, and only in Eukaryotes.
The earliest widely accepted multicellular Eukaryotic fossils, filaments and spherical groups of cells, appear around the Mesoproterozoic-Neoproterozoic boundary, while filamentous Prokaryotes are known from the Archaean. Early multicellular Eukaryotes include Bangiomorpha pubescens, a putative Red Alga from 1050 million-year-old deposits in the Canadian Arctic, Eosolena loculosa, a Eukaryote of uncertain affinities from 1030 million-year-old deposits in Siberia, Arctacellularia tetragonala, another species of uncertain affinities from 1000 million-year-old deposits in the Democratic Republic of Congo, Proterocladus antiquus, a possible Green Alga from 950 million-year-old deposits in North China, Archaeochaeta guncho, another species of uncertain affinities from 950 million-year-old deposits in northwestern Canada, and Ourasphaira giraldae, a possible Fungi from 890 million year old deposits in the Canadian Arctic.
Some putative multicellular Eukaryotes have also been recorded from earlier in the Mesoproterozoic, including Eosolena minuta, from 1500 million-year-old deposits in northern Siberia, or the carbonacious impressions of the Gaoyuzhuang Formation in North China, which can reach tens of centimetres across, or the possible Eukaryotic microfossils from the 1600 million-year-old Tirohan Dolomite of central India. The oldest examples of the coilled microfossil Grypania are currently dated to about 2100 million years before the present (i.e. Late Palaeoproterozoic) although it is debated whether this is a Eukaryote or a Cyanobacterium. Of similar age are the pyritic macrostructures of the Francevillian Biota of Gabon, though there is some debate as to whether there are of biological origins at all.
In 1989, micropalaeontologist Yan Yuzhong published a description of a filamentous Eukaryotic fossil from the 1630 million-year-old Chuanlinggou Formation of North China in the Bulletin of the Tianjin Institute of Geology and Mineral Resources. At this time journals were only available in print, and the Bulletin, which was printed in Chinese, had almost no circulation outside of China. Furthermore, the quality of the images in Yan's paper were rather poor, leading to the publication being largely overlooked.
In a paper published in the journal Science Advances on 24 January 2024, Lanyun Miao and Zongjun Yin of the State Key Laboratory of Palaeobiology and Stratigraphy at the Nanjing Institute of Geology and Palaeontology, Andrew Knoll of the Department of Organismic and Evolutionary Biology at Harvard University, Yuangao Qu of the Institute of Deep-sea Science and Engineering of the Chinese Academy of Sciences, and Maoyan Zhu, again of the State Key Laboratory of Palaeobiology and Stratigraphy at the Nanjing Institute of Geology and Palaeontology, and of the College of Earth and Planetary Sciences of the University of the Chinese Academy of Sciences, re-examine the Chuanlinggou Formation fossils, and discuss the implications of these for the origin of multicellular Eukaryotic life.
Samples of grey shale were collected from the Wengjiazhuang Section of the Chuanlinggou Formation in Kuancheng County of Hebei Province, which has been dated to 1634.8 million years before the present (±6.9 million years) using uranium/lead ratios in zircons. Zircons are minerals formed by the crystallisation of cooling igneous (or in this case, impact) melts. When they form, they often contain trace amounts of uranium, which decays into (amongst other things) lead at a known rate. Since lead will not have been present in the original crystal, it is possible to calculate the age of a zircon crystal from the ratio between these elements. Microfossils were then extracted from these shales by acid maceration.
This technique recovered flattened greyish or pale brown filamentous fossils, which Miao et al. describe as Qingshania magnifica, the name used by Yan Yuzhong for his material. These are not the only filamentous fossils derived from the Chuanlinggou Formation shales, but are significantly larger than other forms, supporting the idea that they are Eukaryotic in origin, while the other forms are Prokaryotic, probably Cyanobacteria. The 278 individual specimens Miao et al. identified ranged from 20 to 194 μm in diameter, with a maximum length of 860 μm. The filaments were straight or curved, and made up of smooth-walled cells, more than 20 of which were present in the longest specimens. These cells are generally cylindrical in shape, with a cell length of 15 to 190 μm. Terminal cells, where preserved, are hemispherical. None of the specimens had any form of external sheath or holdfast.
The specimens show considerable variation, with the largest being ten times as wide as the smallest, individual cells being cylindrical, barrel-shaped, or cup-shaped, and filaments being of even width or tapering towards one end. Despite this variation, Miao et al. treat them all as a single species, suggesting that the variations reflect s different growth or developmental stages within the population.
Some of the filaments have small, round-to-ovoid structures within some of their cells. These structures are faint, but always contained entirely within the cell, making it unlikely that they are separate structures superimposed upon the filaments. Inclusions within cells, from both Proterozoic and Phanerozoic settings, have variously been interpreted as endocysts, collapsed cytoplasm, or organelles. The fossils are interpreted as being compressed cell walls, which makes it likely that structures withing them would be endocysts. A variety of Eukaryotic and Prokaryotic groups produce endospores in response to worsening conditions (such as the end of a growing season), but these tend to have protective envelopes thicker than the outer cell wall, which is not the case with these structures. However, the structures are found only in larger cells, and are only slightly smaller than the smallest cells, which suggests that they may be some form of asexual reproductive spore; similar spores are produced by some extant filamentous Algae, such as Urospora wormskioldii.
Microscale Raman and Fourier transform infrared spectroscopic investigations of the composition of the filaments suggested that the cell walls of Qingshania magnifica were composed largely of aromatic compounds, with a lower proportion of aliphatic compounds, with the aliphatic compounds forming long chains with little branching. This is not sufficient to make any assessment of the taxonomic status of Qingshania magnifica on its own, but is quite distinct from the composition of Cyanobacterial cells found in the same deposits.
The original specimens of Qingshania magnifica described by Yan in 1989 were identified from thin sections of yellowish-green shales, and had a maximum width of about 250 μm and were up to 6000 μm in length. Yan identified these as Green Algae, placing them in the modern family Ulotrichaceae. Miao et al.'s specimens are slightly smaller, but preserve more detail, allowing for a more detailed reconstruction. They interpret Qingshania magnifica as a simple multicellular organism with large cells and a degree of morphological variation, with a life cycle that involved spores produced within cells, which then produced thin filaments, which grew into thicker filaments, which were capable of producing more spores.
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