古老的化石换新颜—谈化石工艺品

化石记载了地球历史上不同时期不同生物的生活和演化。因此,化石是古生物工作者的主要研究对象。通过研究化石,可以确定生物群的面貌,研究生物进化的不同环节,为生物进化论提供证据;可以确定化石产出层位的地质时代,进行不同区域的地层对比,为寻找矿藏服务。化石代表着当时生活在地球表面的生物体,是生物圈的主要成员。同时化石又埋藏在不同时期的沉积物中,是岩石圈的组成分子。可见,化石是地球历史上古环境的当然“见证人”。化石本身就可以成矿,石油和煤都是不折不扣的化石。当今,以化石为原料雕刻的化石工艺品又以新的姿态进入人们的生活之中。     

古生物学领域的新辟园地──分子古生物研究

分子古生物研究是现代分子生物学、有机地球化学和古生物学等领域相交叉、结合而形成的年轻学科方向,是古生物学领域的一个新兴生长点。与传统古生物学相比,分子古生物研究更依赖于现代化的实验、分析技术,它使古生物研究从宏观深入微观。分子古生物研究应当与传统的形态解剖研究相结合,两者的关系是相互补充的。现提出狭义分子化石和广义分子化石的概念,并指出它们与有机地球化学研究中的生理标志化合物等概念的异同。当今分子     

一株岩石放线菌对碳酸钙形貌的影响北大核心CSCD

【目的】微生物诱导成矿是近年来地质微生物学领域研究中的热点之一。采用一株分离自喀斯特地区的岩生放线菌DHS C013^T菌株,研究该菌株及其代谢产物在由NaHCO3、Ca（NO3）2.4H2O组成的成矿体系中对CaCO3生成及形貌的影响。【方法】将在葡萄糖麦芽酵母提取物（MGYP）培养液中培养的放线菌上清液、菌丝球、发酵液以及无菌的MGYP培养液和超纯水分别加入成矿体系,SEM观察不同处理成矿体系中的底部沉淀物。【结果】在超纯水成矿体系中只形成标准菱面体方解石,而在添加放线菌及其代谢产物甚至含有机质的培养液则可形成形态各异的CaCO3晶体,如球形、哑铃以及表面具有鳞片状的柱形。这些特殊形态的CaCO3晶体的形成,可能是在放线菌的菌丝球和菌丝片段以及胞外分泌物上成核和逐渐生长的结果。【结论】放线菌菌丝体及代谢产物对调控和影响CaCO3的晶体形貌有重要作用。研究结果对进一步认识放线菌及其代谢产物诱导生物成矿提供了新的证据。     

分子系统学在微体古生物及相关领域中的应用

核酸（ＤＮＡ和ＲＮＡ）和蛋白等生物大分子（尤其是它们的序列资料）可作为重要的生物性状用于系统分类和演化等主题的研究。相对于形态学性状而言,分子性状不仅是前者的补充,而且具有许多前者无法比拟的优点;比如ＤＮＡ作为遗传信息的直接载体能较准确地反映生物类群之间的系统发生关系、信息量巨大、易于定量化和进行计算机分析等等。分子古生物研究包含两个方面：一、发掘化石生物分子,以提供历史生物界演化过程中的直接遗传学证据以及检验分子演化速率等方面的独特数据;二、利用现代分子生物学数据,探讨化石生物界的系统发生问题。上述两个研究方向均已成为当今演化生物学领域的热点。有孔虫等具有重要化石记录的微体生物的分子系统学研究已经开始。随着现生的和化石的生物分子资料的逐渐积累,预期在不久的将来,分子资料将成为微体古生物研究中不可缺少的重要数据之一。     

来自分子化石的信息

来自分子化石的信息王大锐大千世界,五彩缤纷;化石世界也是多姿多彩,大到恐龙、古象,小到单细胞生物有孔虫、放射虫和。这些都是人类已经认识了上百年的化石种类了。近年来,科学家们又提出了一种叫“分子化石（ｍｏｌｅｃｕｌａｒｆｏｓｓｉｌｓ）的概念。分子化石,...     

化石与宝石

化石作为地壳中石化的生物遗体和遗迹,在自然科学的研究中有着十分重要的价值和意义,对于了解地球发展演化历史,以及研究古地理和古气候及成矿条件有着十分重要的作用。然而某些颜色艳丽、纹饰清晰、形体完整的化石品种同时还有很高的美学价值与经济价值,构成宝石界中...     

热泉微生物化石的识别研究及其科学意义

现代海底热液喷口以及许多陆地热泉周围生活着密集的生物群落。热液生态系统的初级生产者嗜热细菌和古细菌(Archaea),其初级能量来源是由地球深部上升喷出流体提供的化学能。围绕现代热泉微生物及其与地史时期热泉微生物化石的对比研究表明,它们具有相似结构特征。研究微生物成矿机制和微生物化石化作用,以及沉积物中由生物化学作用产生的生物标志,不仅有助于探讨海底热液活动的规律性和成矿机制,也可以为鉴别古老岩石和地外矿物中生命现象提供更多更详细的鉴定标志,对于理解生命起源和地外生命都有重要的理论意义。     

贵州凯里生物群中的宏观藻类化石丘尔藻(Chuaria)及其意义

丘尔藻Chuaria Walcott,1899是宏观藻类中研究最早、延续时限最长的一类宏观藻类化石,它是新元古代赵家山生物群、淮南生物群及西陵峡动物群的代表分子之一.寒武纪第3世凯里生物群中Chuaria化石的产出,不仅是其最年青的代表,也是凯里生物群中宏观藻类组合中较为重要的分子之一.凯里生物群中Chuaria化石呈...     

恐龙化石中蛋白质的研究

恐龙化石中蛋白质的研究孙家明译由分子生物学家和古生物学家组成的研究组已经查明一种保存在恐龙骨骼中的蛋白质，打开了利用古代分子帮助区分恐龙和其他脊椎动物的可能性。长期以来，科学家一直认为在数百万年前的材料中找到的蛋白质不可靠，因为这样的有机分子通常很快...     

多足动物亚门主要类群的谱系发生与谱系年代分析——基于形态、化石和分子的综合数据

多足动物可能是最早入侵陆地环境的无脊椎动物之一。随着泛甲壳动物大类的确立,多足动物亚门的谱系发生问题已成为节肢动物研究中的焦点。基于表型性状的系统学研究中,多足动物亚门在节肢动物门中的系统分类和单系性等问题一直存在争议;化石记录的稀少又使多足动物的起源及演化历史变得迷雾重重。近年来分子系统学的进展为深入探讨多足动物谱系发生问题开辟了新的途径;分子定年的应用为探讨多足动物起源、登陆等早期演化问题提供了新的手段。谱系年代学综合了化石记录和分子定年两方面的优势,为更加精确地讨论多足动物起源、内部类群分歧时间及其地质背景奠定了基础。谱系年代分析显示,多足动物类起源于寒武纪晚期或更早;多足动物内部类群的分歧不晚于奥陶纪;而化石证据显示,多足动物的登陆事件也可能发生在这一时期,多足动物内部类群的分歧事件(及食性分化)可能与登陆过程有关。精确的多足动物亚门谱系发生关系以及谱系年代学细节还有待于进一步综合系统学、多基因和多重化石参照点的合理分析加以深入和完善,进而为早期陆地复杂生态系统的建立提供新的证据。     

Blackened bioclasts and bituminous impregnations in the Koněprusy Limestone (Lower Devonian), the Barrandian area, Czech Republic: implications for basin analysis

Carbonate reef talus facies of the Koněprusy Limestone (Pragian, Lower Devonian, Barrandian) locally exhibit widespread impregnation by organic matter resulting in a partial to complete blackening of the limestones. Two contrasting types of impregnation are recognized: blackening of individual carbonate fossils and bioclastic layers within the limestone originated very early during diagenesis. The blackening is due to finely dispersed organic matter and possibly some iron sulphides and clay minerals that selectively adhered to the outer layers of corals, bryozoans, and crinoid fragments, leaving other fossils unaltered. These darkened fossils are similar to black pebbles—i.e., reworked, dark to black limestone clasts and bioclasts that are known to occur exclusively in shallow-water zones of both ancient and modern carbonates. The alteration of fossil fragments may have taken place in very shallow-water environments, possibly those of saline and reducing back-reef lagoons or supratidal-intertidal zones, with organic matter being derived from decayed algae and microbes, or early vascular terrestrial plant material. Following the coloration, the blackened fossils were removed from their original position by waves or storms and transported into relatively deeper-water reef slope settings to form graded, “salt-and-pepper”-colored bioclastic beds. The presence of blackened fossils in the carbonate succession may point to episodic emergence and indicates a vanished vegetated siliciclastic hinterland that may once have existed to the west or south from the present-day erosive edge of the Barrandian Devonian strata. Subvertical veins cutting the Koněprusy Limestone and filled with black solid bitumen and blackened calcite resulted from a subsequent but substantially later diagenetic event, which is a testament of aqueous and petroleum fluid migration through the succession during deeper burial. Microthermometric characteristics of the aqueous inclusions embedded in vein calcite indicate that the veins were precipitated by brines of low to moderate salinity (0.5–9.5 wt% NaCl equiv.) with temperatures in the range of 87–116°C. The bitumen in the veins is epi-impsonite (R_r?=?0.70–1.90%), which is interpreted as degraded petroleum residuum that experienced thermal alteration at around 120°C. The AFT modeling combined with fluid inclusion microthermometry and wider geological considerations indicate that the veins originated during the Variscan orogeny, most probably upon deep burial of the Lower Paleozoic strata in Carboniferous time.     

Analysis of hopanes and steranes in single oil-bearing fluid inclusions using time-of-flight secondary ion mass spectrometry (ToF-SIMS)

Steranes and hopanes are organic biomarkers used as indicators for the first appearance of eukaryotes and cyanobacteria on Earth. Oil-bearing fluid inclusions may provide a contamination-free source of Precambrian biomarkers, as the oil has been secluded from the environment since the formation of the inclusion. However, analysis of biomarkers in single oil-bearing fluid inclusions, which is often necessary due to the presence of different generations of inclusions, has not been possible due to the small size of most inclusions. Here, we have used time-of-flight secondary ion mass spectrometry (ToF-SIMS) to monitor in real time the opening of individual inclusions trapped in hydrothermal veins of fluorite and calcite and containing oil from Ordovician source rocks. Opening of the inclusions was performed by using a focused C₆₀⁺ ion beam and the in situ content was precisely analysed for C₂₇–C₂₉ steranes and C₂₉–C₃₂ hopanes using Bi₃⁺ as primary ions. The capacity to unambiguously detect these biomarkers in the picoliter amount of crude oil from a single, normal-sized (15–30 μm in diameter) inclusion makes the approach promising in the search of organic biomarkers for life's early evolution on Earth.     

北衙金矿矿石内部可培养细菌多样性初步研究

目的对北衙金矿矿石样品内部的可培养细菌进行分离并对其多样性进行研究。方法采集云南北衙金矿矿石样品,采用固体肉汤培养基、卵黄培养基及厌氧琼脂培养基分离矿石内部的可培养细菌,并利用16SrRNA基因序列构建系统发育树,初步评估细菌多样性。结果北衙金矿矿石内部细菌的主要种群包括厚壁菌门和放线菌门的不同菌属,包括芽孢杆菌属、类芽孢杆菌属、考克菌属及节杆菌属的菌株,其中抗逆性较强的优势菌群为放线菌门的细菌。结论本研究证实北衙金矿矿石内部的确存在大量可培羔±田萧．并且右种群名样件．     

Molecular fossils probe life's origins

Molecular fossils allow evolutionary biologists to look deep into the history of life on the Earth, far beyond the fossil record and possibly to the first living organisms.The fossil record—surviving mineral components or imprints of multicellular life—has provided valuable insights into how animals and plants evolved over millennia, but offers limited scope for discerning the origins of life itself or the separation of organisms into the three domains of eukaryotes, prokaryotes and archaea. The development of new technologies, however, is enabling scientists to analyse molecular fossils, such as the remnants of ancient nucleic acids, sugars, proteins, carbohydrates and lipids, to study the evolution of key metabolic pathways. This knowledge should enable researchers to peer back through time as far as the great oxidation event (GOE) that enabled the emergence of eukaryotic life. In addition to the analysis of the organic molecules themselves, the study of modern genomes to look for ancestral clues might yield knowledge about the protein structures present in the earliest forms of life.The GOE is thought to have occurred when cyanobacteria released free oxygen into the atmosphere through oxidative photosynthesis. While there is reasonable consensus that this occurred around 2.4 billion years ago, there is still uncertainty over how long it took until sufficient free oxygen accumulated to enable oxidative metabolism and, eventually, the emergence of new life forms. Initially, minerals, including iron, which would have been present in metallic form and plentiful, are thought to have taken up the oxygen produced by cyanobacteria. Atmospheric oxidation would not have started until the Earth''s surface minerals had become saturated, but the estimated time to that point ranges from 100 million years to 1 billion years. Because cyanobacteria are widely believed to be one of the first lifeforms because of their ability to thrive in anoxic conditions, resolution of this question could move scientists closer to establishing the origins of life.…molecular fossils provide information about the organisms they are derived from and the biosynthetic pathways in operation at the time of their formationUnlike physical fossils, molecular fossils do not contain material derived directly from the original organism itself, but rather are biomarkers that represent some of its specific chemical composition and provide a ‘signature''. Molecular fossils are embedded in rock or sediment and are altered over time by chemical and physical processes. As such, they can only be dated indirectly by analysis of the surrounding rock or sediment. Although direct dating methods are now considered fairly reliable, indirect methods are controversial because they rely on various assumptions, notably that the sample has not been contaminated and has remained fixed relative to its surroundings. “It is assumed the molecules of the microbes present in these rocks are of similar age,” said Stefan Schouten, an organic geochemist at the Royal Netherlands Institute for Sea Research, Texel, Netherlands. “Generally this assumption is correct, though in recent sediments offsets of up to 5,000 years have been noted.”The molecules are commonly separated from one another by using gas or high-pressure liquid chromatography and identified by mass spectrometry. The surrounding material is dated, usually by using well-established radiometric methods, often combined with stratigraphy: the analysis of rock or sediment formation through the accumulation of successive layers, which assumes that a lower layer must be older than the one above it.Despite the challenges, molecular fossils provide information about the organisms they are derived from and the biosynthetic pathways in operation at the time of their formation. Some of the key biomarkers in old deposits include sesquiterpenes, which indicate that a fossil came from a plant or insect; biphytanes, which point to archaea; hopanes, which suggest bacteria; 2-methylhopanes, which are specifically associated with cyanobacteria; and steranes, which point to eukaryotes. Hopanes, for instance, are derived from hopanoids, which give strength and rigidity to the plasma membranes of bacteria. Sterols fulfil a similar role in eukaryotes and form steranes under the action of sedimentary processes.The most extensive use of molecular fossils to date has been to search for biomarkers […] of the [great oxidation event] and the associated emergence of eukaryotic lifeThe most extensive use of molecular fossils to date has been to search for biomarkers that provide evidence of the GOE and the associated emergence of eukaryotic life. Notable advances have been made, but have raised major controversy over the duration of the GOE. In 1999, Jochen Brocks and colleagues at the University of Sydney, Australia, reported evidence that eukaryotes were present up to 2.7 billion years ago, which is 1 billion years earlier than had previously been believed [1]. In a paper published in Science, the researchers argued that the presence of abundant 2α-methylhopanes, which are characteristic of cyanobacteria, indicated that oxygenic photosynthesis evolved well before the atmosphere became oxidizing. They also wrote that, “the presence of steranes, particularly cholestane and its 28- to 30-carbon analogues, provides persuasive evidence for the existence of eukaryotes 500 million to 1 billion years before the extant fossil record indicates that the lineage arose.”The paper was heralded as a breakthrough and highly cited during the following decade. Brocks, however, discovered that some of the sediment samples that his team had used had been contaminated. In 2008, he coauthored a paper with different colleagues that essentially overturned the findings of the 1999 paper [2]. “The most important point is whether these biomarkers in 2.7 [billion year] old rocks are indeed that old,” Brocks said. “After many years of scientific dispute about them, others embraced the earlier findings and published follow up papers apparently vindicating the original 1999 results, but the community has come to the consensus that these hydrocarbons have entered the Archaean rocks at a later point in time” [3].Simon George, leader of the organic geochemistry group in the Department of Earth and Planetary Sciences at Macquarie University in Sydney, Australia, argues that although the discovery of contamination was a setback for Brocks and others, it does not disprove the validity of all other findings based on samples of an apparently similar age. “Jochen Brocks''s […] inference is that everybody''s work is based on contamination. He''s certainly proven that some of the samples he worked on were affected by contamination, but it''s a bit of a leap to say everyone else''s is,” George explained. He argued that findings of steranes in ancient samples have been repeated in different geographical locations and by a variety of people at several leading institutions. “I''d be surprised if everyone was seeing contamination,” he said.Gordon Love, an organic geochemist at the University of California Riverside, CA, USA, is more cautious. He commented that findings based on archaean rocks are often unreliable because the levels of biomarkers are very low, which makes it harder to sift out contaminants. “The pursuit of Archean lipid biomarkers has always been viewed as a very extreme application of molecular organic geochemistry requiring the most sophisticated and sensitive instrumentation to detect any signals at all,” he said. “We are talking about trace quantities of biomarkers that wouldn''t normally adversely affect ancient biomarker studies or even show up in routine analyses since the absolute yields of these compounds are so low, but which become significant when dealing with highly overmature Archean organic matter [original matter that has been transformed by thermal and chemical processes into oil and gas].” Nevertheless, Love noted that the conclusion that eukaryotes evolved over 2.5 billion years ago might still be correct. “We cannot say that the absence of steranes shows that eukaryotes had not evolved. The most appropriate conclusion, in my strong opinion, is that that organic matter found in Archean rocks has been so thermally transformed that we have no way of knowing whether eukaryotic biomarkers were ever present as original lipid constituents.”Answers might ultimately come from another promising line of research into archaean evolution that relies on the analysis of molecular fossils obtained from ‘fluid inclusions'' within sediment rocks. These are small microscopic bubbles of liquid and gas—typically 0.1–1.0 mm in diameter—trapped within crystals. Because they have been trapped since their formation, they are almost guaranteed to be free from contamination. The problem so far is that they have had to be analysed in bulk to provide enough fluid for separation and mass spectrometry. This requirement makes the work less reliable the further back you go because there is not enough fluid in single samples to date accurately anything that is older than 2.4 billion years.George and colleagues are now applying the well-known technique of ‘time-of-flight'' mass spectrometry to analyse individual fluid inclusions without having to extract them. This technique was developed more than 60 years ago and has long been used in inorganic chemistry. It works by directing ion beams at a sample to identify molecules by their mass-to-charge ratio. “You use beams of ions to drill down into rock and then when you get to the required depth you put the analysis beam on,” George explained. The principle is that the velocity of an ion depends on its mass-to-charge ratio, which can be calculated by measuring the time that it takes for the particle to reach a detector, thus identifying the particle. “It''s a way of assessing very small samples,” George said; adding that a lot more work on instrument development and the interpretation of results will be needed before we can say which biomarkers are present in fluid inclusion zones. “We think there are steranes in there, but it is hard for us to prove it,” George said. “We can definitely see hydrocarbons in there and be sure it is really old material, but because we can''t do gas chromatography separation on these we can''t be sure […] If we are able to prove fluid inclusions are holding this larger chemical record of life, it becomes a very important tool for understanding life''s origins, because we can go back as far as 3.2 billion years.”Love said that analysis of oil-bearing fluid inclusions, as practised by Simon George''s group, is a promising approach because the effects of extreme thermal maturity on organic molecules are often not as acute for migrated petroleum fluids trapped in sandstones under high pressure as they are for rock bitumens found in the parent rocks. But he cautioned that the migration of these fluid inclusions away from the parent rock where they originated introduces some uncertainty over dating. “There will always be some degree of ambiguity concerning the age and stratigraphic position of the parent source rock that actually generated the oil trapped in the inclusion,” Love explained. “At the same time, I look forward to seeing what they will generate from new, cleanly drilled Archean cores using the fluid inclusion approach.”Schouten likewise anticipates the results of this research and suggested that such work could finally settle the question of whether eukaryotes diverged from archaea and prokaryotes during the archaen period. “It is the work of Simon George on fluid inclusions which makes me think that we cannot fully dismiss that possibility yet,” he said.Modern genomes and proteins are also fossils in their own way, littered with evidence of genes that were useful to ancestral organisms but that have been abandoned or adapted in today''s species. The comparative analysis of genomes and surface proteins, for example, could help scientists understand how viruses evolved before the three domains of life split. Viruses are believed to have been around at the time, having coevolved with early life, but their presence is impossible to establish directly because they do not even leave molecular fossils. As such, viral evolution can only be studied through observation of their comparative structures and genetic sequences across life''s three domains. Only recently have virologists been able to look at viral origins and correlate them with the emergence of prokaryotes, eukaryotes and archaea.Sarah Butcher, from the Institute of Biotechnology at the University of Helsinki, Finland, led an international team that made a significant breakthrough in March 2013, based on an archaeal virus found in a salt pan [4]. “The major insight in the paper was to show that the molecular architecture of an archaeal virus is conserved both with the most common bacterial viruses and a wide range of eukaryotic viruses in the herpes family,” Butcher said. The study combined genomic analysis with electron microscopy and computerized image reconstruction to determine that the major coat protein of the isolated archaeal Haloarcula sinaiiensis tailed virus 1 (HSTV-1) has an almost identical structure to that of the bacterial virus Hong Kong 97 (HK97), which is one of the so called head-tailed dsDNA bacteriophages. This similarity had been predicted, but the study provided the first physical proof, backed up by equally compelling genomic evidence, according to Butcher. The analysis revealed that hallmark proteins found in dsDNA bacteriophages, such as terminases and portals, are present in the HSTV-1 genome. Furthermore, the genomes themselves had common structural features.Modern genomes and proteins are also fossils in their own way, littered with evidence of genes that were useful to ancestral organisms…Earlier work had already identified the same fold in herpes viruses, which infect a wide variety of animals, including humans [5]. As Butcher pointed out, the same lineage identified first in the HK97 bacteriophage has now been found in viruses from all three domains of life. Viruses have evolved unique mechanisms for infecting cells dependent on their hosts, but the capsid proteins seem to share common structural features. “The basic argument is that capsid proteins with the same fold share common ancestry, even when, as is often the case, they no longer share any detectable sequence similarity,” commented Roger Hendrix, a viral evolution specialist at the University of Pittsburgh, PA, USA. He explained that the idea is that the adaptation to different host environments led to fundamental sequence changes in ancestral viruses as new functions were acquired, but that there was no corresponding selective pressure to alter the fold of the capsid. “The simple interpretation of this is that there were viruses with some resemblance to modern viruses before cells started to divide into the three current domains and some of the then-existing viruses stuck with and coevolved with each of the three emerging cellular domains,” he said.He cautioned, however, that another explanation for the common fold could be that at some stage in evolution, after the three cellular domains of life split, a virus jumped across the domains, spreading the common fold. This is not likely to have occurred recently because the cells of the different domains have diverged so far that such a viral jump would be difficult, though it could have occurred only shortly after life split into three domains and cannot be ruled out.A third possible explanation for the common fold would be that it coevolved in parallel in each of the three domains, but this is less likely to have happened, according to Hendrix. “Co-evolution is a formal possibility but […] unlikely since I think it would imply that some protein folds are ‘ideal'' and selected in certain viruses of all three domains but not others,” he said. “There are too many successful ways to make a viral capsid for this to make sense, at least to me.”At any rate, the evidence does indicate that diverse viruses were around at the time of the last common ancestor of the three domains of life and contributed to their evolution through their ability to mutate quickly and donate genes to host genomes. As such, the work being done to untangle viral evolution by studying modern viruses is very much related to the advances being made with molecular fossils to probe our way back towards the origins of life.     

世界各地琥珀中的脊椎动物包裹体

琥珀是一种经过石化作用形成的天然树脂化石,亦是一种有机宝石。琥珀在全球范围分布较广,尤其在欧洲的波罗的海、中美洲的多米尼加-墨西哥、亚洲的缅甸等区域有着大量的发现。全球各地琥珀中,昆虫包裹体最为常见,脊椎动物包裹体数量较少。但相对保存于沉积岩中的传统脊椎动物化石来说,保存于琥珀中的脊椎动物包裹体可以额外提供生物体的软组织、原始死亡状态、生存环境等信息,且可以保存比传统骨骼化石更直观、立体、精细的生物形态学信息,为研究生物演化、恢复古环境、古生态和古行为学等方面提供了重要的依据。文中主要总结了来自全球各琥珀产区迄今为止发现的各种脊椎动物包裹体,包括了非鸟恐龙、鸟类、哺乳类、爬行类、两栖类等,以及它们背后隐藏的演化信息,并对未来的研究方向与趋势做了初步的展望。     

Life on Mars: chemical arguments and clues from Martian meteorites

Primitive terrestrial life – defined as a chemical system able to transfer its molecular information via self-replication and to evolve – probably originated from the evolution of reduced organic molecules in liquid water. Several sources have been proposed for the prebiotic organic molecules: terrestrial primitive atmosphere (methane or carbon dioxide), deep-sea hydrothermal systems, and extraterrestrial meteoritic and cometary dust grains. The study of carbonaceous chondrites, which contain up to 5% by weight of organic matter, has allowed close examination of the delivery of extraterrestrial organic material. Eight proteinaceous amino acids have been identified in the Murchison meteorite among more than 70 amino acids. Engel reported that l-alanine was surprisingly more abundant than d-alanine in the Murchison meteorite. Cronin also found excesses of l-enantiomers for nonprotein amino acids. A large collection of micrometeorites has been recently extracted from Antarctic old blue ice. In the 50- to 100-μm size range, carbonaceous micrometeorites represent 80% of the samples and contain 2% of carbon, on average. They might have brought more carbon than that involved in the present surficial biomass. The early histories of Mars and Earth clearly show similarities. Liquid water was once stable on the surface of Mars, attesting the presence of an atmosphere capable of deccelerating C-rich micrometeorites. Therefore, primitive life may have developed on Mars as well and fossilized microorganisms may still be present in the near subsurface. The Viking missions to Mars in 1976 did not find evidence of either contemporary or past life, but the mass spectrometer on the lander aeroshell determined the atmospheric composition, which has allowed a family of meteorites to be identified as Martian. Although these samples are essentially volcanic in origin, it has been recognized that some of them contain carbonate inclusions and even veins that have a carbon isotopic composition indicative of an origin from Martian atmospheric carbon dioxide. The oxygen isotopic composition of these carbonate deposits allows calculation of the temperature regime existing during formation from a fluid that dissolved the carbon dioxide. As the composition of the fluid is unknown, only a temperature range can be estimated, but this falls between 0° and 90°C, which would seem entirely appropriate for life processes. It was such carbonate veins that were found to host putative microfossils. Irrespective of the existence of features that could be considered to be fossils, carbonate-rich portions of Martian meteorites tend to have material, at more than 1000 ppm, that combusts at a low temperature; i.e., it is an organic form of carbon. Unfortunately, this organic matter does not have a diagnostic isotopic signature so it cannot be unambiguously said to be indigenous to the samples. However, many circumstantial arguments can be made to the effect that it is cogenetic with the carbonate and hence Martian. If it could be proved that the organic matter was preterrestrial, then the isotopic fractionation between it and the carbon is in the right sense for a biological origin. Received: January 22, 1998 / Accepted: February 16, 1998     

Microbe‐like inclusions in tree resins and implications for the fossil record of protists in amber

During the past two decades, a plethora of fossil micro‐organisms have been described from various Triassic to Miocene ambers. However, in addition to entrapped microbes, ambers commonly contain microscopic inclusions that sometimes resemble amoebae, ciliates, microfungi, and unicellular algae in size and shape, but do not provide further diagnostic features thereof. For a better assessment of the actual fossil record of unicellular eukaryotes in amber, we studied equivalent inclusions in modern resin of the Araucariaceae; this conifer family comprises important amber‐producers in Earth history. Using time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS), we investigated the chemical nature of the inclusion matter and the resin matrix. Whereas the matrix, as expected, showed a more hydrocarbon/aromatic‐dominated composition, the inclusions contain abundant salt ions and polar organics. However, the absence of signals characteristic for cellular biomass, namely distinctive proteinaceous amino acids and lipid moieties, indicates that the inclusions do not contain microbial cellular matter but salts and hydrophilic organic substances that probably derived from the plant itself. Rather than representing protists or their remains, these microbe‐like inclusions, for which we propose the term ‘pseudoinclusions’, consist of compounds that are immiscible with the terpenoid resin matrix and were probably secreted in small amounts together with the actual resin by the plant tissue. Consequently, reports of protists from amber that are only based on the similarity of the overall shape and size to extant taxa, but do not provide relevant features at light‐microscopical and ultrastructural level, cannot be accepted as unambiguous fossil evidence for these particular groups.     

Exploring the impact of fossil constraints on the divergence time estimates of derived liverworts

In this study, we evaluate the impact of fossil assignments and different models of calibration on divergence time estimates carried out as Bayesian analyses. Estimated ages from preceding studies and liverwort inclusions from Baltic amber are used as constraints on a molecular phylogeny of Cephaloziineae (Jungermanniopsida) obtained from sequences of two chloroplast coding regions: rbcL and psbA. In total, the comparison of 12 different analyses demonstrates that an increased reliability of the chronograms is linked to the number of fossils assigned and to the accuracy of their assignments. Inclusion of fossil constraints leads to older ages of most crown groups, but has no influence on lineage through time plots suggesting a nearly constant accumulation of diversity since the origin of Cephaloziineae in the early to Middle Jurassic. Our results provide a note of caution regarding the interpretation of chronograms derived from DNA sequence variation of extant species based on a single calibration point and/or low accuracy of the assignment of fossils to nodes in the phylogeny.     

Abundance of macroalgal organic matter in biofilms: evidence from n-alkane biomarkers

Biofilm development on titanium panels immersed in the surface waters of Dona Paula Bay was investigated using molecular biomarkers such as n-alkanes and other chemical and biological parameters. Biofilm biomass measured as organic carbon (OC), organic nitrogen (ON), chlorophyll a, diatoms and bacterial numbers on the titanium panels generally increased over the period of immersion. Total lipids and n-alkane concentration also showed similar trends. n-alkanes from C(12) to C(30) were detected in the biofilm samples, which showed a bimodal distribution. The first mode consisted of n-alkanes > C(23) with a strong even over odd predominance. In the second mode, the n-alkanes < C(23) were more abundant with odd carbon number maxima at C(15), C(17) and C(19) and a strong odd over even carbon number predominance (Carbon Preference Index > 2). The predominance of these odd-chain n-alkanes strongly indicates that the organic matter derived from macroalgal sources was the major contributor to the biofilm organic matter developed on the titanium panels over the 15 d period of study. The data suggest that molecular characterization is a useful tool in understanding the sources of biofilm organic matter. The observed abundance of macroalgal organic matter during the 15 d period of biofilm development may play an important role in subsequent fouling by micro- and macrofouling organisms.     

TEM evidence for eukaryotic diversity in mid-Proterozoic oceans

Biomarker molecular fossils in 2770 Ma shales suggest that the Eucarya diverged from other principal domains early in Earth history. Nonetheless, at present, the oldest fossils that can be assigned to an extant eukaryotic clade are filamentous red algae preserved in ca. 1200 Ma cherts from Arctic Canada. Between these records lies a rich assortment of potentially protistan microfossils. Combined light microscopy, scanning electron microscopy, and transmission electron microscopy on 1500‐1400 Ma fossils from the Roper Group, Australia, and broadly coeval rocks from China show that these intermediate assemblages do indeed include a moderate diversity of eukaryotic remains. In particular, preserved cell wall ultrastructure, observed using transmission electron microscopy (TEM), can help to bridge the current stratigraphic gap between the unambiguous eukaryotic morphologies of later Proterozoic assemblages and molecular biomarkers in much older rocks.