Life on land prior to the origin of land plants: fossil evidence and palaeoenvironmental significance

大约5.4亿年前，海洋中的多细胞生物急剧多样化。所谓的“寒武纪大爆发”见证了海洋中丰富、多样和不同的多细胞动物、真菌和藻类的出现和多样化。然而，令人惊讶的是，虽然海洋充满了复杂的生命，但陆地表面被认为基本上是贫瘠的。直到大约7500万年后的奥陶纪中期，陆地植物的第一个证据才出现。为什么在复杂的多细胞生物（植物）入侵陆地之前有如此广泛的滞后，如果有的话，在奥陶纪中期之前生活在陆地上？该项目的目的是研究新发现的古代大陆沉积物化石，这些化石表明，在陆地植物进化之前，陆地表面曾有一种简单的藻类植物。长期以来，人们一直怀疑，在陆地植物起源之前，大陆上可能存在着一种简单的生物群，这种生物群通常被称为“藻类浮渣”。据推测，这种生物群由简单的光合生物如细菌和藻类组成，也可能是分解者如真菌。这种生物群可能栖息在各种环境中，并利用各种生活方式，包括（i）自由生活在淡水（湖泊，河流等）中; (ii)在周期性淹没地区（如河口泥滩）的藻类垫;（iii）在原始土壤内部和表面的生物土壤结皮;（iv）在岩石内部（作为内生岩）。不幸的是，这主要是猜测。除了极其罕见但诱人的简单化石和微弱的地球化学信号的报道外，几乎没有确凿的证据表明存在这样的陆地生命。新发现的化石材料包括简单的球形囊状结构（sphaeromorphs），管和细丝，以及各种其他有机微体化石。微体化石代表了生活在水生环境中的生物，也代表了生活在周围陆地表面（土壤结壳等）的生物。被冲入水生环境中。它们清楚地表明了早期地球生命的存在。但挑战在于了解它们代表什么类型的生物体？该计划是将它们与被认为代表古代陆地生态系统的生物材料进行比较。今天，除了最寒冷的极地地区、高峰和最干燥的沙漠，陆地植物遍布地球的表面。这些不适宜居住的环境中居住着藻类垫，生物土壤结皮和内石器社区，据信代表了陆地植物起源之前的残余社区。这些群落的生命将被收集，从北美和澳大利亚的炎热沙漠和冰岛的寒冷沙漠。它将被实验性地转化为化石材料，有望在化石记录中存活下来。然后将活的和化石材料进行比较，以确定化石实际上代表什么。对从古陆地沉积物中发现的化石的分析将极大地阐明我们对前陆植物陆生生物的性质的理解。显然，一种“藻类浮渣”在陆地表面居住了很长一段时间。这对了解陆地植物起源之前的环境和全球变化的性质具有巨大的进化和古生态学意义。目前陆地植被约占地球生物量的90%，对环境有重大影响。（通过碳固存、土壤形成、风化等影响气候、大气成分和沉积模式）。经常被忽视的前陆植物陆生生物群到底有多重要？它通过早期陆地生物化学循环对环境产生了多大影响？

Approximately 540 million years ago there was a dramatic diversification of multicellular life in the oceans. The so-called 'Cambrian explosion' witnessed the appearance and diversification in the oceans of abundant, diverse and disparate multicellular animals, fungi and algae. Rather surprisingly, however, whilst the oceans teemed with complex life, the land surface is considered to have been essentially barren. It wasn't until the Mid Ordovician, some 75 million years later, that the first evidence for land plants appears. Why was there such an extensive lag before the invasion of the land by complex multicellular organisms (plants) and what, if anything, lived on the land prior to the Mid Ordovician? The aim of this project is to study newly discovered fossils from ancient continental deposits that suggest that the land surface harboured a simple vegetation of algae before the land plants evolved. It has long been suspected that prior to the origin of land plants the continents may have harboured a simple biota often referred to, rather disparagingly, as an 'algal scum'. It has been postulated that this biota consisted of simple photosynthesizing organism such as bacteria and algae, and possibly also decomposers such as fungi. This biota probably inhabited a variety of environments and exploiting a variety of lifestyles including (i) free living in bodies of freshwater (lakes, rivers etc.); (ii) as algal mats on periodically inundated areas (such as estuary mud flats); (iii) within and on the surface of rudimentary soils as biological soil crusts; (iv) within rocks (as endoliths). Sadly this is mostly speculation. There is very little firm evidence for such terrestrial life, barring extremely rare but tantalizing reports of simple fossils and faint geochemical signals. The newly discovered fossil material consists of simple, spherical, sac-like structures (sphaeromorphs), tubes and filaments, and various other organic microfossils. The microfossils represent organisms that inhabited aquatic environments, but also those inhabiting the surrounding land surface (soil crusts etc.) that were washed into the aquatic environments. They clearly indicate the presence of early terrestrial life. But the challenge is understanding what types of organism they represent? The plan is to compare them with living material believed to represent counterparts of the ancient terrestrial ecosystems. Today land plants inhabit all of the Earth's surface except for the coldest polar regions and high peaks and the driest deserts. These inhospitable environments are inhabited by algal mats, biological soil crusts and endolithic communities believed to represent relict communities from before the origin of the land plants. Life from these communities will be collected, from the hot deserts of North America and Australia and the cold deserts of Iceland. It will be experimentally turned into fossil material that might be expected to survive in the fossil record. Then the living and fossil material will be compared to determine what the fossils actually represent. Analysis of fossils recovered from ancient terrestrial deposits will greatly illuminate our understanding of the nature of pre-land plant terrestrial biotas. Clearly an 'algal scum' inhabited the land surface for a very long time. This has immense evolutionary and palaeoecological significance, in addition to enormous implications for understanding the nature of the environment and global change prior to the origin of land plants The current terrestrial vegetation constitutes some 90% of the living biomass of the planet and has a major impact on the environment (affecting climate, atmospheric composition and pattern of sedimentation through carbon sequestration, soil formation, weathering and so on). Just how significant was the oft neglected pre-land plant terrestrial biota and how much did it influence the environment through early terrestrial-based biogeochemical cycles?