Life on Earth

　　Life originated in the early seas less than a billion years after the Earth was formed. Yet another three billion years were to pass before the first plants and animals appeared on the continents. Life’s transition from the sea to the land was perhaps as much of an evolutionary challenge as was the genesis of life.

　　What forms of life were able to make such a drastic change in lifestyle The traditional view of the first terrestrial organisms is based on megafossils--relatively large specimens of essentially whole plants and animals. Vascular plants, related to modern seed plants and ferns, left the first comprehensive megafossil record. Because of this, it has been commonly assumed that the sequence of terrestrialization reflected the evolution of modern terrestrial ecosystems. In this view, primitive vascular plats first colonized the margins of continental waters, followed by animals that fed on the plants, and lastly by animals that preyed on the plant-eaters. Moreover, the megafossils suggest that terrestrial life appeared and diversified explosively near the boundary between the Silurian and the Devonian periods, a little more than 400 million years ago.

　　Recently, however, paleontologists have been taking a closer look at the sediments below this Silurian-Devonian geological boundary. It turns out that some fossils can be extracted from these sediments by putting the rocks in an acid bath. The technique has uncovered new evidence from sediments that were deposited near the shores of the ancient oceans-plant microfossils and microscopic pieces of small animals. In many instances the specimens are less than one-tenth of a millimeter in diameter. Although they were entombed in the rocks for hundreds of millions of years, many of the fossils consist of the organic remains of the organism.

　　These newly discovered fossils have not only revealed the existence of previously unknown organisms, but have also pushed back these dates for the invasion of land by multicellular organisms. Our views about the nature of the early plant and animal communities are now being revised. And with those revisions come new speculations about the first terrestrial life-forms.

　　Key

　　在地球形成不到十亿年的时间后，生命就起源于早期的海洋了。然而(按照以前通行的观点)，要再过30亿年，第一批动植物才出现在大陆上。或许，正如生命的起源一样，生命从海洋向陆地的过渡，同样在进化论上受到质疑。

　　什么形式的生命能使其生活方式发生如此惊人的变化?对于第一批登陆的生物，传统的观点是建立在巨化石基础上的，这些巨化石是一些基本上完整的动植物的较大标本。与现代种子植物和蕨类植物有种属相关的微管植物，留下了一批巨化石记录。因此，人们可以推测，生命登陆的次序反映了现代陆生系统的进化。按照这种观点，维管植物首先移居到大陆水域的周边，其次是以这种植物为食的动物，最后是吃这种食草动物的动物。而且，巨化石也表明(提示)，在四亿多年前的志留纪和盆泥纪之间的期间(在志留纪和盆泥纪之间的期间，即四亿多年前)，陆生生命已经出现，种类也急剧增加。

　　但是最近，古生物学家一直对志留纪-盆泥纪抵制界限以下的积淀物进行更为周密的调查。结果表明，将岩石放在一些盛有酸的容器里，一些化石就能从这些沉淀物中离析出来。这项技术从古海洋岸边的沉积物中又发现了新的证据——植物的微小化石以及在显微镜才可以看得见的小动物的碎片。多数情况下，标本的直径不足1/10微米。尽管他们在岩石中埋藏了数亿年，许多化石仍遗留有生物体的有机物质。

　　这些新发现的化石不仅揭示了先前不为人质的有机体的存在，也将多细胞生物进居陆地的时间向前推移。我们对早期动植物群体(群落)本质的看法正得以修正，随之而来的是对第一批陆生生命的新推断。