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

　　在地球形成不到十億年的時間後，生命就起源於早期的海洋了。然而(按照以前通行的觀點)，要再過30億年，第一批動植物才出現在大陸上。或許，正如生命的起源一樣，生命從海洋向陸地的過渡，同樣在進化論上受到質疑。

　　什麼形式的生命能使其生活方式發生如此驚人的變化?對於第一批登陸的生物，傳統的觀點是建立在巨化石基礎上的，這些巨化石是一些基本上完整的動植物的較大標本。與現代種子植物和蕨類植物有種屬相關的微管植物，留下了一批巨化石記錄。因此，人們可以推測，生命登陸的次序反映了現代陸生係統的進化。按照這種觀點，維管植物首先移居到大陸水域的周邊，其次是以這種植物為食的動物，最後是吃這種食草動物的動物。而且，巨化石也表明(提示)，在四億多年前的誌留紀和盆泥紀之間的期間(在誌留紀和盆泥紀之間的期間，即四億多年前)，陸生生命已經出現，種類也急劇增加。

　　但是最近，古生物學家一直對誌留紀-盆泥紀抵製界限以下的積澱物進行更為周密的調查。結果表明，將岩石放在一些盛有酸的容器裏，一些化石就能從這些沉澱物中離析出來。這項技術從古海洋岸邊的沉積物中又發現了新的證據——植物的微小化石以及在顯微鏡才可以看得見的小動物的碎片。多數情況下，標本的直徑不足1/10微米。盡管他們在岩石中埋藏了數億年，許多化石仍遺留有生物體的有機物質。

　　這些新發現的化石不僅揭示了先前不為人質的有機體的存在，也將多細胞生物進居陸地的時間向前推移。我們對早期動植物群體(群落)本質的看法正得以修正，隨之而來的是對第一批陸生生命的新推斷。