Life originated in the early seas less than a billion years after Earth was formed. Yet another three billion years were to pass before the first plants and animals appeared on the continents

Life originated in the early seas less than a billion years after Earth was formed.Yet another three billion years were to pass before the first plants and animals appearedon 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? Thetraditional view of the first terrestrial organisms is based on megafossils ― relativelylarge specimens of essentially whole plants and animals. Vascular plants, related to modern seed plants and ferns, left the first comprehensive megafossil record. Becauseof this, it has been commonly assumed that the sequence of terrestrialization reflected the evolution of modern terrestrial ecosystems. In this view, primitive vascular plantsfirst 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 megafossilssuggest that terrestrial life appeared and diversified explosively near the boundarybetween 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 sedimentsbelow 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 manyinstances the specimens are less than one-tenth of a millimeter in diameter. Althoughthey were entombed in the rocks for hundreds of millions of years, many of the fossilsconsist 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 bymulticellular organisms. Our views about the nature of the early plant and animal communities are now being revised. And with those revisions come new speculation about the first terrestrial life-forms.

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