The beginning of the Silurian period, about 443 million years ago, happened when some environmental problem, probably ice ages, killed most of the plants and animals on Earth. The creatures that survived this extinction were the ones that were most able to change quickly to adapt to new conditions, and their ability to change speeded up evolution even more than before. By 428 million years ago, the first tiny branching plant, Cooksonia, developed on land. More and more plants lived on land, slowly evolving to be able to away from the sea coasts to the edges of lakes and streams.
Fossil of the extinct early plant
Cooksonia (about 2 cm high)
About 420 million years ago, the big continent of Europe slowly crashed into another continent, the beginnings of North America, pushing big mountain ranges up towards the sky, which then eroded again into the ocean. The Earth had warmed up again since the Ordovician period and was warmer than usual, so there was more water and less ice.
At the end of the Silurian period, about 416 million years ago, a bunch of smaller environmental problems killed off a lot of plants and animals again and started the Devonian period.
To find out more about geology, check out these books from Amazon.com or from your library: