THE LAND INVASION
The colonization of the continents is one of the most important steps in evo-
lutionary history. Strangely, after having been in existence for more than
three-quarters of Earth history, life finally conquered the land some 450 mil-
lion years ago. Part of the reason for such a delay might be the lack of oxygen
during the previous era needed to form an effective ozone screen.The ozone
layer filters out harmful ultraviolet rays from the Sun. The strong ultraviolet
radiation probably kept life in the protective waters of the oceans until the
concentration of ozone was sufficient to make conditions safe to venture out
onto dry land.Figure 84Specimen of a
microcrinoid from the lower
member of the Lawson
Limestone in the Florida
Peninsula, Lafayette
County, Florida.
(Photo by P. L. Applin,
courtesy USGS)
Historical Geology