|Why does the Earth have grass?
This is an interesting question -- "why"
questions are very difficult to answer. While
there are several levels of "why" a person could
ask, ranging from existential/philosophical to
mechanistic, I think we can probably answer the
"why didn't grass go extinct" question. It seems
that grass evolved about 65 million years ago,
proliferating mostly after the Cenozoic period,
which is when the mass extinction of dinosaurs is
thought to have occurred. Grasses are hard to chew
and digest for many animals, which suggests at
least from an evolutionary standpoint why they
flourished: the many herbivores that were left
over after the mass extinction would have a tough
time eating them.
Grass is a plant (actually, it's a family of
plants). Plants grow where plants grow. Grass,
being an herb (a non-woody plant), tends not to
live where it can get shaded by trees, so you
generally get grasses mainly in areas where the
trees are sparse (there are grasses that do
tolerate shade, but most do better in sun).
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