natural to Earth or from space were the usual causes of mass
extinctions, with events classed as to swiftness by the two types which
sometimes are encountered and/or debated for many scientific concepts -
the gradualistic and the catastrophic events
or happenings (ref.#pb2). The earthly causes include phenomena like
climate change, abnormal volcanism, among others.
Most early mass extinctions were gradual events with Earthly causes,
while later events generally were catastrophes (ref.#wc2). Earth's
thinner crust and greater internal heat in older ages could have
started many of the causes for early events (ref.#pa3).
A major mass extinction that is pretty much well known now is what
practically wiped out dinosaurs around
65_million_B.C. wherein about 90%
of Earth's life forms became extinct (ref.#pb1). Catastrophic asteroid
impact primarily caused that event, although a period of gradualistic
changes in environment and climate may also have softened the dinosaurs.
The event was not the only disaster nor the worst that ever happened to
well developed life forms tens of millions of years before the time of
The mass kill around 251.4_million_B.C (Permian-Triassic period) nearly
extinguished all of Earth's living, and was tagged as "the mother of
all mass extinctions" (ref.#wc2).
By the time Man walked erect some million years ago or so, animals of
Earth were much cut down in size and were much easier game (a human
Homo erectus boy was seen around 1.6_million_B.C. in Kenya, about 5'6"
tall and weighing approximately 150 pounds - ref.#pc1 & ref.#pd2).
A sixth major mass extinction is now considered in progress that began
around 0.1_million_B.C. when Man started to change the environment
(ref.#wf1). Earth had five(5) other major mass extinctions before Man