Geoscience Reference
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Age (MYr = million years)
Figure 20.
Estimate of extinction rate for marine life for the last 600 million years.
The extinction rate is the number of families of known marine organisms that became
extinct per unit of time. Spikes represent major extinction events. The dotted line is the
time trend.
Triassic extinction about 250 million years ago wiped out about 90
percent of all species.
The extinction rate of the last 15,000 years has been relatively low
in the chronology for marine life shown in Figure 20. Indeed, many of
the dramatic extinctions in the most recent period were due to human
interventions. For example, more than half of the large mammal spe-
cies of the Americas disappeared in a short period around when humans
fi rst arrived about 13,000 years ago, and they were probably annihilated
by our spear-carrying ancestors. There is evidence that humans have
had a similarly disastrous effect on species on other continents and is-
lands. In earlier periods, preservation of species was of no interest, and
when a species disappeared, as in the case of the dodo bird, that event
was unmourned and sometimes even unremarked.
 
 
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