About 66 million years ago, something terrible happened
to life on our planet. Ecosystems were hit with a double blow as massive volcanic eruptions filled
the atmosphere with carbon dioxide and an asteroid roughly the size
of Manhattan struck the Earth. The dust from the impact reduced or
stopped photosynthesis from many plants, starving herbivores and the carnivores
that preyed on them. Within a short time span, three-quarters of the world's species
disappeared forever, and the giant dinosaurs, flying pterosaurs, shelled squids, and marine reptiles
that had flourished for ages faded into prehistory. It may seem like the dinosaurs
were especially unlucky, but extinctions of various severities have
occurred throughout the Earth's history, and are still happening
all around us today. Environments change, pushing some species
out of their comfort zones while creating new opportunities
for others. Invasive species arrive in new habitats,
outcompeting the natives. And in some cases,
entire species are wiped out as a result of activity
by better adapted organisms. Sometimes, however, massive changes
in the environment occur too quickly
for most living creatures to adapt, causing thousands of species to die off
in a geological instant. We call this a mass extinction event, and although such events may be rare, paleontologists have been able
to identify several of them through dramatic changes
in the fossil record, where lineages that persisted
through several geological layers suddenly disappear. In fact, these mass extinctions are used
to divide the Earth's history into distinct periods. Although the disappearance
of the dinosaurs is the best known mass extinction event, the largest occurred long before
dinosaurs ever existed. 252 million years ago,
between the Permian and Triassic periods, the Earth's land masses gathered together
into the single supercontinent Pangaea. As it coalesced,
its interior was filled with deserts, while the single coastline eliminated
many of the shallow tropical seas where biodiversity thrived. Huge volcanic eruptions
occurred across Siberia, coinciding with very high temperatures, suggesting a massive greenhouse effect. These catastrophes
contributed to the extinction of 95% of species in the ocean, and on land, the strange
reptiles of the Permian gave way to the ancestors of the
far more familiar dinosaurs we know today. But mass extinctions are not just a thing
of the distant past. Over the last few million years, the fluctuation of massive ice sheets
at our planet's poles has caused sea levels to rise and fall, changing weather patterns
and ocean currents along the way. As the ice sheets spread,
retreated, and returned, some animals were either
able to adapt to the changes, or migrate to a more suitable environment. Others, however,
such as giant ground sloths, giant hyenas, and mammoths went extinct. The extinction of these large mammals coincides with changes in the climate
and ecosystem due to the melting ice caps. But there is also
an uncomfortable overlap with the rise of a certain hominid species
originating in Africa 150,000 years ago. In the course of their adaptation
to the new environment, creating new tools and methods
for gathering food and hunting prey, humans may not have single-handedly caused
the extinction of these large animals, as some were able to coexist with us
for thousands of years. But it's clear that today, our tools and methods
have become so effective that humans are no longer reacting
to the environment, but are actively changing it. The extinction of species
is a normal occurrence in the background of ecosystems. But studies suggest
that rates of extinction today for many organisms are hundreds to thousands of times higher
than the normal background. But the same unique ability that makes
humans capable of driving mass extinctions can also enable us to prevent them. By learning about past extinction events, recognizing what is happening today
as environments change, and using this knowledge to lessen
our effect on other species, we can transform humanity's impact
on the world from something as destructive
as a massive asteroid into a collaborative part
of a biologically diverse future.
