Biology Reference
In-Depth Information
more, each tree cut down has a double negative impact, not only releasing carbon but no longer
assimilating it from the atmosphere. Through photosynthesis, trees create carbohydrates from CO
2
and water, synthesizing the carbon molecules with water and releasing oxygen as a waste product.
In the process, the world's remaining tropical forests sequester 20 percent of global carbon emis-
sions from fossil fuels, a number that decreases with logging and the clearing of land even as man-
made carbon emissions rise steadily.
So dramatically have humans transformed the earth that in the early 1980s the American scient-
ist Eugene F. Stoermer proposed the name
Anthropocene
for our current geologic epoch. Zoologists
Guy Cowlishaw and Robin Dunbar write: “Not since the demise of the dinosaurs 65 million years
ago has this planet witnessed changes to the structure and dynamics of its biological communities as
dramatic as those that have occurred over recent millennia, and especially in the past four hundred
years.” Humans have devastated millions of square miles of habitat, and since 1600, eighty-nine of
the planet's approximately five thousand mammal species have gone extinct, with 169 others crit-
ically endangered. More recently, agricultural and industrial revolutions have reshaped the world,
changing the composition of the soil, water, and air, and the estimated current rate of extinction in
rainforests alone, for all organisms—insects, plants, bacteria, and fungi—is 27,000 a year. Despite
the severity of our impact, the entire 250,000 years of human history hardly compares to the dam-
age we have done in the last fifty years, and given our current rate of expansion, hundreds, if not
thousands, more animal species are expected to die off within the century.
In a way, the asteroid strike that most likely ended the dinosaurs' rule 65.5 million years ago
and our current age are bookends, containing a long, largely continuous span of evolution and di-
versification of life that created humans, bonobos, and the rainforests as we currently know them.
After the asteroid's collision, dust and ash filled the atmosphere, blocking sunlight and disrupting
the food chain by killing off photosynthesizing organisms. When herbivorous dinosaurs could no
longer graze, the carnivores that preyed on them also died, eliminating all top predators. The only
creatures that endured were those that could subsist on insects and worms, which themselves bred
in the carrion and detritus. One of the traits that has made us so destructive to our environment al-
lowed our small, rodentlike ancestors to survive: they could eat just about anything.
After that cataclysm, the earth was a relatively quiet place, but over the next ten million years,
it heated up significantly, and mammals thrived, spreading across the globe, speciating to fill ecolo-
gical niches left vacant by the dinosaurs. That the subsequent transformation of the rainforest likely
shaped modern humans reveals how changes in the environment can shift our path, transforming us
from one kind of creature to another, with radically different behavior.
The planet's hot phase could have had a number of causes, from changing ocean currents to vol-
canic venting that released massive quantities of atmospheric carbon. Trees covered the earth nearly
pole to pole, the Canadian Arctic and Greenland host to lush, closed-canopy forests, to alligators,
tapirs, flying lemurs, hippolike mammals, and giant tortoises. Palm trees grew in Wyoming, where
primates left some of their earliest fossil evidence. Though resembling squirrels in both size and ap-
pearance, they had the nails characteristic of primates rather than claws. With the planet so densely
forested, they easily spread across Europe, Asia, and Africa.
By forty-eight million years ago, plant life had sequestered a great deal of atmospheric carbon in
oil and coal deposits, and the planet cooled as a result of the continents' drift away from the equat-
or. Until then, the earth had been in a warm phase, without significant polar ice, alpine glaciers, or
continental ice sheets for 250 million years. The clustering of landmasses in the single massive con-
