Geology Reference
In-Depth Information
The recent rise of
Homo sapiens
may be traced back to the rodentlike survivors of that
rogueManhattan-sizeasteroidof65millionyearsago.Withinafewmillionyearsofthedi-
nosaurs'demise,mammalshadradiatedintovacantecologicalniches,tofieldsandjungles,
mountains and deserts, air and oceans. Even so, the last 65 million years have not been
easy. Many of these strange and wonderful new mammals died in other mass extinctions
56, 37, and 34 million years ago, from causes as yet uncertain.
Humans ultimately descended from the survivors of the last of those catastrophes. Mon-
keys, the great apes, and us—we all point to a common primate ancestor about 30 milli-
on years ago. The first hominids, the evolutionary family that includes primates who walk
erect, arose perhaps 8 million years ago in central Africa.
Meanwhile, a resurgence of glaciation that began around twenty million years ago has
increased in intensity and frequency. Perhaps eight separate times in the past three million
years, ice has spread from the poles to cover great swaths of the high latitudes, reaching
as far south as the upper Midwest. Though not as extreme as the earlier snowball Earth
episodes, these repeated ice ages were each accompanied by drastic drops in sea level by
hundreds of feet. Ice bridges linked Asia and North America, allowing migrations of all
manner of mammals, including mammoths, mastodons, and eventually humans to the New
World.
These ice ages may have led to another surprising evolutionary twist. According to one
intriguing theory, cold temperatures favor the survival of infants who stay close to their
mothers for longer periods, as well as infants with bigger heads (the larger the head, the
lower the heat loss). Big heads mean big brains, while more time with mother means more
time to learn. Perhaps it is not a coincidence that the first human,
Homo habilis,
or “man
the toolmaker,” appeared shortly after one of these great glaciations, 2.5 million years ago.
Throughout the intervening millennia, it has been the human lot to endure and adapt to
repeated change. Frigid ice advances followed by unusually warm “interglacial” periods;
droughts followed by floods; great retreats of the seas followed by equally great advances:
such cycles were for the most part mercifully gradual, spanning many generations, and no-
madic humans had plenty of time to move and survive. Such adaptations are only among
the most recent examples of life responding to the changeable Earth.
Indeed, the last half-billion years of Earth history have seen the most astonishing inter-
play between life and rocks—a coevolution that continues with a vengeance in the age of
technological man.Aeonsagorocks,water,andairmadelife.Life,inturn,madetheatmo-
sphere safe to breathe and made the land green and safe to roam. Life turned the rocks into
soils that have, in turn, nurtured more life and become home to an ever-widening array of
flora and fauna.
