Geology Reference
In-Depth Information
Living Earth
The Origins of Life
Earth's Age: 500 million to 1 billion years
Infant Earth at age five hundred million years gave little hint as to how very precocious
itwouldsoonbecome.Earthboasteddramaticvolcanism,tobesure,butsodidseveralother
planets and moons in our Solar System. Earth was graced by globe-spanning oceans, but so
was Mars in those early days, while Jupiter's giant moons Europa and Callisto were envel-
oped by ice-covered oceans more than fifty miles deep, thus holding far greater proportions
of the precious liquid at the surface. Plate tectonics helped to transform our planet, but in
thoseearlyyears,VenusandpossiblyMarshadtheirownversionsofconvection-driventec-
tonics.
NordidchemistrysetEarthapart.Basaltandgranitewerethefoundationstonesforallthe
rocky planets. Oxygen, silicon, aluminum, magnesium, calcium, and iron dominated their
compositions. Earth had its share of carbon and nitrogen and sulfur, but other worlds in our
Solar System were equally endowed with those vital elements. By almost every measure,
Earth four billion years ago appeared to be a rather ordinary planet.
But Earth was soon to become unique among known worlds. Admittedly, it was already
unique in that at five hundred million years, no other known planet or moon had endured
such sweeping episodes of change; no other planet had altered its outward appearance so
thoroughly and so often. But these metamorphoses were different only in scale, not in kind.
The most dynamic engine of planetary change—what sets Earth apart—was yet to emerge.
OnlyEarthbecamealive.TheoriginandevolutionofthebiospheredistinguishesEarthfrom
all other known planets and moons.
What Is Life?
What does it mean to be alive? What is this phenomenon that makes Earth so different from
the rest of the known cosmos? We might well try to describe life as a set of distinctive inter-
twined traits—a complex structure, coupled with the ability to move, to grow, to adapt, and
to reproduce. We might point to such distinctive cellular attributes as a membrane or long
strands of the genetic molecule DNA. But no matter how long the list of diagnostic traits,
there always seem to be exceptions. Lichens don't move. Mules don't reproduce.
