Geoscience Reference
In-Depth Information
Harvard University documents the changing face of life during the first
four billion years of Earth history. He shows how a variety of biological
innovations, like the invention of oxygen-producing photosynthesis,
for example, fundamentally shaped the history of life. After oxygen-
producing organisms first evolved, other organisms that use oxygen fol-
lowed, and they then prospered, multiplied, and evolved into yet other
oxygen-utilizing life forms. Eventually this led to animals, the most
biologically complex of all organisms on Earth. With no oxygen, there
would be no animals. So, clearly, innovations during biological evolu-
tion have shaped, evened defined, the biosphere. But does evolution
alone explain the bounty of life on our planet?
To consider this question, we quickly compare Earth and Mars. Sci-
entists still hold out for the possibility of life on Mars: after all, Mars is
the same age as Earth and there is some evidence for at least occasional
surface and subsurface water on the planet. Even as I write, NASA's
rover
Curiosity
is probing the Martian surface for signs of water, and for
clues as to how water interacts with the planet's surface environment.
As we will discuss more fully below, and as the tenet goes, where there
is water, there may be life. Yet, if there is life on Mars, it doesn't jump up
and down like the hos in hoville, crying: “We are here, we are here,
we are here!” In contrast, if intergalactic explorers probed Earth as we
presently probe Mars, it would be impossible to miss Earth's abundant
life. The question is, quite simply, why is there so much life on Earth?
To answer this we will for the moment abandon considerations of
evolution and start with a more fundamental question: hat are the
basic ingredients needed for life, at least for life as we know it? As I digest
my lunch of lasagna leftovers, I proclaim that food must be important.
Yes indeed, but not all organisms can eat lasagna, and I'm reminded of
a whole class of creatures who don't eat any kind of organic matter at
all, but rather make their cells from simple inorganic substances. Plants
fit this bill, growing from carbon dioxide and water and using the en-
ergy of the Sun to combine these compounds into cell biomass and
oxygen.
Many other types of organisms also fit the bill, and most of them do
not use the Sun for energy. Rather, they gain their energy from promot-
ing the reaction between inorganic substances in so-called oxidation-
reduction reactions, where electrons are transferred during the reaction.
