chemical reaction found, today, in all plants and plant-like organisms that
contain chlorophyll. Organisms capable of photosynthesis can capture and
store energy directly from sunlight, in a reaction whose main inputs are
carbon dioxide and water and whose main outputs are sugary molecules
such as glucoses, which can store energy, and free oxygen. The ¿ rst
photosynthesizing prokaryotes appeared as early as 3.5 billion years ago.
Photosynthesis marks a fundamental threshold in the history of life because
it enabled living organisms to tap the colossal energy À ows generated in the
core of the Sun by hydrogen fusion.
Today, all plants practice photosynthesis, capturing energy from the Sun using
green molecules of chlorophyll. As plants are consumed by other organisms,
this captured energy diffuses throughout the biosphere via the “food chain.”
Some of the oldest microfossils (3.5 billion years old) are photosynthesizing
algae, like modern “cyanobacteria.” They created coral-like structures called
stromatolites, some of which still exist today. Photosynthesis produces
oxygen as a by-product. By 2.5 billion years ago, free oxygen started building
up in the atmosphere. For many prokaryotes, oxygen was poisonous, which is
why Lynn Margulis and Dorion Sagan described this change as the “oxygen
holocaust” in their book Microcosmos.
This revolutionary change in the
atmosphere provides one marker for
the beginning of the Proterozoic eon,
from about 2.5 billion years ago.
The third crucial transition is the
appearance of “eukaryotic” cells
more than 1 billion years ago, during
the Proterozoic eon. Lynn Margulis
(1938–) showed that eukaryotes
evolved through the merging of
once independent species of prokaryotes. Evidence for this is the presence
in all eukaryotes of internal “organelles,” some of which have their own
DNA, which suggests they had once existed quite independently. Internal
organelles include mitochondria, which can extract energy from oxygen, and
chloroplasts, which can extract energy from sunlight through photosynthesis.
The merging of these entities through “symbiosis” anticipates the later
Photosynthesis marks a
fundamental threshold in
the history of life because it
enabled living organisms to
tap the colossal energy À ows
generated in the core of the
Sun by hydrogen fusion.