Life, origin of WORLD OF MICROBIOLOGY AND IMMUNOLOGY350•
point in this process, the transition from a lifeless collection
of reacting molecules to a living system probably occurred.
The third process following organization into simple living
systems was biological evolution, which ultimately produced
the complex web of modern life.
The underlying biochemical and genetic unity of organ-
isms suggests that life arose only once, or if it arose more than
once, the other life forms must have become rapidly extinct.
All organisms are made of chemicals rich in the same kinds of
carbon-containing, organic compounds. The predominance of
carbon in living matter is a result of its tremendous chemical
versatility compared with all the other elements. Carbon has
the unique ability to form a very large number of compounds
as a result of its capacity to make as many as four highly sta-
ble covalent bonds (including single, double, triple bonds)
combined with its ability to form covalently linked C—C
chains of unlimited length. The same 20 carbon and nitrogen
containing compounds called amino acids combine to make
up the enormous diversity of proteins occurring in living
things. Moreover, all organisms have their genetic blueprint
encoded in nucleic acids, either DNAor RNA. Nucleic acids
contain the information needed to synthesize specific proteins
from their amino acid components. Enzymes, catalytic pro-
teins, which increase the speed of specific chemical reactions,regulate the activity of nucleic acids and other biochemical
functions essential to life, while other proteins provide the
structural framework of cells. These two types of molecules,
nucleic acids and proteins, are essential enough to all organ-
isms that they, or closely related compounds, must also have
been present in the first life forms.
Scientists suspect that the primordial Earth’s atmos-
phere was very different from what it is today. The modern
atmosphere with its 79% nitrogen, 20% oxygen, and trace
quantities of other gases is an oxidizing atmosphere. The pri-
mordial atmosphere is generally believed not to have con-
tained significant quantities of oxygen, having instead rather
small amounts of gases such as carbon monoxide, methane,
ammonia and sulphate in addition to the water, nitrogen and
carbon dioxide, which it still contains today. With these com-
binations of gases, the atmosphere at that time would have
been a reducing atmosphere providing the hydrogen atoms for
the synthesis of compounds needed to create life. In the 1920s,
the Soviet scientist Aleksander Oparin (1894–1980) and the
British scientist J.B.S. Haldane (1892–1964) independently
suggested that ultraviolet (UV) light, which today is largely
absorbed by the ozone layer in the higher atmosphere, or vio-
lent lightning discharges, caused molecules of the primordial
reducing atmosphere to react and form simple organic com-The sea.womi_L 5/6/03 3:32 PM Page 350