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THE ORIGIN OF LIFE 185
and that these in turn combined to form more
primitive atmosphere complex molecules. Then, the complex mole-
H 2 O, N 2 , H 2 , CO, H 2 S
1 cules became separated from the surrounding
medium, and acquired some of the characters
of living organisms. They became able to
small- and medium-sized molecules
sugars, purines, pyrimidines, amino acids, lipids absorb nutrients, to grow, to divide (repro-
2
duce) and so on.
The hydrothermal model is a recently pro-
posed modification to the Oparin–Haldane
large molecules
polysaccharides, nucleic acids, proteins biochemical model (Nisbet & Sleep 2001).
3
According to this view, the last universal
common ancestor of life (sometimes abbrevi-
ated as LUCA) was a hyperthermophile, a
membrane “protocells”
4
simple organism that lived in unusually hot
conditions. The transition from isolated amino
genetic mechanism;
ATP system prokaryotes c. 3500 Ma acids to DNA (Fig. 8.1) may then have hap-
5
pened in a hot-water system associated with
active volcanoes. There are two main kinds of
hot-water systems on Earth today, hot pools
organelles eukaryotes c. 1600–900 Ma
6
and fumaroles fed by rainwater that are found
around active volcanoes, and black smokers
distinct tissue multicellular in the deep ocean. Black smokers arise along
layers c. 1260–950 Ma mid-ocean ridges, where new crust is being
organisms
7
formed from magma welling up as major
Figure 8.1 The biochemical theory for the oceanic plates move apart (see p. 42). Seawa-
origin of life, as proposed by I. A. Oparin and ter leaks down into the crust carrying sulfur
J. B. S. Haldane in the 1920s. Biochemists have as sulfate, mixes with molten magma and
achieved steps 1–3 in the laboratory, but emerges as superheated steam, with the
scientists have so far failed to create life. ATP, sulfur now concentrated as sulfi de. As miner-
adenosine triphosphate. als precipitate in the cooler sea bottom waters,
they color the emerging hot-water plume
black. Black smokers are too hot as a site
a boost in 1996 when David McKay and a for the origin of life, but the other kinds of
team from NASA announced that they had hydrothermal systems are less extreme.
identified fossil bacteria and organic chemical This leaves us the Oparin–Haldane bio-
traces of former life in a Martian meteorite. chemical model as a broad-brush picture of
These findings have, however, been disputed how life might have originated, and the hydro-
vigorously, and the initial excitement has thermal model as a specific aspect. How far
waned. It is hard to see how extraterrestrial/ have scientists been able to test the biochemi-
panspermia models for the origin of life could cal model?
be tested decisively and, in any case, posit-
ing the origin of life on another planet still
leaves open the question of how that life Testing the biochemical model
originated. In cartoons and pop fi ction, the white-coated
The biochemical model for the origin of life scientist is seen in a laboratory full of mysteri-
was developed in the 1920s independently by ous bubbling glass vessels, and he declares,
a Russian biochemist, A. I. Oparin, and a “I’ve just created life”. Could this be true?
British evolutionary biologist, J. B. S. Haldane. How far have the experiments gone along the
They argued that life could have arisen chain of organic synthesis that is postulated
through a series of organic chemical reactions in the biochemical model for the origin of life
that produced ever more complex biochemi- (see Fig. 8.1)?
cal structures (Fig. 8.1). They proposed that It took some years before the fi rst labora-
common gases in the early Earth atmosphere tory results were obtained. The Oparin–
combined to form simple organic chemicals, Haldane biochemical model was proposed in
