50
4 Life Evolves
4.3 Alien Earth and Gaia
The planet where life was emerging was very much unlike the planet we live on.
It was an alien Earth where hardly any organism we know would survive, as there
were only traces of oxygen at best, but plenty of poisonous gases in the atmosphere.
This is the reason why the search for the origins of life drives interest toward extremophiles, microbial life thriving under unusual conditions: in hydrothermal vents
at the Mid Atlantic Ridge, near sulphurous volcanic cauldrons, or buried deep under the rocks, or in the ocean trenches, or in the Antarctic’s icy underground lake.
Various strains of bacteria may be resistant to radiation, get along without oxygen,
or thrive in strongly acidic, alkaline, or salty environments.
Another focus of interest is life on exoplanets; several thousands of those have
been discovered at the time of writing (early 2019), and this number is bound to
grow to five figures by the time this book is printed. We are unlikely ever to be
able to consult alien colleagues, even if they are found somewhere in our close
galactic vicinity, just a few scores of light years away, still less collect bacterial
samples; but detecting the composition of planetary atmospheres is feasible, and
this can give a clue. Erwin Schr¨ odinger (1944) wrote: Living matter evades the
decay to equilibrium. If the atmosphere contains oxygen and gases that can react
with oxygen, for example, methane, this is already a telling sign that one or both of
them are of a biogenic origin, and it could be life, even if primitive, that maintains
the atmosphere in a non-equilibrium state. Closer to home, primitive life might be
possible on Mars, in the higher layers of the atmosphere of Venus, and in the interior
of Jupiter’s and Saturn’s moons.
Palaeontology provides far more information on the early life here on Earth than
exoplanets will ever do. Fossilized microorganisms more than four eons (billions
of years) old have been found in hydrothermal vent precipitates in Quebec, and
evidence of early life on land was discovered in 3.5 eons-old mineral deposits in
Western Australia. This indicates the very early appearance of life forms, soon after
the oceans were formed. Moreover, life may have appeared even earlier than known
records indicate, and may have been extinguished and emerged anew. At the time,
although the Sun was weaker, the young Earth might have been very hot due to frequent impacts of large meteorites and intense radioactivity, and it has therefore been
suggested that the first microorganisms may have been hyperthermophilic, thriving
at temperatures above 80 ◦ C.
Life needs an energy supply – where could energy come from? The ultimate energy source for life as it exists today is sunlight supporting the photosynthesis of
organic matter by plants, algae, and cyanobacteria, with oxygen released as a waste
product. For archaic life on the violent young planet, the energy might have come
from the Earth’s inner heat and the chemical energy of minerals. The relevance
of Miller’s experiment has been questioned, and Oparin’s idea of life emerging in
a “primordial soup” has lost its popularity because of the difficulty in preventing
the dissolution and hydrolysis of organic material. An alternative mechanism going
back to Carl von N¨ ageli (1884) is prebiotic synthesis in a safer environment of adsorbed layers. In N¨ ageli’s words, cited by G¨ unter W¨ achtersh¨ auser (2007), probably
4 Life Evolves
4.3 Alien Earth and Gaia
The planet where life was emerging was very much unlike the planet we live on.
It was an alien Earth where hardly any organism we know would survive, as there
were only traces of oxygen at best, but plenty of poisonous gases in the atmosphere.
This is the reason why the search for the origins of life drives interest toward extremophiles, microbial life thriving under unusual conditions: in hydrothermal vents
at the Mid Atlantic Ridge, near sulphurous volcanic cauldrons, or buried deep under the rocks, or in the ocean trenches, or in the Antarctic’s icy underground lake.
Various strains of bacteria may be resistant to radiation, get along without oxygen,
or thrive in strongly acidic, alkaline, or salty environments.
Another focus of interest is life on exoplanets; several thousands of those have
been discovered at the time of writing (early 2019), and this number is bound to
grow to five figures by the time this book is printed. We are unlikely ever to be
able to consult alien colleagues, even if they are found somewhere in our close
galactic vicinity, just a few scores of light years away, still less collect bacterial
samples; but detecting the composition of planetary atmospheres is feasible, and
this can give a clue. Erwin Schr¨ odinger (1944) wrote: Living matter evades the
decay to equilibrium. If the atmosphere contains oxygen and gases that can react
with oxygen, for example, methane, this is already a telling sign that one or both of
them are of a biogenic origin, and it could be life, even if primitive, that maintains
the atmosphere in a non-equilibrium state. Closer to home, primitive life might be
possible on Mars, in the higher layers of the atmosphere of Venus, and in the interior
of Jupiter’s and Saturn’s moons.
Palaeontology provides far more information on the early life here on Earth than
exoplanets will ever do. Fossilized microorganisms more than four eons (billions
of years) old have been found in hydrothermal vent precipitates in Quebec, and
evidence of early life on land was discovered in 3.5 eons-old mineral deposits in
Western Australia. This indicates the very early appearance of life forms, soon after
the oceans were formed. Moreover, life may have appeared even earlier than known
records indicate, and may have been extinguished and emerged anew. At the time,
although the Sun was weaker, the young Earth might have been very hot due to frequent impacts of large meteorites and intense radioactivity, and it has therefore been
suggested that the first microorganisms may have been hyperthermophilic, thriving
at temperatures above 80 ◦ C.
Life needs an energy supply – where could energy come from? The ultimate energy source for life as it exists today is sunlight supporting the photosynthesis of
organic matter by plants, algae, and cyanobacteria, with oxygen released as a waste
product. For archaic life on the violent young planet, the energy might have come
from the Earth’s inner heat and the chemical energy of minerals. The relevance
of Miller’s experiment has been questioned, and Oparin’s idea of life emerging in
a “primordial soup” has lost its popularity because of the difficulty in preventing
the dissolution and hydrolysis of organic material. An alternative mechanism going
back to Carl von N¨ ageli (1884) is prebiotic synthesis in a safer environment of adsorbed layers. In N¨ ageli’s words, cited by G¨ unter W¨ achtersh¨ auser (2007), probably