The oldest fossils?

McCARTY ET AL: THE STORY OF EARTH AND LIFE, STRUIK , 2005.

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LIFE’S COMMON ANCESTRY

Life as we know it probably began as a selfrepli-
cating organic molecule. Perhaps a distant echo of
that molecule is carried today in all living things.
All life shares the common use of a few molecules,
just four in fact, which are linked together in a
chain to form the blueprint of life - the genetic
code. We use a combination of two digits to store
and manipulate information in our computers: 0
and 1, on and off, the binary system. Life somehow
chose to use a combination of four digits to store
information. Most organisms today use a molecular
chain of these four molecules, called DNA, to carry
the code. All also make use of a simpler version to
read the blueprint and to actually do the work of
running life, especially the manufacture of proteins.
This is RNA. It is surmised that the first organism
probably used this simpler form both to carry the
information for replication and to execute it.

All living things also consist of cells, little sacks
formed by a membrane, which contain the chemi-
cals of life, as well as the DNA molecule that holds
the instructions about what to do with the chemi-
cals. The DNA is capable of replicating itself and
the entire sack. All living things have another pecu-
liar commonality. Amino acids (the building blocks
of proteins), nucleotides (the building blocks of
DNA and RNA) and simple sugars come in two dis-
tinct forms - so-called left-handed and right-hand-
ed forms (like a pair of gloves). All living organisms
use the left-handed form of amino acids and the
right-handed form of nucleotides and sugars in their
metabolism. The common use of the same four mol-
ecules in the genetic code, as well as the use of the
same forms of amino acids, nucleotides and sugars,
suggests that all living things descended from one
single, self-replicating system.

Life probably arose only once. Each one of us
represents the end of an unbroken line of ancestors
extending back over 4 000 million years to that first
self-replicating organism. Every animal, insect and
plant likewise represents the end of an unbroken line
extending back to that same organism. Her name is
LUCA. the last Universal Common Ancestor.

The first replicating RNA could not have had a
blueprint, however, and must have arisen by some
other process. Some scientists believe that replica-
tion may have occurred on the crystalline surface of
a mineral, possibly a clay mineral or iron sulphide
..., which provided the template. Others
speculate that another type of mineral, called zeo-
lite, provided the site for replication. Zeolites have
cavities inside their crystal structures, in which mol-
ecules can come together and unite. They are wide-
ly used as catalysts in the petroleum industry for this
reason. Some scientists maintain that the cell mem-
brane is the more important thing, and must have
come first, because the chemicals of life need to be
contained before they can react with each other. Yet
others believe that proteins rather than RNA came
first, a view originating with a famous experiment
by Stanley Miller and Harold Urey, in which amino
acids were synthesised by electrical discharges
in an assumed early Earth atmosphere (water,
methane, ammonia and hydrogen).

In reality, while the subject is frequently dis-
cussed, no one really knows how life started, what
the essential chemicals were, or where they came
from. Even the most primitive living organism uses an
incredibly sophisticated chemistry, so far removed
from any conceivable, simple, self-replicating
molecule that it is impossible to relate life to such
a molecule. Perhaps, in desperation, the famous British
biologist Sir Francis Crick resurrected the notion
of panspermia, that life on Earth was seeded from outer
space, an idea first proposed by the Swedish chemist
Svante Arrhenius. Of course, this does not solve the
problem of life's origin; it merely defers it elsewhere.


THE EARLIEST LIFE

While the origin of life remains enigmatic, scientists
have learnt much about its subsequent develop-
ment on Earth through the study of living organisms,
many of which appear to be extremely primitive.
These may in fact be living fossils like the coela-
canth or cycads, little different from their ancient
ancestors that formed the first living cells.

The most primitive form of life is considered to
be a group of bacteria known as the archaea or
archaebacteria. These are single-celled organisms
without a nucleus (prolaryotes). Archaebacteria
occur in a variety of habitats. Many are thermo-
philes (heat loving) growing around hot springs in
volcanic regions and thriving in very acidic condi-
tions. Some inhabit the deep oceans, living around
hydrothermal vents along the mid-ocean ridge
system. Here they obtain their energy from the
chemical imbalance between the hot water and
the ocean. They use chemical reactions - such as
the oxidation of hydrogen sulphide emerging from
the vents to sulphate, the oxidation of iron and
manganese, or the reduction of sulphate to sul-
phide - to power their metabolism.

These bacteria have acquired an impressive
name - chemolithoautotrophic hyperthermo-
philes. This translates as follows: chemo = chemi-
cal foods; litho = sourced in rock; autotrophric = to
supply energy without other agencies; philes =
lovers of; hyper = extreme; and thermo = heat.

A branch of these archaebacteria have adapted
to living in extremely saline conditions, the
so-called halobacteria. Another group, the
methanogens, live in very oxygen-deficient, water-
logged environments, either hot or more ambient,
and release methane gas, deriving their energy
from fermentation. These bacteria are common
today in swamp environments, where they pro-
duce the streams of swamp gas that constantly
bubble from the black mud. They also thrive in the
gut of cattle.

Most of the archaebacteria live under oxygen.
deficient conditions, and many prefer hot environ-
ments, perfect adaptations to the hydrothermal
vents of the early Earth. These simple organisms
probably formed the rootstock of all life, including
humans. We may still carry a memory of our
ancient hydrothermal origins in our need for cer-
tain essential trace elements such as copper, zinc,
cobalt, nickel, molybdenum and selenium, which
form part of important molecules called enzymes,
used in protein manufacture. These trace elements
are extremely rare in our everyday environment,
but are relatively common in the hot, sulphur- and
metal-rich water that emerges from hydrothermal
vents. It must have been in this environment that
these enzymes first evolved.

Euan Nisbet of the University of London has
used this line of reasoning to propose that life arose
around hydrothermal vents in island arc environ-
ments because metals such as zinc, copper and
molybdenum are more abundant in these systems.

There is a second group of bacteria, known as
the eubacteria, which evolved from the archae-
bacteria early on in life’s history. The difference
between the eubacteria and the archaehacteria is
subtle, involving differences in the structure of
their DNA. The primitive forms of eubacteria are
also heat lovers and probably evolved in the
hydrothermal arena. However, they have widened
both their characteristics and their domain,
and they played a major part in life's subsequent
history. It is this branch of the family that devel-
oped photosynthesis and the processes of denitri-
fication and nitrogen fixation that changed the
world forever.


PHOTOSYNTHESIS

Photosynthesis is arguably the most important
chemical process on Earth because it powers almost
the entire global ecosystem. The essentials of the
process are simple - carbon dioxide and water
combine to make carbohydrate and oxygen gas -
but the execution is a masterpiece of chemistry.
Carbon dioxide and water will not react with each
other spontaneously and are stable together. Energy
has to be added to get them to react. This energy
comes from light and is captured by the remarkable
and extremely complex molecule chlorophyll.
How this molecule and its associated chemical
pathways arose is another mystery in the story of life.
Euan Nisbet has suggested that it may have original-
ly developed as a heat sensor to protect bacterial
cells in the hydrothermal environment. Another pos-
sibility is that it evolved from compounds used in the
fermentation process. The most primitive form of
photosynthesis involves carbon dioxide and hydro-
gen sulphide, used by the purple sulphur bacteria for
carbohydrate manufacture. The process does not
release oxygen and is termed anaerobic photosyn-
thesis. Energy to drive it comes in the form of
infrared radiation (heat), so it is suited to the hot,
hydrothermal environment where it most probably
evolved. Later, in the cyanobacteria
..., the process came to involve carbon
dioxide and water, using more energetic
radiation in the form of red light. This is
termed aerobic photosynthesis and
produces oxygen as a waste product.

The appearance of aerobic photo-
synthesising bacteria was extremely
important because it meant that life
was no longer restricted to hydrother-
mal vents. Instead of relying on anoxic
sites or sites where there was a natural
chemical disequilibrium, life could
now survive just about anywhere. The
only food required was a steady supply
of carbon dioxide and water, which
were both abundant, and light, which
was freely available from the Sun. As a
consequence, the cyanobacteria multi-
plied and colonised the global oceans.
All life generates waste. In the case of
the cyanobacteria, the waste product was oxygen, a
gas that was poisonous to most of the other bacteria
cohabiting with the cyanobacteria.

Photosynthesising bacteria evolved very early in
the story of life; there is chemical evidence to sug-
gest that they were present on Earth 3 800 million
years ago. The oldest rocks to contain visible signs
of their presence are those from Barberton, where
fossilised bacterial cells have been discovered in
cherts ... In addition, there is evidence
in the rocks at Barberton of bacterial colonial living
in the form of stromatolites (figure 6.4). Fossilised
bacteria have also been found in rocks of the same
age in the Pilbara region of Western Australia.

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