Oxygen and Functional Evolution of Organisms
by Miquel Riera-Codina ([email protected])
Men and animals live thanks to the oxygen in the air. Oxygen is considered to be a
source of life. But the earliest atmosphere did not contain oxygen. How and when did
this gas appear? What influence did it have over the evolution of animals? This article
attempts to point out some aspects which might lead us to think that oxygen had an
important role in the development of the complexity of living organisms, in a similar
way perhaps to the way that the use of oil has facilitated the development of a
complex human society.
In the XVIIIth century, Lavoisier and Laplace thought that life was like combustion, that is to say
that in the same way that wood produces heat when oxidized by air, animals oxidize organic matter
to obtain energy (to heat themselves, grow and move).
æ æ æ æ æ æ ææÆ CO 2 + H 2O + heat (flames )
CnH 2 nOn + O 2 æcombustion
respiration
CnH 2 nOn + O 2 ææ æ æ æ æ æ ææÆ CO 2 + H 2O + energy ( ATP)
photosynthesis
.
CO 2 + H 2O + energy (light)ææ æ æ æ Æ CnH 2 nOn + O 2
The complex structure of living organisms needs energy to sustain it (second principle of the
thermodynamics). Organisms are highly improbable structures which are maintained thanks to
metabolic energy. However, the development of organisms with increasing functional complexity
required the development of energetic systems which were progressively more efficient. Lets try now
to explain how oxygen could condition the evolutionary process in animals.
The first organisms developed without O2.
Respiration, and therefore life forms as we know them now, could not have developed in the earliest
stages of the Earth’s formation since the atmosphere of the primitive Earth did not contain oxygen.
The presence of this gas is a relatively recent fact (from 1,5 to 2 billion years ago). What influence,
therefore, did oxygen have in the development of organisms? Why were complex cells not able to
associate in multicellular organisms before the appearance of oxygen? First let's notice that it is
difficult for us to assimilate and appreciate very long periods of time, so perhaps it would be useful
to consider a new relative time scale much closer to that of our daily life. Imagine then that the entire
history of the Universe has occurred in the space of one year, between, for example, January 1st
and December 31th of 2003. Based on this time scale, which will represent our reference
calendar, on the first of January the Universe was formed, but it would not have been until
September 14 (or approximately 4,6 billions of years ago) that the Earth was formed. By then the
atmosphere was rich in nitrogen, hydrogen, carbon dioxide, methane and water vapour among other,
but did not contain oxygen.
Relative date
Evolutive event
January 1
Universe formation
September 14 Earth
October 15
formation
November 1
Anaerobic photosynthetic bacteria
(do not produce O2 )
November 12 Photosynthesizers that produce O2
Functional evolution
Abiotic (biochemical) evolution
Development of anaerobic
.metabolism
December 1
Development of aerobic metabolism
Development of O2-rich atmosphere
2:
Formation of ozone layer
December 8
First eukaryotes (complex
cells)
First multicellular
animals
Development of organs and tissues
December 19
First vertebrates (primitive
fishes)
Development of sistems of functional
coordination
December
22 to 24
Land colonization: first amphibian,
Reptiles, and flying
insects
Development of functional
complexity
December 26
First birds and mammals
December 29
First primates
Encephalic development.
Centralized functional regulation,
higher
functions
Higher encephalic functions
December 16
De 31 22h 36’ First
hominides
-23h 46’
Man uses fire
-23h 59’ 35’’
First village
-23h 59’ 56’’
Christ is
born
Development of modern science
and industry
-23h 59’ 59’’
Consolidation aerobic metabolism
Cultural development
Ozone hole
.
Contrary to what is shown in the
movies, the landscape would have
had a quite different aspect to that
found in the present arid areas, it
probably had a greyish appearance,
like volcanic zones, because the rocks
were not oxidized on entering in
contact with the atmosphere.
Already in the 1920’s Oparin and Haldane explained that in such conditions important biochemical reactions
would occur that would lead to the formation of organic compounds (abiotic or chemical evolution).
However life did not appear until approximately October the 15th of our calendar (about 3.4 billion years ago).
Some tiny vesicles, which were able to develop inside
coordinated reactions to obtain energy, appeared in the
seas. Thus, the first form of living organism appeared, the
cell. These first cells were very simple (protobionts or
“metabolic vesicles”, and eubions, when incorpored a
genetic system, true life forms) and they evolved by
increasing their internal complexity becoming organisms
not dissimilar to the present bacteria. They had to obtain
the energy necessary to support their internal level of
organization by oxidation of organic compounds
(heterotrophs), by the oxidation of inorganic substances
(chemotrophs) or by the oxidation of the organic
compounds previously synthesized by means of luminous
radiation (autotrophs).
Anyway one thing is clear, the oxidation must have been produced in the absence of O2 (anaerobes), hence
the final acceptor of electrons could either have been an inorganic substance (for example sulphate-reducing
microbes, see Shen et al. in Nature, 410, 77-80, 2001) or a partially oxidated organic compound (fermentation),
but in either case this must have been with a reduced oxidative capacity in comparison with that of the O2.
This type of metabolism produces little energy and therefore these organisms could not evolve toward very
complex forms. In any case, they began to proliferate into the oceans and developed more complex and
efficient chemical reactions (metabolic evolution).
Below is shown a nice model for evolution of primitive cells extracted from http://www.gly.uga.edu/railsback/
1122main.html
The oxygen appears: the first great atmospheric “contamination”
Returning to our calendar, on November 1 some cells begin to use sunlight to obtain energy by photosynthesis
(like the present green and purple sulphur bacteria which are anaerobics and do not produce O2) and later some
of these cells developed an oxygen-producing photosynthesis (like blue-green algae). This mechanism was
more suitable energetically and therefore this type of cell began to proliferate in the seas, this enlarged quickly
the production of oxygen which thus began accumulating in the primitive atmosphere. 1.8 million years ago (by
December 1) the quantity of atmospheric oxygen already was similar to the present.
The accumulation of oxygen in the atmosphere had
a spectacular effect on the landscape. The rocks
began to oxidize and become red and yellowish,
the landscape was filled with colour similar to the
present arid areas. In fact oxygen represented the
first large scale environmental pollution due to the
action of the living organisms.
In addition, in the higher layers of the atmosphere, a gas was formed which derived from the oxygen: ozone.
This gas intensely absorbs the ultraviolet radiation from the sun, which destroys the living organisms; therefore
these primitive cells could reach the upper layers of the seas and have access to oxygen. Some of them
developed metabolic pathways to utilize this new gas. We are not here going to enter into the interesting
molecular adaptations that would need to be produced to metabolize the O2, we only wish to note that by
aerobic respiration cells could obtain much more energy. Organisms with this high energy performance could
evolve into more complex forms. As a result, a new cellular organization appeared, the eukaryotic cell. These
are complex cells, with a nucleus and organelles, with an aerobic metabolism already consolidated which
forms part of all multicellular organisms. This happened approximately 1.4 billion years ago (December 8,
from our calendar).
An interesting hypothesis about the
formation of eukaryotic cells based on
the close association between two
bacteria with complementary
metabolisms has been recently
proposed. First, both cells tend to be
grouped because the hydrogen and the
carbon dioxide produced by the
aerobic bacteria is used by the
autotrophic methane producer. Later,
a close association is promoted
because this favours a better
exchange. Finally, a cell is included
as an organelle (mitochondria) in the
other cell. The two metabolisms are
ultimately fused in a final, more
efficient, aerobic metabolism.
Organic
compounds
Aaerobic bacteria
Anaerobic bacteria
Glucose
Glucose
ATP
H2
Piruvate
ATP
CO2
O2
Anaerobiosis
ATP
Methane producer
CH4
H2O
CO2
Process of close association
Organic
compounds
Glucose
Glucose
ATP
H2
Piruvate
O2
ATP
CO2
ATP
CO2
Anaerobiosis
Methane producer
H2O
“Eukaryotic” cell (with mitochondries)
CH4
Organic
compounds
Glucose
Mitochondria
ATP
Piruvate
Piruvate
O2
Cytoplasm
ATP
CO2
H2 O
ATP
Residual
pathway
The aerobic metabolism is consolidated in eukaryotic cells: the road to multicellular forms is open.
Eukariotes having a greater energy performance could develop locomotion and systems relation. Near the
surface of the seas, where there was more oxygen, cells may have begun to associate in coordinated groups,
forming the first multicellular-like organisms.
Coelenterates are very simple
animals, their cells are not
differentiated in specialized
tissues.
Multicellulars are more complex and thus more energy is needed for maintaining their level of organization: the
development of an aerobic metabolism would have been crucial for their formation. These new organisms took
advantage of the efficiency of aerobiosis, began to proliferate and evolve to greater size and more complex
forms, and developed specialized tissues and organs. The first multicellulares appeared 670 million years ago.
550 million years ago invertebrates appeared with the skeletal structure that gave rise to the great proliferation
of forms during the Cambrian period. Very soon after the first vertebrates appeared (primitive fish: Agnatha
and Placoderms) although they were not diversified until the Devonian period (400 million years ago). All this
occurred only in approximately a week of our calendar (12-20 December).
The pressure of the sea population stimulates the development toward land and air respiration. The air
has a high oxygen content.
Then another notable moment in the evolution of organisms occurred. The explosion of living forms filled
those oceanic areas more suitable for life, so some animals were pushed to less stable zones, such as marshes,
rivers, lakes and flooded areas. Warmer and rainier periods enlarged the extensions of flooded land in the form
of mangrove swamps and marshy zones. These areas contain hot water which is very poor in oxygen, so
animals that lived there had to adapt to the use oxygen from the air, thus facilitating the transition of life
toward land.
The terrestrial environment is much more changable than the marine one, thus animals had to develop an air
based respiration system, and mechanisms to insulate them from environmental variations, and this involved
the development of mechanisms of physiological regulation. However, animals had access to a higher energy
source since air contains much more O2 than water. Again, therefore, it was the oxygen that provided the
energy to allow animals to adapt to changing environments and to develop to new evolution stages: the
development of complex regulation systems. Thus, the amphibians appear and a little later the reptiles colonize
the land (from Permian to Cretácico, 280 to 80 million years ago). In the Triásico already primitive mammals
existed although they would not diversify and proliferate until Cretácico (140 million years ago).
Mammalian (the first mammals were small but developed important homeostatic
mechanisms for functional regulation) and birds developed large functional regulatory
and coordination centres in the encephalus (Prosencephalus) and these allowed them to
attain greater independence from the changing environment. Their encephalus acquired
functions of auto-control and conscious conduct.
From the appearance of the first hominids in Africa 11 million years ago (31 from December at 10 o'clock
hours and 30 minutes of the night of our calendar) things began to move rapidly. Thus at eleven o'clock in the
evening our ancestors had already learned to work with stone tools, by four minutes to twelve they had learned
to use the fire, the first village appeared at half a minute to midnight, at 4 seconds before the stroke of twelve
Christ was born, and one second before man developed the scientific method and began the modern industrial
revolution. Mankind has made use, for good or for ill, of this last second to develop all we now know as
science and modern industry.
The modern industry release to atmosphere chlorofluocarbons.
One of the most important consequences of industrial development has been the destruction of the ozone layer
through accumulation of chlorofluocarbon compounds (CFC) produced by modern industry. It was thought
initially that these compounds would be ideal since they appeared not to react and therefore not to pollute. So
they were produced and released into the atmosphere in large quantities. But slowly the CFC ascended through
the atmosphere and accumulated in the stratosphere.
There, in extreme temperature conditions ( more
than 60-80ºC below zero) and intense sunlight
(present in the Antarctic during relatively long
periods of time), these gases become reactive
and break down the ozone in oxygen. The
problem is that a cyclic process can be occur
and thus each molecule of CFC continuously
destroys the ozone.
Photolysis of CFC
Cyclic process of ozone destruction
2O3
2O2
CF2Cl2 (CFC) + light Æ CF2Cl + Cl
2Cl
2ClO
O2
ClO2
Cl2O2
It would be paradoxical if an organism which in part has arisen thanks to the formation of the ozone layer in
ancestral periods of the Earth, now returns the planet to its initial conditions.
In the CFC case international measures have been taken and it seems that the reduction in emission of CFCs is
under control. We should keep in mind, nevertheless, that the process has great inertia and at present large
quantities of these compounds released by the industry of the eighties are still arriving in the stratosphere . Will
we be able to really stop the destruction of the ozone layer by the CFCs? Will we be able, know how to or
want to act to avoid future alterations in the environment? Keeping in mind the giddy changes that are being
produced in the environment we should be cautious in introducing any alterations that can induce effects with
great inertia. Which history will we be able to write for January 1, 2004 of our calendar?
Related links
• Cellular evolution (Chapter 11). In EVOLUTION FACTS, INC. BOX 300 - ALTAMONT, TN. 37301 .
http://evolution-facts.org
• The Progenote. Draft of an article to appear in the ENCYCLOPEDIA OF MOLECULAR BIOLOGY.
http://www.sp.uconn.edu/~gogarten/progenote/progenote.htm
• Webpage for Dr. Bruce Railsback's lecture section. In Earth's History of Global Change, University of
Georgia, Department of Geology. http://www.gly.uga.edu/railsback/ 1122main.html
• Cellular genesis. In The Harbinger symposium, "Religion & Science," Alabama Humanities Foundation,
http://www.theharbinger.org/articles/ rel_sci/fox.html
• The Origin of Higher Life Forms. http://www.amazingdiscoveries.org/ lifeforms.html
Books
•
•
•
•
•
Avers, Charlotte J. Cap. 18 Cellular and molecular evolution. In Molecular Cell Biology. Ed. AddisonWesley (1986)
Montero, F. Y Morán, F. Biofísica. Procesos de autoorganización en Biología. Ed. EUDEMA, 1992.
Losada, M., Vargas, M. A., De la Rosa, M.A., Florencio, F.J. Los elementos y moléculas de la vida.
Introducción a la química biológica y biología molecular. Ed. Rueda, 1999.
Ecología y Evolución. En História Natural. Ed. Carroggio. 1990
Tomo 15 Registre fòssil. En Història natural dels països catalans. Ed. Fundació Enciclopèdia Catalana,
1989.
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Martin, W. & Müller, M. The hydrogen hypothesis for the first eukaryote. Nature, 392, 37-41 (1998)
Castresana, J. & Saraste, M. Evolution of energetic metabolism: respiration-early hypothesis. TIBS, 20,
443-447 (1995)
López-García, P. & Moreira, D. Metabolic symbiosis at the origin of eukaryotes. TIBS, 24, 88-93 (1999)
Solomon, S. Progress towards a quantitative understanding of Antarctic ozone depletion. Nature, 347, 347353 (1990)