The earth formed 4.5 billion years
ago. Ancestral archaea (archaebacteria) evolved from undersea hydrothermal vents
3.8 billion years ago when the earth was hot and fiery. The atmosphere was full
of methane and carbon-dioxide (CO2). Archaea are the only organisms which
produce methane, even now (apart from petroleum breakdown deep under the earth).
Eubacteria evolved from ancestral archebacteria some 3.2 billion years ago but caused no climate
change. For 1.3 billion years there was no oxygen in the air and the only life
was bacteria. Until 2.5 billion years ago when one type of eubacteria evolved
to acquire thylakoid membranes and photosynthesis, becoming Cyanobacteria.
This amazing thing has happened
just once in the history of this earth. Flight for example has evolved four
times independently (insects, pterodactyl, birds, bats) not counting many types
of gliding.
This singular event of
photosynthesis changed the face of the earth causing “The Great Oxygenation
Event”. All the methane combined with oxygen to form CO2, which again got used
up by Cyanobacteria. The oxygen killed off most of the bacterial types. With
greenhouse gases (methane, CO2) gone, the earth cooled drastically. The whole
of earth was like a ball of ice for 300 million years or so, from 2400 to 2100
million years ago (Mya), called the Huronian glaciation (from Lake Huron). Even
the cyanobacteria were frozen and most of life stopped.
Oxygen caused the first climate
catastrophy and a “Snowball earth”.
The Huronian glaciation ended 2100 Mya.
It escaped that icy deep freeze because volcanos and moving tectonic plates
released enough carbondioxide to warm the earth and life flourished again. A
supercontinent called Columbia formed, as a result of these geological changes,
lasting from 2000 to 1800 Mya. Only to break up again. The air was polluted
with oxygen but life found a way and evolved to use oxygen now. An archaebacterial
ancestor had evolved into eukaryotes, which unlike all bacteria, have a nucleus.
Eukaryotes absorbed (a process called endosymbiosis) a type of eubacteria into
the cytoplasm, which could efficiently use oxygen. These are now called mitochondria,
being present in every cell and help in efficient oxygen utilization. Some
eukaryotes further had endosymbiosis of cyanobacteria within them which turned
into plastids, the chlorophyll substance of plants. These two types of still
unicellular eukaryotes lived along with the archebacteria, eubacteria and
cyanobacteria for 1.4 billion years (2100-700 Mya), evolving but without significant
change.
Rodinia, another supercontinent
formed 1100 Mya started breaking up by moving tectonic plates 700 Mya. Weather
patterns changed, there was more rain and it froze causing 100 million years of
deep freeze. The glaciation ended around 600 Mya due to volcanic eruptions. But
a drastic change happened to life.
600 Mya, as life bloomed again
after the freeze, multicellular organisms evolved from different types of
eukaryotes in multiple independent events to create plants (which has plastids
derived from cyanobacterial thylakoids), animals (invertebrate and vertebrate)
and others like fungi. Plants control cell growth using plant hormones. Animals
control cell growth using apoptosis, a programmed cell death. Fungi grow with
food, portions die without food to retreat and sporulate. All different means
to become multicellular. To start with plants were small but when they evolved
to acquire a hard protein called lignin 450 Mya, it made huge trees possible. Massive
rainforests covered the earth in the carboniferous era from 350 to 290 Mya. CO2
levels were 800 ppm. This era converted enormous quantities of carbon into coal
via fossilisation of trees. Ambhibians and Insects predominated the land
animals.
It ended as before when plate
tectonics changed the climate. The earth cooled. The “Carboniferous Rainforest
Collapse” occurred. Thick forests turned into open tree fern covered land.
Amphibians died in large numbers, Reptiles evolved and life moved on.
Going over 4 billion years of the
earth’s life one sees four significant events.
a. 1. Photosynthesis
by Cyanobacteria caused a snowball earth 2400 Mya
b. 2. Melting
glaciation (Huronic) caused evolution of Eukaryotes 2100 Mya
c. 3. Second
melting glaciation caused evolution of multicellular organisms 600 Mya
d. 4. A newly
evolved protein called lignin (450 Mya) laid the groundwork of making the coal
which today threatens yet another modern climate catastrophy
Each event is associated with
climate change. Evolve or perish is the situation repeatedly created by our
planet. Every time, life has evolved and not perished. These 4 billion years
also created the present biomass balance where 80% is plants, Eubacteria including
cyanobacteria 15%, Fungi 2%, Archaea 1%, Animals and rest 2%.
In that single last line is the
secret to climate change and the balance of life which we as humans need to find. The future projected
a thousand years from today is crystal clear. Either humans learn to fine tune
earth’s climate. Or the earth turns into another snowball for thousands of
years, because the end result of global warming is once again a cooling catastrophy.
Ref:
1. 1. Kopp RE,
Kirschvink JL, Hillburn IA, Nash CZ.The Paleoproterozoic snowball Earth:A
climate disaster triggered by the evolution of oxygenic photosynthesis. PNAS 2005;102:11131-11136
2. 2. Bar-On YM,
Pholips R, Milo R. The biomass distribution on earth. PNAS 2018;115:6505-6511.
3. 3. Wikipedia
for everything else as on 18.7.20
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