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3,800 mya: Bacteria diversify
Single-celled bacteria are the oldest-known living organisms. Most
bacteria reproduce by making carbon copies of themselves through a process called fission.
While this might suggest that all bacteria must be alike, bacteria display staggering
diversity, probably from the start. From scientific study, we know that basic mutations
allow bacteria to pass genes, or sections of genes, between cells of the same bacterial
type as well as between different bacterial types. In this way, "lateral transfer" of
genetic information allows for tremendous variation.
3,700 mya: Photosynthesizing bacteria
Early microbes thrive in an oxygen-free environment, feeding
on organic molecules like glucose and producing energy in a process called fermentation.
As populations grow and food supplies become scarce, bacteria that can generate their
own food and energy evolve. Many of them use a process known as photosynthesis, converting
energy radiated from the sun into chemical energy the organism can store and use.
Photosynthesizing bacteria grow larger and reproduce faster than their competitors.
3,500 mya: Oldest fossils
Stromatolites appear offshore as distinctive mounds or dome-shaped
formations and are the oldest conclusive evidence of bacterial life. As photosynthesizing
bacteria thrive near the water's surface, they form dense mats. Living in layers below
these light-loving bacteria are other bacteria, which prefer darker conditions and feed
off the dying remains of the bacteria above them. Over time, as fine sediment is trapped
between these layers, the mats bind together and harden. Stromatolites, which continue
to form today in the coastal waters of northwest Australia and South Africa, create the
first extensive reef systems.
3,400 mya: Small continents form
As several chunks of smaller land blocks collide, the earliest
continents form.
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The changing face of the planet (3,400-2,500 mya)
The lithosphere, the rocky outer shell that covers Earth, is
broken into several rigid slabs known as tectonic plates. These plates float atop
a softer interior layer and are moved by currents caused by the release of Earth's
intense interior heat. As they move, plates might meet head-on, slide past one
another, or move away from one another. Jostling between plates can build mountains,
cause earthquakes, and produce volcanoes.
By 2,600 mya, the major period of continent building ends. About
85 percent of Earth's total landmass has been created. The continents are larger
and stronger now, with shallow, wide underwater shelves. Stromatolite colonies, the
first highly visible signs of life, thrive in these offshore shelf habitats and
expand across the planet. Their reef formations mark shallow-water boundaries.
About 2,500 mya, the continents join together to form the first
supercontinent. Scientific evidence suggests the planet's major continents have
come together only four or five times. Each time this happens, terrain changes
remarkably -- mountain ranges form, inland seas appear -- and climate and species
migration patterns can be greatly affected.
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3,100 mya: Continents begin shifting
Driven by heat deep within Earth, the continents' underlying plates
begin to shift.
2,700 mya: First eukaryotes
In 1999, geologists in northwest Australia discovered traces of oil
in rock that formed 2,700 mya. The oil contained sterols, which are fatty acids that
hold cell membranes together. Because sterols are only produced by eukaryotes --
organisms whose cells contain a nucleus -- researchers conclude that these sterols
were made by primitive eukaryotes. Plants, animals, fungi, and amoeba -- all of which
are eukaryotes -- descended from such early organisms.
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A win-win proposition (2,700 mya)
Scientists hypothesize the first eukaryotes evolve from
endosymbiosis, a relationship in which two different kinds of single-celled
organisms join together in a manner that benefits both parties. In this case,
certain free-living bacteria live inside host bacteria and carry out a task for
which the hosts are not well suited: cellular respiration. The engulfed bacteria,
now called organelles, use oxygen to convert organic food supplies (sugars) into
energy for the host.
How do we know that organelles were once free-living bacteria?
For one thing, their DNA differs from that found in the host cell's nucleus. In
addition, these bacteria can only reproduce on their own. In other words, they cannot
be made by the host cell.
Life's first recorded merger proves successful, and the early
eukaryotes multiply. As oxygen becomes more abundant over time, cellular respiration
is more productive and eukaryotes grow larger and larger.
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2,600 mya: Bacteria on land
Microbial mats -- networks of photosynthetic bacteria -- appear on soils
as centimeter-thick layers. Their existence suggests that a primitive ozone layer, which
protects life from the Sun's harmful ultraviolet radiation, has begun to form by this time.
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