477
Earth’s First Life
of-life researchers consider this scenario
highly improbable as the scalding temperatures would have destroyed any early types
of self-replicating molecules. Their supposition is that life would have first appeared
along sheltered stretches of ancient
beaches, where waves and tides brought
together various organic materials formed
in the Precambrian oceans.
Regardless of where or how life
originated, it is clear that the journey
from “then” to “now” involved change
(FIGURE 19.21). The first known organisms
were single-cell bacteria called prokaryotes,
which means their genetic material (DNA)
is not separated from the rest of the cell by
a nucleus. Because oxygen was absent
from Earth’ s early atmosphere and oceans,
the first organisms employed anaerobic
(without oxygen) metabolism to extract
energy from “food.” Their food source
was likely organic molecules in their
surroundings, but the supply was very
limited. Later, bacteria evolved that used
solar energy to synthesize organic compounds (sugars). This event was an important turning point in evolution—for the
first time organisms had the capability of
producing food for themselves as well as
for other life-forms.
Recall that photosynthesis by ancient
cyanobacteria, a type of prokaryote,
contributed to the gradual rise in the
level of oxygen, first in the ocean and
later in the atmosphere. Thus, these early
organisms dramatically transformed our
planet. Fossil evidence for the existence
of these microscopic bacteria includes
distinctively layered mounds of
calcium carbonate, called stromatolites
(FIGURE 19.22A). Stromatolites are limestone
mats built up by lime-accreting bacteria.
What is known about these ancient fossils
comes mainly from modern stromatolites
like those found in Shark Bay, Australia
(FIGURE 19.22B).
The oldest fossils of more advanced
organisms, called eukaryotes, are about
2.1 billion years old. The first eukaryotes
were microscopic, water-dwelling organisms. Unlike prokaryotes, the cellular
structures of eukaryotes contain nuclei.
This distinctive structure is what all multicellular organisms that now inhabit our
planet—trees, birds, fishes, reptiles, and
humans—have in common.
ongoing, phase of Earth’ s history, in which humans evolved, the action of glacial ice, wind,
and running water added the finishing touches to our planet’ s long, complex geologic history.
C O N C E P T C H E C K 1 9 . 5
During which period of geologic history did the supercontinent of Pangaea
come into existence?
During which period of geologic history did Pangaea begin to break apart?
Describe the climate during the early Jurassic period.
Earth’ s First Life
The oldest fossils provide evidence that life on Earth was established at least 3.5 billion
years ago. Microscopic fossils similar to modern cyanobacteria (formerly known as bluegreen algae) have been found in silica-rich chert deposits worldwide. Notable examples
include southern Africa, where rocks date to more than 3.1 billion years, and the Lake
Superior region of western Ontario and northern Minnesota, where the Gunflint Chert contains some fossils that are older than 2 billion years. Chemical traces of organic matter in
rocks of greater age have led paleontologists to strongly suggest that life may have existed
3.8 billion years ago.
How did life begin? This question sparks considerable debate, and hypotheses abound.
Requirements for life, assuming the presence of a hospitable environment, include the
chemical raw materials that form the essential molecules of DNA, RNA, and proteins. These
substances require organic compounds called amino acids. The first amino acids may have
been synthesized from methane and ammonia, both of which were plentiful in Earth’ s primitive atmosphere. Some scientists suspect these gases could have been easily reorganized
into useful organic molecules by ultraviolet light, while others consider lightning to have
been the impetus, as the well-known experiments conducted by Stanley Miller and Harold
Urey attempted to demonstrate.
Other researchers suggest that amino acids arrived “ready-made,” delivered by asteroids
or comets that collided with a young Earth. A group of meteorites (debris from asteroids
and comets that strike Earth) called carbonaceous chrondrites known to contain amino acidlike organic compounds, led some to hypothesize that early life may have had an
extraterrestrial beginning.
Yet another hypothesis proposes that the organic material needed
for life came from the methane and hydrogen sulfide that spews
from deep-sea hydrothermal vents (black smokers). Is
it also possible that life originated within a hot
spring similar to those in Yellowstone
National Park? Some origin3
2
1
Earth’s First Life
of-life researchers consider this scenario
highly improbable as the scalding temperatures would have destroyed any early types
of self-replicating molecules. Their supposition is that life would have first appeared
along sheltered stretches of ancient
beaches, where waves and tides brought
together various organic materials formed
in the Precambrian oceans.
Regardless of where or how life
originated, it is clear that the journey
from “then” to “now” involved change
(FIGURE 19.21). The first known organisms
were single-cell bacteria called prokaryotes,
which means their genetic material (DNA)
is not separated from the rest of the cell by
a nucleus. Because oxygen was absent
from Earth’ s early atmosphere and oceans,
the first organisms employed anaerobic
(without oxygen) metabolism to extract
energy from “food.” Their food source
was likely organic molecules in their
surroundings, but the supply was very
limited. Later, bacteria evolved that used
solar energy to synthesize organic compounds (sugars). This event was an important turning point in evolution—for the
first time organisms had the capability of
producing food for themselves as well as
for other life-forms.
Recall that photosynthesis by ancient
cyanobacteria, a type of prokaryote,
contributed to the gradual rise in the
level of oxygen, first in the ocean and
later in the atmosphere. Thus, these early
organisms dramatically transformed our
planet. Fossil evidence for the existence
of these microscopic bacteria includes
distinctively layered mounds of
calcium carbonate, called stromatolites
(FIGURE 19.22A). Stromatolites are limestone
mats built up by lime-accreting bacteria.
What is known about these ancient fossils
comes mainly from modern stromatolites
like those found in Shark Bay, Australia
(FIGURE 19.22B).
The oldest fossils of more advanced
organisms, called eukaryotes, are about
2.1 billion years old. The first eukaryotes
were microscopic, water-dwelling organisms. Unlike prokaryotes, the cellular
structures of eukaryotes contain nuclei.
This distinctive structure is what all multicellular organisms that now inhabit our
planet—trees, birds, fishes, reptiles, and
humans—have in common.
ongoing, phase of Earth’ s history, in which humans evolved, the action of glacial ice, wind,
and running water added the finishing touches to our planet’ s long, complex geologic history.
C O N C E P T C H E C K 1 9 . 5
During which period of geologic history did the supercontinent of Pangaea
come into existence?
During which period of geologic history did Pangaea begin to break apart?
Describe the climate during the early Jurassic period.
Earth’ s First Life
The oldest fossils provide evidence that life on Earth was established at least 3.5 billion
years ago. Microscopic fossils similar to modern cyanobacteria (formerly known as bluegreen algae) have been found in silica-rich chert deposits worldwide. Notable examples
include southern Africa, where rocks date to more than 3.1 billion years, and the Lake
Superior region of western Ontario and northern Minnesota, where the Gunflint Chert contains some fossils that are older than 2 billion years. Chemical traces of organic matter in
rocks of greater age have led paleontologists to strongly suggest that life may have existed
3.8 billion years ago.
How did life begin? This question sparks considerable debate, and hypotheses abound.
Requirements for life, assuming the presence of a hospitable environment, include the
chemical raw materials that form the essential molecules of DNA, RNA, and proteins. These
substances require organic compounds called amino acids. The first amino acids may have
been synthesized from methane and ammonia, both of which were plentiful in Earth’ s primitive atmosphere. Some scientists suspect these gases could have been easily reorganized
into useful organic molecules by ultraviolet light, while others consider lightning to have
been the impetus, as the well-known experiments conducted by Stanley Miller and Harold
Urey attempted to demonstrate.
Other researchers suggest that amino acids arrived “ready-made,” delivered by asteroids
or comets that collided with a young Earth. A group of meteorites (debris from asteroids
and comets that strike Earth) called carbonaceous chrondrites known to contain amino acidlike organic compounds, led some to hypothesize that early life may have had an
extraterrestrial beginning.
Yet another hypothesis proposes that the organic material needed
for life came from the methane and hydrogen sulfide that spews
from deep-sea hydrothermal vents (black smokers). Is
it also possible that life originated within a hot
spring similar to those in Yellowstone
National Park? Some origin3
2
1