CONCEPTUAL LIFE SCIENCE
Evolution
THE HETEROTROPH HYPOTHESIS
According to the heterotroph hypothesis, the first organisms to develop were
heterotrophs. The autotrophic organisms developed after the heterotrophic ones.
Primitive conditions on the Earth
The age of the Earth is thought to be 4.5 x 109 (4.5 billion) years. Evidence for
this estimate comes from measurements of the decay of Uranium. This time is divided
into four eons as listed in Table XV. In the earliest eon, there was a reducing
atmosphere, much heat, lightning (electrical discharge), ultraviolet (UV) light from the
Sun and radiation.
There was no ozone layer in the earliest days of the Earth. To form an ozone
layer requires oxygen. Oxygen is the waste product of photosynthesis. Photosynthesis
did not develop until after the formation of the first eukaryotic cells, about 2.5 billion
years ago. Therefore, without an ozone layer, UV light came all the way down to the
surface.
There are no rocks found so far that go back to the formation of the Earth.
Geological processes of erosion, transport and deposition have worn away the original
rocks. There have been rocks found in Canada and Australia that are dated in the range
of 3.9 – 4.1 billion years.
Table XV. Eons of Time
Eon
Hadian
Archean
Proterozoic
Phanerozoic
Duration (Millions of Years)
4500 – 4000
4000 – 2500
2500 - 600
600 – Today
Synthesis reactions
A reducing atmosphere of ammonia (NH3), methane (CH4), hydrogen (H2) and
water (H2O) has been shown to be able to produce such molecules as urea, hydrogen
cyanide, organic acids and other molecules under laboratory conditions by random
synthesis reactions with each other. There was no oxygen in the early atmosphere but
there was some CO2 present. The molecules produced experimentally in Miller’s spark
discharge apparatus were similar to biological molecules. For example, amino acids were
found.
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Aggregates of organic molecules
Aggregates are collections of organic molecules that may have formed along the
shorelines of primitive seas where the molecules were exposed to ultraviolet light.
Radiation on rocks causes them to heat up. There is evidence that when organic
molecules are heated together dry, they react. Fox took dry amino acids and heated them
together. Water was released that condensed on the inside of the test tube at the top.
This condensation resulted from reactions between the water molecules. Thus the origin
of the term “condensation reaction.”
Reproduction of molecular aggregates and coascervates
Certain larger aggregates (coascervates) might have begun to divide after reaching
a certain size. Experiments have shown that CH4, NH3 and H2O can yield bases like
those found in DNA.
Anaerobic respiration
Fermentation is found in all cells, not only the anaerobic bacteria but even those
that use aerobic respiration. So, anaerobic respiration must have developed first. The
first organisms probably got their energy by fermenting organic molecules that were
dissolved in the water.
Development of autotrophs
Some organisms must have developed the means by which to perform
photosynthesis. The resulting release of oxygen over time led to the formation of the
oxidizing atmosphere we have today.
Aerobic respiration
Those organisms that could tolerate and use oxygen went on to become aerobic.
It is thought that eukaryotic cells developed as a result of one kind of prokaryote living
inside another.
Endosymbiosis
This explanation is called the endosymbiotic theory. According to this
explanation, mitochondria that we have today were originally a form of prokaryote that
had a knack for producing energy. Similarly, chloroplasts are thought to have resulted
from other prokaryotes that could trap solar energy. Evidence for this theory comes from
the fact that mitochondria and chloroplasts have their own DNA molecules, independent
of the DNA in the chromosomes of the nucleus of the cell. This DNA is closely related
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to bacterial DNA, not nuclear DNA. Also, mitochondria and chloroplasts have their own
ribosomes. These ribosomes are closely related to bacterial ribosomes and are distinctly
different from the cytoplasmic ribosomes found in the eukaryotic cell.
THE GEOLOGIC TIME SCALE
Life began in the proterozoic eon, about 2.5 billion years ago. This is also known
as the Pre-Cambrian era. As shown in Table XVI, an era is a subdivision of an eon.
Each era is further subdivided into periods. The periods correspond to major divisions in
geologic (and biologic) history. Geologists use biology as evidence for different periods.
Up until the discovery of radioactivity in 1895, there was uncertainty about the age of the
Earth or different layers of rocks. Geologists then and now use fossils to indicate
information about the age and history of various rocks. The most recent period, the one
we are in now, is known as the Quaternary Period.
Glaciations of the Quaternary period
There were four major glaciations (ice ages) in the Quaternary period. The
earliest ice age began 600,000 years ago. Others occurred beginning at 500,000 years
and 250,000 years. The last one began at 70,000 years and ended 20,000 years ago.
THE FOSSIL RECORD
The oldest rocks are over 3.5 billion years old. Fossils are the remains of living
things or are objects that were made by living things. These are left in sand or mud that
will eventually be turned into sedimentary rock. Teeth and bones make good fossils as
well as tools of primitive humans and worm burrows.
In undisturbed rock layers, the oldest fossils (indicating the oldest rocks) should
be found in the oldest layers. This is the case in the Grand Canyon that exposes
approximately 2.5 billion years of Earth history in one place. Relatedness between
fossils is based on comparative anatomy.
EARLY MAMMALS
The first mammals came into existence as early as 1.5 x 108 (150,000,000) years
ago. The earliest primates developed about 70,000,000 years ago.
The earliest monkeys came into being some 40,000,000 years ago. The most
primitive of them already had the same dental pattern as modern humans.
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Table XVI. Geologic time scale.
ERA
Cenozoic
PERIOD
Quaternary
Tertiary
Mesozoic
Cretaceous
Jurassic
Triassic
Paleozoic
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
Pre-Cambrian
EVENTS
Age of humans
Four major ice ages
Increase in mammals
Evolution of higher animals
Increase
in diversity
of plants
Extinction of dinosaurs
Diversity of flowering plants
Origin of primates
Origin of mammals and birds
Dominance of dinosaurs
Beginning of dinosaurs
Dominance of gymnosperms
Rise of flowering plants
Reptiles spread and develop
Abundant amphibians
Reptiles appear
Growth of great coal forests
Age of fishes
First amphibians
First large forests on land
First land plants
First fishes and vertebrates
Age of marine invertebrates
Beginning of life—at least 5x
longer than all geologic time
following
BEGAN*
0.01
2
7
25
36
58
65
135
EPOCH
Holocene
Pleistocene
Pliocene
Miocene
Oligocene
Eocene
Paleocene
180
225
270
350
400
440
500
600
*Dates are in millions of years.
Fossil record of early humans
Information about the early ancestors of humans is derived from the study of
fossil bones and skulls. In the Miocene epoch lived Proconsul. Proconsul may have
been the common ancestor of chimpanzees, gorillas and humans. It is 25 million years
old and had teeth like those of apes.
In the Pliocene epoch lived Ramapithecus. This animal had teeth more like those
of the hominids. It may have been the earliest hominid (ape-man). It is 12 million years
old. The hominid family includes humans. All hominids other than humans are extinct.
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Early Pleistocene hominids
Early Pleistocene hominids lived in the Old Stone Age. Australopithecus
afarensis (southern ape of the Afar region in Africa) lived in East Africa at least 3 million
years ago. It then evolved into the gracile A. africanus and the robust A. transvaaliensis
at about 2 million years ago. Gracile organisms have delicate boning whereas robust
organisms have very heavy boning. The robust ones lived for about 1 million years then
died out. Humans are gracile. The gracile ancestors evolved into the genus Homo.
More recently in the Pleistocene lived Homo habilis. It dates from about 1.8 to 2
million years ago. Homo is the genus that includes humans. All members of the genus
Homo, other than Homo sapiens sapiens are extinct. Homo habilis, like Australopithecus
was small but had distinct differences in the skull bones and teeth.
Middle Pleistocene humans
Homo erectus had a more advanced Stone Age culture called Pre-Mousterian. It
had the ability to use fire and create simple tools. Some specimens have been given
names such as Java Man. Homo erectus developed in Africa about 1.6 million years ago.
Homo erectus and Australopithecus lived in the same general areas of Africa and
Europe. Australopithecus may have died out because Homo erectus was more advanced
and could compete more easily.
The earliest prototype Homo sapiens is dated at about 300,000 years ago. During
the last ice age it evolved into two sub-species, Homo sapiens neanderthalis and Homo
sapiens sapiens. Steinheim Man is of the Neanderthal type. It had Mousterian culture
which included ritual burial.
The late Pleistocene
Homo sapiens neanderthalis lived between 130,000 and 35,000 years ago. In
Europe it lived alongside Homo sapiens sapiens. The two sub-species eventually
interbred to form the modern human, Homo sapiens sapiens.
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Table XVII. Evolutionary Trends in Primates
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Primitive five-digit hand and foot pattern
Free mobility of thumb and big toe
Flattened nails instead of sharp claws
Increased facial angle
Perfection of visual apparatus
Reduction in apparatus of smell
Increase in size and complexity of the brain
Development of uterus and more efficient nourishment of the fetus in pregnancy
Erect posture and development of bipedalism
Prolongation of post-natal life periods