The nebularThe nebular
hypothesis maintainshypothesis maintains
that the Solarthat the Solar
System formed fromSystem formed from
the gravitationalthe gravitational
collapse of acollapse of a
fragment of a giantfragment of a giant
molecular cloudmolecular cloud. The. The
further collapse offurther collapse of
the fragments led tothe fragments led to
the formation ofthe formation of
dense cores One ofdense cores One of
these collapsingthese collapsing
fragments (known asfragments (known as
thethe pre-solar nebulapre-solar nebula))
would form whatwould form what
became the Solarbecame the Solar
The earth was very
hot. The energy from
heated the surface of
minerals and from
the decay of
heated the interior of
spewed lava and
gases – relieving
some pressure in
form Earths early
contained no free
with other bodies.
outer layer of the
planet cooled to
form a solid crust
when water began
Around 4.4 billion years
ago earth cooled enough
for water in the
atmosphere to condense
leading to millions of
years of rainstorms with
lightening – enough for
the rain to fill
Miller and Urey in 1953
i. Tested the Oparin-Haldane hypothesis by creating
conditions in which there was an
- Atmosphere above warmed sea water that contained H2
, and NH3
- Electrodes that simulated lightning.
- From this setup, they obtained organic compounds such as
amino acids that were collected in cooled water.
The experiment - organic molecules could be created out of
So…….why don’t we see this happening in today’s world?
Any organic molecules that are now formed would be
used up by living organisms.
If microorganisms were created from these organic
molecules in the early earth’s water bodies, this would have
been an example of spontaneous creation!
For much of history, man believed that living organisms could
be created spontaneously from non-living material (e.g. flies
from dead meat, geese from barnacles, etc.)
This idea was refuted by Louis Pasteur in the 1860’s.
3. RNA was probably the first hereditary material
a. Today, genetic information is usually stored as DNA, but
some organisms such as viruses use RNA to store info.
Geologic Time Scale with Key EventsGeologic Time Scale with Key Events
Glaciations; mammals increased; humans
Mammals diversified; grasses
Aquatic reptiles diversified; flowering plants; mass extinction
Dinosaurs diversified; birds
Dinosaurs; small mammals; cone-bearing plants
Reptiles diversified; seed plants; mass extinction
Reptiles; winged insects diversified; coal swamps
Fishes diversified; land vertebrates (primitive amphibians)
Land plants; land animals (arthropods)
Aquatic arthropods; mollusks; vertebrates (jawless fishes)
Marine invertebrates diversified; most animal phyla evolved
Anaerobic, then photosynthetic prokaryotes; eukaryotes,
then multicellular life
Key EventsEra Period Time
Summary of Evolution of Life
in the seas
(1 billion years)
(3.7 billion years)
EVIDENCES IN FAVOUR OF
1. EVIDENCES FROM MORPHOLOGY AND
2. EVIDENCES FROM EMBRYOLOGY
3. EVIDENCES FROM PALEONTOLOGY
EVIDENCES FROM MORPHOLOGY AND
Organs which have similar structure and embryonic origin but are
different in function are known as homologous organs and the
phenomenon is called homology.
This is due to their common ancestry. Homology is exhibited by
every organ system of vertebrates from lowest to highest including
The limbs of all vertebrates have a common structural plan. For
example forelimbs of variety of vertebrates like whale (flippers), bat
(patagia), birds (wings), horse (legs), man (hands).
The forelimb of man
The forelimb of man, used for handling things
The forelimb of frog
frog which is used for hopping
The forelimb of rabbit
rabbit which is used for leaping
The forelimb of whale
flippers of whale or seal which are used for swimming,
The forelimb of bat
wings of bat and bird used for flying.
Though forelimbs in these vertebrates are modified
for a variety of uses they are constructed on same
The pentadactlye plan
The skeleton of forelimb of all the vertebrates
contains humerus, radio-ulna, carpals,
metacarpals and phalanges.
The existence of homologous anatomical organ
structures implies a common evolutionary origin of
amphibians, reptiles, birds and mammals.
from astructure or
Trait of their
Certain organs which perform similar functions but are different in
development and origin are called analogous organs and the
phenomenon is called analogy.
For example the wing of an insect, the wing of a bat and the wing of
bird serve the same purpose of flying but their basic structure is
totally different. The wings of bat and birds are in fact modified forms
of forelegs and internally have the bones, nerves and blood vessels.
While the wings of insects are membranous, made up of thin flap of
chitin and supported by hollow tubes called veins.
Analogous organs show the convergent evolution. This means that
they evolved from different ancestry but developed similar
characteristics when put in similar habitats.
The evolution of
Vestigeal organs are those organs which are rudimentary and non-
functional in one group of animals while same organs are well
developed and functional in another group of animals.
Presence of vestigial organ is a vital evidence of organic evolution.
From the evolutionary point of view, vestigial organs are structures
that were well- developed, functional and necessary in ancestors
but are now in process of disappearance.
EVIDENCE FROM EMBRYOLOGY
Embryology is the science that deals with the study of the development of
embryo. A comparative study of embryology of different groups of animals
reveals certain features which provide evidence for organic evolution.
All multicellular organisms exhibit common pattern of development.
Comparative study of embryological development of different vertebrate like
fish, frog, lizard, chick, rabbit, human etc. shows similarity in early stages.
All vertebrates begin their life history as a single celled zygotezygote..
The zygote in all organisms undergoes series of division to produce
hollow ball of cells called blastulablastula..
Blastula further develops into gastrula.gastrula.
Gastrula in all the vertebrates has the same three primary germ layers,
namely ectoderm, mesoderm and endoderm.ectoderm, mesoderm and endoderm.
The young ones of fishes resemble very closely to the tadpole larvae of
amphibians. This shows the origin of amphibians from fish-like
The embryos of all the vertebrates at one stage or the other possess
pharyngeal gill slitspharyngeal gill slits, two chambered heartheart,, tailtail etc. Even human
embryo at eight weeks develops the gill slits and tail. But as the
development proceeds, gill clefts persist only in fishes and get closed in
The heart remains two chambered as in adult fish, but it becomes three
chambered in amphibians and most reptiles and four chambered in
crocodiles, birds and mammals.
The fish have a linear heart (sinus venosus, atrium, ventricle and conus arteriosus,
in that order input and output)
The heart of amphibians is modified from the heart of the fish, which was a linear
heart (sinus venosus, atrium, ventricle and conus arteriosus, in that order entry and
exit). The structure of the heart of amphibians is highly variable and has been
modified into a three-chambered heart with two receptacles, and one exit.
In reptiles, changes occurred in the transition environment, an aquatic to land. The
development of this movement involves changes in the structure of the heart to
adapt to new habitats. Reptiles has three chambers, two atria (right and left) and a
ventricle. The ventricle is incompletely subdivided into two chambers.
The circulatory system of birds is composed of a heart and a complex system of
veins and arteries. The major evolutionary advance that arise regarding their
relatives the reptiles is that the heart has four chambers, two atria and two ventricles.
Basically the operation of the mammalian heart is almost the same as birds. Their
hearts are divided into four compartments, two atria and two ventricles.
Comparative Study Of Heart
EVIDENCES FROM PALAEONTOLOGY
Palaeontology deals with the study of fossils.
What are fossils?
Usually plants and animals after death are decomposed by bacteria. But
sometimes they are preserved as fossils.
Fossils are any form of preserved remains or traces, thought to be derived
from living organisms. They provide concrete proof to the fact that variety of
animals and plants had lived in various geological ages of earth.
Examples of fossils
Dinosaurs: They existed on earth some 200 million years ago in the Jurassic
and Triassic periods. The dinosaurs known as Tyrannosaurs were the largest
living carnivore ever. It stood 20ft tall and was about 50 ft in length. It had
dagger like teeth measuring about six inches.
Archaeopteryx: Commonly known as reptile-bird, was a primitive bird-like
form. Its fossil was found in Germany in the rocks of Jurassic period, of about
150 million years ago. It was about the size of a crow and represented both the
characters of birds and reptiles.
characters of birds and reptiles
Like birds it had wings with feathers but, wings had clawed digits like
It also had beak like in birds. But, beaks had conical teeth like in
It had feathery tail like in birds. But, Tail was long like lizards and had
Body was covered with feathers like in birds but also had scales like in
It skeletal framework resembled more with reptiles than the birds.
It is probable that it used its forelimb for flying as well as climbing.
ArchaeopteryxArchaeopteryx provides a connecting link between reptile and birds.
Evolution of horse:
The history of evolution dates back some 60 m.y.a. in Eocene epoch and
involves about 20 genera. The phylogeny of horse starts with Eohippus, a small
fox like creature with longer head. It had shorter legs with four toes on each front
foot and three on each hind foot. It was a forest dweller, feeding upon soft
vegetations. It passed through evolutionary line to give rise to modern day horse.
5.Equus (modern day horse)5.Equus (modern day horse)
Major changes seen in modern horse are:Major changes seen in modern horse are:
•Enlargement and elongation of third digit
•Loss of other digits
•Elongation of fore part of skull
•Lengthening of limbs
•Increase in the length and mobility of neck
•Development of premolars and molars into high crowned
•Increase in body size
• There is variation in all species
• Some variation better adapted for the environment
• Natural selection (survival of the fitter): Individuals
with characteristics better adapted for the
environment will survive and have more viable
offspring than non adapted individuals.
Lamarck’s evidence and inference
species and fossils:
lines of descendents
• Use and disuse
• Inheritance of
What exactly is a theory?
• Explain which picture describes Lamarck’s
view and which pictures describes Darwin’s