XII BIOLOGY
CH-5 Origin
and Evolution of Life
Evidences of organic evolution- Human
Evolution.
The
process
of
evolution
is
supported by evidences provided
by
various branches of biology
Such
as -Palaeontology, comparative anatomy,
embryology and molecular biology .
A)Palaeontology : The study of
ancient life with help of fossils is called palaeontology. Fossils are the dead
remains of plants and animals that lived in past in various geological layers.
The study of fossils provides the most convincing and direct evidence of
evolution.
Fossils are formed in sedimentary rocks, amber (yellowish fossils resin),
ice, peat bogs etc.
During fossilization, the primitive forms of organisms occupy the older,
lower layers and the advanced forms occupy the upper, more recent layers of the
earth.
Types of
fossils :
1.Actual remains : These are most common type of
fossils. The plants, animals and human bodies got embedded in permafrost of
arctic or alpine snow remain preserved in the actual state, e.g. Wooly Mammoth
in Siberia. Amber or hardened resin contains preserved bodies of many insects
and arthropods
2.Moulds : These are the hardened encasements
formed in the outer parts of organic remains which later decayed leaving
cavities. Body parts of plants or animals later decays but the impression still
remains and becomes permanent. For example Foot – prints are formed in this
manner.
3.Cast : They are hardened pieces of mineral
matter deposited in the cavities of moulds.
4.Compressions : Internal structure is absent but a
thin carbon film indicates the outline of external features.
Significance
of Palaeontology :
•
It is useful in reconstruction of phylogeny.
•
It helps in studying various forms and structures of
extinct animals.
•
It provides
record of missing link between two groups of organsims.
•
It helps in the study of habits of extinct
organisms.
•
Palaeontology provides the following types of
evidences.
Connecting
link (missing link) :
•
These
are fossil forms
transitional or intermediate between
two groups of organisms. It shows some
characters to both
the groups. Thus it indicate the evolutionary
line Seymouria (between
amphibians and reptiles). e.g. Archaeopteryx (between reptiles
and birds).
Archaeopteryx
lithographica :
It is fossilized crow size toothed bird found from
jurassic rocks in Germany. It is known as missing link between reptiles and
birds because it shows characters of both.
Reptilian
characters :
•
Presence of long tail, claws and scales on the body.
•
Single headed ribs.
•
Abdominal ribs are present which look like ribs of
crocodile.
•
Jaws with homodont teeth.
•
Sternum without keel.
•
Bones are solid (nonpneumatic).
•
Hind limbs had four clawed digits.
Avian
characters :
•
Feathery exoskeleton.
•
Forelimbs are modified into wings.
•
Jaws are modified into beak.
•
Skull bone is completely fused.
•
Large rounded cranium.
•
Cranium with large orbits and a single condyle.
•
Limb bones are bird like.
•
Hind limbs with four toes first toe is opposible.
Thus from the above study it is very clear that birds evolved from reptiles. Huxley justified this by calling birds as glorified reptiles.
B) Morphology :
Morphology deals with study of external structures
while, anatomy deals with study of internal structures. From comparative study
of morphology and anatomy we can understand the evolutionary aspects in the
form of homologous, analogous and vestigeal organs.
a) Homologous organs :
Homologous
organs are those organs, which are structurally similar but perform different
functions.
For example :
1)Forelimbs of vertebrates such as lizzard, bird,
bat, horse, whale and man,all of them have humerus, radius-ulna, carpals, metacarpals
and phalanges in
their forelimbs. Forelimbs
of these vertebrates are
structurally similar but perform different functions.
2)Vertebrate heart and brain.
3) In plants, thorns of Bougainvillea and tendrils of
cucurbita represent homology.
The structural similarities between
the homologous organs indicates that they have a common ancestory.
Differences in homologous organs are examples of divergent evolution or adaptive radiation.
Analogous organs are those which are structurally dissimilar but functionally similar. These organs have external superficial similarity due to similar functions but they are different anatomically.
For e.g. wings of butterfly (insects) and of birds look superficially alike but they are no anatomically similar structures though they perform similar functions.
Other examples of analogous organs.
1. Eye of the octopus (mollusca) and of mammals. They differ in their retinal position, structure of lens and origin of different eye parts.
2. The flippers of penguins (birds) and dolphins (mammals).
3. Sweet potato (root modification) and potato (stem modification) store food in form of starch.
Analogous organs leads to convergent evolution i.e. different organisms shows same superficial structural similarities due to similar functions or habitat. These organs do not help to trace the common ancestry. Thus analogous organs do not have significant role in evolution .
Vestigeal organs are imperfectly developed and
non-functional, degenerate structures which were functional in some related and
other animals or in ancestors. The vestigeal organs are no longer required by
the organism but indicate the relationship with those organisms were these
organs are fully developed.
Examples
: Human beings
show some vestigeal organs like-
1. Presence of vestigeal nictititating membranes.
2. Presence of wisdom teeth (third molars).
3. Coccyx (tail bone) : It is greatly reduced in man
since the tail is of no use due to erect posture.
4. Vermiform appendix and the caecum. It is
functional in herbivorous mammals for digestion of cellulose. In man due to
eating of cooked food it has lost its function.
Presence of these vestigeal organs provide evidence that man has (evolved) descended from simple primates.
Cell is the basic structural and functional unit of
life in all organisms.
Similarities in proteins and genetic material
performing a similar function among diverse organisms gives evidence of a
common ancestry.
Basic metabolic activities also occur in a similar
manner in all organisms.ATP is the energy source in all living organisms.
Speciation :The process of
formation of a
new species from the per-existing species is called speciation.Species is
a group of
similar organisms that can
interbreed and produce
a fertile offspring in nature.
New species are formed by the following modes.
a) Intraspecific Speciation :
i)Allopatric
speciation :
Formation of a new species due to separation
of a segment of population from the original population by distanced or a geographical
barrier cutting across the species range.
e.g. creeping
glaciers, development of mountains. Migration
of individual also causes allopatric speciation. The mode
of evolution here is called adaptive radiatione.g. 14
different species of
finches in Galapagos islands
and several marsupial species in the Australian
continent.
ii)Sympatric
speciation:
Formation
of species within single population without geographical isolation. These are
formed due to reproductive isolation. e.g. Cichlid fishes in Lake
Victoria. Mutations are helpful in sympatric speciation.
b)Interspecific Speciation : Hybridisation :
Two
different species on crossing may give rise
to a new
species. e.g. Mule
is a hybrid produced by interbreeding between
a male donkey and a female horse.
Hinny is offspring of male horse and female donkey.
The planet earth with its present biodiversity was
not so when it was born. Study of fossils tells us that life forms were not the
same millions of years ago (MYA). Geological time scale is used to understand
the sequence of events that to
place on the
earth in different
ages over a period
of time. It
is divided into
six major ‘Eras’ Eras
ended with major
environmental changes on earth resulting into extinction and emergence
of new species. The eras are further divided into periods and epochs based on
minor but landmark events
in each era.
Table 5.15 shows the geological
time scale at a glance.
•
The
first life appeared
on the earth
some 2000 million years ago. It took billions of years for this process
to take place, from protenoids to first cells
the transition is
still a mystery. Once formed the living forms
diversified into various groups.
•
Life began in
the sea water andplants were the first living beings to adapt to terrestrial
life. Fishes evolved and diversified. The lobefin group of fishes too got
diversified.
•
Some
developed stout and strong fins and could go to land and come back to water.
•
The coelecanth
was considered a
living fossils. It was thought that lobefins are extinct
•
but the variety
of the lobefin
fish, called coelacanth was
caught in 1938 in South Africa.
•
Reptiles evolved from amphibians. They are the first true land vertebrates.
They do not have to go
to water for
reproduction. (Hint : think
of amphibian and
reptilian eggs).
But about 200
million years ago (mya) some reptiles moved back to
aquatic mode of life and gainedfish like form as seen in Ichthyosaurs.
•
Thegiant reptiles like Dinosaurs once
dominated the earth but are now extinct.
When was this?
•
Around
65 million years ago! why it must have happened? Can we give affirmative reason
for this extinction? At around the same time giant ferns were present on earth.
However, they also became extinct and got converted to fossil fuels.
•
How
this must have happened? Decline of giant reptiles marked the begining of
dominance of mammals. These viviporous organisms were more intelligent.
•
They
could avoid danger. Early mammals were small shrew like organisms, but this
group diversified. Whales, dolphins,
seals and sea cow live in water, bats are the flying mammals, Kangaroo rats are fossorial, lemurs are arboreal.
•
Major
physical disturbances led
to phenomenon like the continental
drift i.e. continents moved from
their original place.
•
As
a consequence when south America joined north America, ancestral forms of
horse,hippos, rabbits, etc. native to south america were dominated by north
American animals.
•
At
the same time, marsupial diversified into different habitats in Australia.
These surviveddue to lack of competition.
•
From
the fossil records we can trace complete evolutionary history of
horse,elephant, dog, etc. Human beings are the most evolved animals on the
earth.
Human Evolution:
1)It
has been traced that the human evolved from a tree dwelling shrew like animal.
2)This
process began in Paleocene epoch.
3)During
this period, dwindling forests forced arboreal mammals to adapt to life on
land.
4)
This descent must have been the driving force. In the following chart, it can
be seen that we are most closely related to gibbons, chimpanzees and gorillas.
5)The
major evolutionary trends in transition from ape to man are considered further.
.
6)Special
characteristics have been acquired by man in the course of evolution.
7))Major
changes that took place in evolution of man include increase in size and complexity
of brain and enhanced intelligence, increase in cranial capacity, bipedal
locomotion, opposable thumb
8)erect
posture, shortening of forelimbs and lengthening of hind limbs, development of
chin, broadening of pelvic girdle, development of lumbar curvature, social and
cultural development (articulated speech, art, development of tools, etc.).
9)Cranial
capacity of human begins increased over a period of time and large size of
frontal lobe helped in development of high forehead.
10)Increase
in intelligence necessitated physical development so that body and brain could
be used effectively and productively.
11)Freedom
of forelimbs from locomotory function and opposable thumb led to better
utilization of hands for holding objects effectively and development of motor
skills etc.
12)Bipedal
locomotion, upright posture coupled with stereoscopic vision helped man to move
around safely on land.
13)Evolutionary
history of man was traced with the help of fossil remains found over a period
of time. , erect posture, shortening of forelimbs and lengthening of hind
limbs, development of chin, broadening of pelvic girdle, .
14)Some
of our ancestors and their evolutionary history is shown in the table. .
15)The above table clearly shows the gradual increase in cranial capacity, shape of skull and dentition of the ancestral humans till date. our journey of evolution still continues…
Click for here PDF open